Evolutionary Forensic Psychology

EVOLUTIONARY FORENSIC PSYCHOLOGY
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EVOLUTIONARY FORENSIC
PSYCHOLOGY
Darwinian Foundations of
Crime and Law
Edited by
Joshua D. Duntley and Todd K. Shackelford
2008
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Library of Congress Cataloging-in-Publication Data

Evolutionary forensic psychology : Darwinian foundations of crime and law/
edited by Joshua D. Duntley, Todd K. Shackelford.
p. ; cm.
Includes bibliographical references and index.
ISBN: 978-0-19-532518-8
1. Forensic psychology. 2. Evolutionary psychology. I. Duntley, Joshua. II. Shackelford,
Todd K. (Todd Kennedy), 1971–
[DNLM: 1. Evolution. 2. Forensic Psychiatry. 3. Adaptation, Psychological.
4. Crime—psychology. 5. Crime Victims. 6. Genetics, Behavioral. W 740 E93 2008]
RA1148.E98 2008
614'.15—dc22 2007051354
9 8 7 6 5 4 3 2 1
Printed in the United States of America
on acid-free paper
Dedications
Joshua Duntley dedicates this volume to Helena and Paula. Todd Shackelford
dedicates this volume to Viviana, Helayna, Avelina, Rex, Ethan, and Mackenzie.
v
Preface
Evolutionary Forensic Psychology is not a completely accurate title for this volume.
It may suggest to some readers that the topics explored here are representative of
a subdiscipline of the field of forensic psychology. This is not the case. Evolution by
natural selection is the only known process capable of generating the complex adap-
tations that compose the human mind. Because all psychological mechanisms owe
their existence to evolutionary processes, there is no such thing as a nonevolution-
ary forensic psychology. All forensic psychology is inherently evolutionary, whether
or not it is explicitly acknowledged. Our understanding of forensic psychology can
benefit from knowledge of the causal processes that designed our psychological
mechanisms.
The book is titled Evolutionary Forensic Psychology because the profoundly im-
portant insights that evolutionary perspectives provide are relatively new. This vol-
ume presents a critical introduction to the application of evolutionary perspectives
to prominent issues in forensic psychology, exploring theories and research findings
that will help to move the field of forensic psychology rapidly forward.
Forensic psychology encompasses a large and diverse range of issues. In con-
structing this volume, we sought contributions from experts on topics that are of the
greatest relevance to the field. Each chapter demonstrates how evolutionary logic has
enriched our understanding of topics and generated new hypotheses and research
findings, progress that would not have occurred without the unique contribution of
a Darwinian perspective.
Although most of the chapters explore the nature of psychological mechanisms
that produce criminal behavior, an evolutionary perspective has the power to inform
research across domains of forensic psychology. It can help us to differentiate between
crimes resulting from psychopathology and those that are the product of cognitive
adaptations functioning as they were designed to function. It can help us to under-
stand the origins and evolved functions of cognitive adaptations that produce crime
and the psychological mechanisms that generate the perception that some behaviors
are criminal. An evolutionary perspective also can inform our understanding of why
some crimes are considered to be worse than others, why some people are thought
to deserve longer sentences than others who committed the same crime, and why
vii
viii Preface
sex differences are pervasive in the commission and perceptions of crime. Current
and future forensic psychological research informed by an evolutionary perspective
will have an impact on the prevention of crime, how laws are written and enforced,
how clinical forensic psychologists and forensic psychiatrists evaluate criminals, the
selection of juries and the methods of presenting information to them, and the kinds
and structure of punishments in the penal system. The ultimate goal of this volume
is much more ambitious than to provide information about how evolutionary theory
can inform forensic psychology—we hope it will provide a spark that will ignite theo-
retical innovation and new programs of research in this important area.
Acknowledgments
We would like to thank the contributors for their brilliant work in the outstanding
chapters they wrote for this volume. Our special thanks go to Professor David M.
Buss for his mentorship in evolutionary psychology and its application to the dark
side of human nature. We would also like to thank Andy Thomson for his insightful
feedback during the conception of the volume and on early drafts of the introduc-
tory chapter of the book. We extend our gratitude to our editor, Lori Handelman,
assistant editor, Jenna Hocut, and production editor, Angelique Rondeau, at Oxford
University Press for their insights and support during the production of the volume.
Finally, we would like to thank our families for their enduring patience and support,
which made it easier for us to spend hours away from them to complete work on this
book.
ix
Contents
Contributors xiii
Part One Introduction and Overview

1 Evolutionary Forensic Psychology 3
TODD K. SHACKELFORD AND JOSHUA D. DUNTLEY
2 The Promise of Evolutionary Psychology for

Criminology: The Examples of Gender and Age 20
ANTHONY WALSH AND KEVIN M. BEAVER
Part Two Adaptation and Violent Crimes

3 The Origins of Homicide 41
JOSHUA D. DUNTLEY AND DAVID M. BUSS
4 Intimate Partner Violence 65

AARON T. GOETZ, TODD K. SHACKELFORD,
VALERIE G. STARRATT, AND WILLIAM F. MCKIBBIN
Part Three Adaptation and Sex Crimes

5 The Evolutionary Psychology of Sexual Harassment 81
KINGSLEY R. BROWNE
6 Evolutionary Psychological Perspectives on Rape 101

WILLIAM F. MCKIBBIN, TODD K. SHACKELFORD,
AARON T. GOETZ, AND VALERIE G. STARRATT
7 The World’s Oldest Profession: Evolutionary

Insights into Prostitution 121
CATHERINE SALMON
xi
xii Contents
Part Four Adaptation and the Production of Criminal Behavior

8 Risk-Taking, Antisocial Behavior, and Life Histories 139
SANDEEP MISHRA AND MARTIN L. LALUMIÈRE
9 Theft 160
SATOSHI KANAZAWA
10 In Cold Blood: The Evolution of Psychopathy 176

MARTIN L. LALUMIÈRE, SANDEEP MISHRA, AND GRANT T. HARRIS
Part Five Victims of Crime

11 Victim Adaptations 201
JOSHUA D. DUNTLEY AND TODD K. SHACKELFORD
12 The Evolution of a Sense of Justice 230

DENNIS L. KREBS
Part Six Applications and Future Directions

13 Reducing Crime Evolutionarily 249
LEE ELLIS
14 Did the Victim Deserve to Die? Darwin Goes to Court 268

J. ANDERSON THOMSON JR.
Author Index 287

Subject Index 301
Contributors
KEVIN BEAVER SATOSHI KANAZAWA

College of Criminology and Interdisciplinary Institute
Criminal Justice, Florida State of Management, London
University, Tallahassee, Florida School of Economics and
Political Science, London,
KINGSLEY BROWNE
England
Wayne State University Law
School, Wayne State University, DENNIS KREBS
Detroit, Michigan Department of Psychology,
Simon Fraser University,
DAVID BUSS Burnaby, British Columbia,
Department of Psychology, Canada
University of Texas at Austin,
Austin, Texas MARTIN LALUMIÈRE
Department of Psychology,
JOSHUA D. DUNTLEY University of Lethbridge,
Criminal Justice Program, Lethbridge, Alberta,
Richard Stockton College of Canada
New Jersey, Pomona, New Jersey
WILLIAM F. MCKIBBIN
LEE ELLIS Department of Psychology,
Department of Sociology, Florida Atlantic University,
Minot State University, Davie, Florida
Minot, North Dakota
SANDEEP MISHRA
AARON T. GOETZ Department of Psychology,
Department of Psychology, University of Lethbridge,
California State University- Lethbridge, Alberta,
Fullerton, Fullerton, California Canada
GRANT T. HARRIS CATHERINE SALMON
Penetanguishene Mental Department of Psychology,
Health Care, Penetang, University of Redlands,
Ontario, Canada Redlands, California
xiii
xiv Contributors
TODD K. SHACKELFORD ANTHONY WALSH

Department of Psychology, Department of Criminal Justice
Florida Atlantic University, Administration, Boise State
Davie, Florida University, Boise, Idaho
VALERIE G. STARRATT
Department of Psychology,
Florida Atlantic University,
Davie, Florida
Center for the Study of Mind
and Human Interaction,
University of Virginia,
Charlottesville, Virginia
PART ONE
INTRODUCTION AND OVERVIEW

1
Evolutionary Forensic Psychology
TODD K. SHACKELFORD AND JOSHUA D. DUNTLEY
Forensic psychology is a burgeoning field in the social and behavioral sciences. It

explores the application of the science and the profession of psychology, including
questions and issues relating to the law and legal systems. Research and practice in
forensic psychology have been approached from a broad range of theoretical per-
spectives, from psychoanalytic to behavioral-genetic. It also has explored issues
ranging from the criminal mind to the origins of rules that govern the structure of
societies. Despite these achievements, however, differences in theoretical perspec-
tives in forensic psychology have led to an often splintered and incomplete treat-
ment of the field.
Darwin’s (1859) theory of evolution by natural selection is the theoretical frame-
work that unifies the field of biology. It unites research and understanding of the
development, control, and organization of behavior. It informs domains of research,
including communication, territoriality, parenting, and social behavior. The study
of humans, which includes all of the social sciences, is part of the field of biology.
Evolutionary forensic psychology is a necessary step toward a unified and complete
understanding of psychology and the law.
Why Evolutionary Forensic Psychology?
Evolutionary psychology uses an adaptationist approach to explore the cognitive

foundations of behavior. Over the deep history of humankind, individuals faced spe-
cific recurrent problems, generation after generation, that affected how long they
survived, how well they lived, and, of greatest relevance for natural selection, how
successful they were at reproducing. Some individuals had characteristics that made
them better able to solve these problems than others. The better problem-solvers
were more likely to survive and reproduce. When there was a genetic basis for
3
4 Introduction and Overview
characteristics contributing to better problem solving, the genes that contributed to

the development of those characteristics were passed on in greater numbers than the
genes coding for less successful characteristics. A beneficial characteristic providing
even a 1% advantage in reproduction (fitness advantage) over other, less beneficial
characteristics could completely replace the poorer characteristics in a few thousand
generations (Nilsson & Pelger, 1994). Over the millions of generations of human
evolutionary history, characteristics that helped individuals to solve recurrent prob-
lems that affected their fitness were gradually sculpted into functional adaptations
by the process of natural selection.
Evolutionary processes undoubtedly shaped physiological characteristics to help
solve problems of survival and reproduction. The skin is well adapted to protecting
the vital organs beneath from injury and infection. The lungs, with their vast surface
area and moist membranes, are marvelous adaptations for extracting oxygen and
releasing carbon dioxide. The heart is a powerful pump that functions to circulate
oxygen and other nutrients to cells throughout our bodies. Just as selection shaped
physiological adaptations with specific problem-solving functions, it also shaped the
structure of thoughts, preferences, desires, attitudes, and emotions to guide behav-
iors toward solving historically recurrent problems that affected reproductive fitness.
The adaptationist approach used by evolutionary psychologists uses knowledge of
recurrent ancestral problems to generate hypotheses about the functions and forms
of cognitive mechanisms in human minds.
Humans do not have specialized horns for fighting rivals or teeth for incapacitat-
ing prey. Instead, our minds house a large complement of specialized cognitive adap-
tations that coordinate patterns of behavior capable of solving such problems. Tooby
and DeVore (1987) argue that humans occupy the “cognitive niche” in earth’s eco-
systems. They propose that our place in this unique niche is largely the result of the
importance of social interaction over the course of human evolutionary history. In-
teracting with others can facilitate finding solutions to adaptive problems. However,
sociality can also create unique sources of conflict. There would have been signifi-
cant selection pressure over human evolutionary history in favor of strategies that
coordinate cooperation with others in contexts where working together was more
beneficial than going it alone (Trivers, 1971), such as hunting large game, building
shelter, and defending against attacks from rival groups. There also would have been
significant selection pressure for the evolution of strategies to best others in contexts
of conflict over scarce resources (Buss & Shackelford, 1997a), including competition
for attractive mates and territories.
One general strategy for winning contests over limited resources is inflicting
costs on rivals. Damaging rivals in competition for resources makes the net benefit
of controlling the resources lower for them. Inflicting enough damage can make
the costs of competition for a scarce resource exceed the benefits of controlling the
resource, at which point the most adaptive strategy is to disengage from competi-
tion, leaving the resource to the cost-inflicting individual. The potential benefits of
cost-inflicting strategies in contexts of competition for resources would have created
Evolutionary Forensic Psychology 5
selection pressure for the purposeful infliction of costs as a strategy to outcompete

rivals. A special set of adaptations may have evolved for this purpose in psychopaths
(see Chapter 10 of this volume).
Several sources of conflict between individuals have been recurrent over human
evolutionary history. Understanding the nature of recurrent conflicts between in-
dividuals in our evolutionary past can give us insight into the form and function
of manifest conflicts between people today. In what follows, we explore some of the
most important sources of conflict for our ancestors and briefly discuss their implica-
tions for the field of evolutionary forensic psychology.
Interindividual Conflict
Conflict over Status

One broad context of conflict between individuals is the struggle for status. All avail-
able evidence indicates that men who are high in status have sexual access to a
greater number of women than do men who are low in status (Betzig, 1993; Buss,
2003a; Hill & Hurtado, 1996; Perusse, 1993). Men who are high in status also are
more likely than their low-status rivals to seek out younger and more fertile women
(Grammer, 1992) and to marry women who are more attractive (Taylor & Glenn,
1976; Udry & Eckland, 1984). An individual in a group cannot ascend a status
hierarchy without displacing someone above, bumping that person to a lower posi-
tion and inflicting costs associated with status loss. The potential for large fitness
gains associated with increases in status would have created selection pressures for
specialized cognitive adaptations that lead to hierarchy ascension and other cogni-
tive mechanisms to prevent large status falls. Because a greater number of mating
opportunities enhances the reproductive success of men more than that of women,
there should be greater status striving among men than among women. Research
across the life span has found this to be the case, with men placing greater impor-
tance on coming out ahead and women focusing more on maintaining social har-
mony (Maccoby, 1990; Pratto, 1996; Whiting & Edwards, 1988).
Conflict over Material Resources

A second context of ancestrally recurrent conflict was fighting over material re-
sources, specifically resources that helped to solve recurrent adaptive problems. Such
resources included territory, food, weapons, and tools. There also was conflict to gain
the favor of individuals who were the suppliers of material resources, examples of
which include the conflict between siblings for investment from their parents and
elder kin (Parker, Royle, & Hartley, 2002) and conflict between women for access
to men with resources (Buss, Larsen, & Westen, 1996; Buss, Larsen, Westen, &
Semmelroth, 1992). In general, the scarcer and more valuable the resource in terms
of its contribution to an individual’s reproductive success, the greater the conflict

should be between individuals over access to the resource.
Conflict over Mating Resources

Whereas the minimum obligatory parental investment for women is nine months,
the minimum investment for men can be as little as a few moments. Because
women’s minimum investment in reproduction is greater, the costs of a poor mate
choice are greater for women than for men (Trivers, 1972). As a result, there is con-
flict between the sexes about the timing of sexual activity. Because sex is less costly
for men than for women, they desire sexual activity much earlier in a relationship
than do women (Werner-Wilson, 1998). Men also desire a greater number of sexual
partners than do women (Schmitt, Shackelford, Duntley, Tooke, & Buss, 2001) and
are more amenable to short-term, uncommitted sex (Buss, 2003a). These differences
in men’s and women’s sexual desires are a clear source of evolutionarily recurrent
conflict between the sexes. One hypothesized result of this conflict is sexual harass-
ment, a topic explored by Kingsley Browne in Chapter 5 of this volume. Another is
the existence of female prostitution. In Chapter 7 of this volume, Catherine Salmon
provides insight into the origins of this cross-culturally ubiquitous phenomenon.
There also is conflict within each sex for access to members of the opposite
sex. Women are biologically limited in the number of offspring they can have in
their lifetime. In contrast, men are limited reproductively only by the number of
female partners they can successfully impregnate. Given an equal sex ratio, men
who impregnate more than one woman or who have more than one long-term
partner at any time effectively deprive rivals of a potential mate. Human polygy-
nous mating systems, in which males may have more than one mate at a time, lead
to greater reproductive success for some men and zero reproductive success for
others. Over evolutionary time, the greater reproductive variance among men se-
lected for more extreme male strategies to acquire mates. Daly and Wilson (1988)
argue that “risky strategies” such as the use of violence are one outcome of this
unique selection pressure on men. Over evolutionary time, men who failed to take
risks would have been at a disadvantage in competition for mates and, therefore,
less likely to leave descendants. In Chapter 8 of this volume, Martin Lalumière re-
views theoretical and empirical work on risk tolerance and risk avoidance from a
life history perspective.
Conflict and Kin Selection
Evolutionary researchers have documented that conflict is usually tempered by ge-

netic relatedness. Genetic relatives are less likely to experience conflict over resources
than are nonrelatives. Closer genetic relatives do not experience conflict as often as
or to the degree that more distant relatives do (see Buss, 2004, for a review). This is
argued to be the evolutionary product of kin selection. According to kin selection

theory (Hamilton, 1963; Maynard Smith, 1964), humans and other organisms have
evolved to act more favorably toward their genetic relatives than toward nonrela-
tives. If genes that code for altruism exist in an individual, they also are likely to be
present in the individual’s genetic relatives. Natural selection would favor behaving
altruistically toward genetic kin who can convert that investment into reproduction,
which translates into the production of additional copies of shared genes.
Daly and Wilson (1988) applied the logic of kin selection theory to family rela-
tionships. They hypothesized that genetic relatedness creates a special kind of fam-
ily bond. Genetic relatives, they argue, should behave more altruistically toward
one another than family members who are not genetically related, such as step-
parents and stepchildren. To put it another way, stepfamily members should be in
greater conflict with each other than genetic family members. To test their hypoth-
esis, Daly and Wilson secured homicide records from the United States and Canada.
They used homicide as an assay of conflict between family members. They discov-
ered that children are between 40 and 100 times more likely to be murdered when
they reside in a home in which a stepparent is present than when residing with
two genetic parents. Adult children are also more likely to kill a stepparent than a
genetic parent. Daly and Wilson propose that the greater conflict between stepfam-
ily members is produced by an activation failure of psychological mechanisms that
generate closeness between genetic relatives. In Chapter 4 of this volume, Aaron
Goetz and Todd Shackelford explore the conflicts between intimate partners that
can lead to violence.
Specific Cost-Inflicting Strategies to Outcompete Rivals
Theft
One strategy of cost infliction that may be used to gain an advantage in competition
for resources is theft (see Cohen & Machalek, 1988) or otherwise cheating rivals out
of their resources. A valuable weapon can be stolen and used against its owner. Valu-
able territory can be encroached upon and its vegetation, water, shelter, and wild-
life exploited (Chagnon, 1983). Mates can be poached from an existing relationship
(Buss, 2000; Buss, 2003a; Schmitt & Buss, 2001). Public knowledge that a person
has been cheated or has had valuables stolen also can affect the individual’s reputa-
tion. The person may be viewed by others as someone who is easy to exploit, perhaps
increasing the likelihood that others will attempt to cheat or steal from the person in
the future. An easily exploitable person will likely be less attractive to members of the
other sex. In short, cheating and the theft of resources can be effective strategies of
cost-infliction for the gain of reproductively relevant resources, including material
resources and status. The topic of theft is explored by Satoshi Kanazawa in Chapter 9
of this volume.
Vigilance and Violence in Romantic Relationships

Buss and Shackelford (1997b) found that men and women engage in tactics that
range from vigilance to violence to defend their relationships. Fueled by jealousy, an
emotion absent from contexts of material-resource theft, men’s tactics of defending
against mate poachers were found to be different from women’s. Men are more likely
to conceal their partners, display resources, and resort to threats and violence, espe-
cially against rivals. Men also are more likely to use tactics of submission and self-
abasement, groveling or promising their partner anything to get her to stay. Women
are more likely to enhance their appearance and to induce jealousy in their partners,
demonstrating their desirability by showing that they have other mating options
available to them.
Rape
Rape, a topic explored in Chapter 6, is a strategy aimed directly at obtaining repro-
ductive resources at a cost to the victim. A rapist may benefit from the behavior by
siring offspring that he may not have otherwise produced. Not only does rape inflict
terrible emotional (Block, 1990; Burgess & Holmstrom, 1974) and physical (Geist,
1988) costs on women, it also inflicts fitness costs by bypassing female mate choice
mechanisms (Buss, 2004). Although scholars have concluded that there is not
enough evidence to determine whether men have adaptations to rape (Buss, 2003a,
2004; Symons, 1979), ethnographies and historical records suggest that rape oc-
curs cross-culturally and was recurrent over the deep time of our evolutionary his-
tory (Buss, 2003a). The occurrence of rape would have created selection pressure for
strategies to avoid and resist it.
Violence and Homicide

Unlike some other strategies of inflicting costs, violence and homicide represent more
flexible solutions to conflicts between individuals. Violence can be used to damage the
status of a competitor or as an instrumental measure to facilitate theft. Homicide can
free resources from the control of a rival and permanently eliminate cost-inflicting
competitors.
Using violence to injure rivals can be an effective strategy to remove them from
competition for a valuable resource. A healthy individual can compete more effec-
tively than the rivals he injures. Competitors may be more likely to avoid or to drop
out of competition with an individual who has injured them in the past. An indi-
vidual who is capable of inflicting greater injuries to his competitors than they can
inflict on him may gain a reputation of being difficult to exploit. This reputation may
protect an individual against violent confrontations and grant easier access to re-
sources with less resistance from competitors.
Daly and Wilson (1988), among others (Chagnon, 1988; Ghiglieri, 1999), have
documented that violence and homicide can be outcomes of intraspecific competition.
Competition for mates (Buss, 2000; Weekes-Shackelford, Shackelford, & Buss, 2003),
competition for status (Daly & Wilson, 1996; Shackelford, 2005), and competition
for resources (Daly & Wilson, 1988; Kruger & Nesse, 2004) have been documented to
be sources of violent conflict with the potential to lead to homicide. Even homicides
that result from seemingly trivial altercations between two individuals who did not
previously know one another can be understood through the lens of evolutionary
psychology (Buss, 2005; Daly & Wilson, 1988; Ghiglieri, 1999). For much of our evo-
lutionary history, social reputation carried long-term repercussions that are largely
missing from modern societies. An individual’s social sphere was smaller in the past,
typically consisting of several dozen individuals. The winner of confrontations would
garner a reputation as someone who should not be trifled with, and thus would have
fewer battles to fight in the future. The loser would garner a reputation as a person
who can be exploited and would either have to fight again or cede his resources in
future conflicts. David Buss and Joshua Duntley address homicide in Chapter 3 of this
volume, and Aaron Goetz and Todd Shackelford explore violence in families in par-
ticular in Chapter 4.
Coevolution
From an evolutionary perspective, all crimes can be thought of as strategies that

function to benefit the criminal at the cost of the victim. Evolutionary theories of
crime are fundamentally coevolutionary theories of adaptations that produce crimi-
nal behavior and counteradaptations to defend against being victimized (Buss &
Duntley, 2006; Duntley, 2005). Haldane (1932, 1949a, 1949b) was among the first
to recognize the importance of coevolutionary arms races in his discussion of the
influence of infectious diseases on human evolutionary history. He pointed out that
infectious pathogens possess adaptations that enable them to use human tissues to
reproduce and that we have evolved counterstrategies to prevent our being invaded
by pathogens.
Antagonistic coevolutionary arms races are part of the evolutionary history of
all species. They can occur between species, such as the lynx and the hare, or within
species between competing adaptations in contexts of social conflict. Such coevolu-
tionary arms races have likely shaped a large number of complex adaptations. They
can create massive selection pressures capable of producing quite rapid evolutionary
change (see Altizer, Harvell, & Friedle, 2003; Phillips, Brown, & Shine, 2004).
The Fitness Costs of Being Victimized

It is a truism that victims of crime incur fitness costs. Individuals who are victimized
are at a competitive disadvantage to those who are not. A victim of homicide pro-
vides an extreme example, the fundamental logic of which applies to all forms of vic-
timization. A murder victim’s death has a much larger impact on his or her inclusive
fitness than just the loss of the genes in the person’s body. Death by homicide often
has cascading deleterious effects on a victim’s inclusive fitness, including (a) the loss
of future reproduction; (b) damage to existing children from lack of protection, in-
vestment, or the addition of stepparents; and (c) damage to the victim’s extended kin
group from diminished investment and a reputation for being exploitable.
A murder victim’s fitness losses can potentially be translated into a rival’s fitness
gains. The residual reproductive and parenting value of the mate of a homicide victim
may go to a rival, often at the expense of the victim’s children with that mate, who may
become stepchildren, a condition associated with an increased risk of abuse and homi-
cide (Daly & Wilson, 1988). The murder of a man or woman creates an opening in a
social group’s hierarchy into which a rival can ascend. The children of rivals who had
two surviving genetic parents would thrive relative to the victim’s children, who would
be deprived of the investment, protection, and influence of two genetic parents.
Victim Defenses
The great costs resulting from being the victim of crime would have selected for ad-
aptations to (a) avoid being victimized; (b) punish conspecifics who damage individu-
als’ inclusive fitness by inflicting costs on others, their genetic relatives, mates, or
coalitional allies; and (c) eliminate or otherwise render impotent individuals who
presented a persistent threat of inflicting costs in the future on the larger social
group of which an individual, his kin, and his coalition are a part (e.g., psychopaths,
hostile members of other groups). Inflicting costs on cost-inflicting rivals, including
murdering them, is hypothesized to be part of an evolved strategy to avoid or stanch
the inclusive fitness costs of being victimized by another individual or group (Buss &
Duntley, 2006, in press; Duntley, 2005; Duntley & Buss, 2005).
To avoid being victimized, intended victims must be sensitive to cues indicative of
situations in which someone else might benefit from inflicting costs on them. Insight
into the likelihood that one will be the victim of crime before the crime occurs re-
quires that the majority of crimes be committed in predictable sets of circumstances.
If particular crimes recurrently occurred in response to predictable sets of circum-
stances over our evolutionary history, selection would be for defense mechanisms ca-
pable of recognizing those circumstances and trying to change or avoid them. These
and other aspects of victim adaptations are explored in Chapter 11. The evolution
of such defense mechanisms, in turn, would have selected for strategies that could
circumvent the evolved defenses. In this way, adaptations to avoid being victimized
would have served as selection pressures for the refinement of psychological adapta-
tions for inflicting costs over evolutionary time. These new cost-inflicting adaptations
would have selected for further refinements in defense adaptations—cost-infliction
and defenses against victimization locked in a perpetual, antagonistic, coevolution-
ary arms race across generations, as illustrated in Figure 1.1.
Demonstration of the existence of crime-specific defenses against victimization
that appear to have been designed to defeat corresponding criminal strategies would
Figure 1.1. Adaptations that produce criminal behavior create selection pressure for the
evolution of counter adaptations in victims, which in turn create novel selection pressures
for the evolution of counter-counter adaptations in criminals
provide evidence that (a) the crime was likely a recurrent feature of ancestral envi-
ronments, (b) the criminal strategy occurred in predictable patterns over our evolu-
tionary history and, therefore, (c) there may be adaptations specifically for the crime.
The greater the corresponding specificity of design in the psychologies of crime and
defenses against crime, the stronger the evidence that the two have had a coevolu-
tionary relationship, and the greater the support for the existence of adaptations for
criminal behavior.
There are no perfect solutions to any adaptive problem. Every adaptation is a
compromise between the numerous different adaptive problems an organism faces.
At the same time as an individual selection pressure operates to shape or refine an
adaptation in a certain direction, other selection pressures push and pull on the evo-
lutionary trajectory of its form and precise function, diverting it away from its opti-
mal course for any single adaptive problem. It is unlikely that there would be enough
stability in the selection pressures of a coevolutionary arms race, in combination
with the other adaptive problems of survival and reproduction, for perfect adaptive
solutions to evolve. Therefore, it is unlikely that adaptations that produce criminal
behavior and adaptations to defend against being victimized will lead on every occa-
sion to the outcomes for which they were designed. For selection to favor them, they
need only lead to greater reproductive success than competing designs on average
across the individuals in a population over evolutionary time.
Coevolutionary arms races may involve the competing interests of more than two
individuals. This is particularly apparent in contexts involving mating (Buss, 2003b).
Coevolutionary arms races involving more than two individuals can occur, for exam-
ple, when a woman who is married to one man becomes interested in another man.
There is selection pressure on the woman to be faithful to her husband so as not to lose
his investment or risk violent retaliation for her affair. There is also selection pressure
on the woman to obtain better or different genes from those possessed by her husband
or acquire additional investment from another man. Female adaptations to engage in
infidelity in some contexts would select for male adaptations to stanch women’s infi-
delities, especially when a man and woman are in a long-term mating relationship.
One hypothesized male adaptation for dealing with infidelity is to inflict costs on the
woman—domestic violence, stalking, marital rape, or even murder.
Female adaptations that produce infidelity in certain contexts would select for ad-
aptations in men who are not the woman’s long-term mate to lure women or aid them
in being unfaithful. These male adaptations that promote female infidelity would, in
turn, create selection pressure on men’s long-term mating psychology for adaptations
to prevent other men from poaching away their long-term partners, including the
infliction of costs on the mate poacher, the cheating mate, or both. Any adaptation
that results from what Buss (2003b) refers to as “triadic coevolution” is shaped by se-
lection pressures created by the adaptations of the two other individuals involved, as
illustrated by Figure 1.2. Newly evolved psychological mechanisms that benefit any
one individual in the triadic relationship impose new selection pressures on both of
the other individuals. Adaptations in long-term males that lead to cost-infliction as
a strategy for dealing with a partner’s infidelity, for example, would select for defense
adaptations in both romantic partners and poachers. One possible evolved defense
against being victimized is to anticipate victimization and preemptively inflict costs
on the victimizer. This would select for defense adaptations in victimizers. These de-
fense adaptations are hypothesized to factor into the decision calculus responsible
for motivating or inhibiting cost-inflicting patterns of behavior in men who discover
that their partners have been unfaithful.
The Importance of Time and Opportunity

Time was likely an important and potentially powerful selection pressure on the psy-
chology of criminal behavior and could have been so in at least two ways. First, the
time available to solve a problem may increase or decrease the likelihood with which
criminal behavior will be chosen as a solution. The amount of time that people have
to react to different adaptive problems varies from situation to situation. Solutions to
adaptive problems also vary in terms of how much time they require to be enacted
effectively. The interaction of time with adaptive problems and solutions would have
created selection pressure for psychological mechanisms capable of calculating the
Figure 1.2. When three individuals have conflicting interests in the same adaptive problem
domain, an adaptation in one individual can simultaneously create selection pressure on
two (or more) other individuals. The counter adaptations that evolve in each of the two other
individuals as a result can then create antagonistic selection pressure on the other two. This
triadic coevolutionary process can carry on indefinitely through time, as long as there is
recurrent conflict between those involved for some fitness-relevant resource.
amount of time available to solve a given problem (Buss & Duntley, in press). Esti-
mates of the amount of time available would have been a source of input for making
decisions about which adaptive solution should be employed.
There were likely recurrent contexts of conflict between people who had both
a very large potential fitness impact and a narrow time frame in which to enact a
solution. Such situations could have selected for some of the risky, cost-inflicting
strategies we label as crimes. Examples may include homicides that are committed in
self-defense. A woman who is cornered in the kitchen by her abusive husband may
reach instinctively for a knife to defend her life with—by ending his. In such situa-
tions, homicide may not be the most beneficial possible solution to the problem, but
it is the least costly of available alternatives.
The presentation of rare opportunities that put cost-inflicting competitors at a
significant disadvantage in highly fitness-relevant situations, if recurrent, could also
have acted as selection pressures for the adoption of risky, criminal strategies (Buss &
Duntley, in press). For example, a man who walks in on his wife and a rival in the
act of having sex is simultaneously assaulted with an extremely significant adaptive
problem and presented with a rare opportunity. The rival is naked and distracted,
making him vulnerable to attack. The husband may never again have the rival at
such a disadvantage. It would be surprising if selection did not fashion adaptations

to employ homicide to exploit such rare contexts.
There also may have been recurrent adaptive problems involving social conflict
that required a greater amount of time to effectively enact a strategy involving crimi-
nal behaviors (Buss & Duntley, in press). Cost-inflicting strategies that require the
coordination of the efforts of multiple individuals require more time to deploy than
strategies perpetrated by one person. Examples include contexts of coalitional ag-
gression or tribal warfare. The raids of rival groups perpetrated by the Yanomamo
in order to kidnap women and capture resources (Chagnon, 1988) could not be suc-
cessful without coordination, which requires a larger window of time than many
situations in which individuals commit single murders.
A second way that time could have been an important selection pressure for the
evolution of adaptations that produce criminal behavior rests on the importance
of responding to costly assaults from others in a timely fashion (Buss & Duntley, in
press). Most people are familiar with the proverb, “Revenge is a dish best served cold,”
which suggests that emotional detachment and planning are best for taking revenge.
This may be true for the optimal planning of strategies of revenge. However, there
are clear time limits on the effectiveness of strategies for seeking revenge. Waiting
too long to avenge being wronged can decrease the effectiveness of vengeance in two
ways: first, by allowing more time for a reputation of being exploitable to grow, and
second, by creating a larger window for exploitation to occur. Although revenge may
be a dish that is best served cold, reputation may be an asset that is best defended
by striking while the iron is hot. Inflicting costs on the individual who is the source
of reputational damage, including murdering the person, is one effective strategy
for the defense of reputation (Buss, 2005; Chagnon, 1988). Murder eliminates the
person’s ability to inflict costs in the future and clearly signals to other rivals the price
they will pay for similar assaults.
As explained by Buss and Duntley (in press), the timing of cost-inflicting, crimi-
nal strategies relative to other, complementary strategies is also likely to have been
an important source of selection pressure on the function of mechanisms that pro-
duce criminal behavior. Adaptations that produce criminal behavior likely comprise
a suite of mechanisms designed not only to inflict costs but also to deal with the
probable consequences of victimizing someone. Inflicting costs as the solution to a
primary adaptive problem is likely to create secondary problems, such as retribution
from victims and their genetic relatives. The recurrent costs of secondary problems
would have created selection pressure for the evolution of secondary solutions to
those problems. Some secondary solutions would be best employed after the second-
ary problems they created. For example, a criminal could take steps to (a) cover up
the crime, (b) subsequently avoid victims and their genetic relatives, (c) threaten to
inflict additional costs on them, (d) actually inflict costs on them if they attempt to
retaliate, or (e) marshal a formidable coalition to help make the costs of avenging
the victim’s death too high to be adaptive. Other secondary solutions may be more
appropriately adopted before the primary solution involving the infliction of costs
Figure 1.3. The use of cost-infliction to help solve an adaptive problem can create secondary
problems, such as retribution from victims and their kin. Selection would have operated on
the mind to anticipate likely problems resultant from cost-infliction and shaped a menu of
possible solutions. Solutions to secondary problems created by criminal behavior could be
enacted before, during, or after the crime
takes place. For example, an individual who may, in the future, adopt a strategy that
includes cost-infliction could try to impugn the status and reputation of the person
he or she intends to victimize. An intended criminal might also attempt to drive
wedges between would-be victims and the kin and coalitional allies who would pose
the greatest threat of helping the victims seek revenge, thus eliminating or decreas-
ing the magnitude of secondary problems that will likely result from inflicting costs
on victims. These ideas are illustrated in Figure 1.3. Interestingly, adaptations for
inflicting costs could use information about the effectiveness of secondary solutions
employed in anticipation of the secondary problems that cost-infliction will create
as a source of input for the cost-benefit calculus that determines whether to pursue
one particular criminal strategy over another, or do something else. In addition, if
secondary solutions employed before a cost-inflicting strategy in particular contexts
were recurrent over evolutionary time, selection should have operated on victims’
defense adaptations to recognize the secondary solutions and motivate people to take
action to prevent criminal behavior from occurring.
Implications of Adaptationist Research

on the Psychology of Crime
There is great promise in applying the adaptationist approach to all cognitive and be-
havioral phenomena. Evolutionary theory provides a powerful set of tools for explor-
ing the functions of psychological mechanisms. It suggests specific, novel hypotheses
and provides a logical framework that opens and unites data sources not routinely uti-
lized in psychological research (e.g., comparative, ethnographic, bioarcheological).
If it turns out that cognitive mechanisms that produce criminal behavior are bio-
logically engrained in the human psyche, it does not mean that we should be more
tolerant of crime because people “can’t help themselves.” We are not tolerant of a
number of behaviors that humans may be biologically disposed to engage in, such
as infidelity, spousal violence (Buss, 2000), and violence toward stepchildren (Daly &
Wilson, 1988). In fact, there is substantial evidence to suggest that morality itself has
evolutionary roots (see Chapter 12 of this volume). The existence of adaptations that
produce crime also does not mean that crime is inevitable. Research on homicidal fan-
tasies, for example, demonstrates that the vast majority of murder fantasies are not
translated into homicidal reality (Kenrick & Sheets, 1993). Jones (1997) argues that
our system of laws is designed to act as a lever to move behavior in desired directions.
By gaining a better understanding of how and why our psychology produces criminal
behavior, we may be able to create more effective legal interventions to prevent crimes
from occurring and more effective psychological treatments for offenders, likely of-
fenders, and victims (see Chapter 14 of this volume). Even if the application of evo-
lutionary logic to help understand criminal behavior turns out to be misguided, the
research findings it produces represent a valuable contribution to our understanding
of crime.
In conclusion, evolutionary forensic psychology recognizes that crimes such
as murder, nonlethal violence, rape, theft, and cheating are manifestations of evo-
lutionarily recurrent conflicts between individuals. The cost-inflicting strategies
that we recognize as crimes may have been favored by natural selection when they
gave individuals an advantage in competition for resources (see Chapter 2 of this
volume). Darwin’s theory of evolution by natural selection provides a powerful meta-
theoretical framework that has the potential to unify and energize forensic psychol-
ogy just as it has the biological sciences (see Chapter 13 of this volume). In the future,
we predict that evolutionary psychology will revolutionize the field of forensic psy-
chology, including our understanding of the psychology of crime, the cognition and
behaviors of victims, jury selection, eye-witness testimony, judges’ views of human
nature, insanity, competency, and public policy. It is difficult to predict exactly how
evolutionary psychology will affect the criminal justice system. The accumulation of
research findings grounded in evolutionary theory will refine and change the way
we think about legal systems. New discoveries will also open new directions for in-
quiry and spawn additional research. Evolutionary forensic psychology represents
the beginning of a revolution of thought and discovery that will bring us closer to
the truth of who we are and what our laws are capable of doing.
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2
The Promise of Evolutionary

Psychology for Criminology
The Examples of Gender and Age
ANTHONY WALSH AND KEVIN M. BEAVER
The maladies of sociology are many and grave, so much so that some scholars have
deemed the discipline to be terminally ill (Barkow, 2006; Ellis, 1996; Horowitz, 1993;
Lopreato & Crippen, 1999; Van den Berghe, 1990; Walsh, 2002) or have wondered
if it could, or even should, be saved (Kanazawa, 2006). Sociology is indeed adrift in
a foggy maze of theoretical contradictions, ideological self-righteousness, and non-
sensical postmodern “display prose,” but to declare it beyond hope is premature. The
discipline needs a solid anchor to stabilize it while its crew figures out how to steer it
out of the swamp. That anchor is biology, the science that sociology divorced itself
from by its fundamentalist interpretation of Durkheim’s dictum that the cause of
social facts should be sought only in other social facts (Udry, 1995). Sociology as a
whole took this to mean that there are no other sources of human social behavior,
and as a result many sociologists became not simply oblivious to biology but “mili-
tantly and proudly ignorant” (Van den Berghe, 1990, p. 177). As a subdiscipline of
sociology, criminology is essentially in the same boat, although it seems that more
criminologists than scholars in other areas of sociology have heeded biology’s call, as
evidenced by an avalanche of recent books entirely devoted to or containing signifi-
cant coverage of biosocial approaches (Agnew, 2005; Ellis & Walsh, 2000; Fishbein,
2001; Robinson, 2004; Rowe, 2002; Walsh, 2002; Walsh & Ellis, 2003, 2007).
Sociologist Matthew Robinson has said that “the biological sciences have made
more progress in advancing our understanding about behavior in the past 10 years
than sociology has made in the past 50 years” (2004, p. 4). Robinson is absolutely
correct, although we think he is overly generous to sociology. Talk of biology sends
20
The Promise of Evolutionary Psychology 21
shudders down the spines of traditionally trained criminologists, whose understand-

ing of “biology” barely extends beyond the standard textbook fare of phrenology,
atavism, and the XYY syndrome. The fears of such people might be allayed if they did
just two things: (1) learned a little about that which they fear, and (2) realized that
there is no such thing as a strictly biological theory of crime. All existing theories of
human behavior that integrate biological insights are biosocial. There are three gen-
eral biosocial approaches: evolutionary, behavior genetic, and neurophysiological.
These approaches, while employing different methods, work with different units of
analysis and invoke different levels of causation; they are fully complementary (i.e.,
their principles are conceptually consistent across all three levels of analysis), and
they all recognize the tremendous importance of the environment.
Evolutionary psychology is the most environmentally friendly of the three gen-
eral approaches. It is axiomatic that behavioral patterns of all living things are ul-
timately the result of evolutionary processes and that human nature is the sum of
human adaptations to ancestral environments. There is no scientifically viable alter-
native explanation to evolution by natural and sexual selection for basic behavioral
design. There are still those who deny this, believing that because humans have
developed culture and possess the cognitive skills to override biological dispositions
we have freed ourselves from evolutionary constraints (Ruffie, 1986). John Alcock
(2001) has responded to such quasi-existentialist notions by stating the following:
[T]o say that human behavior and our other attributes cannot be analyzed in evolutionary
terms requires the acceptance of a genuinely bizarre position, namely, that we alone among
animal species have somehow managed to achieve independence from our evolutionary
history, that our genes have for some undefined reason relinquished their influence on the
development of human psychological attributes, that our brain’s capacity to incorporate
learned information has no relation to past selection, that differences in brain functioning
in the past had no impact on the genetic success of people, and many other tenets that
would be considered outlandish if applied to [other animal life forms]. (p. 223)
In short, the behavior of Homo sapiens is subject to the same explanatory frame-
work as the behavior of any other animal. This is decidedly not to say that culture is
irrelevant to understanding human behavior. The human behavioral repertoire may
be composed of evolved adaptations, but adaptations require evolutionarily relevant
triggers from the environment to develop and activate them, and these triggers differ
in different cultural contexts. Because of this, Jerome Barkow (1989, p. 635) assures
us that we will always need the social sciences to fully understand these triggers, but
he also exhorts us not to forget that “psychology underlies culture and society, and
biological evolution underlies psychology.”
The Evolution of Traits Related to Criminal Behavior
If we accept the notion that evolution shaped human psychology and behavior, we
have to accept the companion notion that morally undesirable human traits such
as deception and violence owe their existence to their usefulness in the reproduc-
tive success of the species’ distant ancestors, like the more positive traits such as al-
truism, nurturance, and love. Needless to say, we do not display evolved patterns of
behavior motivated by the desire to maximize genetic fitness: “Evolutionary psychol-
ogy is not a theory of motivation. . . . Fitness consequences are invoked not as goals
in themselves, but rather to explain why certain goals have come to control behavior
at all, and why they are calibrated in one particular way rather than another” (Daly
& Wilson, 1988a, p. 7). Parents nurture and love their children not because they are
motivated by a subconscious genetic voice telling them that if they do they will push
more of their genes into the future but rather because ancestral parents who loved
and nurtured their children saw more of them grow to reproductive age and pass on
those traits down the genetic line. Ancestral parents who neglected and abused their
children compromised their viability and thus reduced the probability of their own
genes being represented in future generations. This is the ultimate (evolutionary)
reason that love and nurturance of offspring are the species norm whereas abuse
and neglect are aberrant.
Although evolutionary psychologists consider criminal behavior to be morally
regrettable, they also consider it biologically normal behavior for which we all have
the potential (Kanazawa, 2003). If behavior we define as criminal today is biologi-
cally normal, it must have conferred some evolutionary advantage on our distant
ancestors; that is, it must have had positive fitness consequences. But how can mor-
ally obnoxious acts such as murder, rape, theft, and assault be evolved adaptations
when they are clearly maladaptive in modern environments due to their tendency
to result in the perpetrators’ being imprisoned, where the opportunities for repro-
ductive success are bleak to say the least? The answer to that is twofold. First, we
must understand that specific criminal behaviors are not themselves adaptations:
“Genes do not code themselves for jimmying a lock or stealing a car . . . the genome
does not waste precious DNA encoding the specifics” (Rowe, 1996, p. 285).
Second, because a behavior is currently maladaptive does not mean that mecha-
nisms underlying it were not designed by natural selection to solve some environmen-
tal problem in the distant past. An adaptation is a current feature with a past, and
a feature that is currently adaptive may or may not have a future. Modern human
environments are different in so many ways from the environments that the species
evolved in that traits selected for their adaptive value then may not be adaptive at all
today, and traits and behaviors that appear to be adaptive today may not have a his-
tory of natural selection (Daly, 1996; Mealey, 1995).
Criminal behavior is a way of acquiring valued resources by force or fraud—that
is, by exploiting and deceiving others. Evolutionary psychologists refer to exploitive
and deceptive behavior as cheating, whether or not it has been culturally defined as
criminal. Although we all have the potential to exploit and deceive others, few of
us ever do so to a criminal extent. We do not because we are a highly social and co-
operative species with minds forged by evolution to form cooperative relationships
built on reciprocal trust. We cooperate with others because we feel good when we
do so, and because it identifies us as reliable and trustworthy, attributes that confer
valued social status on us. In short, cooperation and reciprocal altruism are in the
best interests of every member of a social species. Again, cooperation and altruism
are not engaged in so that the actor can feel good or because he or she is consciously
motivated by the desire for status. Social organisms do so, and are neurologically re-
warded when they do, because their distant ancestors who behaved this way enjoyed
greater reproductive success than those who did not, thus passing on the genes for
the brain structures and neurotransmitters that presumably underlie the propensity
(Barkow, 1997).
If cooperation is so rewarding, why do we find individuals who cheat rather
than cooperate? The short answer is that getting something for nothing is also
rewarding and that cooperative systems create niches for noncooperators to ex-
ploit (Tibbetts, 2003). Cheats are individuals who gain resources from others by
signaling cooperative intentions but then defaulting. In the absence of internal
(self-control, guilt, shame) or external (threats of punishment or ostracism) de-
terrents, it is in an individual’s interests to obtain resources from others under the
assumption of reciprocity and then fail to follow through. Such social parasitism
has been observed among numerous animal species (Alcock, 1998), which im-
plies that it has had positive fitness consequences. In the human species, criminal
behavior may be viewed as an extreme form of defaulting on the rules of coopera-
tion or reciprocity.
The Basic Assumptions of Evolutionary Theories of Crime
All evolutionary theories of criminal and antisocial behavior focus on reproductive

strategies and the behavioral tactics that flow from them (see Walsh, 2006, for a
discussion of these theories). The reproductive strategies of any species can be ap-
portioned according to the resources (time and energy) devoted to parenting versus
mating effort. At one extreme we have species that devote all of their resources to
mating and none at all to parenting (e.g., oysters, who lay many thousands of eggs),
and at the other we have species such as Homo sapiens who devote a huge propor-
tion of resources to parenting effort. Reproductive strategies are underlain by a suite
of evolved traits that facilitate their successful pursuit. Among humans, the suite
of traits useful for focusing on mating effort includes deceitfulness, impulsiveness,
sensation-seeking, and aggression; traits useful for focusing on parenting effort in-
clude empathy, conscientiousness, and altruism.
As is readily deduced, the traits useful for mating effort are also useful in pursu-
ing criminal activity, and the traits useful for parenting effort are associated with
prosocial activity. As David Rowe (1996, p. 270) phrased it, “[C]rime can be identi-
fied with the behaviors that tend to promote mating effort and noncrime with those
that tend to promote parenting effort.” The strength of these traits is arrayed on a
continuum dispersed around an adaptive mean; they are not characteristics one
has or has not. For the great majority of people, resources are expended mostly on
mating effort at some points over the life course and on parenting effort at other
points as reward contingencies shift. The most deceitful, impulsive, aggressive, and
sensation-seeking among us are not constitutionally suited to anything requiring
long-term commitment, including commitment to marriage and parenting, nor
are they suited for pursuing prosocial activities in general. A reproductive strategy
emphasizing mating effort is thus similar to criminal behavior in that direct and
immediate methods are used to procure resources illegitimately with little thought
being given to the consequences. Parenting effort, on the other hand, is embedded
in a prosocial lifestyle in which resource procurement relies on the accumulation of
social and occupational skills (the ability to delay gratification) that are attractive
to females.
The empirical research is unequivocal in its conclusion that an excessive con-
centration on mating effort is linked to criminal behavior. Ellis and Walsh (2000) re-
viewed fifty-one studies examining the relationship between number of sex partners
and criminal behavior and found fifty of them to be positive. The same authors also
reviewed thirty-one other studies and found that age of onset of sexual behavior was
negatively related to criminal behavior (the earlier the age of onset, the greater the
criminal activity) in all thirty-one.
More recent data from a British cohort study of over 1,100 twin pairs found
that 27% of the children were fathered by the most antisocial 10% of males in the
cohort (Jaffee, Moffitt, Caspi, & Taylor, 2003). A U.S. study looking at self-selection
in different family structures (broken versus intact) found that genetic differences
accounted for 94% of the difference on an antisocial subscale between the most
at-risk group (single mothers of half-siblings, a structure indicative of mating ef-
fort) and the least at-risk group (two-parent family with full siblings, indicative of
parenting effort). While the researchers were more concerned with genetics than
evolutionary psychology, they concluded, “Although temperament, personality,
or cognitive bias toward sexual variety may be proximate causes of single par-
enthood or multiple matings, they may also comprise components of an overall
reproductive strategy that emphasizes mating over parenting effort” (Cleveland,
Wiebe, van den Oord, & Rowe, 2000, pp. 744–745). Finally, anthropologists have
found striking differences in behavior between cultures that emphasize different
reproductive strategies. In cultures emphasizing mating effort significantly more
than parenting effort, members exhibit behaviors such as low-level parental care,
hypermasculinity, violent competitiveness, and transient bonding, all of which
are considered antisocial in Western societies (Harpending & Draper, 1988; Ember
& Ember, 1998).
In this chapter, we examine large categories of individuals from an evolutionary
perspective rather than individual traits that lead to differences and criminal behav-
ior. The two best demographic predictors of where reproductive effort is focused are
gender and age, which, not coincidently, are also the best demographic predictors of
criminal and other antisocial behaviors.
Gender
In every culture and every historical period, males commit far more crime than fe-
males, and the more serious the crime, the more males are overrepresented. This
so-called gender ratio problem (i.e., why do males and females differ so much in their
propensity to commit crimes?) has been identified as one of the key issues in feminist
criminology (Daly & Chesney-Lind, 1996). Male and female crime rates are highly
correlated (the high 0.80s to low 0.90s) across different nations, states, and cities
(Campbell, 1999). These correlations indicate that females respond to the same en-
vironmental conditions as males, albeit far less frequently and seriously. Similarly,
most female offenders are found in the same social situations as most of their male
counterparts—that is, among single-parent families located in poor, socially disorga-
nized neighborhoods. The similarity of environmental conditions coupled with the
large differences in criminal activity between the sexes led Daly and Chesney-Lind to
ask, “Why do similar processes produce a distinctive, gender-based structure to crime
and delinquency?” (1996, p. 349).
Criminologists have attempted to answer this question in a variety of ways, but
almost always under the assumption of the psychic identity of the sexes. If males and
females are psychically identical, then it makes sense to explain differences between
them as products of differential socialization (e.g., men are socialized to be assertive,
aggressive, and dominant, and women are socialized to be nurturing, passive, and
family oriented).
An example of this thinking is Mears, Ploeger, and Warr’s (1998) contention that
the gender ratio is the result of the differential exposure of the genders to delinquent
peers, the fact that males are more influenced by delinquent peers than females, and
the fact that females have greater inhibitory morality than males. This “explanation”
says little beyond claiming that boys will be boys and girls will be girls, and it begs the
questions of why males are more exposed and more influenced by delinquent peers
than females, and why females have a stronger sense of morality (Walsh, 2002).
The assumption inherent in the traditional sociological view is that if females were
socialized in the same way as males and had similar roles and experiences, their rates
of criminal offending would be roughly the same. This is pure nonsense undeserving
of additional comment save to quote Dianna Fishbein’s (1992, p. 100) summation of
the gender ratio issue: “[C]ross cultural studies do not support the prominent role of
structural and cultural influences of gender-specific crime rates as the type and ex-
tent of male versus female crime remains consistent across cultures.”
Males and Mating Effort
So-called radical feminists, on the other hand, reason that because the magnitude of
the gender gap varies across time and space and yet still remains constantly wide at all
times and in all places, biological factors must play a major role. They further note that
robust sex differences in dominance and aggression are seen in all human cultures
from the earliest days of life, are underscored during the teen years, and are observed
in all primate and most mammalian species for which no one would evoke socializa-
tion as an explanation (Archer, 1996; Geary, 1998). Neuroscience has long informed
us that gender-typical behavior is the result of hormones that organize the brain in
male or female directions during sensitive prenatal periods (Amateau & McCarthy,
2004). This neurohormonal process organizes male brains in ways such that males
become more vulnerable to the various traits associated with antisocial behavior via
the regulation of brain chemistry (Ellis, 2003; Lopreato & Crippen, 1999).
Thus the sexes come into this world with “differently wired brains,” and these
brain differences “make it almost impossible to evaluate the effects of experience [the
socialization process] independent of physiological predisposition” (Kimura, 1992,
p. 119). The major biological factor organizing the male brain along male lines and
which underlies gender differences in dominance, aggression, violence, and general
antisocial behavior is testosterone (Archer, 1996; Kanazawa, 2003). No one claims
that testosterone is a major or even minor cause of crime and general mayhem,
only that it is the major factor that underlies gender differences in crime and general
mayhem.1
While neurohormonal differences provide a scientifically robust explanation
of the genders’ different responses to environmental instigation, we also need to
know why these differences exist in the first place. Talk of brains and hormones
invokes mechanisms operating in real time that explain how one thing leads to
another. Disciplines such as behavior genetics and neurophysiology are most in-
terested in such proximate-level causal mechanisms. Evolutionary psychology is
interested in ultimate-level why causes, causes that lead via an extended period
of selection to the adaptations we call proximate causes. To answer questions
about why causes, we are required to consider the different selection pressures
that confronted our distant male and female ancestors with respect to reproduc-
tive considerations.
As Lopreato and Crippen (1999, p. 114) point out, “The two sexes are endowed
with differing reproductive strategies, and from this difference arise various behav-
ioral tendencies.” There is much more variability among males than females in terms
of reproductive success, with some males leaving no offspring and others fathering
large numbers. The nature of female physiology ensures that females have a lower
potential reproductive ceiling than males, but few will be reproductive failures rela-
tive to males. The major strategy throughout our evolutionary history for increasing
a female’s reproductive success has been for her to secure and hold on to the assis-
tance of a mate to raise her offspring. Given lower variation but greater reproductive
certainty, females have evolved a mating strategy inclining them to be choosier than
males about whom they will mate with; indiscriminate mating would have had nega-
tive reproductive utility for females (Badcock, 2000; Buss, 1994; Cartwright, 2000).
Given the lower reproductive ceiling of females, traits that maximized the probabil-
ity that existing children would survive (parenting effort) evolved rather than traits
designed to maximize mating effort. Simply put, females have more strongly evolved
neurohormonal mechanisms that underlie the traits conducive to successful parent-
ing effort than males, and because these traits are essentially prosocial traits, females
are less likely to commit crimes.
The only limitation to male reproductive success is access to females; the more
females a male can have sex with, the greater his fitness potential. Males have an
evolved desire for multiple partners because in fitness terms, there is much to gain and
little to lose following this strategy. However, every male is in competition with every
other male for access to females, a situation that in evolutionary environments has
often resulted in violence. Even in modern times, most nonstate violence in the world
is male-on-male violence generated, to a great extent, by sexual competition (Daly,
1996).
In addition to overcoming competition from other males, males also have to re-
spond to the more restrained female reproductive strategy. They can comply with
the female preference and commit to a single female and assist her to raise their off-
spring, or they can trick or force a female to have sex and then move on to the next
conquest. These two strategies have been called “Cad vs. Dad” (Cashdan, 1993). To
successfully pursue a cad strategy, it would be counterproductive to be distracted by
emotional signals, either by guilt, shame, or anxiety from within or the fear and dis-
gust of a potential victim. It would therefore be useful to have mechanisms that mute
the neurohormonal regulators of the social emotions so that one is less likely to feel
guilt, shame, anxiety, and sympathy (Dugatkin, 1992; Nesse & Lloyd, 1992).
The extreme of the cad strategy is, of course, the psychopath. The greatly re-
duced ability to experience the social emotions of shame, embarrassment, guilt, em-
pathy, and love has marked psychopaths across time and cultures. One of the most
consistent physiological findings about psychopaths is their inability to “tie” the
brain’s cognitive and emotional networks together (Patrick, 1994; Scarpa & Raine,
2003). David Rowe (2002, pp. 62–63) provides a sketch of the traits useful in sup-
porting the male cad mating strategy, which is incidentally an excellent description
of the psychopath:
A strong sexual drive and attraction to novelty of new sexual partners is clearly one com-
ponent of mating effort. An ability to appear charming and superficially interested in
women while courting them would be useful. The emotional attachment, however, must
be an insincere one, to prevent emotional bonding to a girlfriend or spouse. The cad may
be aggressive, to coerce sex from partly willing partners and to deter rival men. He feels
little remorse about lying or cheating. Impulsivity could be advantageous in a cad be-
cause mating decisions must be made quickly and without prolonged deliberation; the
unconscious aim is many partners, not a high-quality partner.
Almost all heterosexual males have probably used “cad” tactics (falsely pro-
claiming love and fidelity or the use of some form of coercion, perhaps even force)
to obtain sex at some time or another, although the vast majority will eventually
settle down and assist a female in raising their young. A small minority, however, will
continue to exploit females across the life course, assisted by the traits that facilitate
mating effort, which are, as we have seen, the same traits that facilitate criminal be-
havior. Males have thus evolved to be more risk-taking, violent, and manipulative in
competitive situations than females.
Females and Parenting Effort
We have thus far examined only the male half of the equation—that is, why are males
more crime prone? We now turn to the other half: why females are less crime prone.
The best evolutionary explanation for all of the sex differences in traits and their neu-
rohormonal bases that make females both more inclined toward parenting effort and
less inclined toward criminal behavior is Anne Campbell’s (1999) staying alive/low
fear hypothesis. This hypothesis has to do with the selection pressures faced by ances-
tral females with regard to parental investment and status striving. The obligatory pa-
rental investment of males is limited to a few pelvic thrusts, after which they can be on
their way. The obligatory parental investment of females is enormously greater. Only
after months of gestation and years of lactation can she contemplate further children,
which means that her reproductive success is far more tied to children she already has
than is that of a male. The greater dependence of the infant on the mother renders
a mother’s presence more critical to offspring survival (and hence to the mother’s
reproductive success) than is the presence of a father. In ancestral environments the
care of nursing infants meant that females always kept them in close proximity, and
this posed an elevated risk of injury to the child as well as the mother if the mother
placed herself in risky situations (Beckerman, 1999). Because female survival is more
critical to female reproductive success (in terms of maximizing the probability that
offspring will survive) than is male survival, Campbell’s hypothesis is that females
have evolved a propensity to avoid engaging in behaviors that pose survival risks.
Campbell proposes that the evolved mechanism underlying this propensity is a
physiology that responds to many different risky situations and that is subjectively
experienced as fear. There are essentially no sex differences in fearfulness across a
number of contexts, unless a situation contains a significant risk of physical injury.
The greater fear response under such circumstances accounts for the greater ten-
dency of females to avoid potentially violent situations and to employ indirect and
low-risk strategies in competition and dispute resolution relative to males. In simple
terms, the ancestral females who avoided or removed themselves from situations in-
volving a high risk of physical injury or death were more likely to survive, and their
survival increased the probability that their offspring would survive and that their
genetic lineage would be perpetuated.
The staying alive/low fear hypothesis also has implications for sex differences in
status seeking. Because males have greater variance in reproductive success than fe-
males but less parental certainty, they too gain greater fitness benefits by engaging in
intrasexual competition for mating opportunities. High-status and dominant males
always attract more females than low-status, subservient males (Mazur, 2005). Sta-
tus and dominance striving is often risky business (and certainly was in evolutionary
times), and because attaining status is less reproductively consequential for females
than for males, there has been less pressure for the selection of mechanisms useful
in that endeavor for females. In environments of evolutionary adaptation, a male’s
reproductive success often rested on involving himself in risky situations in which
high fear levels would have been a definite handicap.
Campbell points out that although females engage in intrasexual competition
for mates, it is rarely in the form of violence and aggression, with most of it being
low key, low risk, and chronic as opposed to the high key, high risk, and acute na-
ture of male intrasexual competition. Females cannot compete for the female assets
most pertinent to attracting a committed mate such as youth and beauty, which a
woman either possesses or does not. Male assets that attract females, unlike youth
and beauty, can be achieved in competition with other males. Males who are most
willing to incur risks to achieve status and dominance gain the resources that come
with them and thus potentially gain access to more females.
Women do commit crime, of course; but, as Campbell notes, when they do, their
crimes rarely involve risk of physical injury and are almost always committed for
instrumental reasons. For instance, Campbell points out that although robbery and
larceny/theft involve expropriating resources from others, females constitute about
43% of arrests for larceny/theft and only about 7% of arrests for robbery, a crime
carrying a relatively high risk for personal injury. Campbell (1999, p. 210) notes
that while women do aggress and do steal, “they rarely do both at the same time
because the equation of resources and status reflects a particularly masculine logic.”
Robbery, and flaunting the material trappings signaling its successful pursuit, is seen
ultimately as a campaign for respect and status in the street culture from which most
robbers come (Jacobs & Wright, 1999). Studies of female robbers provide no men-
tion that they crave the additional payoffs of dominance that male robbers do, or
seek reputations as “hardasses” (Messerschmidt, 1993). Aggressive and dominant
females are not particularly desirable as mates, and certainly a woman with a repu-
tation as a “hardass” would be most unattractive.
It is not sex per se that exerts pressure for the selection of the mechanisms that
underlie these traits and behaviors; it is parental investment. In some bird and fish spe-
cies, males contribute greater parental investment than females (e.g., incubating the
eggs and feeding the young), and in these species the sexes have evolved many behav-
ioral characteristics that are the opposite of the characteristics of males and females
in mammalian species in which females assume all or most of the burden of parent-
ing. In these “sex-role reversal” species, females are bigger and more aggressive, they
have more testosterone, and they are more promiscuous risk-takers in intrasexual
competition for mates than males (Betzig, 1999). Species exhibiting sex-role reversal
provide support for Campbell’s thesis that parental investment, not simple biological
sex, accounts for traits supporting different reproductive strategies and underline the
usefulness of cross-species comparisons for understanding human behavior.
Age and Crime
Age is almost as strong a predictor of criminal offending as gender. Across time and
space we consistently observe a rapid increase in delinquency at puberty and then a
slow decline after reaching its peak between 16 and 18 years of age (Ellis & Walsh,
2000). Why this is so has long been a mystery to criminologists. Some take a stab
at explaining it by pointing to the increase in peer involvement in adolescence and
the decline in antisocial behavior thereafter resulting from the decreasing influence
of peers and the increasing influence of girlfriends, wives, children, and employers
(Warr, 2002). It escapes their attention that this simply describes situations that
co-occur with the usual beginning and the end of delinquent behavior; it does not
explain why the period between these events is so filled with such behavior or why
associations with peers lead to negative behavior more often than to positive behav-
ior. Long ago, Shavit and Rattner (1988, p. 1457) pointed out that the age peak in
delinquency is “unexplained by any known set of sociological variables.” This view is
shared by Gottfredson and Hirschi (1990), who basically conclude that because the
age effect is constant across time and place, criminologists should accept it as a fact
and go on from there, perhaps reasoning that age in this context is simply an index of
a certain developmental stage (puberty) we all go through. But this is a message of de-
feat; there must be something special going on during this period of life that temporar-
ily increases the probability of antisocial behavior, and which demands explanation.
The 2003 New York Academy of Sciences conference on adolescent brain devel-
opment provided some key points relevant to the age effect (White, 2004, p. 4):
1. Much of the behavior characterizing adolescence is rooted in biology inter-
mingling with environmental influences to cause teens to conflict with their
parents, take more risks, and experience wide swings in emotion.
2. The lack of synchrony between a physically mature body and a still maturing
nervous system may explain these behaviors.
3. Adolescents’ sensitivities to rewards appear to be different from those of
adults, prompting them to seek higher levels of novelty and stimulation to
achieve the same feeling of pleasure.
Adolescence starts with puberty, a stage in human development that marks the
onset of the transition from childhood to adulthood, during which our bodies pre-
pare for procreation. This transition is not without its problems, as we observe that
“many happy and loveable children suddenly morph into malcontents acting like
they should be in pampers rather than pants” (Walsh & Ellis, 2007, p. 230). Puberty
is a series of biological events, and adolescence is a process that begins at puberty and
ends with adulthood. Adulthood typically means taking on socially responsible roles
such as acquiring a full-time job and settling down and taking on family life, roles
that define us as independent members of society. In many respects, adolescence is
a period in limbo because, although we no longer need parental care, we are not yet
ready to take on the roles and responsibilities of adulthood (Moffitt, 1993). Adoles-
cence is a normal and necessary period in the human life span in which one can
experiment with a variety of social skills before having to put them into practice as
an adult (Bogin, 1993).
Testosterone begins playing its role by organizing the male brain during the sec-
ond trimester of pregnancy so that it will respond in male-typical ways when the
brain is activated in that direction at puberty (Ellis, 2003). After sex-specific brain
organization takes place, there is little difference in levels of male and female testos-
terone until puberty, at which time males have approximately ten times the female
level (Felson & Haynie, 2002). Testosterone is most responsible for the development
of male characteristics, including behavioral characteristics such as aggression and
dominance-seeking (Quadango, 2003).
At the same time as they are experiencing hormonal surges, adolescents’ brains
are undergoing changes in the ratio of excitatory to inhibitory neurotransmitters.
Dopamine and another excitatory transmitter called glutamate peak during adoles-
cence, while the inhibitory transmitters gamma-aminobutyric acid and serotonin
are reduced (Collins, 2004; Spear, 2000; Walker, 2002). Additionally, the adoles-
cent brain goes through an intense period of physical restructuring as hormonal
surges prompt the increase of gene expression initiating the process of slowly refin-
ing neural circuitry to its adult form (Walker, 2002). A series of magnetic resonance
imaging studies have revealed that the prefrontal cortex (PFC) undergoes a wave of
synaptic overproduction just prior to puberty, followed by a period of pruning dur-
ing adolescence and early adulthood (Giedd et al., 1999; Sowell, Thompson, Holmes,
Jernigan, & Toga, 1999).
In addition to all the synaptic modifications occurring in the PFC, the adolescent
PFC is also less completely myelinated (myelin is the fatty substance that coats and
insulates axons) than the adult PFC (Sowell et al., 1999). A less myelinated brain
means less efficient message transmission and a larger time lapse between the onset
of an emotional event in the limbic system and the PFC’s rational judgment of it. All
this amounts to the conclusion that there are physical reasons for the greater ratio
of emotional to rational responses often observed in teenagers. Adolescents are es-
sentially operating with a brain on “go slow” superimposed on a hormone-driven
physiology on “fast forward.” This explains why many teenagers find it difficult to ac-
curately gauge the meanings and intentions of others and why they experience more
stimuli as aversive during adolescence than they did as children and will when they
are adults (Walsh, 2002, p. 143). Richard Restak (2001, p. 76) perhaps put it best
when he wrote, “The immaturity of the adolescent’s behavior is perfectly mirrored
by the immaturity of the adolescent’s brain.”2
The implications for antisocial behavior in all this are obvious in that the neu-
rohormonal modifications and adjustments going on facilitate a tendency to assign
faulty attributions to situations and to the intentions of others. As Agnew (2005)
points out, a greater sensitivity to stressors leads to an increase in irritability and
a decrease in self-control, which in turn lead to a greater probability of antisocial
behaviors. This would be particularly true in so-called honor subcultures, which are
defined as “communities in which young men are hypersensitive to insult, rushing

to defend their reputation in dominance contests” (Mazur & Booth, 1998, p. 362).
Such subcultures develop when there is high risk of one’s resources being expro-
priated by thieves and in which the governing body is too weak (or not trusted) to
prevent and punish such theft (Anderson, 1999; Shackelford, 2005). According to
Quinsey (2002, p. 3), the intense and often deadly “in your face” rivalry among poor
inner-city youths supports the evolutionary contention that “crime is functionally
related to inter male competition that has its ultimate roots in reproductive rivalry.”
Males in honor subcultures are behaving as natural selection designed them and,
incidentally, in historically normative ways (honor subcultures in which dueling was
an accepted way to settle disputes have existed throughout history [Mazur & Booth,
1998]). This biological and historical normativeness does not, of course, make such
behavior morally acceptable.
Adolescence and the behaviors manifested during the period must therefore be
viewed as adaptations (Spear, 2000; White, 2004). Evolutionary biologists stress that
natural selection favors the most adventurous and dominant males because such
characteristics typically result in more mating opportunities and thus greater repro-
ductive success. As among all primate species, mid-adolescence and early adulthood
are periods of intense competition among males, particularly where social controls
are lacking, for dominance and status aimed ultimately at securing more mating op-
portunities than the next male (White, 2004, p. 7). As Martin Daly (1996, p. 193)
put it, “There are many reasons to think that we’ve been designed [by natural selec-
tion] to be maximally competitive and conflictual in young adulthood.”
Mercifully, adolescence is short-lived. Around the age of 20, the ratio of excit-
atory transmitters to inhibitory transmitters becomes more balanced as the former
start to decrease and the latter start to increase (Collins, 2004). With more bio-
balanced brain signals, more adult-like personality traits emerge. Findings from five
different countries show age-related decreases in personality traits positively related
to antisocial behavior (e.g., neuroticism, extraversion) and increases in personality
traits positively related to prosocial behavior (e.g., agreeableness, conscientiousness)
(McCrae et al., 2000). The fine-tuning of neurological and endocrine systems lays
the foundations for the acquisition of responsible social roles that help us stay on
the straight and narrow and correlate with the steep declines in antisocial behavior
noted everywhere crime statistics are gathered.
Conclusion
It should be clear from the preceding that evolutionary psychology shares with
mainstream sociology the belief that Homo sapiens are social beings who desire to fol-
low social rules. However, evolutionary psychology is Hobbesian rather than Rous-
seauesque in that it tells us this desire comes from the yearning for self-preservation
and not from a romanticized notion that we are inherently good beings who will
commit antisocial acts only when forced to do so by “society.” Evolutionary psychol-

ogy agrees with Durkheim (cited in Walsh, 2002, p. 99) that society is the moral
good guy because “it alone has the power necessary to stipulate law and to set the
point beyond which the passions must not go.”
We have evolved to be reciprocal altruists who know that we can realize our
self-interests more often by following rules than by not following them. The apparent
paradox of social beings committing antisocial acts is resolved when we realize that
our desire to cooperate with our fellows provides opportunities for noncooperators
to victimize us. The individuals most likely to do so are those who are disadvantaged
in the competition for wealth, power, and status, which is what most mainstream
criminological theories express. There is agreement between mainstream theories
and evolutionary psychology on many aspects of crime and criminality. Adding evo-
lutionary explanatory concepts to these theories would not only enrich and broaden
their repertoire of concepts; it would also ground them in the one existing theory
that has the potential to add unity and coherence across all disciplines that study
the behavior of living things. Evolutionary psychology highlights the kinds of envi-
ronments in which those behaviors that trouble us most are likely to emerge, and it
is the only extant metatheory capable of uniting, integrating, and making sense of
the disparate data on human behavior coming to us from many theories and many
disciplines. Criminology will ignore this perspective at its peril.
Notes
1. Ellis (2005) notes that the average correlation between testosterone and criminality
is a modest 0.20 to 0.25, although Mazur (2005) indicates that this correlation is higher for
behavioral measures than for self-report measures.
2. Several studies show generally that the earlier the onset of puberty, the greater the
level of problem behavior for both girls and boys (Beaver & Wright, 2005; Caspi et al., 1994;
Felson & Haynie, 2002). Juveniles who enter puberty significantly earlier than their peers
must confront their “raging hormones” with a brain that is no more mature than those of
their peers. In one study, testosterone level predicted future problem behavior, but only for
boys who entered puberty early (Drigotas & Udry, 1993). Felson and Haynie (2002) found
that boys who experienced early onset of puberty were more likely to commit a number of de-
linquent and other antisocial acts than other boys, but that they were also more autonomous
and better psychologically adjusted and had more friends.
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PART TWO
ADAPTATION AND
VIOLENT CRIMES
3
The Origins of Homicide
JOSHUA D. DUNTLEY AND DAVID M. BUSS
Why people kill their fellow human beings is a question whose answer has thus
far eluded a comprehensive scientific explanation. This chapter describes homicide
adaptation theory, a recent theoretical contender that offers an evolutionary psy-
chological explanation of the most common forms of homicide. We begin by reviewing
some key statistics about homicide. We discuss examples of the unique selection
pressures created by human cognitive adaptations for social exchange that are hy-
pothesized to have selected for homicide. We explore the coevolutionary arms race
between adaptations for homicide and defenses against being killed. We compare ho-
micide adaptation theory to nonadaptationist explanations for conspecific killings in
humans. Finally, we explore how an evolutionary perspective sheds light on why the
law does not treat all forms and contexts of homicide the same.
In the United States, you are ten times more likely to be murdered on the day
you are born than at any other time during your life (Centers for Disease Control,
2006). If you survive your first day, you still have a greater risk of being murdered
during your first year of life than in any other year of childhood (Overpeck, Brenner,
Trumble, Trifiletti, & Berendes, 2002).
Homicide in modern societies is less rare than is often believed. Crime rates are
typically reported as the number of incidents per 100,000 people per year. In the
United States, for example, there were 17,034 homicides in 2006, which translates
to a homicide rate of 5.7 per 100,000 people that year (U.S. Department of Justice,
2007). At first glance, this may make homicide seem like a fairly rare event. How-
ever, if you compute this risk over the average life span of a U.S. citizen (77.8 years), it
translates to roughly a 1 in 225 lifetime risk of being a victim of homicide.
In 2004, homicide ranked fifteenth among the leading causes of death for men and
women of all ages (Centers for Disease Control, 2006). Women and men, however, did
not have the same likelihood of being killed by someone else. For men, homicide was the
thirteenth leading cause of death. For women, homicide didn’t crack the top twenty.
41
42 Adaptation and Violent Crimes
The likelihood of being killed also differed as a function of age and ethnicity. For all men
between the ages of 15 and 24 years, homicide was the second leading cause of death,
but for black men in the same age group it was the number one cause of death.
We propose that homicide rates are not accurate indicators of the number of
times that people adopt strategies of lethal aggression against others. The homicide
rates in many nations would undoubtedly be much higher were it not for emergency
medical interventions that were not available to our ancestors for most of our evolu-
tionary history. Researchers in the United States found that faster ambulances and
better emergency room care, much of which was developed during the first Gulf
War between the United States and Iraq from 1990 to 1991, are largely responsible
for much of the decrease in homicide rates over the last three decades in the United
States. It has been estimated that there would be 30,000 to 50,000 additional killings
in the United States each year—at least tripling or quadrupling the current homi-
cide rate—without the advances in emergency care technology that have occurred
during the last thirty years (Harris, Thomas, Fisher, & Hirsch, 2002). Thirty thou-
sand more homicides each year would translate into a 1-in-81 lifetime risk of being
murdered. Fifty thousand additional homicides per year would create a 1-in-57
lifetime murder risk in the United States. If we eliminated medical advances that oc-
curred before the 1990s, such as the advent of antibiotics and other important in-
novations, the lifetime homicide risk would reach much higher levels.
Homicide rates vary predictably from culture to culture. Some cultural variation
has been traced to factors such as resource discrepancy (Wilson & Daly, 1997) and
the relative costs and benefits of killing a conspecific versus other strategies for solv-
ing problems (Buss & Duntley, 2006), suggesting that humans may possess decision
rules that guide the implementation of homicidal behaviors (Duntley & Buss, 2005).
This should lead to predictably different rates of conspecific killing wherever there are
differences in the costs and benefits of eliminating conspecifics. Regional differences
in homicide rates have been well documented. In the United States, the rates of killing
are much higher than in many industrialized nations, exceeding those in the United
Kingdom and Japan by a factor of ten; exceeding those in France, Austria, Sweden,
and Germany by a factor of nine; and exceeding the rates in Canada, Italy, Portugal,
Korea, and Belgium by a factor of five. But the homicide rates in many other countries
are equivalent to or exceed those in the United States (United Nations, 1998). The
lifetime probability of being a homicide victim in Venezuela and Moldova is 1 in 90,
twice that of the United States. In Estonia and Puerto Rico, the likelihood is 1 in 60,
three times that of the United States. And in Colombia and South Africa, the likelihood
is greater than 1 in 20 that a person will die at the hands of a killer, more than ten times
the lifetime homicide risk in the United States. Even among those nations that currently
exhibit low homicide rates, much higher frequencies of conspecific killing were a consis-
tent part of their histories. Historical evidence suggests that the relatively low homicide
rates in many modern societies is a recent phenomenon (e.g., Dower & George, 1995;
Ruff, 2001). Additionally, the rates of homicide recorded by nations typically do not
include casualties of warfare or genocide.
The Origins of Homicide 43
The homicide rates in industrialized nations pale in comparison to the risk of

being killed by a competitor in many preindustrial cultures. Including deaths result-
ing from lethal raids and tribal warfare, homicides account for roughly one in ten
deaths of adult men among the Huli; one in four deaths among the Mae Enga; and
one in three deaths among the Dugum Dani and Yanomamo (Chagnon, 1988). Even
among the so-called gentle people or peaceful !Kung San of Botswana, there were
twenty-two homicides over a twenty-five-year period in a population of 1,500, more
than four times the rate of killing in a typical year in the United States (Lee, 1984).
For our understanding of homicide to be complete, we must explain observed
patterns of conspecific killing, including (a) why men are vastly overrepresented
among killers across cultures (87%); (b) why men are also overrepresented among
homicide victims across cultures (75%); (c) why women across cultures commit
some kinds of homicide more than men (e.g., infanticide of their genetic children
soon after birth); (d) why people in every culture kill in qualitatively distinct con-
ditions, leading to predictable infanticides, stepchild killings, men killing women,
women killing men, intrasexual rivalry homicides, and warfare killings; and (e) why
people experience homicidal fantasies in circumstances that correspond closely
to the contexts in which people actually commit murder (Buss & Duntley, under
review).
Homicide Adaptation Theory
Buss and Duntley (1998, 1999, 2003, 2004, under review) have proposed a theory
that humans possess adaptations designed specifically for killing conspecifics. Psy-
chological adaptations for homicide are argued to be the outcome of the process of
natural selection. Like all adaptations, they were favored when they contributed bet-
ter solutions to recurrent ancestral problems, on average, than competing adaptive
mechanisms. Information processing adaptations evolved to scrutinize and some-
times produce homicidal behavior in adaptive problem contexts recurrently solvable
by homicide in the past. Although some have suggested the possibility of adaptations
for homicide (Ghiglieri, 1999; Pinker, 1997) and others have argued that humans
may have an instinct to kill (e.g., Chagnon, 1988), no other theorists have gone into
depth in exploring the design of adaptations for homicide (see a notable exception
dealing with warfare: Tooby & Cosmides, 1988).
Buss and Duntley (under review) hypothesize that homicide was functional in
solving a variety of adaptive problems. Specifically, the killing of a conspecific could
have contributed to (a) preventing the exploitation, injury, rape, or killing of self, kin,
mates, and coalitional allies by conspecifics in the present and future; (b) reputa-
tion management against being perceived as easily exploited, injured, raped, or killed
by conspecifics; (c) protecting resources, territory, shelter, and food from competi-
tors; (d) eliminating resource-absorbing or costly individuals who were not geneti-
cally related (e.g., stepchildren); and (e) eliminating genetic relatives who interfered
with investment in other vehicles better able to translate resource investment into
genetic fitness (e.g., deformed infants, the chronically ill or infirm).
Homicide is a unique and potentially powerful strategy with dramatic fitness con-
sequences for both perpetrator and victim. It is reasonable to hypothesize that conspe-
cific killing has been subjected to evolution by natural and sexual selection. Homicide is
different from other strategies for inflicting costs because it leads to the absolute end of
direct conflict or competition between individuals. People who are killed can no longer
compete with or inflict costs on their killers. Dead competitors can no longer directly
influence the environment or social context that they shared with their killers. These
distinct outcomes of homicide would have created unique selection pressures to shape
psychological mechanisms to produce homicidal behavior in contexts in which the
elimination of a conspecific yielded better fitness outcomes than other available strate-
gies (Buss & Duntley, under review; Duntley, 2005).
Adaptations for homicide would be more likely to evolve when they reliably con-
tributed to the solution of an adaptive problem with a high impact on individual fit-
ness, such as preventing a rival from killing one’s child. Adaptations for homicide also
would be more likely to evolve when a large number of different adaptive problems
could be solved, or at least partially solved, by eliminating a conspecific. Consider, for
example, the intrasexual rival of a man who was preventing his ascension in a sta-
tus hierarchy, attempting to poach away the man’s mate, monopolizing a scarce and
valuable shelter as winter approaches, and who took every opportunity to publicly
humiliate the man’s brother. A large number of fitness costs are being inflicted by a
single individual, and a significant amount of benefits could be gained through his
elimination. The greater the fitness costs that a rival imposes on an individual, and
the greater the benefits that would become available if the rival died, the heavier the
weight of selection pressure would be for the evolution of homicidal strategies.
Different ancestral problems required different specific solutions. Homicide ad-
aptation theory proposes that there are multiple, different psychological adaptations
for homicide, each of which is devoted to the solution of different kinds of adaptive
problems. By this logic, psychological design for infanticide is distinct from psycho-
logical design for warfare; psychological design for mate homicide in men is distinct
from psychological design for mate killing in women. Some information processing
mechanisms are undoubtedly shared between the different adaptations for homicide
and with adaptations for the solution of other domains of adaptive problems. Selec-
tion would favor the sharing of subroutines performing the same function over rein-
venting them anew for each psychological adaptation. To be capable of addressing
the unique combinations of characteristics inherent in different adaptive problems,
however, each homicide adaptation must have at least one design feature that is dis-
tinct from other adaptations. In short, homicide adaptation theory proposes that se-
lection has fashioned a number of specialized psychological adaptations in humans
to solve distinct and historically recurrent adaptive problems.
Homicide adaptation theory is a coevolutionary theory. Just as killers obtained
large ancestral benefits from the use of homicide in some contexts, victims and their
genetic kin suffered extraordinary costs. The costs are hypothesized to have cre-
ated selection pressure for the evolution of defenses against becoming a victim of
homicide and to adaptations in victims’ genetic kin to prevent relatives’ untimely
deaths or to minimize the costs in the aftermath. The evolution of defenses against le-
thal aggression would have made killing less beneficial, creating new selection pres-
sure for design features capable of bypassing victims’ defenses. This coevolutionary
arms race between homicide adaptations and victim defenses is hypothesized to have
contributed to rapid evolutionary change and elaborate design in both.
Homicide adaptation theory does not propose that homicide evolved to be the pre-
ferred strategy for any adaptive problem in all situations. In most circumstances, the
high costs of committing homicide would have outweighed its benefits. However, ho-
micide adaptation theory does propose that homicidal behavior was the best solution
for rare combinations of adaptive problems and circumstances. It is these relatively
rare adaptive problem contexts that provided selection pressure for the evolution of
homicide adaptations. As a result, it is not possible to point to just one feature of
a situation that will activate a psychology of homicide in every instance, in every
person. There are other, mitigating environmental factors (Gartner, 1990), heritable
personality features (Rhee & Waldman, 2002), and hormonal (Niehoff, 1999) and
developmental influences (Dodge, Bates, & Pettit, 1990) that contribute to the adop-
tion of homicidal behavior. These influences were likely part of the selection pres-
sures that shaped homicide adaptations. The presence of these influences, as well as
their magnitudes, can help us to predict when conspecific killing will be more or less
likely to occur. Without complete knowledge of how the various influences interact
with human psychology to produce homicidal behavior, however, it is not possible to
make perfect predictions about whether homicide will occur in any individual case.
This problem is not idiosyncratic to the prediction of homicide. The same is true in
making predictions about any behavior. We hypothesize that psychological mecha-
nisms for homicide steer an individual in the direction of adaptive behaviors that reli-
ably result in the death of another individual. This is accomplished through a variety
of affective, motivational, and computational systems that narrow in on homicide as
the solution to adaptive problems.
The adaptive problems to which we are referring are fluid, unfolding and chang-
ing over time. As time passes and other individuals pursue adaptive strategies, the
nature of adaptive problems changes, and the solution to one set of adaptive prob-
lems may reliably create others. It is the reliable unfolding of adaptive problems that
shaped psychological adaptations in humans over evolutionary time, including those
that end others’ lives.
In sum, homicide adaptation theory proposes a new explanation of homicide:
Over the long expanse of human history, there were recurrent sources of conflict be-
tween individuals, such as conflict over reputation and social status, conflict over re-
sources, and conflict over romantic partners. Killing is hypothesized to be one among
an arsenal of context-contingent strategies shaped by natural selection to win con-
flicts with others. Homicide differs qualitatively from nonlethal solutions to conflict.
Once dead, a person can no longer damage the killer’s reputation, steal his resources,
prevent the killer from attracting a romantic partner, or have sex with the killer’s
spouse. According to homicide adaptation theory, our evolutionary heritage has en-
dowed all of us with a psychology to kill others. These psychological processes lead us
to entertain fantasies of killing and, in rare instances, act on them when we encounter
sources of conflict that were successfully won by homicide in the evolutionary past.
Pathways for the Evolution of Homicide Adaptations

Homicide adaptation theory proposes that there were more numerous selection pres-
sures for the evolution of psychological mechanisms that produce conspecific killings
in humans than there were in other animals, including other social primates. One
source of selection pressure is humans’ unique psychological mechanisms for social
exchange (Cosmides & Tooby, 2005). Human psychology is exquisitely sensitive to
the details of reciprocal exchange relationships. People have long memories for their
histories of social exchange with different individuals. We intuitively represent the
quantity and quality of social goods that change hands, the debts we owe to others,
and the debts that others owe us. We are capable of representing a range of repro-
ductively relevant resources, including tangible items (e.g., food, tools), social favors
(e.g., assistance in preparing food, shelter), and sexual access. The values of some
evolutionarily recurrent and reproductively relevant resources are hypothesized to
have been built into human cognitive systems by selection (e.g., sexual access, items
of food, protection from hostile conspecifics). Our calculus for social exchange intui-
tively represents equivalent values across different types of resources, allowing us to
perceive, for example, the number of arrowheads that would be equivalent in value
to a serving of meat or an introduction to a desirable potential mate. We also can rep-
resent future social debts and balances across resource types, designing near-optimal
plans for future patterns of resource allocation and debt.
We hypothesize that people have evolved to be equally adept at representing
the magnitude of costs inflicted on them by conspecifics. As with social debts and
balances, the values of reproductively relevant, ancestrally recurrent costs are hy-
pothesized to have been built into our evolved psychology by selection. In addition,
humans are hypothesized to have developed the ability to forecast the values of costs
that they may incur at the hands of conspecifics in the future.
Not all social exchange relationships are the same. Every category of social re-
lationship, be it a friendship or a mateship, has, at its foundation, an implicit social
contract—a mutually agreed-upon code of conduct that delineates the majority of
what constitutes acceptable and unacceptable behavior in the context of the relation-
ship. The rules of implicit social contracts are hypothesized to have been shaped by
selection and represent co-evolved compromises between the individual interests of
those involved in relationships. For example, having sex with a romantic partner is
viewed as acceptable conduct in the explicit social contract of long-term relationships
(Shackelford & Buss, 1996). It can be mutually beneficial to the reproductive success
of both participants. Having sex with someone other than one’s long-term romantic
partner, however, is a rule violation. It inflicts costs on the partner who was cheated
on. Different social relationships have different implicit rules of conduct. For example,
although having sex outside of a long-term romantic relationship usually constitutes
a rule violation for long-term mating relationships, it does not constitute a violation of
the rules that comprise the implicit social contracts between close genetic relatives.
These three classes of evolved mechanisms—those that track social debts, those
that represent the values of reproductively relevant resources, and those that form
the foundation of the implicit rules that guide social exchange for each type of an-
cestrally recurrent relationship—are hypothesized to have opened a new adaptive
design space in which natural selection could experiment with a range of behavioral
strategies to best rivals and achieve a fitness advantage. We propose that one of the
strategies was homicide.
Individuals can adopt a range of strategies to obtain benefits, including har-
vesting resources directly from the environment, cooperating with conspecifics to
be effective in this task, or inflicting costs on conspecifics to steal or to gain control
of resources. Because a co-evolved victim strategy against attack is to violently resist
attackers, selection should have favored the use of cost-inflicting strategies to gain
only resources of exceptional fitness value, valuable enough to outweigh the costs of
violent victim resistance and subsequent retaliation (Buss & Duntley, 2005; Duntley,
2005; Duntley & Buss, 2004; also see Chapter 11 of this volume). For resources
with the greatest fitness values, victims’ violent defenses and retaliation would have
evolved to be particularly fierce to protect their resources, defend hard-won reputa-
tions of being difficult to exploit, and reappropriate that which was forcibly taken.
The recurrent use of violence to defend the most valuable reproductively relevant
resources is hypothesized to have provided selection pressure for the only strategy
capable of completely and permanently eliminating the costs of victim resistance
and retaliation—homicide. Ironically, effective victim defenses against cost-inflicting
strategies may have selected for the ultimate cost-inflicting behavior.
Individuals also can adopt a range of behaviors to address costs inflicted by
others when they violate the rules of social contracts. Like strategies to obtain
benefits, a range of strategies is proposed to have evolved to deal with violations of
social contracts. Social contract violations that inflicted the greatest ancestral fit-
ness costs would have selected for evolved strategies powerful enough to stanch the
costs of the violations and prevent their recurrence. We hypothesize that the costs
resulting from some social contract violations were large enough to have favored
the evolution of homicidal strategies to address them. For example, sexual infidel-
ity represents a social contract violation for both men and women in the context
of a long-term mating relationship. For men, the fitness costs of a partner’s sexual
infidelity can be particularly large. Cuckolded men may unknowingly invest in a
rival’s offspring at a cost to their own genetic fitness. Being made a cuckold by an
unfaithful wife causes reputational damage to the husband in all cultures for which
there are relevant data (Daly & Wilson, 1988). Because experiencing deep emotional
bonding with another man is a reason women have affairs (Greiling & Buss, 2000),
unfaithful wives also may share information about their husband’s vulnerabilities,
secrets, and social weaknesses with their affair partners. The costs of being cuck-
olded would be particularly damaging for high-status men, who have the most to
lose from their partner’s affair. Higher-status men also are more protected from
being punished for inflicting costs on others and could more easily replace a mate
than lower-status men. The homicide of an unfaithful wife or the rival with whom
she was unfaithful would have deprived male competitors of her residual reproduc-
tive value, helped to ameliorate reputational damage, contained leaked information
about the husband’s vulnerabilities, and possibly eliminated the prenatal child of
a competitor. For these reasons, we hypothesize that killing by high-status men in
response to their partner’s infidelity was favored by selection.
In sum, we propose that cognitive adaptations that evolved to facilitate social
exchange in human relationships created a design space for the evolution of novel
behavioral strategies to best competitors that was unique among the social primates
and other species. Homicide is one strategy hypothesized to be selected for because
of its distinct evolutionary outcome—the permanent elimination of a strategically
interfering or cost-inflicting conspecific.
Intentionality
As shorthand, the description of how psychological adaptations function to produce
behavior is sometimes phrased as if the content of the cognitive processes is available
to conscious awareness and under the intentional control of the individual. There
are many possible functions of making the content of information processing avail-
able to conscious awareness. First, it may have no function, a by-product of memory
systems or of metacognitive mechanisms (Wegner, 2002). Second, conscious aware-
ness may be a true reflection of the most relevant or important information pro-
cessing that is occurring at a given time and may function to allow an individual
to exert his or her will. Third, conscious awareness may function only to motivate
behavior and not to afford humans “free will” over their actions. By this account,
our conscious experience of the world may or may not be a veridical representation
of events. Consistent with the logic of error management theory (Haselton & Buss,
2000), conscious experience may contain biases that lead to inaccuracies in the
representation of information and function only to motivate individuals to pursue
adaptive strategies. We propose that humans are aware of only a subset of their cog-
nitive machinery dedicated to homicide. Evidence from studies of homicidal ideation
suggests that some of the content of homicide mechanisms is available to conscious
awareness. But conscious awareness of the cognitive processes that motivate homi-
cidal behavior and a conscious intention to kill are not requisite features of adapta-
tions to kill others. All that is required in order to kill is a cognitive program capable
of producing behavior that reliably leads to the death of conspecifics. For some con-
texts, lack of conscious knowledge of the functioning of one’s homicide mechanisms
could be beneficial in a number of ways, including (a) preventing other mechanisms

from derailing or decreasing the likelihood of success of a homicidal strategy, or
(b) allowing killers to more convincingly argue to others that the death they caused
was unintentional, possibly decreasing or eliminating sanctions for the act.
Uncertainty
An important factor hypothesized to increase the complexity of killing others as
part of a strategy to solve adaptive problems is uncertainty. Varying degrees of
uncertainty pervade every aspect of adaptive problems solvable by homicide. There is
uncertainty about the reliability of the environmental cues that activate adaptations
for homicide. For example, is a rival having clandestine sexual encounters with a
person’s mate, or are the two of them just friends who enjoy each other’s company?
Uncertainty also surrounds the estimates of variables entered into calculations of
every aspect of a homicide scenario—from how much physical force a particular
weapon will require to end someone’s life, to how vigorously the victim will fight
back, to how easily the killing could be covered up, to how likely genetic relatives of
the victim will be to seek revenge. Seeking out additional information is one strategy
to decrease uncertainty. A person can test the strength of social alliances, check the
lethality of a weapon, or learn the daily routine of an intended victim to discover
when he or she is most vulnerable. Meticulous planning of every detail of a homicide
informed by additional information may also make killers’ minds more certain of the
outcome of their plans. Some degree of uncertainty, however, always remains.
As a homicidal strategy unfolds over time, some aspects of a situation may occur
in ways that were not anticipated. This can happen for at least three reasons. First,
incorrect knowledge may be entered into the calculations that form the plans for
homicide. For example, assumptions may be made about the formidability of a vic-
tim based only on the person’s size and weight and observations made of the person
in limited contexts. Uncertainty would remain about the range of possible effective
homicidal methods in the absence of observing the victim’s fighting ability. Second,
unanticipated events may confound a plan to kill. For example, a victim may unex-
pectedly bump into a friend while jogging in the evening, an activity he or she usu-
ally does alone. The presence of the friend may be enough to derail a killer’s plans
for the victim’s death. Finally, killers may fail to enter a relevant piece of information
into their homicidal plans. A homicide may be planned for night, for example, after a
victim is asleep in his or her house. Killers may fail to consider the extent to which the
darkness will cripple their ability to navigate through their victim’s home.
It is important to understand how uncertainty can limit the power of homicide-
scenario building for at least two reasons. First, it suggests that cognitive adaptations
for killing others must have evolved ways of dealing with the different kinds of un-
certainty. Second, it illustrates how errors in plans to kill that stem from problems of
uncertainty can derail an attempt at homicide and effectively save a victim’s life. In
many contexts, we propose that the psychology of would-be killers is not absolutely
committed to ending the life of another person rather than doing something else,
even if they have a complete plan for the homicide that they have begun to imple-
ment. Other intervening factors can redirect a killer’s homicidal strategy to nonle-
thal alternatives at any point in time until the moment when the victim is dead.
Homicide Adaptation Theory and Other

Theories of Conspecific Killing
There are many different explanations for homicide. Each has a unique perspective
on killing and seeks to explain different aspects of the psychological and behavioral
phenomena surrounding it. The purpose of homicide adaptation theory is to explain
the origins and functions of the psychological processes that produce reliable patterns
of homicidal behavior. By itself, it does not provide a complete explanation for why
every individual who commits homicide does so. A number of other factors may lead to
individual differences that influence the likelihood that a person will kill, including her-
itable individual differences in personality, exposure to violence during development,
frontal lobe damage, personality disorders, and psychopathology. An evolutionary
perspective on homicide is not at odds with any of these individual difference explana-
tions. Homicide adaptation theory proposes the existence of organized, species-typical
psychological processes responsible for producing homicidal behavior. Individual dif-
ference theories identify sources of variation between individuals that may affect an
individual’s probability of committing violent acts, including killing others.
Different theories of homicide need not be in competition. They are often comple-
mentary, capable of accounting for unique variance in why people kill others in any
individual case. When there is competition between different explanations of homi-
cide, it is most often between different theories at the same level of explanation. Buss
and Duntley (2006; under review) and Daly and Wilson (1988), for example, have
proposed competing theories at the ultimate level of explanation, which focus on the
evolutionary origins, design, and functions of psychological mechanisms involved
in producing homicide. We conclude this section with a discussion of the differences
between these two evolutionary explanations of homicide. Now, we consider a range
of other psychological explanations for why people kill and explore their compatibil-
ity with homicide adaptation theory.
Cultural Theories
Many explanations for homicide have focused on the role of cultural norms (Gelles &
Strauss, 1979; Rummel, 1991). According to these theories, homicide is the result
of exposure to cultural influences that may promote violence and which are incul-
cated into the human psyche. According to cultural theories, those individuals ex-
posed to cultural influences that promote homicide should be more likely to kill others
than those who are not exposed to such influences. Two similar examples of cultural
theories invoked to explain homicide are the subcultures of violence (Wolfgang &
Ferracuti, 1967) and culture of honor (Nisbett, 1993) theories. Designed to explain
why homicide rates vary from culture to culture, these theories propose that, at least
within the United States, some subcultures exist that encourage the use of violence
in settling interpersonal disputes.
These theories may help to explain some of the cultural variability in homicide
rates. For example, there is some evidence that conspecific killing is more common in
the cultures of honor in the southern United States than in cultures in the northern
United States that valorize violence less (Cohen, 1998). However, a limited number
of hypotheses have been derived from these theories, and only a minority have been
confirmed (Avakame, 1997; Hagan, Simpson, & Gillis, 1987; Simpson, 1991). In ad-
dition, Daly and Wilson (1989) have pointed out that many cultural theories are not
complete because they merely describe the cultural differences they are supposed to
explain. Similar arguments criticizing the circular reasoning of these theories have
been made by others (e.g., Hagan, Gillis, & Simpson, 1985).
Social Theories
One of the earliest social theories of crime was proposed by Sutherland (Sutherland
& Cressey, 1974). According to differential association theory, criminal behavior,
including homicide, is just another kind of behavior that is learned from people with
whom an individual interacts. Sutherland also argued that everyone has an equal
potential to learn to be a criminal.
Social learning theory was first proposed as a general explanation of human
behavior (Bandura, 1973) and only subsequently revised to explain some aspects of
homicide (Berkowitz, 1993). Patterson (1982), in another version of social learning
theory, argues that parents, teachers, and peers sometimes unintentionally reinforce
what starts out to be occasional and rather trivial antisocial behavior in children
that later escalates into serious offending behavior during adolescence. Patterson’s
version focuses primarily on experiences in the early years of development and little
on experiences during adolescence as causes of criminality. Social role theory (Eagly,
1995) and socialization theory (Berkowitz, 1993) share many assumptions with so-
cial learning theory. Each of these theories proposes that behaviors originate in the
process of observing and imitating others. Some behaviors are rewarded and oth-
ers are punished, gradually shaping an individual’s range of behaviors. These social
theories have been used to explain sex differences in homicide rates and the imitation
of violent behavior (Daly & Wilson, 1989).
A core assumption of social theories that leads them to predict that men should
be more likely to commit homicide than women is that observing violence in the
world causes violent behavior. Because humans observe more instances of men per-
petrating violent acts in life and in the media, the theories propose, men are more
likely than women to engage in similar behaviors. The causal arrow linking violence
in the world to the violent behavior of individuals, however, need not run in this
direction. For example, evidence shows that boys preferentially seek out violent toys
and media images (Hoyenga & Hoyenga, 1993). When parents encourage their boys
to be tough and their girls to be gentle, they may be responding to existing predispo-
sitions in each sex. Popular media may target boys with more violent programming
than girls to exploit the preferences and desires that each sex naturally possesses.
The imitation of violence in the media is also limited in its explanatory power as
a causative influence of homicide because it cannot explain evidence of killing in the
distant past. One of the earliest pieces of evidence for outright homicide comes from a
site in Shanidar, in Iraqi Kurdistan (Tattersall, 1999). This site, located in the Zagros
Mountains near the Turkish border, was excavated in the 1950s by archeologist Ralph
Solecki, and dates to about 60 to 100 thousand years ago. The human remains at this
site include nine different Neanderthal individuals. Their skeletons show varying de-
grees of trauma, but one stands out. Shanidar 3 is a fragmentary skeleton and includes
a partially healed injury on the top of the left ninth rib. The injury consists of a parallel-
sided groove. Pathologists who have seen it agree that it was caused by a penetrating
wound, about what one would expect if a right-handed individual stabbed Shanidar 3
while the two were standing face-to-face (Trinkhaus & Shipman, 1993).
In sum, cultural and social theories of homicide propose that the process of learn-
ing from the social environment is responsible for differences in homicide rates be-
tween cultures and differences in men’s and women’s propensity to kill. Cultures of
honor valorize violence as a solution to interpersonal disputes, and violence is socially
encouraged in male children but discouraged in female children. Learning is undoubt-
edly important for the adaptive calibration and activation of adaptations for homicide
and the pursuit of homicidal strategies, accounting for some of the variance in why
people kill. However, cultural and learning theories in their present form are too gen-
eral to generate specific hypotheses of how experience affects psychological processes
involved in producing homicide differently from psychological processes involved in ad-
dressing other domains of human experience, such as mating relationships and food
preferences. The addition of an evolutionary perspective to the study of how social
and cultural processes affect individuals’ psychology of homicide has great potential
to suggest fruitful directions for future research and may help to account for many ob-
served patterns of homicide (e.g., infanticide perpetrated primarily by young mothers)
(Gove, 1985). This would allow novel, specific predictions to be generated about trends
in homicide that may be the function of different social environments and help to ex-
plain why people sometimes commit homicide instead of doing something else.
Homicide Adaptation Theory and Cultural

and Social Theories of Homicide
Because ancestral environments were likely recurrently variable in a limited num-
ber of reproductively relevant ways (Tooby & Cosmides, 1990), learning adaptations
probably evolved to be sensitive to relevant patterns of variability in the environment

that provided information about the adaptiveness of employing homicide as the so-
lution to specific problems. Growing up in a country ravaged by war, for example,
may have the effect of leading an individual to be more likely to kill in response to
social conflicts with others than a person whose childhood development occurred
in a more peaceful environment, such as the wealthy suburbs of a city in the United
States. Homicide adaptation theory proposes that mechanisms that function to end
the lives of others should be sensitive to the costs and benefits of killing in the local
environment and should use that information to calibrate the likelihood of adopt-
ing a homicidal strategy. A decreased threshold for killing, in this example, is the
designed, adaptive product of adaptations for homicide. Implicit in this account of
homicide mechanisms is the existence of learning algorithms sensitive to specific en-
vironmental inputs that calibrate the action of homicide adaptations. It is beyond
the scope of this chapter to discuss the nature of evolved learning mechanisms.
From an adaptationist perspective, such mechanisms should have evolved to be as
domain-specific as the other design features of adaptations for homicide proposed in
this book and elsewhere (see Buss & Duntley, under review; Duntley, 2005; Duntley
& Buss, 2005).
Individual Differences as Sources of

Error Leading to Homicide
Individual difference factors may interact with evolved psychological mecha-
nisms for homicide to produce a decreased threshold for conspecific killing by
leading to the inappropriate activation of adaptations for homicide (see Cosmides
& Tooby, 1999, for a general discussion of this topic). The mistaken activation
of adaptations that lead to lethal aggression may have several sources, includ-
ing (a) the presence of evolutionarily novel stimuli in modern environments that
“trick” homicide adaptations into recognizing a problem as potentially solvable
by killing when it is not; (b) errors in the mechanisms that weigh the costs and
benefits of homicide, leading to the underestimation of costs, the overestimation
of benefits, or both; and (c) a failure of some mechanisms that are necessary for
the normal functioning of homicide adaptations to activate, leading to incom-
plete processing and the erroneous motivation of homicidal behaviors. In each
of these cases, the majority of evolved mechanisms for homicide would need to
function properly in order for a person to produce the organized behavior capable
of killing someone else. Thus, despite systematic errors at some levels of cognitive
processing, a complete explanation of killing others that is partially the result
of inappropriate activation of psychological adaptations or other psychological
dysfunction must still include an analysis of the evolved mechanisms involved.
One prominent source of psychological dysfunction is mental illness. A number
of different forms of psychopathology have been implicated in contributing to an
increased likelihood of killing.
Psychopathology Theories
Psychopathology is a factor in many homicides. Molecular genetic studies have begun

to identify the specific genes that may have some involvement in conspecific killing.
In one study of schizophrenics, a genetic polymorphism that led to low catechol
o-methyltransferase activity occurred more frequently among violent than nonvi-
olent schizophrenic patients and also occurred more frequently among homicidal
than nonviolent patients (Kotler et al., 1999).
A prospective study of major mental disorders and criminality conducted using
a birth cohort in Northern Finland found that violent offenses were most prevalent
among males with alcohol-induced psychoses or schizophrenic alcohol users. Those
suffering from depression were least likely to kill (Tiihonen, Isohanni, Rasanen, Koi-
ranen, & Moring, 1997). Research conducted in Australia (Mouzos, 1999) found
a similar trend in the disorders that are most common among killers, which also
include bipolar disorder, psychopathy, dissociative identity disorder, and unipolar
mania. In both studies, people with mental illness were found to be more likely than
people not suffering from a disorder to kill members of their families. Proximity the-
ory (Hindelang, 1976; Hindelang, Gottfredson, & Garofalo, 1978), long dismissed by
homicide researchers as an explanation for homicide (Daly & Wilson, 1988), may be
somewhat compatible with the trends in homicide apparent among the mentally ill.
The percentage of all homicides that can be attributed to psychopathology in a
particular region appears to be linked to the homicide rate. Where homicide is rare,
a higher proportion of killings are committed by people suffering from disorders like
schizophrenia or other psychoses. Where homicide is more frequent, a smaller percent-
age of killers are identified as suffering from major psychopathology. For example, stud-
ies of the perpetrators of homicide in Britain found that 39% of killers suffered from a
mental disorder (Gibson, 1975). In Sweden, 53% of killers were found to be mentally
ill (Lindqvist, 1986), as were 35% of Canadian killers (Cote & Hodgins, 1992). Britain,
Sweden, and Canada have some of the lowest homicide rates in the world (Ghiglieri,
1999). In contrast, only 19% of killers in New York City (Grumberg, Klinger, &
Grumet, 1977), 4.4% of killers in Detroit (Boudouris, 1974), and 4.4% of homicidal
offenders in Australia (Wallace et al., 1998) were found to suffer from mental illness.
These differences may provide insight into the evolved functioning of adaptations
for lethal aggression. In both the United States and Australia, there may be environ-
mental cues that are more likely to activate psychological adaptations for homicide
as the solution to adaptive problems faced by individuals. The global criminological
literature is not in a state for precise comparisons of the circumstances that may lead
to cultural differences in the cost-benefit calculus of whether an individual should kill
(Ellis & Walsh, 2000). However, some factors are possible candidates. The magnitude
of discrepancy in resources between the rich and the poor may be important (Wilson
& Daly, 1997). A greater resource discrepancy may lead to greater average payoffs for
adopting risky strategies. Cross-cultural differences in the reputational damage suf-
fered as a result of committing homicide versus the reputational benefit from being
known as a killer may also be important. The reputations of gang members in Los
Angeles and New York have been shown to improve after killing a member of a rival
gang (Alvarez & Bachman, 2002; Vigil, 2003). Higher-status gang members have
more mating partners (Ghiglieri, 1999). The reputational effects of committing ho-
micide are similar among the Yanomamo of Venezuela (Chagnon, 1988).
In sum, there is evidence that psychopathology is a contributing factor in some
(albeit a minority of ) homicides, and that these killers are more likely to manifest
symptoms of mental disease by engaging in nonadaptive forms of homicide, such as
eliminating genetic relatives. This does not mean, however, that psychopathology is
the sole cause of such homicides. Psychopathology and likely most personality dif-
ferences do not add additional information-processing capabilities to the adaptations
that produce homicide. These sources of individual differences more likely distort
cognitive adaptations, sometimes affecting the likelihood that a person will kill. An
individual with schizophrenia who has delusions that his mother is an extraterrestrial
who has plans to eliminate all of humanity, for example, obviously has errors in the
interpretation of information from the environment. Despite these errors in interpret-
ing input, the activation of psychological mechanisms to produce homicide may be
appropriate and adaptive if indeed his mother were an extraterrestrial. It is difficult to
kill someone. The production of a sequence of behaviors capable of successfully end-
ing another person’s life requires a large number of calculations that cognitive system
errors, by themselves, would be incapable of producing. One reasonable hypothesis is
that psychopathology leads to the inappropriate activation of patterned mechanisms
capable of producing successful homicidal behavior. A challenge for future research
is the identification of how, specifically, different forms of psychopathology interact
with the psychological processes that produce homicide to lead to the inappropriate
motivation of lethal aggression.
Homicide as a By-product of Other Adaptations

Adaptations for homicide need not be involved in the production of all homicidal
behavior. Another evolutionary explanation of killing was proposed first by Daly and
Wilson in their book Homicide (1988). According to Daly and Wilson, homicide may
be considered an overreactive mistake, the by-product of psychological adaptations
designed for nonlethal outcomes. They argue that homicide should only be used “as
a sort of ‘assay’ of the evolved psychology of interpersonal conflict” and that it “does
not presuppose that killing per se is or ever was adaptive” (Wilson, Daly, & Daniele,
1995, p. 12). For example, the behavior of a teenage mother who abandons her new-
born in a dumpster to die may be explained by the failure of her psychological mech-
anisms for parenting to engage. Similarly, in the case of a husband who kills his wife
for being sexually unfaithful, Daly and Wilson have argued that male mechanisms
for sexual jealousy and the coercion and control of their mates may mistakenly
overreact, leading the man to kill his wife. Despite their contention that homicide
is a maladaptive by-product of psychological adaptations, Daly and Wilson (1989)
emphasize that an evolutionary account of homicidal behavior is extremely impor-

tant: “[W]hat is needed is a Darwinian psychology that uses evolutionary ideas as
a metatheory for the postulation of cognitive/emotional/motivational mechanisms
and strategies” (pp. 108–109).
There are reasons to question whether Daly & Wilson’s (1988) theory is an ad-
equate explanation of all homicides. First, if lethal aggression has never been adap-
tive, as Daly and Wilson propose it may not have been, then selection could not have
fashioned adaptations for homicide. The only remaining possibilities are that homi-
cide was neutral in terms of selection or that it had negative selective consequences.
In contexts in which homicide yielded recurrently negative fitness consequences,
there would have been active selection pressure against killing others. Yet currently,
homicide continues to take place. In some cultures, the lifetime risk of being killed is
as high as 1 in 3 for men (Ghiglieri, 1999). Daly and Wilson do not explain how a be-
havior with negative selective consequences could be maintained over our evolution-
ary history, but there are at least two possible explanations. First, the overall benefits
of psychological adaptations that sometimes produce homicide as a by-product may
have outweighed the occasional costs associated with killing a conspecific over our
evolutionary history. One other, related possibility is that selection has operated to
eliminate by-product homicides in contexts where such killing was too costly, modi-
fying or fashioning new psychological mechanisms for this purpose. This explana-
tion, however, is no longer a strict by-product hypothesis of the origins of homicide.
It suggests that selection has acted to inhibit homicide in some contexts while allow-
ing it to persist in others. Instead of an argument against adaptations for homicide,
this seems a plausible explanation for the origins of homicide adaptations—through
the gradual selection of mechanisms to produce homicide in the rare subset of situ-
ations in which killing leads to greater benefits than costs.
Second, the by-product explanation of homicide fails to identify the specific
overreactive cognitive mistakes that lead people to kill. In fact, the by-product the-
ory does not explore how information is processed in any of the adaptations shaped
for their nonlethal consequences that sometimes lead people to kill. Without under-
standing their normal function, it is impossible to determine how these mechanisms
may malfunction to produce homicide. Third, the by-product theory of homicide
has difficulty explaining the double standard it applies to conspecific killing in other
species and homicide in humans. Humans are not the only species that kill their
own kind.
Numerous species kill conspecifics in predictable contexts. Among insects (in-
cluding mantids, black widow spiders, jumping spiders, and scorpions), females com-
monly end the lives of their male mates when subsequent consumption of the male
leads to a greater number or increased viability of offspring. The males of these spe-
cies do not sacrifice themselves willingly. In the sexually cannibalistic black widow
spider Latrodectus mactans, for example, males that escape their cannibalistic mates
can often fertilize multiple other partners (Breene & Sweet, 1985). Males across sexu-
ally cannibalistic species use a diverse array of strategies to decrease their chances of
being eaten by their mates: male scorpions sometimes sting their mates after deposit-
ing their spermatophore (Polis & Farley, 1979); male crab spiders (Bristowe, 1958)
and black widows (Gould, 1984) sometimes wrap up females in silk before mating
with them.
Among mammals, there are many well-documented patterns of conspecific kill-
ing. Male lions, wolves, hyenas, cougars, and cheetahs have been observed to kill the
offspring of rival males (Ghiglieri, 1999). Killer lions often benefit because the mothers
of the infants that are killed go into estrus sooner, allowing the infanticidal males to
impregnate them earlier. Among primate species, infanticides have been documented
in similar contexts, including langur monkeys (Hrdy, 1977), red howler monkeys
(Crockett & Sekulic, 1984), mountain gorillas (Fossey, 1984), chimpanzees (Bygott,
1972), and others (Hausfater & Hrdy, 1984). The killing of rival adult males has also
been well documented among mountain gorillas (Fossey, 1984) and the chimpanzees
of Gombe (Wrangham & Peterson, 1996), two of our closest genetic relatives. In all
of these species, the researchers concluded that the conspecific killings were evolved
strategies shaped by selection to produce lethal consequences. Without marshalling
any empirical evidence in support of its contention, the by-product theory of homi-
cide argues that humans are different from all other animal species.
Finally, the by-product theory of homicide has difficulty accounting for premedi-
tated killings—those perpetrated by people who have planned out their deadly act
for weeks, months, or even years. Premeditated homicides are likely only the tip of
the iceberg of cognitive effort devoted to killing. The majority of male and female un-
dergraduates report having at least one homicidal fantasy in their lifetime (Kenrick &
Sheets, 1993). The by-product theory of homicide has no explanation for the exis-
tence of homicidal ideation and no explanation for why people devote a significant
amount of time and cognitive energy to building scenarios about ending the life of
another individual. The by-product theory also does not specify whether homicidal
ideations are also by-products of mechanisms selected for their nonlethal conse-
quences or if they may be adaptive. Advocates of the by-product theory have not
addressed the topic of homicidal ideations at all.
From the adaptationist perspective of homicide adaptation theory, the contexts
that produce homicide as a by-product are unlikely to be contexts for which homicide
evolved to be a possible solution. True by-product homicides should not be associated
with circumstances that could be adaptively addressed with lethal aggression. For
example, a single woman at a wedding who dies as a result of being pushed against
a wall by other women seeking to obtain the bouquet tossed by the bride has not
died as a result of adaptations for killing. Her death was more likely the result of
adaptations for social or mate competition selected for their nonlethal outcomes. If
the death of an individual could have led to a net benefit, on average, over our evo-
lutionary history, it is plausible that the killing could be the functional output of
adaptations for homicide. Homicide adaptation theory proposes that the majority
of killings are the functional outputs of adaptations to produce lethal aggression
(Buss & Duntley, under review; 2006).
By this logic, there are no random homicides. Even those killings that are influ-
enced by severe psychological disorders may have random or inappropriate targets,
but random cognitive processes are unlikely to be able to produce the highly pat-
terned sequences of behaviors that might lead to a person’s death. Even homicides
that may be true by-products of adaptations designed for their nonlethal outcomes
are likely not to be patterned randomly but instead to be highly correlated with spe-
cific categories of interpersonal conflict over reproductively relevant resources.
In sum, nonadaptationist explanations of homicide may be able to predict some
variance in who is likely to become a killer and to identify some broad features of
contexts that may trigger homicidal behavior. When considered individually, they
all share similar weaknesses, which include (a) a failure to provide a comprehensive
explanation of the patterns of homicide; (b) not making predictions about when ho-
micide, instead of some other criminal behavior, is likely to occur; (c) not offering ex-
planations for a large number of the observed patterns of homicide; (d) not specifying
whether homicide is a kind of criminal behavior that could have ever been adaptive
during our evolutionary history; (e) failure to provide an explanation for why people
who are not pursuing a general strategy of criminality would ever commit homicide;
(f) an inability to explain why the majority of normal people report experiencing hom-
icidal fantasies; and (g) failure to explain the patterns of people’s homicidal fantasies.
Homicide Adaptations and the Law
Homicide adaptation theory proposes that humans have evolved adaptations to kill
to solve a variety of adaptive problems, from self-defense homicide to warfare killings.
From a legal perspective, however, some homicides are viewed as warranting more se-
vere punishment than others. Some contexts of homicide are viewed as murder, oth-
ers manslaughter, and some not as crimes at all (e.g., self-defense homicide; defense
of kin). A “crime of passion” in which a man kills his wife or a mate poacher when he
finds them in bed together often gets treated with special leniency in the courtroom
compared to a man who has a long, detailed, premeditated plan to kill his wife or the
poacher. From our adaptationist perspective, both killings are the products of evolved
psychological circuitry, and both have the same outcome. Legally, however, they are
treated differently. Why?
Like Krebs (Chapter 12, this volume), we hypothesize that the answer can be
found in our human evolutionary history. Like people today, ancestral humans were
extraordinarily social. For most of our uniquely human evolutionary history, our
ancestors lived in groups of between roughly 50 and 150 individuals.They depended
on a web of relationships with others to survive and reproduce. Conflicts between
individuals that led to homicide would have directly affected the killer and the victim,
but also would have impacted everyone else in the social group.
Some homicides would have delivered an average benefit to members of the so-
cial group, while others would have delivered a cost. For example, a killing might
free up resources that benefit a large number of individuals in the group or eliminate
a cost-inflicting, exploitative bully. Alternatively, homicide could be used as a tac-
tic of exploitation to monopolize resources or assert greater social dominance. We
hypothesize that homicides that recurrently delivered benefits to individual group
members would have selected for cognitive biases leading people to consider killing in
such contexts to be justified. Homicides that recurrently led to the incursion of costs
among individual group members, however, would not be viewed as justified. This
perspective helps to clarify why many warfare homicides, killing members of rival
groups, are not typically viewed as crimes. When groups compete with one another
over reproductively relevant resources such as territory or women, killing those in
the rival groups typically weakens the victimized groups, allowing resources to flow
more freely to the group that successfully kills.
This is, clearly, a simplified account. Who receives benefits and who incurs costs
largely would have been a function of individuals’ relationships to the killer and the
victim. Allies of the exploitative bully might suffer from decreased access to resources
and a loss of protection. Allies of a person who kills to maintain dominance and mo-
nopolize resources would stand to benefit from their association. Thus, people’s biases
about whether homicide is justified are hypothesized to be highly context-sensitive.
Essentially, however, the same logic applies. Individuals should view homicides as
more justified when the resources that flow to them as a result are greater.
Our ancestors benefited from stable, cooperative relationships with others in
their social group. Many reproductively relevant resources can be obtained more
easily and efficiently with the assistance of others. The killing of one individual by
another had the potential to polarize ancestral communities and destabilize coopera-
tive alliances. Family members and close allies of both the killer and the victim likely
would have appealed to others in the group for support. Individuals would have ben-
efited from predicting how particular contexts of homicide would be viewed by oth-
ers in their group. This would have allowed them to strategically distance themselves
from killers more likely to be ostracized or otherwise punished by the group.
Cosmides and Tooby (1999) have proposed that humans have evolved adapta-
tions for social exchange, including psychological mechanisms to detect cheaters.
Cheaters fail to reciprocate. They are individuals who accept a benefit from someone
else without fulfilling the agreed-upon requirement for it. The magnitude of cheat-
ing is a product of the discrepancy between the values of the resources exchanged.
A person who is paid $100 an hour and sleeps on the job is a bigger cheater than a
person who is paid $5 an hour and sleeps on the job.
We hypothesize that humans apply rules of cheater detection to evaluate the
degree of justification or evilness of different contexts of homicide. Contexts of ho-
micide in which a killing occurs in response to or to prevent heavy cost-infliction by
the victim will be viewed as more justified than contexts of killing in which the costs
from the victim are lower. For example, a husband who kills his wife after she has
sex with another man would be viewed as more justified and less evil than a hus-
band who kills his wife after she kisses another man. A husband whose wife has sex
with another man incurs greater evolutionary costs because he may end up invest-
ing in another man’s child. A woman who kills a badly deformed newborn would be
viewed as more justified than a woman who kills a healthy baby of the same age. A
deformed newborn is unlikely to reproduce and represents a fitness cost to its mother.
A healthy newborn could reproduce and deliver fitness benefits to its mother. In sum,
our sense of whether a homicide is justified or evil is hypothesized to be a function of
the magnitude of discrepancy between the costs to the victim of being killed and the
past or likely future costs inflicted on the killer by the victim. Greater discrepancies
will be viewed as less justified and more evil.
Many aspects of human legal systems have been argued to represent the codi-
fication of our evolved sense of morality and justice (see Chapter 11). When it
comes to homicide, we propose that patterns in the severity of homicide charges
and punishments result from evolved patterns in our representations of the costs
incurred by victims and killers. When people kill in response to or to prevent high
fitness costs, their penalties tend to be lower (Costanzo, 2003). Penalties also tend to
be lower when victims suffer lower fitness costs by losing their lives.
Conclusion
In this chapter, we outlined homicide adaptation theory and explored its fundamen-
tal logic. We discussed examples of the unique selection pressures created by human
cognitive adaptations for social exchange that are hypothesized to have selected for
homicide. We compared homicide adaptation theory to nonadaptationist explana-
tions for conspecific killings in humans. Finally, we explored why the law has a range
of treatments for different contexts of homicide. There is much work to be done be-
fore we have a complete understanding of the causes of homicide. Given the avail-
able evidence, we are confident that homicide adaptation theory is significant step
in the right direction. It provides a framework for viewing homicide not as a unitary
phenomenon but as a collection of diverse phenomena. It leads to a host of novel
hypotheses about the psychological mechanisms tributary to killing. And it parsi-
moniously accounts for existing patterns of homicide that are inexplicable on more
domain-general and adaptation-agnostic theories of homicide.
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4
Intimate Partner Violence
AARON T. GOETZ, TODD K. SHACKELFORD, VALERIE G. STARRATT,

AND WILLIAM F. MCKIBBIN
Violence in Families
The theory of evolution by natural selection revolutionized the study of biology.

So too is it revolutionizing the study of human psychology and behavior. Charles
Darwin himself predicted, “Psychology will be based on a new foundation, that of
the necessary acquirement of each mental power and capacity by gradation. Light
will be thrown on the origin of man and his history” (1859, p. 488). Modern evo-
lutionary psychological perspectives have been used to predict and understand a di-
verse array of human behaviors, such as altruism, mating, and violence. In the past
few decades, many psychologists have begun to recognize the value of using an evo-
lutionary perspective to guide their research. With a focus on evolved mechanisms
and associated information-processing features, evolutionary psychology has risen
as a powerful heuristic tool for the study of human psychology and behavior. Evolu-
tionary psychology leads researchers to look at old phenomena in a different light.
Such a new perspective potentially offers powerful insights into human psychology
and behavior. In this chapter, we use the tools provided by evolutionary theory to ex-
plore why violence and abuse occur between intimate partners. Specifically, we focus
our discussion on physical and sexual intimate partner violence.
Paternal Uncertainty and the Function

of Male Sexual Jealousy
Jealousy is an emotion that is experienced when a valued relationship is threatened
by a real or imagined rival, and it generates responses aimed at stifling the threat.
Jealousy functions to maintain relationships by motivating behaviors that deter
65
rivals from mate poaching and deter intimate partners from infidelity or outright
departure from the relationship (Buss, Larsen, Westen, & Semmelroth, 1992; Daly,
Wilson, & Weghorst, 1982; Symons, 1979). Because ancestral men and women
recurrently faced the adaptive problems of retaining partners and maintaining
relationships over human evolutionary history, men and women today do not dif-
fer in the frequency or intensity of experienced jealousy (Shackelford, LeBlanc,
& Drass, 2000; White, 1981). A sex difference, however, is evident when consid-
ering two basic types of jealousy: emotional and sexual. This sex difference coin-
cides with sex differences in the adaptive problems that ancestral men and women
recurrently had to solve over human evolutionary history in the context of their
mating relationships (Buss, 2000; Symons, 1979). Ancestral women’s adaptive
problem of securing the paternal investment needed to raise offspring exerted a se-
lection pressure for women to be more sensitive to and more distressed by cues as-
sociated with a partner’s emotional infidelity. Ancestral men’s adaptive problem of
paternal uncertainty (i.e., uncertainty regarding biological parenthood), however,
exerted a selection pressure for men to be more sensitive to and more distressed by
cues associated with a partner’s sexual infidelity. Because emotional infidelity and
sexual infidelity were highly correlated throughout evolutionary history (i.e., if an
individual were engaging in one type of infidelity, he or she was often engaging in
the other type of infidelity as well), researchers studying sex differences in jealousy
have used forced-choice methods in which participants are asked to select which
type of partner infidelity upsets them more. Recently, however, some researchers,
such as Sagarin, Becker, Guadagno, Nicastle, and Millevoi (2003) and Wiederman
and Allgeier (1993), also have found a sex difference in jealousy using continu-
ous measures. At least two dozen studies have provided evidence of this sex differ-
ence in jealousy, documenting that men experience greater jealousy in response to
the sexual aspects of an intimate partner’s infidelity whereas women experience
greater jealousy in response to the emotional aspects of an intimate partner’s in-
fidelity. These results are corroborated by experimental data (e.g., Schützwohl &
Koch, 2004), physiological data (Buss et al., 1992), patterns of divorce (Betzig,
1989), and the behavioral output of jealousy, such as mate retention behaviors
(e.g., Buss & Shackelford, 1997).
Men’s sensitivity to and distress as a result of a partner’s sexual infidelity
are not surprising given the severe reproductive costs to men of cuckoldry (the
unwitting investment of resources in genetically unrelated offspring). Some of
the costs of cuckoldry include the potential misdirection of a man’s resources
to a rival’s genetic offspring, his partner’s investment in a rival’s genetic off-
spring, and reputational damage if the cuckoldry becomes known to others (see
Buss, 2000; Platek & Shackelford, 2006). Perhaps with the exception of death,
cuckoldry is associated with the most severe reproductive costs for an individual
man. It is therefore likely that selection will have resulted in the evolution of
male strategies and tactics aimed at avoiding cuckoldry and decreasing paternal
uncertainty.
Intimate Partner Violence 67
Intimate Partner Violence and Sexual Jealousy

Male sexual jealousy is one of the most frequently cited causes of intimate partner
violence (e.g., Buss, 2000; Daly & Wilson, 1988; Daly et al., 1982; Dobash & Dobash,
1979; Dutton, 1998; Dutton & Golant, 1995; Frieze, 1983; Gage & Hutchinson,
2006; Russell, 1982; Walker, 1979). Intimate partner violence is a tactic used by
men to restrict a partner’s sexual behavior (Buss & Malamuth, 1996; Daly & Wilson,
1988; Wilson & Daly, 1996) and may be best understood as a behavioral output of
male sexual jealousy. A man may afford his partner many freedoms, but these free-
doms rarely include sexual activity with other men (Buss, 1996, 2000). Men are
hypothesized to have evolved mechanisms dedicated to generating risk assessments
of a partner’s sexual infidelity. These mechanisms include, for example, assessments
of the time spent apart from his partner (i.e., time during which she might have been
sexually unfaithful), the presence of potential mate poachers, his partner’s repro-
ductive value (i.e., expected future reproduction) and fertility (i.e., current likeli-
hood of conceiving), and his partner’s likelihood of committing infidelity (e.g., Goetz
& Shackelford, 2006; Peters, Shackelford, & Buss, 2002; Schmitt & Buss, 2001;
Shackelford & Buss, 1997; Shackelford et al., 2002; Trivers, 1972; Wilson & Daly,
1993). Moreover, the male mind may be designed to be hypersensitive to cues of
his partner’s sexual infidelity, motivating more false positives than false negatives
because the benefits of the former outweigh the costs of the latter (Haselton & Nettle,
2006). Together with assessments of the likelihood of a partner’s sexual infidelity,
contextual factors—such as social and reputational costs, proximity of the partner’s
adult male kin (who might be motivated to retaliate for a man’s violence against his
partner), and economic dependency (Figueredo & McClosky, 1993; Wilson & Daly,
1993)—are processed by mechanisms of the male mind to inhibit or motivate men
to inflict violence on their partners.
Occasionally, men’s use of violence against their partners is lethal. As with non-
lethal partner violence, male sexual jealousy is a frequently cited cause of intimate
partner homicide across cultures (Daly & Wilson, 1988; Serran & Firestone, 2004).
Killing an intimate partner is costly. But under specific circumstances, might the ben-
efits have outweighed the costs enough for selection to produce a psychology that mo-
tivates partner killing? According to Daly and Wilson (Daly & Wilson, 1988; Wilson
& Daly, 1998; Wilson, Daly, & Daniele, 1995), killing an intimate partner is not the
designed product of evolved mechanisms but instead is a by-product of mechanisms
selected for their nonlethal outcomes. This by-product or “slip-up” hypothesis states
that men who kill their partners have “slipped up” in that their violence—which was
intended to control an intimate partner’s sexual behavior—inadvertently resulted in
their partner’s death.
The by-product hypothesis is attractive in that it would seem too costly to kill
an intimate partner. Why kill a partner and risk the enormous costs that often flow
from such actions when a man could simply end the relationship with the woman
he suspects of sexual infidelity? But consider this: If killing an intimate partner is
a slip-up or accident, as argued by Daly and Wilson, why are so many partner ho-
micides apparently premeditated? Hiring someone to kill a partner, aiming at and
shooting a partner with a firearm, and slitting a partner’s throat appear to be in-
tentional killings, not accidental killings. Although some partner homicides may
be accidental, too many seem premeditated and intended. This is one observation
that led Buss and Duntley (1998, 2003; see also Buss, 2005) to propose that many
intimate partner homicides are motivated by evolved mechanisms designed to moti-
vate killing under certain conditions. Discovering a partner’s sexual infidelity, Buss
and Duntley argue, may be a special circumstance that motivates partner homicide.
This “homicide adaptation theory” does not argue that discovering a partner’s infi-
delity inevitably leads to homicide, but it does suggest that this circumstance would
activate mechanisms associated with weighing the costs and benefits of homicide,
and that under certain circumstances partner killing might be the designed out-
come (for a fuller treatment, see Buss, 2005).
Daly and Wilson’s (1988; Wilson & Daly, 1998; Wilson et al., 1995) and Buss
and Duntley’s (1998, 2003; Buss, 2005) competing hypotheses have not yet been
examined concurrently to determine which hypothesis best accounts for the data
(but see Shackelford, Buss, & Weekes-Shackelford, 2003). Our intention is not to crit-
ically evaluate these competing hypotheses. We intend to argue that intimate part-
ner homicide, by design or as a by-product, is often the behavioral output of male
sexual jealousy stemming from paternal uncertainty.
Men’s “mate retention” or “mate guarding” behavior is another example of the
behavioral output of jealousy. Buss (1988) identified specific mate guarding behav-
iors, such as vigilance (e.g., dropping by unexpectedly to check up on a partner) and
concealment of mate (e.g., taking a partner away from a social gathering where
other men are present). These mate guarding behaviors vary in ways that suggest
they are produced by mechanisms that evolved as paternity guards. For example,
a man guards his partner more intensely when she is of greater reproductive value
(as indexed by her youth and attractiveness) and when the perceived probability of
her sexual infidelity is greater (Buss & Shackelford, 1997). In addition, men who are
partnered to women who have characteristics that make them more likely to com-
mit sexual infidelity guard their partners more intensely (Goetz et al., 2005). Men
also guard their partners more intensely after spending a greater proportion of time
apart from them—a situation that inherently increases the possibility of sexual in-
fidelity (Starratt, Shackelford, Goetz, & McKibbin, in press)—and when she is near
ovulation, a time when an extra-pair copulation or sexual infidelity would be most
costly for the in-pair man (Gangestad, Thornhill, & Garver, 2002).
Recognizing that men’s mate retention behaviors are manifestations of jealousy,
Shackelford, Goetz, Buss, Euler, and Hoier (2005) investigated the relationships
between men’s mate retention behaviors and intimate partner violence, specifi-
cally whether some mate retention behaviors and seemingly innocuous romantic
gestures may be harbingers of violence. Securing self-reports from men, partner-
reports from women, and cross-spouse reports from married couples, Shackelford
and his colleagues found that men’s use of particular mate retention behaviors was
related to partner violence in predictable ways. For example, men who dropped by
unexpectedly to see what their partner was doing or who told their partner that she
would “die” if she ever left him were most likely to use serious violence against their
partners, whereas men who attempted to retain their partners by expressing affec-
tion and displaying resources were least likely to use violence against their partners.
These findings corroborated the results of research conducted by Wilson, Johnson,
and Daly (1995), who found that women who affirmed statements such as, “He in-
sists on knowing who you are with and where you are at all times” and “He tries to
limit your contact with family or friends” were twice as likely to have experienced
serious violence by their partners.
Sexual Violence in Intimate Relationships

and Sexual Jealousy
Between 10% and 26% of women experience rape in marriage (Abrahams, Jewkes,
Hoffman, & Laubscher, 2004; Dunkle et al., 2004; Finkelhor & Yllo, 1985; Hadi, 2000;
Painter & Farrington, 1999; Russell, 1982; Watts, Keough, Ndlovu, & Kwaramba,
1998). Rape also occurs in nonmarital intimate relationships. Goetz and Shackelford
(2006) secured prevalence estimates of rape in intimate relationships from a sample
of young men and from an independent sample of young women in a committed
relationship. They documented that 7.3% of men admitted to raping their current
partner at least once, and 9.1% of women admitted that they had experienced at
least one rape by their current partner. Rape by physical force is just one form of
sexual coercion in intimate relationships (Koss & Oros, 1982; Weis & Borges, 1973).
Pressure may take the form of threats of violence, physical force, or intoxication but
also may include more subtle tactics such as emotional manipulation (Shackelford &
Goetz, 2004). Questions concerning sexual coercion and rape in relationships often
do not encompass this wide range of behaviors; they also are emotionally loaded,
and may be subject to social desirability concerns. These percentages therefore may
be underestimates of the prevalence of rape in intimate relationships among young
men and women who are not married.
Many hypotheses have been generated to explain why, across cultures, women
are sexually coerced by their partners. Some researchers have hypothesized that
sexual coercion in intimate relationships is motivated by men’s attempts to domi-
nate and control their partners (e.g., Basile, 2002; Bergen, 1996; Frieze, 1983;
Gage & Hutchinson, 2006; Gelles, 1977; Meyer, Vivian, and O’Leary, 1998; Watts
et al., 1998) and that this expression of power is the product of men’s social roles
(e.g., Brownmiller, 1975; Johnson, 1995; Yllo & Straus, 1990). Results relevant to
this hypothesis are mixed. Several studies have found that physically abusive men are
more likely than nonabusive men to sexually coerce their partners (Apt & Hurlbert, 1993;
DeMaris, 1997; Donnelly, 1993; Finkelhor & Yllo, 1985; Koziol-McLain, Coates, &
Lowenstein, 2001; Shackelford & Goetz, 2004), a result that is consistent with the
domination and control hypothesis. Gage and Hutchinson (2006), however, found
that women’s risk of sexual coercion by their partners is not related to measures
assessing the relative dimensions of power in a relationship, such as who has more
control over decision making. That is, women partnered to men who hold the domi-
nant position in the relationship are not more likely to experience sexual coercion
by their partners than women partnered to men who do not maintain the dominant
position in the relationship, a result that does not support the domination and con-
trol hypothesis. Although many researchers agree that individual men may sexually
coerce their partners to gain or maintain dominance and control in the relation-
ship, proponents of the domination and control hypothesis often argue that men are
motivated as a group to exercise “patriarchal power” or “patriarchal terrorism” over
women (e.g., Brownmiller, 1975; Johnson, 1995; Yllo & Straus, 1990).
An alternative hypothesis has been advanced by researchers studying sexual co-
ercion from an evolutionary perspective: sexual coercion in intimate relationships
may be related to paternal uncertainty, with the occurrence of sexual coercion re-
lated to a man’s suspicions of his partner’s sexual infidelity (Camilleri, 2004; Goetz &
Shackelford, 2006; Lalumière, Harris, Quinsey, & Rice, 2005; Thornhill & Thornhill,
1992; Wilson & Daly, 1992). Sexual coercion in response to cues of his partner’s
sexual infidelity might function to introduce a male’s sperm into his partner’s repro-
ductive tract at a time when there is a high risk of cuckoldry (i.e., when his partner
has recently been inseminated by a rival male). This sperm competition hypothesis
was proposed following recognition that forced in-pair copulation (i.e., partner rape)
in nonhuman species followed female extra-pair copulations (sexual infidelities; e.g.,
Barash, 1977; Cheng, Burns, & McKinney, 1983; Lalumière et al., 2005; McKinney,
Cheng, & Bruggers, 1984) and that sexual coercion and rape in human intimate
relationships often followed men’s accusations of their partners’ sexual infidelity
(e.g., Finkelhor & Yllo, 1985; Russell, 1982). Before considering the case of partner
rape in humans, we review briefly the animal literature on forced in-pair copulation.
Examining the adaptive problems and evolved solutions to these problems in non-
human animals may provide insight into the adaptive problems and evolved solu-
tions in humans (and vice versa). Shackelford and Goetz (2006), for example, argued
that because humans share with some avian species a similar mating system (social
monogamy) and similar adaptive problems (e.g., paternal uncertainty, paternal in-
vestment in offspring, cuckoldry), humans and some birds may have evolved similar
solutions to these adaptive problems. Identifying the contexts and circumstances in
which forced in-pair copulation occurs in nonhuman species may help us to under-
stand why forced in-pair copulation occurs in humans.
Forced In-Pair Copulation in Nonhuman Animals

Instances of forced in-pair copulation are relatively rare in the animal kingdom,
primarily because males and females of most species (over 95%) do not form long-
term pair-bonds (Andersson, 1994). Without the formation of a pair-bond, forced
in-pair copulation, by definition, cannot occur. Many avian species form long-term
pair-bonds, and researchers have documented forced in-pair copulation in several
of these species (Bailey, Seymour, & Stewart, 1978; Barash, 1977; Birkhead,
Hunter, & Pellatt, 1989; Cheng et al., 1983; Goodwin, 1955; McKinney et al.,
1984; McKinney & Stolen, 1982). Forced in-pair copulation reliably occurs im-
mediately after female extra-pair copulations, intrusions by rival males, and female
absence in many species of waterfowl (e.g., Bailey et al., 1978; Barash, 1977; Cheng
et al., 1983; McKinney, Derrickson, & Mineau, 1983; McKinney & Stolen, 1982;
Seymour & Titman, 1979) and other avian species (e.g., Birkhead et al., 1989; Good-
win, 1955; Valera, Hoi, & Kristin, 2003). Forced in-pair copulation following observed
or suspected extra-pair copulation in these avian species is often interpreted as a sperm
competition tactic (Barash, 1977; Cheng et al., 1983; Lalumière et al., 2005; McKinney
et al., 1984).
Sperm competition is a form of male-male postcopulatory competition. Sperm
competition occurs when the sperm of two or more males concurrently occupy the
reproductive tract of a female and compete to fertilize her egg(s) (Parker, 1970). Males
can compete for mates, but if two or more males have copulated with a female within
a sufficiently short period of time, males must compete for fertilizations. Thus, the
observation that in many avian species forced in-pair copulation immediately fol-
lows female extra-pair copulations has been interpreted as a sperm competition tactic
because the in-pair male’s forced in-pair copulation functions to place his sperm in
competition with sperm from an extra-pair male (Birkhead et al., 1989; Cheng et al.,
1983). Reports of forced in-pair copulation in nonhuman species make it difficult to
claim that males rape their partners to humiliate, punish, or control them—as is often
argued by some social scientists who study rape in humans (e.g., Pagelow, 1988).
Mounting evidence suggests that sperm competition has been a recurrent and
important feature of human evolutionary history. Psychological, behavioral, physi-
ological, anatomical, and genetic evidence indicates that ancestral women some-
times mated with multiple men within sufficiently short time periods so that sperm
from two or more males concurrently occupied the reproductive tract of a woman
(Baker & Bellis, 1993; Gallup et al., 2003; Goetz et al., 2005; Kilgallon & Simmons,
2005; Pound, 2002; Shackelford & Goetz, in press; Shackelford & Pound, 2006;
Shackelford, Pound, & Goetz , 2005; Shackelford et al., 2002; Smith, 1984; Wyckoff,
Wang, & Wu, 2000). This adaptive problem led to the evolution of adaptive solu-
tions to sperm competition. For example, men display copulatory urgency, perform
semen-displacing behaviors, and adjust their ejaculates to include more sperm when
the likelihood of female infidelity is higher (Baker & Bellis, 1993; Goetz et al., 2005;
Shackelford et al., 2002).
The selective importance of sperm competition in humans, however, is an issue
of scholarly debate. Those questioning the application of sperm competition to hu-
mans (e.g., Birkhead, 2000; Dixson, 1998; Gomendio, Harcourt, & Roldán, 1998)
do not contend that sperm competition in humans is not possible or unlikely but
rather that it may not be as intense as in other species with adaptations to sperm
competition. When considering all the evidence of adaptations to sperm competition

in men and current nonpaternity rates, which range from 1% to 30% (see Anderson,
2006; Bellis, Hughes, Hughes, & Ashton, 2005), it is reasonable to conclude that
sperm competition may have been a recurrent and selectively important feature of
human evolutionary history. Below, we discuss theory and research related to forced
in-pair copulation in humans. In keeping with the established animal literature and
a comparative evolutionary perspective, we often refer to partner rape in humans as
forced in-pair copulation—the forceful act of sexual intercourse by a man against
his partner’s will.
Forced In-Pair Copulation in Humans

Noting that instances of forced in-pair copulation follow extra-pair copulations in
waterfowl and documentation that forced in-pair copulation in humans often fol-
lows accusations of female infidelity (e.g., Finkelhor & Yllo, 1985; Russell, 1982),
Wilson and Daly (1992) suggested in a footnote that “sexual insistence” in the context
of a relationship might act as a sperm competition tactic in humans as well. Sexual
coercion in response to cues of his partner’s sexual infidelity might function to intro-
duce a male’s sperm into his partner’s reproductive tract at a time when there is a
high risk of cuckoldry.
Thornhill and Thornhill (1992) also hypothesized that forced in-pair copula-
tion may function as an anti-cuckoldry tactic designed over human evolutionary
history by selective pressures associated with sperm competition. Thornhill and
Thornhill argued that a woman who resists or avoids copulating with her partner
might thereby be signaling to him that she has been sexually unfaithful and that the
forced in-pair copulation functions to decrease his paternal uncertainty. Thornhill
and Thornhill argued that the fact that the rape of a woman by her partner is more
likely to occur during or after a breakup—times in which men express greatest con-
cern about female sexual infidelity—provides preliminary support for the hypoth-
esis. For example, they cited research by Frieze (1983) indicating that women who
were physically abused and raped by their husbands rated them to be more sexually
jealous than did women who were abused but not raped. Similar arguments were
presented by Thornhill and Palmer (2000), and Lalumière et al. (2005) suggested
that antisocial men who suspect that their female partner has been sexually unfaith-
ful may be motivated to engage in forced in-pair copulation.
Both indirect and direct empirical evidence supporting this hypothesis has been
documented. Frieze (1983) and Gage and Hutchinson (2006), for example, found
that husbands who raped their wives were more sexually jealous than husbands who
did not rape their wives. Shields and Hanneke (1983) documented that victims of
forced in-pair copulation were more likely to have reported engaging in extramarital
sex than women who were not raped by their in-pair partner. Studying men’s partner-
directed insults, Starratt, Goetz, Shackelford, McKibbin, and Stewart-Williams (under
review) found in two studies that a reliable predictor of a man’s sexual coercion is
his accusations of his partner’s sexual infidelity. Specifically, men who accuse their
partners of being unfaithful (endorsing items such as “I accused my partner of hav-
ing sex with many other men” and “I called my partner a ‘whore’ or a ‘slut’ ”) were
more likely to sexually coerce them.
Direct empirical evidence supporting this hypothesis is accumulating.
Camilleri (2004), for example, found that the risk of a partner’s infidelity pre-
dicted sexual coercion among male participants but not female participants. It is
biologically impossible for women to be cuckolded, so one would not expect women
to have a sperm competition psychology that would generate sexually coercive be-
havior in response to their male partner’s sexual infidelity. Goetz and Shackelford
(2006) documented in two studies that a man’s sexual coercion in the context of
an intimate relationship is related positively to his partner’s infidelities. According
to men’s self-reports and women’s partner-reports, men who used more sexual
coercion in their relationship were partnered to women who had been or were
likely to be unfaithful, and these men also were likely to use more mate retention
behaviors.
Because cuckoldry is associated with substantial reproductive costs for males
of paternally investing species, men are expected to have evolved adaptations to
address the adaptive problem of paternal uncertainty. One such adaptation may be
a sperm competition tactic whereby sexual coercion and forced in-pair copulation
function to increase the likelihood that the in-pair male, and not a rival male, sires
the offspring that his partner might produce. It may be that a proportion of sexu-
ally coercive behaviors (in the context of an intimate relationship) are performed
by antisocial men who aim to punish, humiliate, or control their partners indepen-
dent of their perception of cuckoldry risk. We are not arguing that all sexual coercion
and forced in-pair copulations are the output of evolved mechanisms designed to
reduce the risk of being cuckolded. Instead, we are suggesting that sexual coercion
might sometimes be the result of male evolved psychology associated with male
sexual jealousy.
Conclusion
It is possible to study intimate partner violence with little or no knowledge of evo-

lution. Most do. Those who study intimate partner violence from an evolutionary
perspective often ask questions that are different from those asked by most clinical
and forensic psychologists. Evolutionary psychologists are interested in ultimate (or
distal) explanations, referring to the evolved function of a trait, behavior, or mecha-
nism. This is in contrast to proximate explanations, which refer to the immediate
causes of a trait, behavior, or mechanism. Although the explanations are different,
they are compatible and equally important (Sherman & Alcock, 1994). A fuller un-
derstanding of intimate partner violence will be reached when both ultimate and
proximate explanations are empirically supported.
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PART THREE
ADAPTATION AND SEX CRIMES

5
The Evolutionary Psychology

of Sexual Harassment
KINGSLEY R. BROWNE
One of the most dramatic social changes to have occurred in Western society in the
past half-century is the tremendous increase in women’s participation in the labor
force. Women now work side by side with men and compete for status with men
in the same hierarchies (Browne, 2002). The results of workplace integration have
not always been as some hoped, however, because men and women turn out not to
be simply interchangeable. Despite the assumption that prohibitions of discrimina-
tion would lead to economic parity between the sexes, for example, men—in large
part for reasons traceable to our evolutionary heritage—tend to engage in behaviors
that result in their earning more money than women and occupying the highest
organizational positions at disproportionate rates. Although men and women have
somewhat different occupational preferences (Browne, 2006), men and women mix
in the workplace far more than in the past. One effect of the breakdown of the sex-
ual division of labor is the expansion of opportunities for sexual conflict to occur in
the workplace. Much of this conflict is today labeled “sexual harassment” (Browne,
1997).
Sexual harassment has been called “one of the most damaging and ubiquitous
barriers to career success and satisfaction for women” (Willness, Steel, & Lee, 2007,
pp. 73–74), though estimates of its incidence vary widely (Gutek, Murphy, & Douma,
2004). By some counts, 90% of all women have faced some form of workplace ha-
rassment (Terpstra & Baker, 1986), yet surveys also reveal that most women do not
think that it is a problem in their own workplaces (Gutek, 1985).
The huge disparities in frequency estimates result, in part, from the diversity of
definitions of sexual harassment. Conduct that has been included within the concept
ranges from comments that make someone feel uncomfortable to forcible rape. Legal
and lay meanings are often different, so that some researchers claim many women
81
82 Adaptation and Sex Crimes
have been subjected to illegal sexual harassment even though the women themselves
do not count themselves as victims (Magley, Hulin, Fitzgerald, & DeNardo, 1999),
while women also may label certain conduct “sexual harassment” even though it is
insufficiently egregious to satisfy the legal definition.
Because the conduct labeled sexual harassment is so diverse—having a wide
array of motivations and effects—developing a unitary view of its causes and, nec-
essarily, of its cures is impossible. Courts have declared, for example, that all of the
following conduct may constitute sexual harassment: forcible rape, extorting sex for
job benefits, sexual or romantic overtures, sexual jokes, sexually suggestive pictures
or cartoons, sexist comments, vulgar language, harassing actions of a nonsexual
form, and even “well-intended compliments” (Browne, 1997). A category of conduct
that encompasses such a wide range of behavior is unlikely to have a single explana-
tion. Nonetheless, some researchers continue to seek unitary causes. Berdahl (2007,
p. 425), for example, poses the question of whether sexual harassment “is motivated
by sexual desire or by sexist antipathy,” as if it must all be either one or the other.
Despite the multitude of behaviors that may constitute sexual harassment, some
patterns recur. The purpose of this chapter is to examine these patterns through the
lens of evolutionary psychology, a perspective that makes better sense of this constel-
lation of behavior than its purely sociocultural competitors. As one forensic psychol-
ogy manual puts it, “Familiarity with sexual harassment research findings enables a
forensic psychologist to place a specific case within the larger context of the phenom-
enon of sexual harassment” (Foote & Goodman-Delahunty, 2005, p. 13). The better
one understands socially undesirable conduct, after all, the better armed one is to
deal with it, whether in the context of workplace prevention or in court proceedings
seeking a remedy for a putative victim.
What Is Sexual Harassment?
Sexual harassment is defined as a form of sex discrimination under the laws of the
United States (Browne, 1997), the United Kingdom (Kelly, 2000), and the European
Union (Defeis, 2004). Title VII of the Civil Rights Act of 1964—the principal U.S.
law governing sex discrimination in the workplace—does not even mention the term
sexual harassment. Rather, case law has identified some forms of sexual harassment
as covered by the general prohibition against discrimination “because of sex.” The
concept’s origin in discrimination law has had substantial influence on the course of
its development and has led to some oddities and uncertainties in the doctrine.
Two relatively distinct categories of sexual harassment have been identified in
the case law and the academic literature (Browne, 1997). The first, known as “quid
pro quo harassment,” is perhaps the archetypal form. It entails a claim that an em-
ployee was required to submit to sexual advances as a condition of either obtain-
ing a benefit, such as being promoted, or avoiding a burden, such as being fired. A
threat of “Sleep with me or you’re fired” is a classic case, although courts may find
Evolutionary Psychology of Sexual Harassment 83
an implied threat in less explicit language. The rationale for viewing such conduct
as sex discrimination, as opposed to mere swinish behavior, is that the sexual de-
mand would not have been made had the employee been of the other sex. Female
employees, or at least some of them, may thus be faced with burdensome conditions
of employment that male employees are not, or, if the offending male supervisor is
homosexual, male employees may be subjected to conditions not faced by women.
The case of the bisexual supervisor was a merely hypothetical objection to this
rationale until a federal court of appeals actually faced such a case in 2000. The
court ruled that a supervisor who had imposed sexual demands on both a husband
and a wife had not engaged in unlawful sexual harassment because the harassment
was not “discriminatory” on the basis of sex (Holman v. Indiana, 2000). The bisexual-
supervisor case challenges the notion that sexual harassment is wrong because it
constitutes sex discrimination. Most people probably do not believe that a supervisor
who extorts sex from male and female employees alike is on a higher moral plane
than a supervisor who is less catholic in his tastes. It is the use of the supervisor’s
workplace power to extort sex that is the primary wrong, not the fact that his targets
happen to be of one sex or the other. Of course, litigants must use the tools at their
disposal, and the antidiscrimination laws have some characteristics that made them
particularly attractive for the task, including the liability of employers for the acts of
their employees and the availability of attorneys’ fees to the successful plaintiff.
The second form of harassment is “hostile environment” harassment, which
involves a claim that the work environment is permeated with sexuality or “discrimi-
natory intimidation, ridicule, and insult” (Meritor Savings Bank v. Vinson, 1986).
A complaining employee must show that she (or he) was subjected to “unwelcome”
conduct, based upon sex, that was “sufficiently severe or pervasive to alter the
conditions of the victim’s employment and create an abusive working environment.”
The “severe or pervasive” requirement is intended to preclude liability for isolated
incidents or behavior that is “merely offensive.” The complainant must also show
not only that she perceived the environment to be abusive but also that a hypotheti-
cal “reasonable person” or “reasonable woman” (more about this distinction later)
would have found it so as well, in order to avoid holding employers hostage to the
hypersensitivity of a particular employee.
Hostile-environment harassment consists of a more varied range of behaviors
than quid pro quo harassment. A hostile environment can be created by sexual ad-
vances that are not tied to tangible aspects of the job; these might come from super-
visors, co-workers, or even subordinates or customers. These cases are perceived as
discrimination for the same reason that quid pro quo cases are—namely, that the
advances were “because of ” the target’s sex. Other cases may involve harassment
of either a sexual or nonsexual form that is directed at a woman because of either
sexual desire or hostility to her sex, so they also fit easily within a discrimination ra-
tionale, although not necessarily a definition of sexual harassment that requires the
conduct to be “of a sexual nature.” Other hostile-environment cases are not so easily
fit into the discrimination model, however. Many involve complaints that the work
atmosphere is generally “sexualized”—filled with sexually provocative pictures, sex-

ual jokes, sexist comments, and the like. Unlike quid pro quo cases, there may be no
intended “target” of this harassment at all, and the sexualized atmosphere may have
predated the entry of women into that particular workplace. A plaintiff in such a
case is not saying that she was treated differently because of her sex but rather that
the environment is discriminatory because sexualized environments are inherently
more burdensome to women than to men.
This chapter will examine three sexual harassment issues that have been either
erroneously or incompletely analyzed because of failure to consider the findings
of evolutionary psychology. The first is whether the hostility of an environment
should be judged from the perspective of the “reasonable person” or from that of the
“reasonable woman” (as in a large portion of cases a woman is the complainant).
A perspective that takes seriously the notion that humans are products of natural
selection—with the attendant differences in selective pressures to which the sexes
have been exposed—suggests that when it comes to matters of sex, there is no such
thing as a “reasonable person.” There are only “reasonable men” and “reasonable
women.” An average of the two—a “reasonable androgyne,” in the words of Lionel
Tiger (1997)—is simply nonsensical.
The second issue involves the frequently repeated but seldom examined asser-
tion that sexual harassment is “not about sex but about power.” Under this view,
men in quid pro quo cases use sex instrumentally to obtain and retain power over
women. An evolutionary perspective does not deny the linkage between power and
sex—indeed, it recognizes the link to be a powerful one—but suggests that the direc-
tion of causation is misperceived. Rather than using sex to obtain power, it is much
more accurate to say that men use power to obtain sex.
The final issue is the accuracy of the assumption that abuse that takes a sexual
form, such as sexual epithets or hazing that has sexual overtones, is necessarily di-
rected at the target “because of sex.” Even prior to the entry of women into the
workforce, men directed such conduct toward one another. When the goal is either
to offend or to test a person, the actor is likely to select a form of conduct to which
he believes the target will be especially sensitive. For both women and men, that
conduct is likely to have sexual overtones. In many of these cases, it would not be
inappropriate to say that this conduct really is “about power”—in the sense of being
related to men’s attempt to achieve status and dominance generally—rather than
sex, but these cases are often assumed to be inherently more sexual than they actu-
ally are.
The “Reasonable Person” or the “Reasonable Woman”?
One of the major unresolved issues in sexual harassment law concerns the appropri-
ate perspective by which to judge whether a work environment is sufficiently hostile
as to be illegal. Specifically, the question is whether the “victim’s perspective” should
take account of sex—that is, whether the environment should be viewed from the
perspective of the “reasonable person” or that of the “reasonable woman.”
The argument for a “reasonable person” reflects concern that a “reasonable
woman” standard is paternalistic and imposes an obligation on men to conform to a
standard of conduct that they cannot understand (Adler & Peirce, 1993). One court,
in rejecting the “reasonable woman” standard, explained that it “may reinforce the
notion that women are ‘different’ from men and therefore need special treatment—a
notion that has disenfranchised women in the workplace” (Radtke v. Everett, 1993).
In contrast, courts adopting the reasonable woman standard have relied upon per-
ceptions of just the differences that other courts have been reluctant to reinforce.
As one court stated, “conduct that many men consider unobjectionable may offend
many women” (Ellison v. Brady, 1991). That court acknowledged that women are
not uniform in their viewpoints but noted that they “share common concerns which
men do not necessarily share” and that “women who are victims of mild forms of
sexual harassment may understandably worry whether a harasser’s conduct is
merely a prelude to violent sexual assault.” Which of these two standards should
be adopted depends in part on just how different men’s and women’s perspectives
actually are.
“Error Management Theory” and

Sex Differences in Perceptions
Men and women, it turns out, have substantially divergent views about sexual mat-
ters in general and sexual harassment in particular. Women tend to view more kinds
of sex-related behavior as harassment, although the sexes differ little in their views
of the most serious forms of harassment, such as coerced sex (Corr & Jackson, 2001;
Rotundo, Nguyen, & Sackett, 2001). Women are more likely than men to perceive
touching or sexual comments to be sexual harassment. One widely reported find-
ing is that a substantial majority of women would be offended by sexual overtures
at work, whereas a substantial majority of men would be flattered (Gutek, 1985).
A study of attitudes about workplace e-mails similarly found that men rated e-mails
containing sexual propositions to be “somewhat enjoyable” whereas women found
them to be very offensive (Khoo & Senn, 2004, p. 210). Thus, it seems, where a
man might see “opportunity,” a woman sees “danger,” as illustrated by Struckman-
Johnson and Struckman-Johnson’s (1994, p. 401) finding that a substantial number
of men “viewed an advance by a good looking woman who threatened harm or held
a knife as a positive sexual opportunity.”
Men inhabit a more sexualized world than women do, and they devote a greater
share of their reproductive effort to short-term mating. Because men see the world
“through sexual glasses,” they tend to see situations as more sexually oriented than
women do. In their interactions with women, men often perceive sexual interest
where women perceive only friendliness (Abbey, 1987, 1982). The tendency of men
to interpret friendly behavior as a reflection of sexual interest coupled with women’s
tendency to interpret sexually interested behavior as mere friendliness leaves much

room for misunderstanding. A woman who has no interest in a sexual relationship
with a man may initially act in a friendly manner, which the man may interpret as
a sign of sexual interest, leading him to respond flirtatiously. The woman’s friendly
response in return may be taken by the man as a positive indicator that his interest
is reciprocated, which may prompt him to respond with sexual advances that are in
fact unwelcome.
The converse of men’s bias toward perceiving sexual interest on the part of a
woman appears to be women’s bias toward perceiving sexual threat on the part of
men in circumstances in which a risk of coercion exists. Discomfort should begin
well before an overt attempt at physical coercion is made, however, since by then
it may be too late. Thus the same behavior that may be perceived as friendly in an
unthreatening atmosphere may be viewed as threatening when the possibilities of
escape are diminished (whether or not a threat is intended).
The differences in perception that lead to miscommunication are easily under-
stood from the perspective of “error management theory” (Haselton & Buss, 2000).
In making judgments under uncertainty, the human mind is biased toward mini-
mizing the reproductive costs of error. As Buss (1994, p. 145) has observed, a male
tendency to infer sexual interest would have been selected for “if over evolutionary
history even a tiny fraction of these ‘misperceptions’ led to sex.” A man who waits to
make advances until he is certain that a woman is sexually interested would tend to be
less reproductively successful than a man who tries as long as there is a chance that
the woman will be receptive. Men are thus predisposed toward false positives in this
context (inferring interest that does not exist) rather than false negatives (failing
to pick up on genuine interest). Similarly, because of the substantial fitness costs
to a woman who loses control over her choice of sexual partner and the timing of
reproduction, natural selection would favor a woman’s cautiousness about sexual
coercion (Thornhill & Palmer, 2000, pp. 100–103). It is better to infer a threat of
sexual coercion that does not exist than to ignore one that does, so women in this
context—especially in the fertile phase of the menstrual cycle (Chavanne & Gallup,
1998)—should be biased toward false positives. As Duntley and Shackelford have
noted (see Chapter 11), natural selection would favor defense mechanisms against
reproductive threats that would have been common in our ancestral environment,
and sexual coercion of women by men has been a threat for a very long time.
Misperceptions, Miscommunication,
and Sexual Harassment
The varying perceptions of men and women create ample opportunity for miscom-
munication in the workplace, as the Safeway corporation learned after it implemented
its “superior customer service” program in 1998. Under this program, clerks were di-
rected to smile at customers, make eye contact, and call them by name (Ream, 2000).
A number of female clerks ultimately filed charges of sexual harassment, claiming
that the overtly friendly behavior required by the policy prompted some male cus-
tomers to interpret their behavior as flirtatious, leading to sexual comments, propo-
sitions, and even stalking. Exacerbating the problem of miscommunication, the
Safeway policy did not permit employees to discontinue the friendly behavior when
customers responded inappropriately, which resulted in further encouragement of
the unwelcome attention. The harassment charges were dropped when Safeway
agreed with some of its unions to modify the policy.
The fact that cues typically employed in courtship are inherently ambiguous
virtually guarantees that miscommunication will happen with some frequency
(Stockdale, 1993). Features of the workplace—such as the need for continued future
association—especially encourage ambiguity. As Yagil and colleagues have noted,
“to the degree that the target’s message is ambiguous it leaves an opening for the
perpetrator to interpret the behavior as welcome” (Yagil, Karnieli-Miller, Eisikovits,
& Enosh, 2006). When a woman tells a male co-worker that she cannot go out with
him because she is busy that night, she may be thinking, “I hope he takes the hint
and leaves me alone,” while he may be thinking, “Great, she’s busy this time, but she
didn’t reject me altogether; I’ll try again and hope she’s not busy next time.” Some-
times women would have it both ways. They do not want to be explicit about their
rejection, in order to avoid creating conflict, but at the same time they resent men for
not taking their attempt to spare feelings (“maybe some other time”) as a no. This
lack of clarity may result in the “persistent requests for a date after repeated refusals”
that is often defined as harassment.
The risk of miscommunication is exacerbated by the perception of many men
that women often are just “playing hard to get” and often mean “yes” even if they say
“no.” Although this notion is often referred to as a “myth” (Semonsky & Rosenfeld,
1994, p. 515), there is substantial evidence that some women do employ this tactic
(Muehlenhard & Hollabaugh, 1988). As Mealey (1992) noted, the fact that “females
are selected to be coy will mean that sometimes ‘no’ really does mean ‘try a little
harder.’ ” Because many men know that “no” can be a prelude to “yes,” they may
persist even when “no” actually does mean “no.” Because of the inherent ambiguity
of many such situations, it is naive to assert that “sexual harassment allegations are
either true or false” (O’Donohue & Bowers, 2006, p. 56), in the way that one might
make such an assertion about the claim of a party to an automobile accident lawsuit
about the color of the traffic light.
When sex differences in perspective lead to miscommunication—that is, when
the man reasonably (from the perspective of the reasonable man) makes sexual
overtures that a woman reasonably (from the perspective of the reasonable woman)
finds disturbing or even threatening—who, if anyone, is to blame? The usual an-
swer is that the man is responsible; after all, he has made a sexual advance that was
“unwelcome,” and sexual harassment doctrine, at least in the United States, does
not make the man’s intent particularly relevant (Browne, 1997). However, when
a person reasonably receives a message different from the one that the sender
reasonably intended to convey, both subjects are engaging in miscommunication.
Yet commentators commonly dismiss with disdain any suggestion that women bear
some responsibility for avoiding such situations (Ehrenreich, 1990, p. 1208 n.114;
Oshige, 1995, p. 578).
The fact that some—though by no means all—sexually harassing behavior
results from miscommunication suggests that sexual harassment training might
abandon its usual exclusive focus on male behavior and focus as well on educating
women that some of their behavior might be misunderstood and that even if they
feel threatened, the men may not actually be threatening them. Instead, sexual ha-
rassment training is often aimed more at heightening sensitivities than educating
recipients to avoid miscommunication. Indeed, it is commonplace in the literature
for the success of sexual harassment training programs to be judged by the increase
in employees’ labeling of particular conduct as sexual harassment (York, Barclay,
& Zajack, 1997).
If a biological perspective can contribute anything to the sexual harassment dis-
cussion, it must be the insight that a “reasonable person” standard is meaningless.
When it comes to matters of sex and sexuality, there are no “reasonable persons,”
only “reasonable men” and “reasonable women.” The discrete sexual natures of men
and women cannot be blended into a one-size-fits-all “human sexual nature” that is
instantiated in a sexless—or hermaphroditic—“reasonable person.” This is not to
suggest, of course, that all men and all women agree among themselves about what
is abusive. Women differ among themselves about the offensiveness of sexual materi-
als and behaviors in the workplace. For example, those adhering to a feminist ideol-
ogy are especially likely to find them offensive (Brooks & Perot, 1991).
Although interesting from an academic perspective, there is mixed empirical
evidence on the question of whether judges and juries are affected in their decisions
by whether they are applying a reasonable woman or reasonable person standard.
Laboratory studies typically show that the choice of standard has a modest effect in
some kinds of cases, with subjects using a reasonable woman standard being some-
what more likely to label particular conduct as harassing (Blumenthal, 1998; Gutek
et al., 1999; Wiener & Hurt, 2000). A study of all reported federal cases in the United
States over a ten-year period, however, found no statistically significant difference
in outcomes between cases explicitly relying on a reasonable woman standard and
those employing a reasonable person standard (Juliano & Schwab, 2001). The fact
that most cases did not identify the standard being employed suggests caution in
drawing too much from the null results. Another study examining factors in decided
cases found that courts deciding cases in “reasonable woman” jurisdictions were
slightly more likely to find for the plaintiff (Perry, Kulik, & Bourhis, 2004).
Power versus Sex
Many who write about sexual harassment are certain that sexual harassment is not
“about sex” at all but “about power” (Avner, 1994; Bravo & Cassedy, 1992)—echoing
equivalent claims often made about the motivations of rapists (see discussion in Palmer
& Thornhill, 2003). In support of this suggestion, they argue (or at least imply) that
victims are not selected according to criteria of sexual attractiveness but rather chosen
more or less at random to be victims of a male need to oppress women. For example,
Gutek (1985, p. 54) asserts that sexual harassment “is likely to happen to almost any
female worker,” but on the next page she points out that victims tend to be young and
either single or divorced. Another device is to set up an extreme straw man and, in re-
jecting it, leave a misleading impression. Thus, Workman and Johnson (1991, p. 766)
note that “some individuals believe only attractive women are sexually harassed” but
that “empirical studies do not support this belief, since women in all ranges of attrac-
tiveness have reported harassment.” Although this statement may leave the casual
reader with the impression that unattractive women are as likely to be targets as at-
tractive women, all the writers have actually said is that not all victims are attractive
(although, for all we know, they may have been the most attractive victims available to
their harassers).
Of course, not all sexual harassment is directed toward obtaining sex, and some
may have no real target at all. A work environment saturated with sexual pictures
and coarse language might be viewed as sexually harassing to all women who find
themselves present, and some harassment may be driven by dominance motivations
or hostility rather than sexual desire. Nonetheless, quid pro quo harassment—when
a superior extorts sex from a subordinate—has clear sexual motives, even if it is the
harasser’s power that makes such extortion possible.
Because of the centrality of sexual behavior to reproductive fitness, an evolu-
tionary perspective should lead to acute skepticism about a claim that activities that
result in sexual intercourse are not “about sex.” This skepticism is especially war-
ranted when the claim is that power and sex are unrelated, as dominance and sexu-
ality share similar roots. As Dabbs (2000, p. 10) has noted, “the major social effect
of testosterone is to orient us toward issues of sex and power.” Sexual coercion is not
a cultural invention of humans born of an ideology of patriarchy but rather a wide-
spread pattern across the animal kingdom (Clutton-Brock & Parker, 1995).
Throughout human history, men have used power as a way of obtaining sex,
whether coercively or through making themselves attractive as mates. Men with the
most power in history—despots whose subjects lived at their sufferance—routinely
surrounded themselves with nubile women whose favors they could command at
their pleasure (Betzig, 1986). Genghis Khan had hundreds of wives and concubines
(Ratchnevsky, 1991), and the Y-chromosome of Genghis and his male relatives is
now found in approximately 8% of males throughout a swath of Asia largely con-
gruent with the boundary of the Mongol empire when Genghis died (Zerjal et al.,
2003). Quite clearly, he reaped substantial reproductive rewards from his power.
Male “despots” in the workplace sometimes adopt a similar strategy, with far less
extravagant success, of course. Many litigated cases involve allegations of explicit
threats of adverse action if the target refuses to engage in sexual activity (Browne,
1997, pp. 48–49 n. 227). There is little reason to think that the motivations of these
supervisors are any less sexual than those of an Eastern emperor. Thus, even the
sexual harassment cases that most conspicuously involve power—explicit quid pro
quo cases—are about both power and sex: a supervisor is using his workplace power
to extort sexual compliance. To say that it is only about power makes no more sense
than saying that bank robbery is only about guns and not about money.
Premature Rejection of the “Natural/Biological” Model

A study commonly invoked to support the argument that sexual harassment is
not about sex was conducted by Tangri, Burt, and Johnson (1982). They proposed
and tested three models of sexual harassment: the “natural/biological” model,
which views harassment as a consequence of natural physical attraction; the
“organizational” model, which views harassment as a consequence of organiza-
tional hierarchy, allowing individuals to use their organizational power to oppress
their subordinates; and the “sociocultural” model, which views sexual harassment
as a result of sex-role socialization and the differential distribution of power in the
larger society. They concluded that there was evidence to support the latter two
models but little to support the first (the explanations are not mutually exclusive, of
course). Following the Tangri study, the idea that there is any significant biological
contribution to harassment is usually mentioned just to be dismissed.
The rejection of the natural/biological model resulted from the failure of the data
to satisfy the predictions that the researchers derived from the model. They had pre-
dicted that if this model were correct, harassers and victims would be of both sexes;
victims would be similar to their harassers in age, race, and occupational status; both
harasser and victim would be unmarried; and the harasser would direct his attention
only toward the victim. They also predicted that the behaviors would resemble court-
ship behaviors, that they would stop once the victim indicated a lack of interest, and
that victims would be “flattered” by the behaviors (although why a woman should be
expected to be “flattered” by behavior she viewed as harassment is hard to fathom).
Because their data did not satisfy those expectations, they rejected the model.
Tangri and associates oddly concluded that the tendency of individuals with
greater degrees of personal vulnerability and dependence on their job to experience
more harassment was some of the “strongest evidence available in these data against
the natural model” (p. 52). Their apparent view was that young, unattached women
are particularly vulnerable and that it is simply coincidental that such women would
also be sexually attractive to a potential harasser (although they did not explain why
a young, single woman is more vulnerable than, say, a 55-year-old woman who has
worked for the same employer for thirty years but has no pension). It is not clear,
however, why a finding that victims were vulnerable would undermine the natural/
biological model. If the harasser’s strategy is to convert his workplace power into sat-
isfaction of his sexual urges—which is the essence of quid pro quo harassment—he
must focus on targets susceptible to the exercise of that power. It is not just attractive-
ness that is important to him; it is attractiveness plus accessibility.
The test of a model is meaningful only if the predictions the researchers de-
rive from the model logically follow from it. This study was not constructed to test
whether harassers were motivated by sexual attraction, however, but rather whether
they were looking for long-term, exclusive mates. But no one has suggested that sex-
ual harassment is mostly “about marriage.” What the researchers should have tested
was whether the actor and the target tend to possess traits that would be relevant to
either long-term or short-term mating.
A subsequent study by Studd and Gattiker (1991)—informed by evolutionary
psychology—analyzed patterns of sexual harassment and concluded that the demo-
graphic profiles of targets were largely what would be expected if harassers were
employing short-term sexual strategies (see Buss & Schmitt, 1993). The strongest
prediction is that the harasser is male and the victim is female, since men are usu-
ally the sexual initiators in both long-term and short-term mating. Other predictions
are that the target will be of reproductive age, physically attractive, and not involved
in a serious long-term relationship (and therefore lacking a male protector). These
predictions are largely satisfied. Less than 1% of federal cases over a ten-year period
involved sexually based behavior aimed at a male employee by a female supervisor
(Juliano & Schwab, 2001). The overwhelming proportion of victims are single, di-
vorced, or separated women under the age of 35 (Studd & Gattiker, 1991). Studd
and Gattiker concluded that the motivation of most men involved in coercive sex in
the workplace was indeed sexual (although not romantic). It is worth noting that in
laboratory studies, subjects seem to assume that harassers’ motives are sexual, as
they are substantially more likely to find that sexual harassment occurred when the
plaintiff is attractive (Madera, Podratz, King, & Hebl, 2007).
Conflicting Predictions about Status and Unwelcomeness

There is some confusion in the literature about what predictions one should make
concerning the effect of a man’s status on a woman’s reaction to sexual advances in
the workplace. For example, Buss (1999, p. 319; 2004, p. 318) has suggested that
“The degree of chagrin that women experience after sexual advances . . . depends in
part on the status of the harasser,” with women being less upset by advances from
higher-status men. Bourgeois and Perkins (2003) claim to have “overwhelmingly
refuted” Buss’s prediction through their finding that women report imagining greater
upset if someone higher in their organization persisted in asking them out on a date
despite their repeated refusals than if the requests came from someone with lower
status. Thus, they assert, their findings support the sociocultural explanation and
refute the evolutionary explanation. It is critical to note, however, that Bourgeois and
Perkins’s study, unlike the study Buss was referring to, placed the high-status man
above the woman in the organization. Bourgeois and Perkins do acknowledge that
absent power differentials, “the evolutionary hypothesis seems to apply” (p. 349).
Rather than refuting the evolutionary psychology account, the Bourgeois
and Perkins results are actually predicted by it. Two well-documented findings are
relevant to these predictions. The first is that women tend to prefer high-status men
to low-status men (Buss, 2004, pp. 110–115). Thus, all else being equal, they are
likely to find advances by the former more welcome than advances by the latter. The
second finding is that women are strongly averse to sexual coercion (Thornhill &
Palmer, 2000). Thus, women will suffer more distress when the possibility of sexual
coercion is high than when it is low. These findings yield two predictions. First, women
are likely to find advances by high-status men in their own organizations to be more
welcome than advances by low-status men. Second, if the advances are not welcome,
women are likely to be more upset by persistent advances by their superiors—who
have the organizational power to coerce them—than by their peers or subordinates,
who likely lack that power. These predictions were tested by Colarelli and Haaland
(2002), whose study varied the man’s power and status separately. They found that
power and status interacted, with harassment ratings increasing as power increased
and status decreased. Thus, advances by a relatively low-status man who held power
over the woman were most distressing of all.
Weaknesses of a Theory that Neglects Biology

An approach that focuses solely on power without resort to sex differences in sexual
psychology cannot explain a number of features of sexual harassment. For exam-
ple, why do women almost never coerce sex from their subordinates? Some argue
that one seldom sees coercion by female superiors because women usually lack the
necessary power (Tangri et al., 1982; Fitzgerald & Weitzman, 1990). However, large
numbers of women hold management and supervisory positions in organizations
and faculty positions in colleges and universities. Nonetheless, reported instances
of sexual coercion by female managers and professors are relatively rare. Although
one might argue that because of the readiness of many men to engage in casual
sex, women have no need to coerce them, that response itself rests on the different
sexual psychologies of men and women. There is little evidence that women super-
visors engage in frequent voluntary sexual relations with their subordinates, either,
and women’s preference for higher-status mates would suggest that this would be a
relatively uncommon occurrence.
One variant of the sociocultural theory holds that sexual harassment is an at-
tempt by men to exert power because of their fear that women constitute a threat to
men’s economic or social standing (Gutek, 1992). Such an argument suggests an
inverse relationship between male societal power and sexual coercion. Yet the most
pervasive coercive sex in the history of the master-servant relationship is not between
men and women in the modern workplace—where women are participating in the
workplace as equals like never before—but rather between a slave owner and his
slaves. Female slaves did not constitute a threat to their owner’s economic or social
standing; indeed, they were a reflection of it. Nonetheless, sexual relations between
slave and owner were extremely common, and that phenomenon was one of the
principal objections of many abolitionists to the institution of slavery. The historical
record is clear that slave owners did not seek slave women at random for sexual rela-
tions. Rather, they preferred those who possessed the attributes that men typically
value in sexual partners: reproductive value as demonstrated by youth and beauty.
This preference was reflected in price, as a prime fieldhand would sell in New Orleans
for $1,800, a top-quality blacksmith would go for $2,500, and a “particularly beau-
tiful girl or young woman might bring $5,000” (Genovese, 1976, p. 416).
One recurrent, yet implausible, theme in the literature is that sexual harassment
represents an implicit conspiracy through which men combine to oppress women
(Farley, 1978, p. xvi). Some researchers have suggested that the reason that married
women are less likely to be harassed is not that men are looking for mates but rather
that harassers are honoring the “property rights” of other men (Gutek, 1985, p. 57;
Lafontaine & Tredeau, 1986), as if men have a pact among themselves that they will
sexually coerce each other’s daughters and sisters but not their wives. Under this
view, male harassers (the majority of whom are married) are more willing to honor
the marital vows of other men than they are their own. This “property rights” argu-
ment rests uneasily with Schneider’s (1982) finding that “closeted” lesbians, who
might have a male partner for all the harasser knows, are subjected to more sexual
advances than “open” lesbians, whose partners are known to be women—a finding
suggesting that predicted receptivity is a factor influencing men’s overtures.
Power and the Priming of Sexual Psychology

The relationship between power and sexual harassment is considerably more subtle
than is often appreciated. Bargh and Raymond (1995) have suggested that many
men in supervisory positions do not realize they are exploiting their power, because
of an unconscious link between power and sex. When such a man is in a position
of power over a woman, an “automatic power–sex association” becomes activated,
which enhances both the likelihood that he will perceive sexual interest on the wom-
an’s part and his perceptions of her attractiveness (also Bargh, Raymond, Pryor &
Strack, 1995; Zurbriggen, 2000). The man may see a sexual situation in which the
attraction seems to be reciprocated whereas the woman is simply being deferential
and friendly to a man who has power over her.
The finding that many men have an automatic association of power and sex sug-
gests that modification of sexual harassment training may be appropriate. Much of
that training is currently focused on warning men that they should not exploit their
power over subordinates to coerce sex or, more generally, that sexual relationships
between supervisors and subordinates are inappropriate. Neither of these messages
is likely to be terribly effective in modifying the behavior of a man having the power/
sex association. Such a man would not tend to view his conduct as exploitive if he
is unaware that it is his power that creates the attraction. Moreover, if he perceives
the relationship as one of mutual attraction, he is less likely to abide by institutional
strictures against supervisor–subordinate relationships. Perhaps a better strategy is
to educate men specifically that being in a position of power will sometimes result
in erroneous perceptions, especially in light of Bargh and Raymond’s estimate that

three-quarters of harassers do not realize that they are engaging in harassment.
Power is unquestionably an important component of some kinds of sexual ha-
rassment. It is an essential ingredient of quid pro quo harassment, since the harasser
must have the apparent power to carry through on his threat if sexual access is de-
nied, and therefore vulnerability to the exercise of that power will be a typical feature
of extortionate harassment. But the claim that “the goal of sexual harassment is not
sexual pleasure but gaining power over another” (Bravo & Cassedy, 1992) gets the
relationship exactly backward. The focus on power to the exclusion of sex appears to
be an unfortunate side effect of the fact that most of the scholarship on harassment
has been from the woman’s, if not the feminist’s, point of view. From the perspective
of the victim, it may seem like all power and no sex. But if the goal of the law is to
regulate the harasser’s actions rather than simply to provide a remedy to the victim,
it is his perspective that must be understood rather than hers.
“Because of Sex”
Although many commentators underestimate the sexual component of quid pro

quo harassment, many also overestimate the sexual component of some hostile-
environment harassment. When the hostile environment consists of sexual expres-
sion or conduct, courts generally view that fact as conclusive proof that the actions
were motivated by hostility on the basis of sex (Browne, 1997). Such motivations
may in fact exist, but not all hostility or harassment directed toward a woman flows
from sex-based animus even if it is expressed in a sex-based way.
Sex-based Language May Have a Variety of Motivations

Women may be called vulgar sexual names, and men may make crude overtures to
women that on their face look like “sexual advances.” However, when a man says
something like “give me some of that stuff,” his “request” is not a “sexual advance”
in the sense that he is acting in the hope that the woman will respond favorably;
instead it is typically a form of insult. In many cases, the insult may arise out of hos-
tility toward women, hostility that is sometimes activated by entry of women into
traditionally all-male workplaces. If it is, then the man’s behavior would constitute
sex discrimination under any definition. On the other hand, the conduct may actu-
ally be more about dominance, which may have nothing to do with the sex of the
target, or hostility, which is only sometimes based on sex.
Insulting language is seldom sex-neutral in nature. Few of the myriad vulgar
epithets that flow like water in today’s culture are characteristically applied indis-
criminately to both sexes. Indeed, a study that asked subjects to identify the worst
things that one could call a man and the worst things that one could call a woman
found no overlap in the most frequently named insults (Preston & Stanley, 1987).
Insults to women often impugn their chastity, whereas those directed toward men
often challenge their masculinity. Even when the same word is used toward individu-
als of different sexes, the meaning may be different (for example, calling a woman
a “bitch” or a “whore” means something quite different from directing those same
epithets at a man).
Many people (perhaps especially men) are prone to cruel and aggressive be-
havior toward those they dislike or perceive to be vulnerable. Where they see weak-
ness, they may attack. Their dislike may or may not spring from sex-based animus,
but regardless of whether it does, their behavior may have sexual overtones, both
because of the sexualized worldview that men tend to possess and the fact that
attackers will choose language to which they believe the target is particularly sen-
sitive. It is important to remember that men’s quest for dominance has not been
primarily about attaining dominance over women but rather achieving domi-
nance over other men (Buss, 1996), a fact that may explain Gutek’s (1985, p. 32)
finding that in the workplace “women are less often treated disrespectfully than
men are.”
Is Everything “Because of Sex”?

Law professor Julie Seaman has recently argued that even when heterosexual men
direct sexual behavior toward other men, their conduct is necessarily “because of
sex” (Seaman, 2005, pp. 394–395). Her argument is not limited to circumstances in
which men make homosexual advances toward other men, which are unproblemati-
cally “because of sex.” Rather, she argues that when men gang up on another man
and subject him to unwelcome horseplay of a sexual nature, they are engaging in
coalition building and dominance activities that are explained “by virtue of the sex
of the object of the behavior” and therefore “closely tied” to the sex of the victim
(pp. 397, 401).
The only same-sex harassment case decided by the U.S. Supreme Court—Oncale
v. Sundowner Offshore Services, Inc. (1998)—was a case of the sort that Seaman
envisions. The plaintiff alleged that he was repeatedly subjected to humiliating sex-
related actions, assaulted in a sexual manner (his supervisor allegedly pushed a bar
of soap into his anus), and threatened with rape (although there was no indication
that the harassers were homosexual). The Court rejected the lower court’s categori-
cal holding that same-sex harassment can never constitute sex discrimination and
sent the case back to the lower court to decide whether the conduct was “because
of sex,” a matter about which the Supreme Court expressed no view. Seaman would
view this conduct as clearly “because of sex” and therefore as sex discrimination. In
a sense, of course, it is. It is extremely unlikely that the same cruel conduct would
have been directed toward a female employee, and the harassers were certainly en-
gaged in coalitionary behavior that is a product of their evolved male psychologies.
Moreover, they were directing their aggression against another man—the primary
target of men’s coalitionary aggression.
The problem with Seaman’s argument is that there is no logical basis for limiting
it to conduct of a sexual nature. Ultimately, her argument implies that all conflict,
whether intersexual or intrasexual, is “because of sex.” Federal harassment law does
not distinguish between sexual and nonsexual conduct. Thus, nonsexual conduct
aimed by men toward women because of their sex is as much sex discrimination
(that is, “because of sex”) as sexual behavior that flows from the same motivation.
Under Seaman’s view, ordinary dominance behaviors directed by men against other
men are also “because of sex” and therefore in violation of Title VII, but, inexplica-
bly, only if they take a sexual form. By similar reasoning, conflict between women is
also in many cases “because of sex,” as women’s conflicts with other women differ
from their conflicts with men. It has often been noted, for example, that across a
variety of professions, “women are the first to attack a woman who gets promoted”
(Benenson & Schinazi, 2004, p. 329), a phenomenon sometimes labeled the “Queen
Bee Syndrome” (Cooper, 1997). Thus conflict arising because one woman was pro-
moted over another—which might be ascribed to simple jealousy—can also be seen,
under Seaman’s analysis, as “because of sex.” Once sexual harassment is defined
as workplace conflict of a sexual or nonsexual nature directed at either same-sex
or opposite-sex co-workers, the definition expands to include virtually all workplace
conflict. That may or may not be a good policy choice, but such an interpretation
would “transform Title VII into a general civility code for the American workplace,”
a course that the Supreme Court has been unwilling to chart (Oncale v. Sundowner
Offshore Services, Inc., 1998, pp. 80–81).
Conclusion
The utopian workplace imagined by some—in which men and women are equally
represented in all occupations and at all hierarchical levels and behave in the same
desexualized, yet fundamentally feminine, manner—is not one likely to be created
by our evolved minds. The tabula rasa perspective of human nature—the view that
sex is just a “social construct”—has encouraged many to believe that people (espe-
cially men) can simply be educated to leave their sexual psychologies behind them
and enter a workplace in which they adopt “work roles” wholly disconnected from
their psyches. This same perspective has led to the adoption of a sexless “reason-
able person” standard in sexual harassment law—an “ideal” androgynous blend of
male and female psychologies. Failure to understand male psychology has led many
women to assert that they just want to be treated like men when, in fact, for very
fundamental reasons, men often do not treat each other very well.
Although many have urged a “desexualization” of the workplace, it is not clear
that this is either a practical or desirable goal. A realistic view of human nature sug-
gests that as long as men and women inhabit the same workplaces, they will interact
as human beings. Part of the interaction among human beings is sexual and roman-
tic. Although sexual harassment surveys ask whether women have ever received
unwanted sexual advances in the workplace, the surveys seldom ask whether women
have ever received welcome ones. Given the large number of workers who find their
romantic partners at work (Hoffman, Clinebell, & Kilpatrick, 1997), the answer for
many would probably be in the affirmative.
An understanding of evolved sex differences in sexual psychologies is essential
to an understanding of the behaviors produced by those psychologies and can also
assist in their management. Sexual harassment training might more productively
focus on educating men and women about sex differences in perspectives to avoid
miscommunication rather than simply heightening female employees’ inclination
to be offended. Similarly, because of the association that many men have between
power and sex, educating male supervisors about the risk of oversexualized percep-
tions of interactions when they are in dominant positions over women may forestall
much unwelcome sexual attention. Expert witnesses in sexual harassment cases—
who to date have virtually all come from the “social construction of gender” school
(see O’Connor, 2006)—might be of more assistance to the jury if they incorporated
a more robust theoretical perspective.
Recognition of the fact that sexual harassment is a manifestation of our evolved
psychologies does not mean that sexual harassment is either good or inevitable. Many
behaviors having origins in our evolved psychologies are recognized to be social pa-
thologies even if they do not reflect psychological pathologies (see Buss, 2005). Be-
haviors are susceptible to modification, even if our underlying psychologies are not.
Finally, it should be remembered that our evolved psychologies are the source not
only of sexual harassment but also of our desire to combat it.
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6
Evolutionary Psychological
Perspectives on Rape
WILLIAM F. MCKIBBIN, TODD K. SHACKELFORD, AARON T. GOETZ,

AND VALERIE G. STARRATT
Rape is a fact of life across cultures (Broude & Greene, 1978; Rozée, 1993; Sanday,
1981). In U.S. samples, estimates of the prevalence of rape are as high as 13% for
women (Kilpatrick, Edmunds, & Seymour, 1992; Resnick, Kilpatrick, Dansky, Saun-
ders, & Best, 1993). Rape is likely more common, however, because rapes often go
unreported (Kilpatrick et al., 1992). Although other forms of rape occur (e.g., male–
male rape), this chapter focuses on the rape of women by men. Definitions of rape
vary. It is typically defined, and will be defined in this chapter, as the use or threat of
force to achieve sexual penile-vaginal penetration of a woman without her consent
(Kilpatrick et al., 1992; Thornhill & Palmer, 2000).
Rape became a public and academic focus following the publication of Brown-
miller’s (1975) book, Against Our Will: Men, Women, and Rape. Brownmiller argued
that rape is “a conscious process of intimidation by which all men keep all women in a
state of fear” (p. 15, emphasis in original). Since then, feminist theories of rape have
dominated the rape research literature. A prominent version of feminist theory con-
tends that rape is the result of social traditions in which men have dominated politi-
cal, economic, and other sources of power (Ellis, 1989). Feminist theorists inspired
by Brownmiller often interpret rape as a method by which men maintain this power
and dominance over women. Moreover, feminist theorists have argued explicitly
that rape is not about sexual gratification and often seem more focused on making
ideological rather than scientific statements about human psychology and behavior
(Thornhill & Palmer, 2000).
This chapter reviews the topic of rape from a modern evolutionary psychologi-
cal perspective (see, e.g., Barkow, Cosmides, & Tooby, 1992; Buss, 2004). Evolution-
ary psychology is a powerful heuristic tool that can be used to generate new, testable
101
hypotheses across all domains of psychology. Evolutionary psychology rests on several

key premises (Buss, 2004). The first premise states that natural selection is the only
known process capable of producing complex functional systems such as the human
brain. The complexity of human behavior can only be understood completely by tak-
ing into account human evolutionary history and natural selection. Second, behav-
ior depends on evolved psychological mechanisms. These are information-processing
mechanisms housed in the brain that register and process specific information
and generate as output specific behaviors, physiological activity, or input relayed
to other psychological mechanisms. Third, evolved psychological mechanisms are
functionally specialized to perform a specific task or to solve a specific problem that
recurrently affected reproductive success over evolutionary history. This premise is
often referred to as domain specificity. Finally, the numerousness premise states that
human brains consist of many specific evolved psychological mechanisms that work
together to produce behavior. Together with a number of other theoretical tools and
heuristics provided by modern evolutionary theory, these premises are used to gener-
ate evolutionary theories of psychology and behavior.
One such heuristic tool that informs evolutionary psychology is parental invest-
ment theory (Trivers, 1972). Parental investment theory consists of two important
premises. First, in sexually reproducing species, the sex that invests more in offspring
(typically the female) will be more discriminating about mating. Second, the sex that
invests less in offspring (typically the male) will be more intrasexually competitive for
sexual access to the higher-investing sex. These premises have been supported in re-
search with numerous species, including humans. Human females, like the females
of most biparental species, invest more in offspring whereas males invest more in
mating effort. These sex differences are greatest in short-term mating contexts (Buss,
1994a, 1994b, 2004).
Misconceptions about Evolutionary Psychology
Some scholars believe that evolutionary psychological research is conducted to

justify racism, sexism, or other undesirable “-isms.” For example, Tang-Martinez
(1997, p. 116) describes a common feminist view that evolutionary psychology is
“inherently misogynistic and provides a justification for the oppression of women.”
However, the feminists to whom Tang-Martinez refers are committing what is known
as the naturalistic fallacy: the error of deriving what ought to be from what is. This
error can be demonstrated clearly with an example: No sensible person would argue
that a scientist researching the causes of cancer is thereby justifying or promoting
cancer. Yet some people continue to argue that investigating rape from an evolution-
ary perspective justifies or legitimizes rape (e.g., Baron, 1985; Marshall & Barrett,
1990, cited in Thornhill & Palmer, 2000).
Related to the naturalistic fallacy is the false belief of genetic determinism—the
idea that behavior is unalterable, programmed, or otherwise unchangeable. This
Evolutionary Psychological Perspectives on Rape 103
argument has been debunked numerous times. Biologist John Maynard Smith noted
that genetic determinism is “an incorrect idea that is largely irrelevant, because it is
not held by anyone, or at least not by any competent evolutionary biologist” (1997,
p. 524). No evolutionary psychologist would argue that because rape is produced by
evolved mechanisms, it cannot be prevented or we should simply accept its occur-
rence. The goal of evolutionary psychology, like the goal of any science, is to further
our understanding of the phenomenon of interest, which in this case is rape. Re-
searching rape from an evolutionary psychological perspective does not justify or
promote this heinous act. Whether evolutionary psychological hypotheses about
rape are correct, new perspectives often allow researchers to gain new insights into
the targeted phenomenon. Gaining a greater understanding about why rape occurs
is fundamental to decreasing its occurrence.
Finally, researchers using an evolutionary psychological perspective often frame
hypotheses in terms of the costs and benefits to an organism of performing a par-
ticular behavior. These costs and benefits refer to the effects on reproductive success
over evolutionary time—that is, costs decreased the probability of successful repro-
duction whereas benefits increased the probability of successful reproduction. These
terms are sometimes misconstrued as referring to a more general idea of perceived
costs and benefits to the individual or to society. However, these terms carry no moral
or ethical meaning and are used only in the context of naturally selected biological
functioning.
Comparative Psychology of Sexual Coercion and Rape
Sexual coercion and rape occur in many species. In fact, evolutionary metatheory
has been used to generate the hypotheses that sexual coercion and rape occur in
species in which males are more aggressive, more eager to mate, more sexually asser-
tive, and less discriminating in choosing a mate (Thornhill & Palmer, 2000). Sexual
coercion and rape occur in insects (Dunn, Crean, & Gilburn, 2002; Linder & Rice,
2005; Thornhill, 1980, 1981, 1987; Vahed, 2002), amphibians, reptiles (Olsson,
1995; Reyer, Frei, & Som, 1999, Shine, Langkilde, & Mason, 2003; Sztatecsny, Jehle,
Burke, & Hödl, 2006), fish (Magurran, 2001; Plath, Parzefall, & Schlupp, 2003),
birds (Gowaty & Buschhaus, 1998; McKinney, Derrickson, & Mineau, 1983; Pizzari &
Birkhead, 2000), and primates (Robbins, 1999; Smuts & Smuts, 1993; Wrangham
& Peterson, 1996), among other species.
Two species in particular provide clear examples of adaptations in males to
sexually coerce and rape females. A large body of evidence demonstrates that male
scorpionflies (Panorpa vulgaris) have an anatomical feature that is designed to facili-
tate sexual access to a female in a coercive fashion—that is, rape. They possess a
notal organ that is used specifically and exclusively for rape (Thornhill, 1980, 1981,
1987; Thornhill & Sauer, 1991). Scorpionfly males do not always secure copulations
through rape. Instead, males display conditional mating strategies. Males that are
able to produce a nuptial gift of food for the female are allowed to mate without coer-
cion. Males that are not able to do so resort to the conditional rape strategy and use
of the notal organ (Thornhill, 1980, 1981, 1987; Thornhill & Palmer, 2000). Thus,
male scorpionflies exhibit evidence of specific anatomical traits that evolved to facili-
tate rape. They also exhibit evidence of a conditional strategy of sexual coercion.
Male orangutans (Pongo pygmaeus) also deploy conditional strategies of sexual
coercion and rape. Orangutans are unique among apes in that they live solitary lives
rather than in groups. Females therefore do not have mates or kin that may deter
or prevent rape (Wrangham & Peterson, 1996). This fact alone makes rape a more
viable strategy for male orangutans. Forced copulations account for up to half of
all copulations (Mitani, 1985; Wrangham & Peterson, 1996). These forced copula-
tions seem to be performed primarily by a subset of males. Wrangham and Peterson
(1996) reviewed evidence indicating that male orangutans exist as one of two dis-
tinct morphs or behavioral types. The large morphs weigh significantly more, move
much slower, and are typically able to find females willing to mate. The small morphs
are typically unable to find females willing to mate with them. These small morphs
are more likely to chase down and rape females. This represents a conditional strat-
egy. If the smaller males are unable to gain sexual access to females through intra-
sexual competition and by being attractive to females, they may use the conditional
strategy of chasing down and raping a female.
Comparative evidence indicates that males of many species have evolved strate-
gies to sexually coerce and rape females. Rape in humans must also reflect adapta-
tions constructed over evolutionary time. Although numerous explanations have
been offered to explain rape in humans (e.g., learning or enculturation, mental
illness, personality differences, drug and alcohol use, and other factors; Bergen &
Bukovec, 2006; Brecklin & Ullman, 2001; Dean & Malamuth, 1997; Lalumiére &
Quinsey, 1996), these factors alone cannot explain the existence of such seemingly
complex behavior. At best, these factors may increase the likelihood of rape, but they
cannot explain the complex organized behavior seen in rape. Only two explanations
are likely to be true: that rape is the product of specialized psychological adaptation,
or that it is a by-product of other adaptations in the male mind (Palmer & Thornhill,
2003a, 2003b; Thornhill & Palmer, 2000). What evidence supports the hypothesis
that rape is the result of an adaptation?
Evidence of Human Adaptations for Sexual Coercion and Rape
For rape to be produced by evolved psychological mechanisms, it must have recur-

rently generated reproductive benefits for ancestral rapists. These benefits must have
outweighed the significant costs that men may incur if they attempt or successfully
complete a rape. Despite the costs, there is evidence that rape may have increased
the number of women with whom ancestral men copulated and, therefore, the re-
productive success of rapist males (Gottschall & Gottschall, 2003; Holmes, Resnick,
Kilpatrick, & Best, 1996; Krueger, 1988; Shields & Shields, 1983; Thornhill, 1999;
Thornhill & Palmer, 2000).
Men do not exhibit morphological features analogous to the notal clamp of male
scorpionflies. Any rape adaptations that men possess are likely to occur in the form
of psychological mechanisms. Researchers, particularly Thornhill and Thornhill
(1992; see also Thornhill, 1999; Thornhill & Palmer, 2000) have identified several
possible rape adaptations. These adaptations are proposed to be universal features
of male psychology that are activated under specific circumstances. Empirical sup-
port for evolutionary psychological theories of rape has been mixed. For example, the
“loser” or mate deprivation model of sexual coercion, in which men with limited or
no sexual access to females rape for lack of other options, typically has not been sup-
ported (Malamuth, Huppin, & Paul, 2005; but see also later in this chapter).
A hypothesized design feature of rape adaptations involves mechanisms that
cause men to evaluate the sexual attractiveness of rape victims differently than that
of consensual partners. Specifically, a rapist might be more successful reproductively
by maximizing the chance that a one-time forced copulation will result in pregnancy.
According to this hypothesis, a would-be rapist may be more likely to target a highly
fertile woman than a woman who is less fertile (Thornhill & Palmer, 2000). Human
female fertility (current likelihood of conception per copulation) peaks in the early
to mid-20s. Therefore, if women in this age range are overrepresented in reports
of rape, it is possible that this reflects a male adaptation that leads to raping fertile
women more often than nonfertile women. Numerous studies have documented that
young women are most often targeted by rapists, and that women of peak fertility
are overrepresented in reported and unreported rapes (Ghiglieri, 2000; Greenfield,
1997; Kilpatrick et al., 1992; Shields & Shields, 1983; Thornhill & Palmer, 2000;
Thornhill & Thornhill, 1983). This evidence does not support exclusively rape-specific
adaptation, however, because men exhibit a preference for sexually attractive part-
ners in general, not just in contexts of rape (see, e.g., Buss, 1994a, 1994b, 2004).
We, like others (e.g., Thornhill & Palmer, 2000), propose that rape is a conditional
strategy that may potentially be deployed by any man. Shields and Shields (1983) ar-
gued that men use a conditional mating strategy consisting of many mating tactics,
including rape. At least one-third of men admit they would rape under specific condi-
tions, and many men report coercive sexual fantasies (see Malamuth et al., 2005, for
a review). Such evidence suggests that rape adaptations might be universal features
of male psychology. Empirical support for evolutionary psychological hypotheses of
rape has been mixed. For example, the mate deprivation model of sexual coercion,
in which men with limited or no sexual access to females rape for lack of other op-
tions, typically has not been supported (Malamuth et al., 2005; but see also later in
this chapter). This mixed support may reflect a lack of appreciation that there may
be several distinct types of rapists. For example, Mealey (1995) proposed that men
with psychopathy represent a genetically distinct morph different from “normal”
men without psychopathy. Lalumière, Harris, Quinsey, and Rice (2005) presented in
a related argument that a small proportion of antisocial men who are more likely to
rape form a qualitatively distinct portion of the population. Similarly, as a heuristic

strategy, we have defined several rapist types. Specifying these types may generate
new insights and testable hypotheses. Other researchers have suggested that defin-
ing subtypes of rapists can be potentially valuable (Malamuth et al., 2005).
Our view of rape may be a more nuanced view of rape than has previously been
explored. We hypothesize that rape may represent a conditional mating strategy,
present in all men, that may result from several qualitatively different ancestral con-
texts combined with individual difference factors among men. Specifically, we pro-
pose five types of rapists (or contexts of rape): (1) disadvantaged men who resort
to rape, (2) “specialized” rapists who are sexually aroused by violent sex, (3) men
who rape opportunistically, (4) high-mating-effort men who are dominant and often
psychopathic, and (5) partner-rapists motivated by assessments of increased risk of
sperm competition. We next discuss evidence for each of these types of rapists.
The Disadvantaged Male

The first hypothesized rapist type is characterized by men who are motivated to rape
if they have no other means of securing copulations. This may be referred to as the
disadvantaged male hypothesis. This hypothesis has previously been referred to as the
mate deprivation hypothesis (Lalumiére, Chalmers, Quinsey, & Seto, 1996). It is sup-
ported by data indicating that rapes are committed disproportionately by men with
low socioeconomic status (Kalichman, Williams, Cherry, Belcher, & Nachimson,
1998; Thornhill & Thornhill, 1983). Furthermore, Krill, Lake, and Platek (2006)
presented evidence that men convicted of rape display lower facial symmetry, an
indicator of poor genetic quality. Facial symmetry is linked positively with physical
and psychological health (Shackelford & Larsen, 1997), and men with lower facial
symmetry are perceived as less attractive and as less desirable mates (Gangestad,
Thornhill, & Yeo, 1994; Gangestad & Thornhill, 1999; Sugiyama, 2005). Deprived
of mates by normal means, some men may resort to rape. Identification of such a
rapist type, however, would not necessarily imply a conditional strategy for rape. One
can imagine that when reproductive opportunities are dismal, some men might be
motivated to take more risks in all domains, with one domain being sexual assertive-
ness, which might lead to rape.
The Specialized Rapist

Another type of rapist may be the specialized rapist. Men in this group are distin-
guished by being sexually aroused by violent sexual stimuli. These men may possess
a psychology that produces differences in sexual arousal in response to depictions of
rapes versus depictions of consensual sex. Because rape carries high potential costs
for the rapist, particularly if caught in the act, rapists with a psychology that moti-
vated quicker arousal and ejaculation during rape might have been more successful
than men who did not possess such a psychology (Thornhill & Palmer, 2000).
Support for the existence of this hypothesized group has been generated by in-
vestigating whether men are aroused by depictions of rape versus depictions of casual
sex. Meta-analyses indicate that convicted rapists demonstrate greater sexual arousal
to scenes of sexual coercion involving force than do nonrapists (Hall, Shondrick, &
Hirschman, 1993; Lalumiére & Quinsey, 1994; Lohr, Adams, & Davis, 1997; Thorn-
hill & Thornhill, 1992).
Specialized rapists also might possess mechanisms that cause them to evaluate
the sexual attractiveness of rape victims differently than the sexual attractiveness
of consensual partners. According to this hypothesis, a rapist will be more likely to
rape a highly fertile woman than a woman who is less fertile (Thornhill & Palmer,
2000). Research has demonstrated support for this hypothesis (see earlier sections
for details). However, it is unclear whether this reflects a specialized rape adaptation
or a more generalized male mating strategy. Future research might test the hypoth-
esis that men evaluate the sexual attractiveness of rape victims differently from the
sexual attractiveness of consensual partners by examining whether men target for
rape reproductive-aged women who are in the most fertile phases of their menstrual
cycles. Such a finding would provide stronger support for this rapist type.
If a rape is a one-time event, it might make adaptive sense for the rapist to in-
seminate the woman with an ejaculate that contains a high sperm count or that
otherwise increases the chance of successful fertilization. Indeed, Thornhill and
Palmer (2000) have hypothesized that some rapists may be capable of producing
a high-sperm-count ejaculate that would increase the chance of fertilization. Men
seem to be capable of unconsciously adjusting sperm number in ejaculates, such as
in response to a greater risk of sperm competition (Baker & Bellis, 1989, 1993), but
it is unknown whether rapists adjust sperm numbers during rape. Evidence for this
would lend support to the specialized rapist type.
Researchers have argued that premature ejaculation might have been adaptive
ancestrally, perhaps by minimizing the chances of predation or detection by jealous
mates (Hong, 1984; see also Gallup & Burch, 2004). It also might make adaptive
sense for a rapist to ejaculate as soon as possible after achieving copulation. This
would reduce the chances of being injured or retaliated against. Therefore, it is pos-
sible that selection may have acted to minimize the time it takes for a man to ejacu-
late during a rape. Research is needed to test this hypothesis. For example, one might
compare the average pre-ejaculatory copulation length during rape versus during
consensual copulation.
There is indirect evidence corroborating the hypothesis that rapists’ ejaculates
are more competitive than those of nonrapists. Gottschall and Gottschall (2003) es-
timated that pregnancy rates resulting from rape were two times that of consensual
per-incident rates. That is, approximately 6% of rapes result in pregnancy compared
to approximately 3% of consensual copulations. Even after controlling statistically
for the age of the woman, the researchers identified a higher conception rate for
rapes than for consensual sex. This evidence suggests that there may be something
different about rapists’ psychology or the competitiveness of their ejaculates. Further
research is needed, however. One promising area of research is the study of semen
chemistry. Burch and Gallup (2006) hypothesized that men may have an adaptation
that functions to adjust semen chemistry to cause ovulation immediately following
a rape. Future research could profitably test this hypothesis, perhaps by comparing
chemical constituents of ejaculates produced by men exposed experimentally to a
coercive sexual scenario with ejaculates produced by men exposed experimentally to
a noncoercive sexual scenario.
Opportunistic Rapists
The third hypothesized rapist type is that of the opportunistic rapist. These men gen-
erally seek out receptive women, but they might shift to sexual coercion and rape if
women are not receptive or if the associated benefits of coercive sex outweigh the
costs—for example, if the chances of injury or retaliation by the victim, the victim’s
family, or society are particularly low. All rapists are predicted to be attuned to a
potential victim’s vulnerability, but an opportunistic rapist is especially so. The uni-
versality of laws and societal norms prohibiting rape (wife rape being a special excep-
tion; see further on) indicates an appreciation that men are more likely to rape when
the costs are low (Palmer, 1989; Thornhill & Palmer, 2000). The fact that rapes reg-
ularly occur during wartime has been presented as evidence of the assessment of
victim vulnerability and decreased likelihood of detection (e.g., Gottschall, 2004).
Men in war are likely to assume lowered costs of committing rape because punish-
ment or retaliation is less likely.
The evidence for the existence of this type of rapist, however, is minimal. Theft
also is common during war, and for the same reason: punishment or retaliation is
unlikely. Support for this hypothesized type may be seen in research demonstrating
that women with family members, particularly adult male family members, living
nearby are much less likely to be physically assaulted by their partner (Figueredo
et al., 2001; Kanin, 1957). This suggests that potential rapists are attending to the
probability of retaliation by a victim’s adult male family members.
High Mating-effort Rapists

A fourth hypothesized type is the high mating-effort rapist. High mating-effort rapists,
in contrast to other types, such as disadvantaged rapists, appear to be more sexually
experienced (Lalumière & Quinsey, 1996). Rapists of this type may be characterized as
aggressive, dominant, and having high self-esteem. These men often are the perpetra-
tors of date or acquaintance rape. Research evidence appears to support this rapist
type. Such rapists often may be characterized as psychopathic (Lalumière et al., 2005).
Lalumière et al. argue that high mating effort is an important facet of psychopathy.
They claim that although most men appear to deploy mating strategies according to en-
vironmental contexts, psychopathic men deploy a high mating-effort strategy in most
contexts, pursuing many partners with little investment and using coercion and rape
when noncoercive tactics fail. There is evidence that psychopathic men display lower
fluctuating asymmetry, an index of overall fitness (Lalumière, Harris, & Rice, 2001),
further distinguishing this rapist type from others, such as the disadvantaged rapist.
Research evidence corroborates the plausibility of this rapist type. Dean and
Malamuth (1997), for example, found that men who scored high on a Sexual Ex-
perience measure “were more likely to report sexual coercion if they were also self-
centered as opposed to nurturant” (p. 74). Premarital sexual coercion is associated
with sexual promiscuity, earlier onset of sexual activity, and greater sexual experi-
ence (Christopher, Owens, & Stecker, 1993; Lalumière et al., 2005). Lalumière and
Quinsey (1996) found that a strong indicator of past sexual coercion is positive
self-perceived mating success and an extensive history of uncommitted sexual re-
lationships. Finally, the risk of date rape is greater when the man initiated the date,
spent money on the woman, and provided transportation (Muehlenhard & Linton,
1987). Perceived relative deprivation, in which an individual’s (high) expectations
about having sex are not satisfied (Malamuth et al., 2005), also may play a role in
the sexually coercive behavior of high mating-effort men. For example, men who
report a greater likelihood of committing rape tend to endorse statements express-
ing an increased perception of mate deprivation but do not report an overall fewer
number of sexual opportunities (Glick & Fiske, 1996; Lonsway & Fitzgerald, 1995).
More research must be conducted to test this hypothesized rapist type. For example,
researchers might test whether men convicted of date rape or sexual assault score
higher on measures of psychopathy.
Partner Rapists
A final hypothesized rapist type includes men motivated to rape their partners under
conditions of increased sperm competition risk. Sperm competition is the compe-
tition that can occur between males for each to have his sperm fertilize a female
(Parker, 1970). The outcome of sperm competition is favored toward males who pro-
duce greater numbers of sperm (Parker, 1970, 1982; Pound, Shackelford, & Goetz,
2006). Rape in response to risk of sperm competition is most likely to occur when
a man learns or suspects that his long-term partner recently has been sexually un-
faithful (Thornhill & Thornhill, 1992).
Partner rapes account for a substantial proportion of reported rapes (Bergen,
1996; Kilpatrick et al., 1992; Russell, 1990). Between 10% and 26% of women re-
port experiencing rape in marriage (Finkelhor & Yllo, 1985; Hadi, 2000; Painter &
Farrington, 1999; Russell, 1990; Watts, Keough, Ndlovu, & Kwaramba, 1998).
Women are particularly likely to be raped by their partner during a breakup insti-
gated by men’s concerns about their partner’s infidelity (Thornhill & Palmer, 2000).
Until very recently in Western society, it was not considered a crime if a man forced
his wife to have sex with him. The right of men to sexual access to their partner was
considered absolute, and only relatively recently in the United States have men been
prosecuted for raping their wives (Bergen, 1996; Russell, 1990).
Studying men’s psychological reactions to risk of sperm competition is another

possible method for testing the hypothesis that men are motivated to rape their part-
ners under conditions of sperm competition. If men exhibit psychological reactions
to risk of sperm competition in noncoercive contexts, it is also possible that they do
so in coercive or rape contexts. Research evidence indicates that men do display such
psychological reactions. For example, men are more aroused by and prefer sexually
explicit images that suggest the occurrence of sperm competition than by sexually
explicit images that do not suggest the occurrence of sperm competition (Kilgallon &
Simmons, 2005; Pound, 2002). Furthermore, men who spend a greater proportion
of time apart from their partners since the couple’s last copulation (and therefore
face a higher risk of sperm competition) report that they find their partner more at-
tractive, are more interested in copulating with their partner, and believe that their
partner is more interested in copulating with them (Shackelford, Goetz, McKibbin,
& Starratt, 2007; Shackelford et al., 2002). These results are independent of rela-
tionship satisfaction, total time since last copulation, and total time spent apart. The
psychological mechanisms that lead men to experience greater interest in copulation
and to believe their partner is interested in copulation with them also may be part of
the suite of mechanisms that lead men to sexually coerce or rape their partners.
Finally, in a direct test of the hypothesis that men may rape their partners under
conditions of sperm competition, Goetz and Shackelford (2006) documented in two
studies that men’s sexually coercive behavior is positively related to their partner’s
infidelities, that is, to the risk of sperm competition. Men with partners who com-
mitted infidelities or who suspected that their partner had committed infidelities (in-
dicating increased risk of sperm competition) were more likely to perform sexually
coercive behaviors, including rape. These findings lend support to the hypothesized
psychological mechanisms that motivate men to commit partner rape in response to
risk of sperm competition.
In summary, it may be useful to characterize rapists as falling into one of sev-
eral categories or types, specifically (1) disadvantaged men, (2) specialized rapists,
(3) opportunistic rapists, (4) high mating-effort men, and (5) partner rapists.
Although future research is needed to test the hypothesized types of rapists, prior
studies offer some preliminary support for this model. We have identified potential
unique ancestral contexts and individual differences that may have selected for
conditional rape strategies. But these contexts and individual differences can be
overlapping. This is to be expected, however, as we argue that all men may possess
adaptations to rape. For example, a high mating-effort context and an opportunity
context are not mutually exclusive: a man who devotes much of his time and energy
to gaining short-term matings may be even more likely to commit rape when circum-
stances (such as wartime) allow him to do so at decreased cost (e.g., when there is a
low chance of retaliation).
Again, it is important to note that the existence of adaptations to rape does not
mean that rape is inevitable or justified. Like any psychological mechanism, rape
mechanisms require functioning genetic and environmental components. Rape is
predicted to occur only under specific environmental circumstances that activate

men’s evolved psychology. Furthermore, because rape behaviors may have a ge-
netic component does not mean that men cannot control their behavior. Just as men
thwart their evolved psychology every time they choose less calorically dense food
over more calorically dense food (as when one is on a diet), so too can men thwart
evolved mechanisms that may lead them to sexually coerce or rape. Only through
thorough research and a broad understanding of sexual coercion, including its
evolved basis, can we hope to reduce or prevent rape.
Women’s Defenses against Rape
Rape is a traumatic event that is likely to have been a recurrent problem for women
over evolutionary history. Rape often leads to many negative consequences for
women; therefore, women may have evolved psychological mechanisms designed to
motivate rape avoidance behaviors. There are several reasons that rape is traumatic
for women. These include disrupting a woman’s parental care, causing a woman’s
partner to abandon her, and causing a woman serious physical injury (Thornhill &
Palmer, 2000) or death. Women are sometimes killed after being raped (Shackelford,
2002a, 2002b). Aside from death, perhaps the greatest cost to women who are raped
is the circumvention of their mate choice (Wilson, Daly, & Scheib, 1997). This is be-
cause anything that circumvents women’s choice in mating can severely jeopardize
their reproductive success (Symons, 1979).
Researchers have speculated that a variety of female traits evolved to reduce the
risks of being raped. Smuts (1992) argued that women form alliances with groups of
men and other women for protection against would-be rapists. Similarly, Wilson and
Mesnick (1997) proposed and found support for the bodyguard hypothesis: women’s
mate preferences for physically and socially dominant men may reflect anti-rape ad-
aptation. Of course, women may form alliances or prefer dominant mates for reasons
other than to avoid rape. Alliances offer protection from such dangers as assault or
predation, and dominant mates may possess higher-quality genes, for example. Fi-
nally, Davis and Gallup (2006) proposed the intriguing possibility that preeclampsia
and spontaneous abortion may be adaptations that function to terminate pregnan-
cies not in the woman’s best reproductive interests, such as those resulting from rape.
Relatively little empirical work has been conducted to identify specific psychological
mechanisms that evolved to solve the recurrent problem of rape avoidance.
Thornhill and Thornhill (1990a, 1990b, 1990c, 1991) have demonstrated that
the psychological pain that women experience after being raped may be produced by
evolved mechanisms designed to focus women’s attention on the circumstances of
the rape, particularly the social cirumstances that resulted in the rape. Thornhill and
Thornhill (1990a, 1990b, 1990c, 1991) argue that, like physical pain, psychologi-
cal pain motivates individuals to attend to the circumstances that led to the pain and
to avoid those circumstances in the future. Victims of rape who have more to lose in
terms of future reproductive success will also experience more psychological pain
relative to women with less to lose in terms of future reproductive success (Thornhill
& Thornhill, 1983, 1990a; Thornhill & Palmer, 2000). For example, women of re-
productive age are hypothesized to experience more psychological pain due to the
greater risk of conception. Thornhill and Thornhill (1990a) demonstrated support
for this hypothesis, documenting that reproductive-aged women are more trau-
matized by rape than are post-reproductive-aged women or pre-reproductive-aged
girls.
The research conducted by Thornhill and Thornhill focuses on the aftereffects
of being raped and on the psychological pain that may motivate women to avoid
the circumstances leading to the rape. Very little research, however, has been con-
ducted to identify the specific behaviors women may deploy to avoid being raped.
Scheppele and Bart (1983) conducted interviews of women who had been raped or
who had been attacked and successfully avoided being raped. Some of these women
described “rules of rape avoidance” (p. 64) and how they followed them—for exam-
ple, “I would never be alone on the street” and “I would watch what I wear” (p. 65).
These qualitative data provide preliminary evidence for rape avoidance adaptations
in women.
Petralia and Gallup (2002) examined whether a woman’s capacity to resist
rape varies across the menstrual cycle. Women in the fertile phase of their men-
strual cycle showed an increase in handgrip strength, but only when presented with
a sexual coercion scenario. Women not in their fertile phase did not show an in-
crease in handgrip strength. Furthermore, women in all other conditions, including
women in the fertile phase who were presented with the neutral control scenario,
showed a decrease in hand strength post-test. This provides evidence for specialized
mechanisms designed to motivate women to behave in ways that cause them to
be less likely to be raped. Women who experience increased strength during their
fertile phase would be better equipped to defend themselves from would-be rapists.
The research by Petralia and Gallup (2002) provides evidence consistent with the
hypothesis that women have evolved mechanisms that motivate rape avoidance be-
haviors.
Chavanne and Gallup (1998) investigated the performance of risky behav-
iors by women in the fertile phase of their menstrual cycles. A sample of women
were asked where they were in their menstrual cycles and to indicate whether
they had performed a range of behaviors in the past twenty-four hours. Behaviors
were ranked by women in a previous study according to how likely they thought
performing the behaviors might be to result in a woman being sexually assaulted,
with riskier behaviors given higher risk scores. Individuals’ risky behavior was es-
timated by taking the summed composite score of all performed activities. Women
in the fertile phase of their menstrual cycle reported performing fewer behaviors
representing a greater risk of being raped. There was no difference in the likelihood
of performing low-risk behaviors between women in their fertile phase and women
outside their fertile phase. This research has some methodological problems that
prevent firm conclusions, however. First, the researchers used only one method
(i.e., the forward-cycle method) to assess women’s menstrual status. Also, Cha-
vanne and Gallup do not specify how the inventory of risky behaviors was devel-
oped, noting only that a preliminary sample of women rated the riskiness of the
behaviors. In addition, the dependent variable may be confounded by diversity
of activity. For example, a woman who performed ten non-risky behaviors (each
scored as a 1 on the riskiness scale) could receive the same score as a woman who
performed two high-risk behaviors (each scored as a 5 on the riskiness scale; see
Bröder and Hohmann, 2003, for discussion). Despite these methodological issues,
this research documented a significant decrease in performance of risky behaviors
by women in the fertile phase of their menstrual cycle. This evidence is consistent
with the hypothesized function of rape avoidance mechanisms, particularly when
women are fertile.
Chavanne and Gallup’s (1998) study was replicated by Bröder and Hohmann
(2003) using a within-subjects design. Twenty-six women who did not use oral con-
traceptives were tested weekly for four successive weeks. The results indicated that
women in the fertile phase of their cycle selectively inhibit behaviors that would ex-
pose them to a higher risk of being raped while performing more non-risky behaviors.
These results provide a conceptual replication of the results reported by Chavanne
and Gallup. Women perform fewer risky behaviors when they are fertile, while still
demonstrating a higher overall activity level (Morris & Udry, 1970) and even while
engaging in more consensual sex (Morris & Udry, 1982). This selective behavior indi-
cates that women may have evolved specialized psychological mechanisms designed
to motivate behaviors that decrease the risk of being raped. Although this study ad-
dressed many of the issues in the Chavanne and Gallup research, there is still no
indication of how risky behaviors were identified. This study also used the somewhat
problematic forward- and reverse-cycle counting methods for identifying the fertile
phase of the menstrual cycle, both of which depend on the potentially unreliable self-
reports of participants (Bröder & Hohmann, 2003).
A recent study by Garver-Apgar, Gangestad, and Simpson (2007) tested the hy-
pothesis that women are more attuned to signs of a man’s potential sexual coercive-
ness during the fertile phase and are able to more accurately detect sexually coercive
men during the fertile phase. A sample of 169 normally ovulating women watched
short segments of videotaped interviews of men. The women were then asked to
rate the men on several items that were summed to create an overall coerciveness
rating. Average coerciveness ratings for each man were computed. Finally, women’s
menstrual status was estimated using the reverse-cycle counting method. The re-
sults indicated that women in the fertile phase of their menstrual cycle rated the men
as more sexually coercive. This suggests that women at greater risk of conception
may be more attuned to signs of male sexual coerciveness than women at lesser risk
of conception. This may represent an evolved cognitive error management bias (see
Haselton, Nettle, & Andrews, 2005, for an overview) toward identifying men as sex-
ually coercive, which might serve to protect women from being raped. This research
provides more evidence that women may have evolved psychological mechanisms
that motivate behaviors to guard against men’s sexual coercion and rape. We note,
however, that the participants viewed videos of strangers. Studies demonstrate that
women have a greater fear of stranger rape than of being raped by someone they
know (Thornhill & Thornhill, 1990b), which suggests that stranger rape was the
greater adaptive problem. This is despite modern patterns of rape, which indicate that
women are more likely to be raped by someone they know (Kilpatrick et al., 1992;
Resnick et al., 1993). These results may reflect the greater potential costs associated
with stranger rape, such as a decreased likelihood of investment by the genetic father
of resulting offspring. Would similar results be found by testing women’s coercive-
ness ratings of acquaintances or other familiar men? Future research is needed to
explore these effects in greater detail. For example, researchers might ask women to
rate the coerciveness of familiar faces of classmates or celebrities.
In summary, limited previous work suggests that women may have evolved psy-
chological mechanisms that motivate them to avoid being raped. These studies have
not assessed specific behaviors performed to avoid rape. Rather, the results of these
studies suggest that women may have evolved mechanisms that motivate them to
assess the risk of sexual coercion, such as the riskiness of walking in a dark parking
lot alone and the coerciveness of a particular man.
Conclusion
Evolutionary psychology is a powerful heuristic tool that allows researchers to con-

sider rape in a new light. Researchers have argued that men possess evolved psycho-
logical mechanisms that motivate them to rape in specific contexts. Although some
accumulating evidence is consistent with this hypothesis, more research must be
conducted before we can conclude that men possess specific adaptations for rape.
Furthermore, we propose that a more nuanced view of rapists is needed, in which
rapists may be characterized as belonging to one of several types distinguished by
the contexts in which they are predicted to commit a rape. Researchers also have
hypothesized that women have evolved mechanisms that motivate behaviors to
avoid being raped. Some evidence supports this hypothesis. Researchers also must
continue to investigate women’s evolved rape avoidance mechanisms before gener-
ating conclusions. Future research should continue to investigate the psychological
mechanisms that may motivate men’s rape behavior and women’s rape avoidance
behavior. Only through continued scientific study of the etiology of rape can we
hope to prevent it.
ACKNOWLEDGMENTS
This chapter was adapted from McKibbin, W. F., Shackelford, T. K., Goetz, A. T., & Starratt,
V. G. (2008). Why do men rape? An evolutionary psychological perspective. Review of General
Psychology, 12, 86–97. The authors thank Joshua Duntley for insightful comments.
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7
The World’s Oldest Profession

Evolutionary Insights into Prostitution
CATHERINE SALMON
Oh, Harlot, you servant of Men

Kings and Princes shall love you
Young Men release their belts
While the old smile in their beards.
For riches you shall both make and destroy
For you, the fertile wife will be foresaken
While priests shall wed you to the Gods.
—Foster, 2001
Prostitution, sometimes referred to as the world’s oldest profession, arouses strong

sentiment. It is defined as “the act or practice of engaging in sexual activity for
money or its equivalent” (Garner, 1999, p. 1238). From a cross-cultural perspec-
tive, this definition can be problematic in that gift giving, of goods or money, often
occurs in the context of courtship, extramarital affairs, and marital relationships. It
is usually males who give such gifts to their sexual partner, even when females have
the same degree of sexual freedom as males (Gebhard, 1971). Nevertheless, in the
United States (except in certain counties in Nevada) it is illegal to be paid for a sex act,
as it is in many other countries. Yet there is strong debate over the nature of the act
and whether it should be considered a crime. In this chapter, I examine the insight
an evolutionary psychological perspective can provide on these issues; address the
reasons for the existence of prostitution; and explore why it is a service almost exclu-
sively provided to men, typically by women.
The History of the Profession
Accounts of prostitution go back to the sexual services provided by priestesses in the

temples of Mesopotamia over 4,000 years ago. Such services were frequently per-
formed as religious or fertility rites, often in conjunction with the grain harvest. Even
121
then, there was a hierarchy of prostitution, with high-status temple prostitutes and
lower-status ones who worked from roadside inns or other locales. Temple priestesses
had greater freedom than the majority of women of the times. Unlike married women,
the priestesses had the right to their own possessions and to buy and sell both prop-
erty and slaves (Ringdal, 2004). Archeology and written accounts suggest a similar
hierarchy in Greek society. Elite, educated prostitutes bought their freedom and were
able to advance their social status. In this society, the sale of sex was the only option
for women who wanted a free life, away from the control of husband and family.
There are many other accounts of prostitution in ancient civilizations. Re-
searchers have found “more than three hundred different words for prostitute in
late Sanskrit, something that signifies both a rich language and a comprehensive sex
market” (Ringdal, 2004, p. 71). The Kama Sutra is known for its frank discussions
of prostitution and sexual behavior, and erotic temple sculptures and paintings are
found in India; temple prostitution remained there until the coming of the British
and Muslims. There are also accounts of prostitution in China in the eighth and
ninth centuries B.C. During the time of the samurai and shoguns in Japan, prostitu-
tion flourished, creating an elegant class of social outcasts with significant freedom,
including the ability to reject customers they did not desire.
By the Middle Ages, Western attitudes toward prostitution had shifted signifi-
cantly. “Medieval law understood prostitution as a commercial enterprise the woman
engaged in for money” (Karras, 1996). During these times, the role of prostitutes
was considered a necessary evil, an outlet for the intense male sex drive that would
otherwise build up and threaten “good” women. Prostitutes were both tolerated
and marginalized, as a woman’s honor depended on her sexual reputation. Church
courts imposed moral order and used public shame as a deterrent. Women could
even be evicted from a town for multiple offenses of prostitution.
The solution to this problem was officially sanctioned brothels, resulting in the
maintenance of public order and protection of respectable women. The brothels ca-
tered primarily to young, unmarried men. Women became prostitutes, for the most
part, voluntarily, due to economic necessity and a lack of reasonable alternatives.
Prostitution substituted for marriage as a form of financial support, as women in the
past had few opportunities for employment outside the family (Bullough, Shelton,
& Slaving, 1988).
The American Old West was also a time and place where prostitution flourished
out of economic necessity (Rutter, 2005). If a woman was not married, there was
little available work, and what there was paid even less than the selling of sexual
services. Prostitution in these circumstances was characterized by many inherent
risks including pregnancy, disease, physical abuse, legal hassles, and social ostra-
cism. There was also a racial hierarchy, with French prostitutes at the top followed
in descending order by Caucasian, Mexican, Indian, black, and Chinese workers.
Working conditions varied (as always) from common brothels to high-end and par-
lor houses that were only for the wealthy customer, with an environment of elegance
The World’s Oldest Profession 123
and pampering that required appointments. Such places were safer and provided
more financial reward for their workers. Some prostitutes also traveled with rail-
roads, mining camps, and military posts, working out of tent towns or wagons.
The modern phenomenon of Western prostitution shares much with its earlier
forms. Since the 1980s, approximately half of Western prostitutes are call girls
while about one-quarter are street workers. Escort services are the main form of
prostitution in the United States. Perhaps surprisingly, a majority of call girls are
from middle-class backgrounds, are young, and are students or have other jobs
in addition to their sex work. Few stay in the business more than five years, dur-
ing which time it serves predominately as a secondary source of income (Ringdal,
2004). In terms of working conditions, street-based sex work is the most danger-
ous. These women are the worst paid, tend to come from backgrounds of lower
socioeconomic status, and are more likely to be arrested than call girls, being much
more visible and often considered a public nuisance. Further up the prostitution
hierarchy are the escorts. Escort, or indoor, work is safer than street work. The
clients are more predictable in number, in repeat business, and often in terms of
the services required. Such work also is more private and pays better, and escorts
are less likely to be arrested than street workers. High-end call girls (in the model
of well-known madam Heidi Fleiss) are the best paid, with usually the best working
conditions. Unsurprisingly, indoor workers tend to be the most satisfied with their
work and lives (Prince, 1986).
One difference between modern prostitution and its earliest forms is that while
a high-class call girl is paid well for her services, she is not accorded the high social
status of the temple priestesses or the high-class courtesans of ancient Greece. Like
the priestesses, though, high-class prostitutes have economic freedom and are typi-
cally well educated.
The Wars over Sex for Pay
The Laws
Different countries have different laws concerning prostitution. Canada punishes
both prostitutes and clients (with jail time, fines, or school for johns), as does the
United States. Prostitution is illegal in both countries. Finland criminalizes only the
client. Italy allows prostitution in the street and in the home but not in a brothel,
while Sweden criminalizes the purchase of sexual services. The Netherlands regu-
lates voluntary prostitution as sex work and prosecutes forced prostitution. Broth-
els and escort agencies are legal in most of Australia. Brothels and their owners
(but not the workers) are subject to licensing, and sex workers employed in legal
prostitution businesses have many of the same rights as other Australian workers
(Sullivan, 2004).
The Players
In many places, the focus seems to be not on eliminating prostitution, despite laws
that make it illegal, but on making it invisible to the public eye. If “laws reflect, legiti-
mate, and reproduce social norms” (Scrambler, 1997), then sex work, like all other
sexual activity, should not be public. But it is important to remember that street
workers are not representative of the majority of sex workers. Approximately 75%
are incall or outcall workers (Sullivan & Simon, 1998). Many prostitutes themselves
actively call for the decriminalization of prostitution in the United States. COYOTE
(Call Off Your Old Tired Ethics) was founded in San Francisco in 1973 by Margo St.
James to improve the image and working conditions of prostitutes. One former pros-
titute, who became an activist for prostitutes’ rights, wrote the following:
A woman has the right to sell sexual services just as much as she has a right to sell her
brains to a law firm when she works as a lawyer, or to sell her creative work to a museum
when she works as an artist, or to sell her image to a photographer when she works as a
model, or to sell her body when she works as a ballerina. Since most people can have sex
without going to jail, there was no reason except old-fashioned prudery to make sex for
money illegal. (French & Lee, 1988)
Under the decriminalization proponents favor, prostitution would become just another
job, subject to normal labor practices like any other occupation. Many prostitutes
also argue that they have a useful function in society, providing emotional support
to some male clients and sexual services for men who are socially or physically dis-
abled; they also claim that they may help prevent marital dissolution by providing an
alternative to an affair, which might lead to a husband leaving his marriage to form
a union with his extra-pair partner (Sanders, 2005).
Although temple priestesses are evidence of early religious support for prostitution,
the majority of active religions today hold rather a different view. Prostitution, like
all sex between unmarried people, has been largely condemned by most Christian
religions. Judeo-Christian faiths hold that sex should take place in the context of mar-
riage, as does the Islamic faith. Hinduism also has prohibitions against sex outside of
marriage (Nath & Nayar, 1997). Many, though not all, of those opposed to prostitu-
tion in the United States today base their arguments on religious or moral grounds.
The argument is usually that prostitution undermines the social and religious insti-
tution of marriage and exploits women. Sex should be reserved for marriage, or at
least, from a moral perspective, to those who are in love. Both religious and moral
opposition to prostitution take a very relationship-focused view of the purpose of
sexual activity. Many Americans have moral reservations about prostitution. In a
2000 national poll, four out of five teenagers stated that the problem of prostitution
was a serious matter to be dealt with (Marcovitz & Snyder, 2004). Such reservations
about prostitution are also one reason for the popularity (particularly among those
employed in the sex industry) of the term sex worker to describe those who work in
the industry of providing sex.
The traditional feminist take on prostitution has been that it is misogynistic,

driven by male contempt for women. Andrea Dworkin was perhaps the best-known
proponent of this view, arguing that prostitution is rape enforced by poverty in which
women exist for the sexual enjoyment of men. As such, it is inherently exploitative.
She once said in a public speech that “when men use women in prostitution, they are
expressing a pure hatred for the human body . . . men use women’s bodies in prostitu-
tion and gang rape to communicate with each other” (Dworkin, 1994). Other femi-
nists have suggested that while women have the right to use their bodies in whatever
way they choose, men should not be allowed to purchase sexual services from women
(Brownmiller, 1975). This is a rather odd argument, as it would seem to imply that
women should only use their bodies in the service of women! Some feminists seem to
believe that in the absence of prostitution and pornography, men will come to want
the exact same relationships and activities and have the same desires as women.
Interestingly, the debate over prostitution, like that over pornography, has made
strange bedfellows of many feminists and members of the religious right. While the
religious right sees it as a threat to morality and the sanctity of marriage and sex,
feminists see prostitution as the product of male domination. Most recently, a schism
has appeared in the women’s movement over sexuality, particularly commercial
sexual representations and activities. Anti-prostitution feminists define prostitution
as a violation of women’s human rights (one reason they also tend to favor punish-
ing the male clients or johns rather than the prostitutes themselves) while the sex
workers’ rights movement argues that it is the state’s repression of prostitutes that is
the human rights violation (Alexander, 1997).
What Light Can an Evolutionary Perspective Shed?
There are several important questions concerning prostitution that have legal and
social implications for which an evolutionary perspective is highly enlightening.
Why does it exist? Why do some men seek it out? Why do some women engage in
it? What are the consequences for those women? And why do other people care so
much about it? I will consider each in turn, though first I want to make clear the
distinction between different levels of explanation. Those who take an evolutionary
perspective on human behavior are usually interested in ultimate, or why, explana-
tions rather than proximate, or how, explanations. Proximate explanations refer to
the immediate factors, such as internal physiology, environmental stimuli, or pre-
vious experience, that produce a particular response. These can be thought of as
conditions that trigger a mechanism that produces a physiological, psychological, or
behavioral response. Ultimate explanations refer to the conditions of the biological,
social, and physical environment that, on an evolutionary time scale, render certain
traits (or mechanisms) adaptive and others nonadaptive (Mayr, 1961). The question
being asked here is, why does this trait or behavioral mechanism exist—what is its
adaptive significance and what advantage did this trait confer?
Consider the example of sexual jealousy in humans. A young man in a bar sees
his girlfriend talking to another man and gets jealous. The proximate explanation for
any resulting behavior might invoke the fact that he actually saw his girlfriend taking
an interest in another man or that he has a history of being cheated on that makes
him sensitive to the situation. The ultimate explanation has a different focus. Where
does the mechanism that produced this feeling of jealousy come from? Humans, as
opposed to other species of primates, have relatively concealed ovulation. There are
no obvious signs when a woman is ovulating and able to conceive. In species where
this is obvious, males can monopolize sexual access to females through mate guarding
when the females are in their fertile period. In humans, this is not possible, as it would
have to take place across the cycle because the timing of ovulation is not unambigu-
ously advertised—though recent work by Haselton and Gangestad (2006) suggests
that some men increase their mate retention tactics when their partner is near ovula-
tion. One human solution to this problem is marriage, which results in increased pa-
ternity certainty (Alexander & Noonan, 1979; Strassman, 1981). But there is always
a risk of infidelity, and if a man were not sensitive to such cues, he would pay a fitness
cost in being cuckolded. As a result, men prefer a lack of promiscuity in their choice
of long-term mates (Thompson, 1983; Weiss & Slosnerick, 1981). Such a focus on
the fidelity of their wives has been demonstrated cross-culturally among men (Betzig,
1989; Daly & Wilson, 1988; Tracy & Crawford, 1999).
Ancestral men who failed to adequately address the problem of paternity uncer-
tainty risked not only direct reproductive losses but also loss of status and reputa-
tion, which could have had a serious negative impact on their ability to attract other
mates. Sexual jealousy is one psychological mechanism that has evolved in men to
combat the potential costs of being cuckolded (Buss, 1988; Buss & Shackelford, 1997;
Buss, Shackelford, Choe, Buunk, & Dijkstra, 2000; Daly, Wilson, & Weghorst, 1982;
Symons, 1979). Considering both ultimate and proximate levels of explanation may
deepen our understanding of the development and significance of a behavior, whether
that behavior is sexual jealousy or prostitution.
So why does prostitution exist? There are millions of prostitutes working world-
wide, both male and female, and in all times and places the majority have and con-
tinue to service men. Some suggest that prostitution is rooted in men’s feelings about
or attitudes toward women, such as disrespect, hostility, or contempt. But such a
broad claim should raise questions. If contempt for women is the primary motiva-
tion, why does homosexual male prostitution exist? And why are there no significant
differences between homosexual and heterosexual prostitution? Prostitution “is not
a window into men’s feelings about or attitudes toward women; it is a window into
the nature of male sexuality” (Salmon & Symons, 2001, p. 48). What is important to
remember is that men who purchase the services of a prostitute are not just paying for
sex. They are paying for “just sex”—sex without commitment, obligation, and court-
ship. Of course, that begs the question of why they would want to pay for “just sex.”
The reasons are a reflection of basic differences in male and female sexual psy-
chologies. In the environment of evolutionary adaptedness, what would have been
the kind of mating strategies that led to reproductive success, the passing on of one’s
own genes, traits, and predispositions to offspring? If most successful reproduction
across human evolutionary history occurred within marriages (or some form of per-
manent union) and most marriages were monogamous, why is there a male market
demand for prostitution and a female willingness to meet it?
Psychological and biological evidence suggests that different strategies led to re-
productive success for men and women. Ancestral men and women differed in some
of the adaptive problems they encountered in the mating arena. However similar
men’s and women’s typical parental investments may have been, their minimum
possible investments differed significantly. If a man fathered a child in whom he did
not invest, this reproduction would have occurred at almost no cost. Even if such
opportunities did not come along frequently in ancestral human populations, taking
advantage of them when they did was adaptive enough that males evolved a sexual
psychology that makes low-cost sex with new women exciting both to imagine (fan-
tasy and pornography) and to engage in (one-night stands and prostitutes), and that
motivates men to seek out such sexual opportunities (Clark & Hatfield, 1989; Ellis &
Symons, 1990; Salmon & Symons, 2001; Townsend, 1995; Wright & Reise, 1997).
Compared to females, human males typically invest less in parental investment, are
less discriminating in their choice of sexual partners, engage in more low-cost sex-
ual situations, and have sexual encounters with more numerous partners (Clutton-
Brock & Parker, 1992; Low, 1989). As a result of different reproductive payoffs, male
and female sexual psychologies can be expected to be as different as male and female
bodies (Bailey, Gaulin, Agyei, & Gladue, 1994; Symons, 1979).
The vast majority of prostitutes’ clients are heterosexual men, often military
or business men—those who travel a great deal and are either separated from their
sexual partner or do not have one (O’Connell Davidson, 1998). National Health and
Social Life Survey data suggest that 36% of those serving in the military have paid
for sex (Sullivan & Simon, 1994). Clients themselves suggest several motivations in-
cluding the thrill, specific types of services not provided by their partner, loneliness,
wanting a variety of women, sexual urges, and convenience. The most frequently re-
quested services of prostitutes are vaginal-penile sex, oral sex, and hand jobs, though
the specialized fetish market has become more common. The majority of clients in
one study were white collar, and more than half were married (Boyle, 1994). Many
prostitutes also have regular clients who are socially or physically disabled and have
had difficulty finding a regular partner (Sanders, 2005). One could sum up the major
conscious motivations as variety, no other partner, cheaper, and less risky than hav-
ing an affair. This inclination to seek out the services of prostitutes in the pursuit of
variety (of partner type and sexual activity) is consistent with the Coolidge effect;
this is a phenomenon in which males of a variety of species, including rats, sheep,
rhesus monkeys, and humans, who have copulated to the point of satiation with one
female can be aroused again in a short period of time if given access to a novel female
(Alcock, 2005; Plaud, Gaither, Amato Henderson, & Devitt, 1997). Evolutionary
psychologists often point out that, in contrast to males, ancestral females had little
to gain and much to lose from engaging in impersonal sex with random strangers
and from seeking sexual variety for its own sake, and that they had a great deal to
gain from choosing their mates carefully. As such, it is unsurprising that they rarely
seek the services of prostitutes, and when they engage in affairs or short-term mat-
ing it is not just for the sake of variety. Men and women possess different facultative
mating strategies; their mating repertoires differ in adaptive ways (Buss & Schmitt,
1993; Gangestad & Simpson, 2000). This is typically articulated in terms of the con-
ditions that influence the behavioral output of psychological mating mechanisms or
the conditions that activate short-term or long-term mating strategies. So why do
prostitutes do it? What conditions produce this type of mating strategy on the part
of women?
In early Sumerian times, prostitutes were described as “wise women, able to edu-
cate, civilize and tame men” (Alexander, 1997, p. 86). A noble pursuit! But such
goals are rarely attributed to modern prostitution. The majority engage in sex work
for economic reasons. In some cases, it may be a woman’s only option, a better op-
tion than others, or an appealing option. “Prostitution is like any other work in that
some do it because it is the only job available to them, while others do it because it
is a good job or because it provides them with money when they need it, or because
they enjoy it” (Perkins & Bennett, 1985, p. 213). Many are supporting children in
the total absence of the father or in the absence of sufficient financial support. For
some young middle-class women, the financial rewards of being a high-priced call
girl are appealing, especially in combination with flexible working hours. Essentially,
there is a market for sex, and women who need or want to take advantage of that
market do so.
Sex differences in the nature of male and female arousal also facilitate the ex-
istence of prostitution and explain some of its patterns. Much attention has been
paid to the visual nature of male arousal and how this has allowed the pornography
industry to flourish (Salmon & Symons, 2001). Among married couples, sex differ-
ences in the motivations for extra-pair sexual relations influence the male-oriented
nature of prostitution. Married women engaging in short-term mating typically do
so in the pursuit of good genes or because they are emotionally unsatisfied in their
marriage (Schmitt, 2005). Neither goal would be well served by the services of a
prostitute, though they might be by a well-chosen one-night stand or affair. Married
men engaging in such mating are more likely to be simply in the pursuit of variety.
They often report being quite satisfied with their marital partner (Schmitt, 2005).
Thus, their goal is well served through the services of a prostitute.
Another way to consider prostitution is in light of an evolutionary model of
courtship framed by social exchange theory. This theory suggests that cooperation
between individuals occurs for mutual benefit. From this perspective, parental in-
vestment by males is exchanged for sexual access to females. There is an exchange
of reproductive goods and services. “Males may have provided food, protection from
predators and other males, some parental care, and sex. Females may have provided
sex, parental care for children, and labor to gather foods” (Crawford & Johnston,
1999, p. 188). Mechanisms evolved to mediate such social exchanges and in a mod-
ern context may result in the exchange of money for service known as prostitution.
Resource acquisition mechanisms may also facilitate the prostitution industry.
Why do some people care so much about the sex other people are having? There
are a number of reasons. Some see prostitution, like any sex out of marriage, as
something bad, something that leads to moral decay. Others may oppose prostitu-
tion on the grounds that it is bad for the prostitutes themselves (citing risks of physi-
cal and emotional abuse, sexually transmitted disease, and other dangers) and for
women as a group. And it is certainly true that there can be significant hazards to
sex work, particularly among those who work outcall, including verbal and physical
abuse, drugs, depression, disease, and harassment by the police. One might also as-
sume that if men generally value fidelity in a long-term mate, working as a prostitute
might impair a woman’s likelihood of finding a quality partner. But there is a great
deal of evidence that men and women have historically engaged and currently en-
gage in short-term (Gangestad & Thornhill, 1997; Greiling & Buss, 2000) and long-
term (Ellis, 1992) mating strategies and have evolved psychological adaptations for
enabling these flexible strategies.
Just one factor stands out to distinguish those who live well with no loss of self-esteem,
from those who may find sex work a difficult or even damaging career choice. Most of the
former have sufficient sex information and are sex-positive. . . . Most of the latter have
internalized negative attitudes about sex. (Queen, 1997, p. 129)
Interestingly, with regard to disease, the data suggest that there is no evidence
that sexually transmitted disease rates are higher among prostitutes than in the gen-
eral population (Pyett, 1996). Another perspective is stated by a prostitute herself:
“I think straight women see us as a threat as we take money for something they do
for free” (Boyle, 1994, p. 88). In many ways, laws concerning prostitution are a tool
for controlling female behavior (Karras, 1996). One imagines men having various
reasons to want to do so: religion, to protect “good” women, to provide a sexual out-
let for unmarried men. But the strongest opposition to prostitution recently has often
been from women, some with genuine concern for the welfare of prostitutes, others
with the desire to protect what they have earned through marriage and what may be
put at risk by husbands who may divert resources away from their wife to prostitutes.
As Symons (1979, p. 259) notes, “to the extent that heterosexual men purchase
the services of prostitutes and pornographic masturbatory aids, the market for the
sexual services of non-prostitute women is diminished and their bargaining position
vis-a-vis men is weakened.”
Legal and Public Policy Implications
There are many ways in which evolutionary psychology can inform public policy,
but perhaps the most useful is the way it informs about what is or what can be. Much
public policy (if not most) is concerned with the way “we” would like the world to be.
It is used to try to shape the world (or a particular society or country) into an “ideal”
form. In other words, policy is a purposive course of action followed by an actor or set
of actors in dealing with a problem or matter of concern (Anderson, 1975). Once an
ideal is decided on, the question is whether it will be easy or very difficult to achieve.
This is a question that evolutionary psychology is well equipped to address.
It is important to note that evolutionary perspectives give us insight into what
is as opposed to what “ought” to be. Using what is to justify what ought to be (rape is
natural and therefore should be accepted) is committing the naturalistic fallacy. The
fact that ancestral females and males exchanged sex for resources and protection
(Symons, 1979) does not imply that women and men today ought to do the same.
But an evolutionary examination of the ancestral problems that made it an adaptive
solution gives us insight into how it functions today and under what circumstances
it may occur with frequency or not at all. The majority of evolutionarily informed
work does not make the mistake of the naturalistic fallacy. It can also help us to avoid
the moralistic fallacy, or the notion that what ought to be can be. A more complete
understanding of human nature, one derived from an evolutionary perspective, can
help us to better realize what can really be achieved and at what costs (Crawford,
2004). The policy process can be thought of in the following simple way: People
have assumptions about human nature and how the world works. They act on these
assumptions, and the outcomes of their decisions will be successful when their as-
sumptions about human nature are correct. When these assumptions are wrong,
there will be costs to these decisions, costs that can sometimes, although not always,
be unreasonably high.
People have a strong inclination to punish those behaviors they disapprove of as
a way of changing or eliminating them. There may be some validity to this approach
in that adaptations evolved in response to the costs and benefits of the behaviors they
produced in the ancestral environment. As a result, increasing the costs of current
behaviors may influence how likely they are to occur (Thornhill & Palmer, 2000).
The majority of legal systems rely on a system centered on punishment, and pretty
much all such punishments have a fitness cost associated with them. And yet there
are problems with a system of punishment as public policy. It’s very expensive (Hutsler,
1995).
It can also be difficult to implement severe punishments, especially in democratic
societies. Infanticide has been common throughout human history. Some hunter-
gatherer societies regard it as a maternal right (Scrimshaw, 1984), while Western
European societies regard it as sinful, immoral, and illegal. Historically, many women
in Europe were hanged for this crime. However, even all-male British juries were un-
willing to convict young, poor, unmarried women for killing their babies when the
punishment was death. Judicial authorities were outraged, and for centuries British
lawmakers made changes in the law in an attempt to obtain more convictions for
infanticide (Hoffer & Hull, 1981). None was successful, and eventually the law was
medicalized. Infanticide remains a difficult legal, social, and medical issue.
Crawford and Anderson (1989) have suggested that prostitution can be consid-
ered a pseudopathology—a behavior that has its origin in adaptations that evolved
in response to problems human ancestors encountered but which, for one reason or
another, is no longer healthy, morally acceptable, or culturally valued. It may have
been common for ancestral females and males to exchange sex for resources and pro-
tection (Symons, 1979). If so, then modern prostitution may be an exaggerated form
of this exchange that occurs when some women need resources and protection and
some men lack sexual access to women through normal courtship. If prostitution
has its origin in ancestral trading of sex for resources and protection, extensive legal
attempts to eliminate it may have other undesirable consequences, such as increased
shoplifting and petty robbery by women, increased use of pornography by men, and
so forth (Crawford, 2004).
Conclusion
From an evolutionary psychology perspective, all behaviors are either the direct or
indirect effects of evolved adaptations. As a result, programs for changing current
behaviors should be based on an understanding of how ancestral environmental
conditions involved in the development and functioning of the relevant adaptation
relate to present environmental circumstances (Crawford, 2000). So, in a sense, tak-
ing an evolutionary perspective toward public policy is very practical. Evolved pref-
erences can suggest values and goals, but they will also enlighten us as to evolved
constraints on people’s preferences, emotions, and behaviors, all factors that will
strongly influence the outcome of policy decisions.
What are U.S. prostitution laws (with the exception of those of some Nevada
counties) attempting to do? What is their goal? Is it the eradication of sex for pay?
The ways the laws are enforced seems for the most part to target street workers who
do not make up the majority of prostitutes in the United States. Even if street work-
ers were eliminated, that would leave close to 75% of sex workers still in business. Is
the point to reduce the incidence of sexually transmitted disease? As there is no dif-
ference in the disease rates among sex workers and the rest of the population in the
United States (Elias, Bullough, Elias, & Brewer, 1998), this seems misguided. More
attention to the sexual practices of those most at risk of catching sexually transmit-
ted diseases might seem more useful, particularly when teenagers seem to be one of
those groups. Are the laws to protect women? If so, they fail miserably with respect
to the prostitutes themselves, who are marginalized, socially ostracized, and denied
the protection of the police that most citizens can count on. The decriminalization of
prostitution that is called for by organizations such as COYOTE would leave prostitu-
tion legal and unregulated—a novel situation, as it is legal and controlled in many
other countries (Sullivan, 2004).
If, however, the prostitution laws are designed to try to control female sexual be-
havior, they have an impact in the sense that they make the negatives of some forms
of sex for pay very high. But they have not eliminated it and are unlikely to do so as
long as there are men who are willing to pay for sex and women who need or want
the financial rewards of providing that service. People often point to the relatively low
rates of prostitution in the United States in recent times and say that this is evidence
that the laws do work to discourage it. But others have pointed out that prostitution
flourishes under certain social conditions and languishes under others, and those
conditions have much more to do with the willingness of unmarried women in gen-
eral to have sex with unmarried men. In our currently sexually permissive times, the
majority of people have had sex before marriage and the majority of women did not
need to be paid to do so; dinner and a date were sufficient. Under such circumstances,
fewer men need the services of a prostitute than they did in times when women were
less willing to share their sexual favors.
It is also important to remember that prostitution laws are not without signifi-
cant cost, both in the monetary sense and in terms of other consequences. An inordi-
nate amount of taxpayer money goes to policing such laws and arresting prostitutes
and clients. There are also personal costs suffered by prostitutes and clients, both
financial and social. And if prostitution serves some need, what will be the costs of
denying that need? Might women commit other, more violent crimes to get access
to resources? Might men who have few sexual opportunities become more coercive?
Maybe both, maybe neither, but such consequences should be considered. If a serious
attempt is to be made to eliminate, reduce, regulate, or decriminalize prostitution, it
will only be successful through an understanding of the functioning of the evolved
specialized psychological mechanisms producing the behavior as well as how envi-
ronmental interactions can influence the functioning of these mechanisms.
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PART FOUR
ADAPTATION AND THE

PRODUCTION OF CRIMINAL
BEHAVIOR
8
Risk-Taking, Antisocial Behavior,

and Life Histories
SANDEEP MISHRA AND MARTIN L. LALUMIÈRE
In this chapter we examine the ultimate causes of risk-taking and antisocial behavior. We
discuss risk-taking and antisocial behavior together because they have much in com-
mon: both often involve impulsive, reckless, immediately rewarding, and self-serving
behavior. We examine them together for ease of exposition but also for empirical rea-
sons: many forms of risk-taking (e.g., speeding while driving, promiscuous sex) are
tightly associated with antisocial behavior, at both the individual and aggregate levels
(reviewed in Mishra & Lalumière, 2008). This observation has led some to propose
such broad-spectrum constructs as “taste for risk” (Daly & Wilson, 2001), “general-
ity of deviance” (Osgood, Johnston, O’Malley, & Bachman, 1988), “problem behavior
syndrome” (Jessor, 1991), and “low self-control” (Gottfredson & Hirschi, 1990) to re-
flect the connection between general risk-taking and antisocial behavior. Just as most
criminologists have realized the futility of crime-specific etiological explanations, it is
becoming clear that the correct target of explanation is something more biologically
relevant than legally defined actions or even socially undesirable behaviors.
Longitudinal studies have uncovered reliable developmental trends in antisocial
behavior. This research has been mostly descriptive, and explanatory questions have
focused largely on proximal factors. In this chapter, we begin by describing three gen-
eral developmental pathways to antisocial tendencies. We then introduce life history
analysis to provide a framework for ultimate explanatory questions about the devel-
opment of both general risk-taking and antisocial tendencies. In particular, we ex-
plore the notion of risky and antisocial behaviors as adapted responses to particular
conditions encountered by individuals during their lifetimes; by “adapted,” we mean
selected over generations because of positive impact on fitness, regardless of current
fitness effects. We describe fundamental crossroads all organisms must face—those
having to do with growth, maintenance, and reproduction—and examine how these
139
140 Adaptation and Criminal Behavior
choices are linked to the three developmental pathways. We end with an application
of life history analysis to a contemporary criminological phenomenon, the sudden
drop in rates of risk-taking and criminal behavior in the 1990s.
The Development of Antisocial Tendencies: Three Pathways
Quinsey, Skilling, Lalumière, and Craig (2004) proposed that there are three key
developmental pathways that describe different patterns of antisocial and risky
behavior over the life course. A pattern of delinquent behaviors concentrated in
adolescence and early adulthood is termed adolescence-limited delinquency (after Moffitt,
1993). Antisocial behavior associated with neuropathology and social adversity
beginning early in life and persisting over the life span is termed life-course-persistent
offending (Moffitt, 1993). A third pattern of antisocial behavior is similar to
life-course-persistent offending, in that it is characterized by early onset of antisocial
behavior and persistence throughout the life span, but individuals do not show any
evidence of early neuropathology or social adversity. This pattern is termed psychopa-
thy (Harris, Rice, & Quinsey, 1994; Harris, Skilling, & Rice, 2001). There are surely
other, less common or more specific pathways (e.g., White, Bates, & Buyske, 2001),
but here we concentrate our discussion on these three general pathways.
Delinquent behavior limited to the periods of adolescence and early adulthood is
normative, and data suggest that it is somewhat anomalous to refrain from any an-
tisocial behavior during this time (reviewed in Moffitt, 1993). Late adolescence and
early adulthood are also rife with other high-risk behaviors, such as speeding while
driving, drug use, and promiscuous sexual activity (Gottfredson & Hirschi, 1990).
Developmental scientists typically invoke learning theory to explain the ubiquity
of adolescence-limited delinquency. For example, adolescents are said to mimic life-
course-persistent peers because risky and antisocial behavior in adolescence often
leads to desirable outcomes (money, reputation, sex) that cannot be obtained as eas-
ily through prosocial means (Moffitt, 1993). Adolescent-limited delinquents desist
from antisocial conduct in early adulthood as the benefits of engaging in such behav-
ior diminish relative to rising costs.
Life-course-persistent offending represents a vastly different developmental pat-
tern of antisocial behavior. Life-course-persistent offenders begin engaging in prob-
lem behavior early in childhood (e.g., they are hyperactive, aggressive) and display
antisocial behavior throughout adulthood. Life-course-persistent offenders are often
raised in disadvantaged environments and exhibit evidence of neurodevelopmental
perturbations experienced early in life. A number of factors possibly causing neu-
rodevelopmental perturbations have been linked with antisocial behaviors, such
as brain injury, malnutrition, maternal smoking during pregnancy, and obstetri-
cal complications (reviewed in Anderson, 2007; Harris, Rice, & Lalumière, 2001).
These factors typically interact with other social factors related to delinquency, such
as a single-parent upbringing, low socioeconomic status, or parental abuse (Rutter,
1997). Together, neurodevelopmental problems and poor social environments are
Risk-Taking, Antisocial Behavior, Life Histories 141
suggested to decrease intellectual (especially verbal) abilities, leading to increased

impulsivity, decreased sensitivity to punishment, and impaired development of pro-
social skills, all of which lead to snowballing adjustment problems.
The third pattern of antisocial behavior is psychopathy. Psychopaths exhibit
chronic and extreme antisocial behavior throughout their lifetime and across various
contexts. They are similar to life-course-persistent offenders in that they experience
early onset of antisocial behavior that perseveres throughout the life span, but they
exhibit important differences in severity of offending, affect and physiology, interper-
sonal relations, and lack of neurodevelopmental pathology (reviewed in Harris et al.,
2001; Lalumière, Harris, Quinsey, & Rice, 2005; Quinsey et al., 2004). Research
suggests that psychopaths do not exhibit signs of neurodevelopmental perturba-
tions experienced by life-course-persistent offenders, and social factors do not seem
to have any influence on the development of their persistent antisociality. Compared
to other life-course-persistent offenders, psychopaths tend to have broader and more
extensive criminal histories, exhibit increased likelihood of reoffending after incar-
ceration, commit more violent crimes, are more likely to engage in instrumental
(goal-directed) violence, and are more likely to select strangers as victims. Affectively
and physiologically, psychopaths also show differences: they exhibit shorter delay of
gratification, show less sensitivity to punishment, and are less physiologically reac-
tive when exposed to cues of distress, fear, or other aversive signs.
The risk-taking and antisocial behaviors described above for the three pathways
are typically seen as a pathological aberration from normal prosocial behavior. Im-
plicit in this view is the assumption that engaging in risky and antisocial behavior is
irrational or counter-productive and self-destructive. This assumption stems from the
perception that risky or antisocial behavior conflicts with an individual’s best interests
due to its high cost (e.g., physical injuries, jail time). An alternative view is that risky
and antisocial behavior may represent an adapted response to particular environmen-
tal or situational conditions, an optimal behavioral option in some circumstances,
maximizing the likelihood of certain outcomes that were statistically related to fit-
ness in ancestral environments. Although risky behavior may lead to costly outcomes
(e.g., foreshortening one’s life expectancy), it may also have benefits (e.g., increased
mating opportunities). When gains and losses are seen in the currency of reproduc-
tive success, a tendency to sacrifice life span in exchange for reproduction under some
circumstances does not seem so irrational. In the context of adaptive decision making
in humans and in light of evolutionary theorizing, when is it beneficial to engage in
risky and antisocial behaviors when other alternatives are available? Why are there
three general developmental patterns of risky and antisocial tendencies?
Life History Analysis: A Brief Introduction
The central tenet of evolution by natural selection is differential reproductive suc-

cess, or fitness, of individuals possessing certain heritable characteristics. There
is enormous variability in the ways that organisms can maximize reproductive
success. Life history analysis represents a way of conceptualizing the evolution of

allocation of effort or energy toward such fitness-relevant characteristics as age of
first reproduction, number of offspring, size of offspring, or length of reproductive
life span. Life history analysis seeks to understand the selective pressures that influ-
ence both the timing and expenditure of limited energy resources under different
conditions.
A key concept in life history analysis is trade-off: organisms must allocate a finite
amount of effort or energy to endeavors that constrain each other, such as num-
ber and size of offspring (Stearns, 1992). Natural selection favors an allocation of
effort or energy that optimally maximizes fitness given the features of a particular
environment (Kaplan & Gangestad, 2005). A number of trade-offs have been docu-
mented in many animal species, including trade-offs between size and number of
offspring, mating and parenting, and growth and early reproduction. These trade-
offs are relevant in the evolution of a diversity of life histories both between species
and within species. Some important trade-offs are discussed in the context of risk-
taking and antisocial behavior, particularly in how they affect potentially adaptive
risk acceptance in different environmental and situational contexts. We also discuss
the relevance of life history analysis for the three general developmental patterns of
antisocial behavior.
Survival, the Present, and the Future
Organisms have limited effort and energetic resources to allocate to different essen-
tial reproductive activities. This limitation forces trade-offs, such as that between
current and future reproduction. Current reproduction incurs costs such as compro-
mised immune function, reduced chance of survival, and lowered expected future
reproduction. Delaying current reproduction incurs the risk of not reproducing at
all because of potential mortality before other reproductive opportunity (Kaplan &
Gangestad, 2005). Organisms “decide” on a schedule of reproduction based on op-
timality of allocation of resources in a particular environment. If the mortality rate
due to extrinsic factors (e.g., predation, accidents) is high, it makes little sense for an
organism to delay reproduction given the potentially severe costs of not reproduc-
ing at all in a dangerous environment. Consequently, effort and energy in this par-
ticular environment should be allocated toward earlier reproduction, minimizing the
chances of death without reproducing.
The time horizon of an organism, determined from environmental cues to life
expectancy, has a powerful influence on life history. Under the eye of selection, or-
ganisms that most accurately assess their time horizons based on internal and ex-
ternal mortality cues and exhibit an appropriate behavioral response would obtain
higher fitness and pass this ability on to future generations. In the context of deci-
sion making, a limited time horizon leads organisms to value immediate, short-term
rewards more highly than larger distal future rewards, a pattern known as future
discounting. Discounting of the future may appear on the surface to be a brash strat-
egy, but choosing such a strategy when faced with certain environmental conditions
may represent an optimal decision. Harvey and Zammuto (1985) showed that age
of first reproduction in females is related to life expectancy at birth both within and
across various species of wild mammals.
Humans appear to exhibit similar sensitivity to time horizons by discounting the
future if time horizons appear to be short or if the projected quality of one’s future
is perceived to be poor. In an interesting study, Phillips, Ruth, and Wagner (1993)
examined the death rates of Chinese Americans who believed that certain years of
birth are associated with susceptibility for certain diseases (e.g., fire years are as-
sociated with the heart and therefore heart diseases). Results showed that Chinese
Americans died significantly earlier if they contracted a disease associated with their
birth year compared to Chinese Americans who contracted a disease not associated
with their birth year. Chinese Americans who were less likely to hold traditional be-
liefs (e.g., not born in China) showed fewer years of life lost if there was a match
between disease and year of birth. White Americans, who were not expected to hold
such beliefs, did not show an association between year of birth and death rates for
any diseases. The findings for Chinese Americans held for all major causes of death
and could not be explained by cohort effects, marital status, or changes in the explicit
outward behavior of patients, doctors, or other healers. One interpretation of these
findings, in light of sensitivity to time horizons, is that certain beliefs about future
outcomes implicitly affect physiology and behavior.
Returning to antisocial and risky behavior, the decision to engage in such be-
havior may be contingent on an individual’s assessment (not necessarily conscious)
of his own need and how the available present and future behavioral options might
meet that need. For example, individuals experiencing economic hardship, poor
health, or low social status may rightly perceive that their expected outcome (e.g.,
a minimum-wage job) will be poor. Such people often appear to engage in behav-
iors reflective of future discounting, such as competitive risk-taking or interpersonal
violence, perhaps because their perceived need for social, reproductive, or financial
advancement exceeds the mean advancement resulting from lower-risk behaviors.
Engaging in such behaviors may lead to at least short-term gains in reputation, re-
productive opportunities, or material resources. Potentially costly risky or antisocial
behavior becomes an appealing option, as the benefits may far outweigh the costs for
an individual with poor future prospects and a shortened time horizon.
Following this logic, age-specific risk rates reflect different valuations in short-
versus long-term rewards. The relative valuation of safety and survival changes
in reference to life stages, particularly in comparison to other potential immediate
rewards such as mating access (Daly & Wilson, 2001; Hill & Chow, 2002; Rogers,
1994). Young males aged 18 to 24 are particularly likely to engage in antisocial and
risk-accepting behavior because competition for status, mates, and resources in that
period of time reaches its peak (Wilson & Daly, 1985). Young males compete not only
with each other but also with older males who have had more time to accumulate
skills, resources, and status, features important to female mate choice. This period of
time allows for the potential of high variance in gains, as the costs of not obtaining
resources and a mate are significant in a fitness sense, and risk-taking may be neces-
sary to obtain favorable outcomes (Hill & Chow, 2002).
There are also situations in which the future is less discounted and behavior is
focused on maximizing long-term outcomes. Other aspects of life history have been
associated with decreases in risky behavior, such as obtaining status, gaining a long-
term mate, and parenthood (Hill & Chow, 2002). For individuals who have already
obtained a mate through competition and are the progenitors of offspring, effort and
energy may shift from competition and mating effort to parental effort, facilitating a
decrease in risky behavior.
Interpersonal conflict can be thought of as an outcome of steep future discount-
ing and a risk-accepting response to social competition, and many homicides occur
as a direct result of male–male competition over status or mates (e.g., Daly & Wilson,
1998, 1990; Wilson & Daly, 1985, 1992, 1997). Wilson and Daly examined homi-
cide rates and reproductive timing as a function of economic inequality and local life
expectancy. In the context of limited life expectancy, it is likely that males would esca-
late competition, and indeed, Wilson and Daly found that the homicide rate increased
as local life expectancy decreased, even after statistically removing the effects of ho-
micide on life expectancy. Similarly, a shift toward immediate reproduction trading
off with later reproduction should be observed in situations of short time horizons,
because the likelihood of successfully reproducing in the future is diminished in such
a situation. Wilson and Daly found support for this hypothesis, demonstrating that
disproportionately high birth rates among younger mothers were observed in neigh-
borhoods with the lowest life expectancies.
The predictability of environmental cues should also play an important role
in the modulation of risky behavior. Risk acceptance may constitute a more effec-
tive strategy when future prospects are unknown. If an individual were to utilize
a “safe,” risk-averse strategy in the face of uncertainty, he might not survive to repro-
duce again if conditions became particularly bad. In the face of unpredictability, a
risky strategy, although paradoxical, may have been associated with greater repro-
ductive success in an ancestral environment. Following from this prediction, Hill,
Ross, and Low (1997) found that reported risk-taking behavior in various domains
(e.g., sexual, health, financial) was higher in college students who exhibited higher
future-unpredictability beliefs and shorter life span estimates. Further research dem-
onstrated that early cues of unpredictability, such as parental divorce or family unre-
liability, are associated with a risk-accepting life history strategy (Ross & Hill, 2002).
It is also possible that divorced or unreliable parents genetically passed on traits that
predispose their offspring toward risky behavior, and previous research has shown
that there is some genetic transmission of antisocial tendencies from parents to off-
spring (e.g., Cadoret, Yates, Troughton, Woodworth, & Stewart, 1995). Behavioral
genetic studies have also shown, however, that the environment accounts for a large
proportion of variance in phenotypes, suggesting that features of the environment
may calibrate or activate evolved mechanisms (e.g., sensitivity to predictability cues)

associated with risk-taking.
In sum, similar trade-offs are seen in humans that are seen in other animals,
wherein short time horizons and unpredictable environments may lead to discount-
ing of the future, as evidenced by greater risky and antisocial behavior in individuals
with shorter or less favorable future prospects.
Mating and Parenting Effort
In most species, males have a higher potential reproductive rate than females and
so can produce more offspring in a given period of time (Clutton-Brock & Vincent,
1991). One mechanism through which this sex difference can manifest is an imbal-
ance in minimal parental investment (Trivers, 1972). In many species, females incur
greater time and energy costs than males in order to produce offspring. In humans,
women must invest at least nine months of time for gestation and must go through
parturition. Except under exceptional circumstances, energy costs are also incurred
in having to provide nourishment for the neonate. In contrast, men need only con-
tribute a single ejaculate to successfully produce an offspring with a fertile woman.
The sex difference in potential reproductive rates creates a situation in which
the “slower” sex, usually female, becomes a valuable resource for which members
of the opposite sex, usually males, compete (Clutton-Brock & Vincent, 1991). Be-
cause a pregnant or lactating female is effectively removed from a pool of potential
mates, the effective (or operational) sex ratio is male-biased, facilitating the evolu-
tion of intense male–male competition for the limited number of available females.
That certain males can monopolize the available pool of females more than others
increases male variance in reproductive success, fueling competition (Clutton-Brock
& Vincent, 1991).
Over evolutionary time, there may have been stronger selection pressure on men
relative to women to seek sexual opportunities, given that an increase in sexual part-
ners likely increased the reproductive success of men more than women over time.
A tendency for men to invest more energy in mating effort than women has been
well established (reviewed in Low, 2000; see also Schmitt, 2005). Although men are
more likely than women to invest in mating relative to parenting effort, there is varia-
tion within the sexes. Some men may be monogamous and invest copious amounts
of time and energy in their offspring; others may attempt to have as many sexual
encounters as possible, never investing in offspring. Similar but smaller variation in
allocation of energy to mating and parental effort is seen in women. In both men and
women, a tendency toward early reproduction and high mating effort is generally
associated with greater risk-taking and antisocial behavior (reviewed in Lalumière
et al., 2005).
In both sexes, risk-taking tendencies are highly associated with being or getting
someone pregnant in adolescence (Bingham & Crockett, 1996; Jessor, Costa, Jessor &
Donovan, 1983). Teenage fathers are more likely to have committed serious crimes
(Stouthamer-Loeber & Wei, 1998) and to have encountered vulnerability factors as-
sociated with antisocial behavior, such as low socioeconomic status or parental anti-
sociality (Fagot, Pears, Capaldi, Cosby, & Leve, 1998). Early pregnancy in females has
also been associated with antisocial behavior, with childhood aggression predicting
early motherhood (Serbin et al., 1998). Other studies have investigated mating ef-
fort more generally in relation to antisocial conduct. Lalumière and Quinsey (1996)
found that variables measuring antisocial tendencies were also related, in men, to a
history of multiple uncommitted sexual relationships. Additionally, antisocial men
are more likely to utilize sexual coercion, aggression, or deception in the pursuit of
mating opportunities. In many studies, age at first intercourse is strongly related to
indicators of antisocial tendencies (e.g., Quinsey, Book, & Lalumière, 2001).
Why are antisocial individuals more likely to engage in potentially costly mating
behaviors, exhibiting high mating effort and low parental effort? Adolescence-limited
delinquents exhibit a peak in antisocial behavior, including increased sexually coer-
cive behavior, after puberty, with a systematic decline occurring sometime thereafter.
As mentioned before, this peak during adolescence may be due to escalated intrasex
competition for mates (Campbell, 1995; Daly & Wilson, 1988; Wilson & Daly, 1985).
Because of sex differences in potential reproductive rate, there is greater variability
in male than in female reproductive success, and thus there are greater fitness ben-
efits bestowed upon males who succeed and greater costs for males who do not.
It should be noted that behaviors that are considered risky may also reflect
hard-to-fake displays of prowess or social status, such as willingness to fight, fear-
lessness, or independence. Adolescent risky and antisocial behavior may thus serve
as an “honest signal” of qualities desirable to females (e.g., health, attractiveness;
Lalumière & Quinsey, 2000; Zahavi & Zahavi, 1997). This notion is supported by
the findings that gang leaders and dominant males enjoy increased access to sexual
partners, and young males are more likely to engage in risky behavior when in the
presence of peers (reviewed in Daly & Wilson, 2001).
Desistance from criminal and risky behavior for most individuals occurs after
adolescence, likely as a function of a shift in allocation of energy from mating to
parenting effort. Marriage, stable work, and aging are all reliable correlates of de-
sistance from risky behavior. A shift from mating to parenting effort (or vice versa)
should be observed when the cost–benefit ratio favors one type of effort over the
other. Investing in a committed relationship with a high-quality mate, for example,
may offer greater fitness benefits in the long term. The relative costs of attempting
to gain multiple mating opportunities—such as time and effort allocated to court-
ing, risks associated with sexual aggression, or retaliatory violence from partner’s
relatives—may be too high compared to the relative benefits of investing in a long-
term relationship with a single partner and allocating effort and energy to children
born of that partnership. Surprinsingly, little research has been done to investigate
the actual effect of having children on the shift from mating to parental effort. We
expect that adolescent-limited delinquents are likely to exhibit diminished risky and
antisocial behavior after having children, whereas life-course-persistent offenders

would not necessarily do so. Lowered testosterone following the onset of fatherhood
has been suggested as a proximal mechanism for this shift (e.g., Gray, Kahlenberg,
Barrett, Lipson, & Ellison, 2002). Only adolescence-limited delinquents exhibit a de-
crease in antisocial behavior with age and so are likely more sensitive to situational
or environmental changes, such as having children. Because life-course-persistent
offenders do not desist from antisocial behavior with age, different mechanisms
are required to explain the persistence of antisocial behavior in this group and in
psychopaths.
In sum, individuals must “decide” to invest energy in mating or parenting. Males
and younger individuals have more to gain and less to lose from engaging in risky
behavior. With age, greater social status, a long-term relationship, and children,
however, the relative valuation of benefits and costs from risky antisocial behavior
changes significantly, and a shift from mating to parenting effort is observed.
Growth, Reproduction, and Competitive Disadvantage
Life-course-persistent offenders do not exhibit desistance from antisocial or criminal

behavior with age, suggesting that a different mechanism is required to explain their
consistently high valuation of antisocial behavior relative to its costs. The construct
of embodied capital, used in human behavioral ecology, is particularly illuminating
of life-course-persistent offending.
Embodied capital refers to intrinsic attributes, such as health, skills, or attrac-
tiveness, that allow for successful competition for resources, mates, and status
(Lalumière et al., 2005). Individuals with low embodied capital may experience an
early and consistent competitive disadvantage, such that a conditional strategy of
persistently risky and antisocial behavior may represent the best chance for obtain-
ing resources, status, or mates. Individuals with low embodied capital would likely
project their future prospects to be poor, thus affecting the cost-benefit ratio of adopt-
ing risky antisocial behavior. The strategy is conditional, in the sense that it faculta-
tively responds to cues of low embodied capital.
Life-course-persistent offenders indeed appear to be at competitive disadvantage
relative to others, suffering early from neurodevelopmental problems, poor academic
success, and poor social support. As a consequence, antisocial behaviors such as the
acquisition of resources through criminal means, establishment of dominance or
higher status through violence, or coercion in attempting to gain mating opportuni-
ties may represent the most beneficial behavioral option. Competitive disadvantage
has been empirically shown to influence rates of antisocial and criminal behavior.
Wilson and Daly (1997) demonstrated that Chicago neighborhoods with higher
local income disparities also experienced higher homicide rates. If one is able to le-
gitimately compete for resources, status, or mates, it is not beneficial to engage in
costly risky or criminal behavior. Low-embodied-capital individuals, however, have
much to gain and often little to lose from discounting the future and engaging in an-
tisocial conduct. The constraints of low embodied capital shift the cost–benefit ratio
of risky and antisocial behaviors, making such behaviors a more beneficial option.
Because low embodied capital may not be easy to remedy, this option remains opti-
mal throughout the life span.
Cues of present or future embodied capital may influence growth trajectories
and the adoption of life-course-persistent antisocial behavior, and these outcomes
may represent consequences of a life history trade-off between investment in long-
term growth and earlier reproduction. Infants exhibit a predictable growth trajec-
tory when they experience typical prenatal conditions. Low birth weight caused by
poor maternal nutrition (Godfrey, Robinson, Barker, Osmond, & Cox, 1996), how-
ever, can often lead to rapid compensatory growth during the early years of a child’s
life, in addition to health problems later in life (Gluckman, Hanson, & Spencer, 2004;
Lummaa, 2003). The experience of poor maternal nutrition in utero may serve as
a cue to the developing fetus that conditions experienced during development (in
this case, limited resource availability) are likely to continue after birth and in the
future. Thus, compensatory growth and accelerated development in the early part of
a child’s life may occur as a preemptive physiological adapted response that is likely
to confer benefits in anticipation of specific future conditions.
Individuals who exhibit compensatory growth may reproduce earlier in life, but
their lack of investment in long-term growth results in an earlier onset of senes-
cence. Such a mechanism may represent an attempt to mature and reproduce earlier
than other potential competitors in a cohort, albeit at the cost of not being able to
reproduce later. Empirical evidence supports a short-term versus long-term growth
life history trade-off in that individuals experiencing early compensatory growth se-
nesce faster and suffer negative reproductive consequences later in life (Lummaa,
2003; Phillips et al., 2001; Eriksson et al., 2001).
Several factors have been implicated in the development of life-course-persistent
offending, including parental abuse, poor nutrition (in utero or during childhood),
neurodevelopmental perturbations, and general developmental instability (Harris
et al., 2001; Lalumière, Harris, & Rice, 2001). Although these factors are typically
seen as disrupting normal developmental processes, another interpretation is pos-
sible. Neurodevelopmental perturbations and poor nutrition may serve as cues of
developmental disadvantage to a mother and her fetus, thus facilitating the develop-
ment of psychological mechanisms calibrated to produce risk-accepting strategies.
Early parental abuse and the subsequent development of persistent antisocial behav-
ior may also reflect the same mechanism, as parental abuse may suggest (analogous
to poor nutrition) low embodied capital and a difficult future. Persistent antisocial
behavior has also been associated with lower life expectancy, consistent with a life
history strategy oriented toward short-term, immediate gains at the cost of long-
term survival (Laub & Vaillant, 2000).
An interesting natural experiment provides information relevant to the suggested
trade-off between growth (or embodied capital more generally) and reproduction.
During World War II, food supplies were limited by the German army in some parts
of the Netherlands, leading to a severe food shortage. Males whose mothers experi-
enced food scarcity during pregnancy had lower birth weight and experienced lower
reproductive success over their lifetime (Lumey & Stein, 1997). These males also
exhibited much higher frequencies of antisocial behavior in early adulthood com-
pared to males whose mothers did not experience food scarcity (Neugebauer, Hoek,
& Susser, 1999).
In sum, persistent antisocial behavior may develop as a conditional life history
strategy based on environmental cues predictive of negative future prospects and
competitive disadvantage (or low embodied capital), with short-term benefits of im-
mediate reproduction and long-term costs of decreased life span. The lack of desis-
tence of antisocial behavior in life-course-persistent offenders can be explained by
low embodied capital and other consequences of compensatory growth (or similar
mechanisms) in response to early predictive environmental cues. In both cases, an
individual has little prospect of improving competitive standing relative to others
in the population and experiences little ability to legitimately acquire a stable job, a
long-term relationship partner, or good social standing—all factors that have been
shown to be associated with the desistence of antisocial behavior in adolescent-limited
delinquents. This framework leads to new expectations regarding the development
of antisocial and risky behavior. For example, fast growth during childhood should
be associated with life-course-persistent offending but not adolescence-limited delin-
quency, unless intense remedial measures are put into place.
Life History Strategies as Personalities
Personality describes an individual’s consistent pattern of emotional, cognitive, and

behavioral responses in multiple contexts (Funder, 2001). Individual differences
manifesting as unique personalities may represent different, consistent patterns of
solving life history problems. Psychopathy may represent an extreme example of a
personality type, in that it is indicative of a consistent pattern of affect, cognition,
and behavior that reflects constant risk acceptance and future discounting. At the
other extreme, exceptionally risk-averse individuals may always choose the “safest”
avenue, whereby risk is avoided and the future is always considered, not discounted.
Some studies have identified other personality traits closely associated with crimi-
nality and antisocial behavior, such as negative emotionality and weak constraint
(Agnew, Brezina, Wright, & Cullen, 2002; Caspi et al., 1994) as well as sensitivity to
rewards (Fonseca & Yule, 1995) and low self-control (Gottfredson & Hirschi, 1990).
Although psychopaths share features with life-course-persistent offenders—
early onset and persistence of antisocial tendencies, for example—there are impor-
tant differences to consider. In particular, psychopaths generally do not seem to have
experienced the same cues of competitive disadvantage as life-course-persistent of-
fenders. They do not exhibit the same neurodevelopmental pathologies, and they
appear to have higher embodied capital. For example, Lalumière et al. (2001) found
that adult psychopaths, compared to other adult offenders, had experienced fewer
obstetrical complications, exhibited lower fluctuating asymmetry (based on ten fea-
tures of the head and body), were less likely to be left-handed (a sign of early neuro-
developmental perturbations), and were rated as more physically attractive. Other
studies have investigated the underlying structure of persistent violence, suggesting
that a factor associated with psychopathy and a factor associated with early devel-
opmental problems (consisting of obstetrical complications, low IQ, problems in in-
fancy, and so on) were unrelated to each other (Harris et al., 2001). This and other
evidence (reviewed in Barr & Quinsey, 2004; Harris et al., 1993; Skilling, Quinsey, &
Craig, 2001) suggests that psychopathy represents a separate subgroup of persistent
offenders ( see Chapter 10 of this volume).
Although engaging in risky behavior is often contingent on fluid environmental
or situational conditions, stable patterns of personality may represent attempts to
establish “niches” in variable environments. Individuals of average embodied capital
who do not suffer from extreme competitive advantage, for example, may engage in
consistently low-risk behavior. Investment in low variance outcomes such as com-
mitment to education throughout early childhood, a stable job, and a long-term re-
lationship in life may reflect a long-term, stable, risk-averse personality.
The introversion–extroversion personality dimension may represent behavioral
patterns that arise from this scenario. Investment in high mating effort and tak-
ing risks that require certain skills may reflect a personality type that is outgoing,
risk-accepting, and extroverted. Nettle (2005) has suggested that the introversion–
extroversion personality dimension reflects different benefits and costs. Extroversion,
for example, was found to be associated with higher mating effort; male extroverts
were more likely to have extrapair partners, and female extroverts were more likely to
leave existing relationships. Costs of extroversion were also hypothesized and found,
with increases in the likelihood of involvement in an accident or illness. In addition,
extroverted women were more likely to expose their children to stepparenting, a
known risk factor for child abuse and murder (Daly & Wilson, 1988).
Some support for the development of different personality “types” that partially
incorporate risk-acceptance and risk-propensity comes from Sulloway’s (1997) in-
vestigation of birth order and personality. Sulloway suggested that firstborns tend
to identify more with their parents, adopting a risk-averse, more conservative strat-
egy over the course of the life span, while later and middle-borns tend to take more
risks. Sulloway’s characterization of birth order and personality is consistent with
the notion that risky or antisocial behavior is an adaptive response contingent on
early cues of future prospects and projected time horizons. Firstborns may develop a
conservative, risk-averse personality because of the greater certainty of their future
resource or status potential derived from parental inheritance. For later-born indi-
viduals, certainty of future resources or status is not guaranteed, and a riskier strat-
egy may be required to gain resources, status, and mates. Some empirical evidence
supports different personalities and risk propensity based on birth order, suggesting
that middle- and later-born offspring are more likely to engage in adolescent anti-
social behavior such as substance use, precocious sexual activity, and criminal be-
havior (Argys, Rees, Averett, & Witoonchart, 2006). Other studies have linked birth
order to personality differences in various domains (e.g., Buunk, 1997; Saroglou &
Fiasse, 2003).
Evidence for animal “personalities” has been accumulating and has been a re-
cent topic of great interest for behavioral ecologists. Wolf, Sander van Doorn, Leimar,
and Weissing (2007) conducted computer simulations that suggest life-history trade-
offs favor the evolution of different personalities (e.g., risk-proneness, aggressiveness,
boldness). Wolf and colleagues argued that intraspecies variation in the valuation of
current versus future fitness returns may lead to polymorphic populations that vary
in their propensity toward short-term- and long-term-oriented life history strategies.
Animal “personalities” may reflect an attempt to establish behavioral “niches” in a
variable environment. Future research using animal models may shed light on the
evolution of personalities in humans.
Life Histories and Heritability
In this chapter, we suggest that people’s life histories vary with conditions encoun-
tered throughout their lifetimes, especially those encountered early in life. Thus,
many life histories, including those involving life-course-persistent offending, are
likely developmentally conditional (see Lalumière et al., 2005, for a thorough dis-
cussion of conditional and obligate strategies associated with antisocial behavior in
humans and other species). It is well accepted, however, that personality in general
and antisocial tendencies in particular show significant heritability in behavior ge-
netic studies (e.g., Mason & Frick, 1994). There are at least three ways to resolve this
apparent inconsistency. First, psychopathy is likely part of a heritable and obligate
life history (see Chapter 10), and psychopathy has not been considered in behavioral
genetics studies of antisocial behavior. Thus, the number of psychopaths in a given
behavioral genetic study of antisocial behavior would directly inflate heritability
estimates. Second, some factors associated with resistance to developmental pertur-
bations must be heritable. Because neurodevelopmental factors are cues to future
competitive disadvantage in our hypothetical model of life-course-persistent offend-
ing, behavioral genetic studies of persistent offending will inevitably obtain nonzero
heritability. Finally, nonzero heritability does not necessarily provide evidence against
the existence of condition-dependent life histories.
For instance, the experience of maltreatment is reliably associated with the de-
velopment of antisocial tendencies (there is now good evidence that this is an envi-
ronmental effect, not simply a genetic transmission effect). Childhood maltreatment
may provide a cue to the quality of current and future environments, and people
may “adjust” their development accordingly. Caspi et al. (2002), however, found
that a genetic polymorphism on the X chromosome associated with the monoamine
oxidase A enzyme (which breaks down some neurotransmitters) moderates this re-
lationship: maltreated individuals with a genotype associated with low expression
of the gene are much more likely to engage in antisocial behavior as adults than
maltreated individuals with a genotype associated with high expression of the gene.
Other gene–environment interactions have been detected using large samples and
sensitive measures (e.g., a serotonin transporter gene and stressful life events on risk
for adult depression; Caspi et al., 2003). Thus, it is likely that the “decision” to adopt
a particular life history is dependent on both the conditions encountered and the
genotype of the individual. For some people, difficult social conditions may not pro-
vide a cue to impending competitive disadvantage because they have the ability to
overcome them.
An Application of Life History Analysis:

Understanding the 1990s Crime Drop
We now turn our attention to the application of life history analysis to understand a
contemporary criminological issue. In the early 1990s, rates for all types of crimes
fell sharply in both Canada and the United States (Blumstein & Wallman, 2005;
Lalumière et al., 2005; Levitt, 2004; Mishra & Lalumière, 2008). A number of
explanations have been offered for the crime drop, including an aging population,
increases in the number of police officers, a stronger economy, and changes in abor-
tion laws in the 1970s (Levitt, 2004). Although each explanation can account for a
small portion of the decline in crime, none appears to explain a significant amount
of the variation in rates of criminal behavior. In addition, many explanations involve
U.S.-specific phenomena, such as increased incarceration, and ignore the parallelism
between the Canadian and U.S. crime data (Ouimet, 2002). It is quite possible that
criminological hypotheses for the decline in crime may be focusing on too narrow a
target of explanation.
Our research suggests that existing explanations of the crime drop have not con-
sidered the broader category of behavior to which most crimes belong, specifically
antisocial behavior and risk-taking (Mishra & Lalumière, 2008). Archival data from
the United States and Canada were used to show that since the early 1990s, antisocial
and risky behaviors in the domains of violence, some types of drug use, accidents,
and sexual behavior have dropped significantly and in a manner that closely parallels
the drop in crime. Our results confirm a strong link between crime, antisocial behav-
ior, and risky behavior and suggest that what requires explanation is not simply the
drop in crime but a more general drop in risk-taking and antisocial behavior.
What facilitated a decrease in criminal and risky behavior in general in the
1990s? We propose in this chapter that antisocial and risk-taking tendencies are af-
fected by people’s time horizons. Here we apply the life history framework presented
in this chapter to suggest potential causes of the crime and risk drop in the hope that
these suggestions may represent fruitful avenues of research. We identify what may
be indicators of a shift from a focus on short-term gains to a focus on long-term gains

in the early 1990s and suggest environmental cues that may have influenced such
a shift.
The significant drop in antisocial behavior that was observed for the entire popu-
lation of the United States and Canada in the early 1990s suggests that time horizons
were perceived to be longer, and future prospects were perceived as more positive.
Therefore, we expect to find indicators of investment in long-term, future outcomes
instead of short-term outcomes and an increase in behaviors suggestive of an opti-
mistic view of the future. In addition, if risk and antisocial behavior are affected by
time horizons and the quality of one’s future, then environmental cues predictive or
indicative of a benevolent future should be observed to precede or accompany the
drop in antisocial behavior in the 1990s.
Preliminary data provide some support for the notion that a shift toward a positive
future orientation was observed over the course of the 1990s. For example, according
to the Youth Risk Behavior Surveillance Survey, teenagers have lived healthier lives
by exercising more and eating more fruits and vegetables since the early 1990s. Visits
to the doctor for tests diagnostic of long-term chronic diseases such as cancer and
diabetes have also increased during that time span despite a drop in the incidence of
many diseases, suggesting that people are investing time in physical maintenance.
Depression rates, which may be reflective of pessimism about future prospects, have
decreased over 25% in Canada since the early 1990s (Patten, 2002). It is important
to interpret such data with caution because many other factors could be responsible
for these changes, such as increased antidepressant prescriptions affecting depres-
sion rates. Together, however, these indicators suggest that as of the early 1990s,
people may have exhibited a greater and more optimistic interest in long-term,
future-oriented behaviors rather than behaviors reflective of short-term, immediate
rewards focused on the present.
Reproductive and parenting behaviors have also changed since the early 1990s.
Investment in high mating effort and attempts at immediate reproduction are as-
sociated with a shorter time horizon and more negative future prospects, whereas
greater investment in parenting and one’s offspring suggests a longer-term and more
future-oriented perspective. Therefore, we should expect that indicators of parent-
ing effort should have increased and indicators of high mating effort should have
decreased since the early 1990s. Since that time, such a shift does appear to have
occurred; mothers have delayed reproduction, with decreases in birth rates observed
for all ages, except for women aged 30 to 44, whose reproductive future is short (data
from the U.S. National Center for Health Statistics, NCHS). There are significantly
fewer teen pregnancies (down more than 20% in both the United States and Canada
since 1991) in addition to fewer live births among teens (NCHS). It would also be
expected that parents allocate more resources to fewer offspring. Even divorce rates
have decreased since the early 1990s, suggesting that people may be investing more
in long-term relationships (NCHS). Collection of more data relevant to reproduc-
tive outcomes and investment in children will provide further tests as to whether
there has been a shift toward long-term strategies involving investing in children,
as opposed to strategies more oriented to short-term mating effort since the early
1990s. One potentially productive avenue of research would be to examine changes
in intensity of parental supervision over the last twenty years (certainly an indicator
of parental effort); parental monitoring is one of the best protective factors for anti-
social behavior (Donovan & Jessor, 1985).
The dramatic increase in obesity rates since the early 1990s is particularly in-
teresting in the context of the life history trade-off between investment in long-term
growth and short-term reproduction (Mokdad et al., 1999). If the decline in crime
and risk-taking since the early 1990s is the result of situational cues signaling fa-
vorable future conditions, we hypothesize that people would invest more in main-
tenance and growth than in immediate reproduction. We are currently analyzing
data at the state level investigating the relationship of reproductive outcomes and
obesity rates since the 1990s. Preliminary results suggest that there is indeed an
inverse relationship between indicators of immediate reproduction, such as teenage
pregnancy, and long-term investment in growth, such as body mass index. We do not
suggest that obesity is adaptive but rather that investment in growth and long-term
health means saving calories rather than spending them. In a modern environment
with easy access to calorie-rich foods, this process leads to obesity. Such results must
obviously be interpreted with caution at present, and further data must be collected,
but these preliminary results suggest that a trade-off between immediate reproduc-
tion and long-term growth may have occurred in concert with the drop in antisocial
and risky behavior in the 1990s. The question still remains, however: What caused
this shift from short-term to long-term strategies?
We described several environmental and situational variables associated with
increases in antisocial behavior and short-term life history strategies in this chapter,
including perceived length of time horizons, projected quality of future prospects,
unpredictability of environments, quality of early environment, intensity of compe-
tition, and competitive disadvantage. Changes in each of these variables may have
preceded or accompanied the drop in antisocial behavior in the early 1990s and
would represent important avenues of investigation.
Life expectancy has been increasing for some time in North America. People per-
ceive the length of their time horizon in more ecologically relevant ways than simply
looking at a calculated national average life expectancy, and so cues such as the pres-
ence of older relatives (parents, grandparents) as well as the presence of older indi-
viduals within smaller local populations (e.g., neighborhoods within a city) would
be indicative of a lengthier expected future. In communities where there are many
sources of extrinsic mortality, such as homicide or accidents, antisocial behaviors
are more often observed (Wilson & Daly, 1997). The recent increase in body mass
index may itself provide a cue to the health of others, generating positive estimates
of one’s (or one’s children’s) future health. A recent study reported that having an
overweight spouse, friends, or siblings increases one’s odds of obesity (Christakis &
Fowler, 2007). Thus it is possible that at the community level, sources of extrinsic
mortality have decreased and cues to future health have increased, leading to more
future-oriented and less antisocial behavior.
Other cues relevant to time horizons and future prospects may include decreases
in perceived inequality, leading to less interpersonal competition and less potential
for individuals to suffer competitive disadvantage. Although inequality between the
richest and the poorest has actually been increasing at the national level since the
early 1990s, it may be possible that communities at a lower level, such as neighbor-
hoods, may have experienced a more egalitarian distribution of wealth, leading to
less inequality and fewer costly antisocial behaviors as a response to lesser competi-
tion. Comparison of different communities since the early 1990s would shed light on
what time-horizon-relevant cues may influence life history strategies.
Conclusion
The application of life history analysis to the development of risky and antisocial be-
havior may provide a useful framework for thinking about both ultimate and proxi-
mal causes, especially hypothetical causes that may not have been postulated under
standard development and learning theories. A consideration of ultimate causes, in
particular, forces us to think differently about the meaning of pathology, the func-
tion of risk and antisocial behavior, the causes of health problems, and how people
respond to difficult early conditions. We hope we have shown that a research pro-
gram informed by life history analysis is a program that may lead to the discovery of
proximal—and thus likely modifiable and preventable—causes.
ACKNOWLEDGMENTS
We wish to thank Josh Duntley, Grant Harris, Danny Krupp, Christine Michell, Michael Seto,
Vern Quinsey, Todd Shackelford, and Tracey Skilling for providing helpful feedback on an ear-
lier version of this chapter. Thanks also to the Social Sciences and Humanities Research Coun-
cil for providing a Doctoral Fellowship to S. M. and a Standard Research Grant to M. L. L.
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9
Theft
SATOSHI KANAZAWA
An Evolutionary Psychological Perspective on Theft
Theft refers to illicit appropriation of resources that rightfully belong to someone else.
Defined as such, the behavior is a cultural universal.1 Interpol’s International Crimi-
nal Statistics (Interpol, various years) reports some incidence of theft and robbery in
each of its 186 member nations every year, so some people steal from and rob others
in every nation. Misappropriation of resources (usually food) from rightful owners is
also observed among other primate species, such as chimpanzees (de Waal, 1989),
bonobos (de Waal, 1992), and capuchin monkeys (de Waal, Luttrell, & Canfield,
1993). That theft is a cultural universal and observed among other species strongly
suggests a biological and evolutionary origin.
An evolutionary psychological perspective on theft begins with a recogni-
tion of the importance of material resources for both survival and reproduction.
Every person needs resources to survive (food, shelter, clothing) and to achieve
reproductive success (parental investment). Since not all individuals are equally
capable of procuring such resources on their own through legitimate means, it
can be expected that some will resort to illicit means to acquire the desired re-
sources.
Besides providing a pan-specific and thus parsimonious explanation for theft
and other property crimes, by explaining similar behavior across many species, an
evolutionary psychological perspective can simultaneously address four empirical
puzzles about theft that no other single criminological theory can: Why do men
commit more theft than women? Why are younger men more likely to commit theft
than older men? Why are the poor more likely to commit theft than the rich? Why
are less intelligent individuals more likely to commit theft than more intelligent
individuals?
160
Theft 161
Empirical Puzzles
1. Why Men and Not Women?

In every human society, men commit an overwhelming majority of both violent and
property crimes (Brown, 1991; Kanazawa & Still, 2000). Worldwide, men commit
more than 90% of all theft and robberies. Why is this?
One relatively unusual feature of the human mating system can account for
the overwhelming male bias toward criminality. Unlike those of most other species,
human males make a large parental investment in their offspring. The unusually
high degree of male parental investment among humans leads to universal human
female mate preference for men with a large amount of resources (Buss, 1989). The
more resources a potential mate has, the more parental investment he can make in
his and his mate’s joint children. Men’s resources increase their children’s chances
of survival and their future reproductive prospects.
Because women prefer men with greater resources as their long-term mates,
men fiercely compete with one another to accumulate resources and attain higher
status. The more resources they possess and the higher the status they occupy, the
greater the reproductive opportunities they have. Wealthier men of high status have
more sex partners and copulate more frequently than poorer men of low status
(Kanazawa, 2003a; Pérusse, 1993). Wealth and status do not have a similar effect
on women’s desirability as long-term mates (Buss, 1989).
From an evolutionary psychological perspective, this is why men account for
an overwhelming majority of thieves and robbers worldwide. Material resources im-
prove men’s reproductive prospects much more than they do women’s. We would
therefore expect men to be much more motivated to accumulate material resources,
either through legitimate or illegitimate means, than women. In fact, not only do
men commit an overwhelming majority of theft and robberies worldwide but they
also make more money through legitimate means, because they are more moti-
vated to do so (Furchtgott-Roth & Stolba, 1999; Kanazawa, 2005a). Men are much
more motivated to accumulate resources, whether through legitimate or illegitimate
means, in order to attract mates.
My suggestion that men steal in order to attract women might at first appear
counterintuitive, because theft, robbery, and other forms of resource malappropria-
tion are universally condemned in human societies (Brown, 1991). It is quite possi-
ble, however, that the psychological mechanism that inclines and predisposes men to
commit property crimes developed in our ancestors in evolutionary history before the
ape–human split (5 to 8 million years ago), and possibly even before the ape–monkey
split (15 to 20 million years ago). In fact, an evolutionary psychological perspective
on theft logically requires that the key psychological mechanism emerge before the
informal norms against theft do; otherwise, resources accumulated through theft will
not lead to higher status and reproductive success for men because men engaging in
such theft will be ostracized for violating norms (unless, of course, the act of theft
goes entirely undetected). I believe that the norms against theft (and other crimes)
might have developed in reaction to the psychological mechanism that inclines men
to steal. The fact that theft appears to be common among our primate cousins who
do not have third-party sanctions against such behavior (de Waal, 1989, 1992; de
Waal, Luttrell, & Canfield, 1993) seems to suggest that our tendency to steal might
have evolved before norms against theft.2
That an overwhelming majority of thieves are men does not mean that women
never steal; they do. However, an evolutionary psychological perspective on female
criminality (Campbell, 1995, 1999, 2002) suggests that men and women may steal
for different reasons.
While men steal not only to satisfy their material needs for food, shelter, and
clothing but also to compete with other men and gain status, women mostly steal
only to satisfy their material needs. Campbell (1999, p. 210) astutely points out that
“theft by women is usually tied to economic needs and occurs as part of their domes-
tic responsibilities for their children” whereas “robbery is the quintessential male
crime, in which violence is used both to extract resources and to gain status.” This is
why, when women do steal, they steal much less, and much less frequently, than men
do. Women steal what they need; men steal partly to show off.
A personal anecdote illustrates this point well. I moved to the London School
of Economics and Political Science in July 2003. Within a month of my arrival in
London, someone broke into my new office and stole two blank checks by carefully
lifting two nonconsecutive checks from the middle of my new checkbook. When I
learned from the bank that the two checks had been cashed for £700 each, I made
the (statistically very unlikely to be true) prediction that the thief must have been a
woman. As it turned out, it was two women. I later found out their identities from the
bank, when they cashed the checks by making them out to themselves in their real
names, perhaps illustrating another point—that criminals are less intelligent than
noncriminals (see later discussion).3
As I had read Campbell’s work before this 2003 incident, it was immediately
obvious to me that the thieves must have been women because it seemed to me that
£700 was rent money, not the kind of money used to show off or to attract women.
It is the kind of money one needs, not the kind of money one wants. I felt that a male
thief would have made out the check for £700,000. Of course, I do not have that kind
of money (nor, I presume, do any of my LSE colleagues). However, from the thief ’s
perspective, if there is at least a 1 in 1,000 chance (0.001%) that the check will clear
for that amount, he can still come out ahead by gambling on £700,000 rather than
making the check out for a safe £700. Given men’s much higher propensity toward
risk-taking, I think a male thief might have taken that chance.
2. Why Younger Men and Not Older Men?

One of the advantages of an evolutionary psychological perspective on theft (and
crime in general) is that it can explain the universal age–crime curve. In their highly
Theft 163
influential 1983 article “Age and Explanation of Crime,” Hirschi and Gottfredson
claim that the relationship between age and crime is invariant across all social and
cultural conditions at all times. In every society, for all social groups, for all races and
both sexes, at all historical times, the tendency to commit crimes and other analo-
gous, risk-taking behavior rapidly increases in early adolescence, peaks in late ado-
lescence and early adulthood, rapidly decreases throughout the 20s and 30s, and
levels off during middle age. Although there have been minor variations observed
around the “invariant” age–crime curve (Greenberg, 1985; Hirschi & Gottfredson,
1985; Steffensmeier, Allan, Harer, & Streifel, 1989), the essential shape of the curve
for serious interpersonal crimes remains uncontested in the criminological litera-
ture. For empirical examples of the invariant age–crime curve, see Blumstein (1995,
Figures 2 and 3), Campbell (1995, Figure 1), Daly and Wilson (1990, Figure 1), and
Hirschi and Gottfredson (1983, Figures 1–8).
While Hirschi and Gottfredson claim that the age–crime curve is invariant and
holds in all societies at all times, they provide no explanations for this universal ob-
servation. They instead argue that no theoretical or empirical variable then available
in criminology (in 1983) could explain it. If the age–crime curve is truly constant
across all populations, any factor that varies across such populations cannot explain
it. Just as a constant cannot explain a variable, a variable cannot explain a constant.
The invariant age–crime curve must be explained by something that is constant
across all societies and cultures. An evolutionary psychological perspective suggests
just such a constant factor (Kanazawa, 2003b; Kanazawa & Still, 2000; Rowe, 2002,
pp. 53–55).
There are reproductive benefits for men of intense competitiveness. Those
who are highly competitive act violently toward their male rivals. Their violence
serves the dual function of protecting their status and honor and discouraging or
altogether eliminating their rivals from competition for mates (Daly & Wilson,
1988, 1990). Their competitiveness also predisposes them to accumulate re-
sources to attract mates by stealing from others (via either theft or robbery). The
same psychological mechanism induces men who cannot gain legitimate access
to women to do so illegitimately through forcible rape (Thornhill & Palmer, 2000).
Figure 9.1(A) presents a hypothetical curve depicting the relationship between
men’s age and their benefit from competition. There are no reproductive benefits
from competition (violence and theft) before puberty because prepubertal males
are not able to translate their competitive edge into reproductive success. With
puberty, however, the benefits of competition skyrocket. Once the men are repro-
ductively capable, every act of violence and theft can potentially increase their
reproductive success. The benefits of competition stay high after puberty for the
remainder of their lives since human males are reproductively capable for most of
their adult lives.
This is not the whole story, however. There are also costs associated with compe-
tition. Acts of violence can easily result in the offender’s own death or injury, and
acts of resource malappropriation can trigger retaliation from the rightful owners of
the resources, as well as from their family and allies. Men’s reproductive success is
Figure 9.1. The benefits and costs of competition and the age crime curve.
Source: Kanazawa & Still, 2000. Used with permission from the American Sociological Association.
obviously reduced if the competitive acts result in their death or injury. Figure 9.1(B)
presents a hypothetical curve depicting the costs of competition as a function of age.
Before men start reproducing, there are few costs of competition. True, being com-
petitive might result in their death or injury, and they might therefore lose in the re-
productive game. However, they also lose by not competing. If they don’t compete for
mates in a polygynous breeding system (which all human societies are; Daly & Wil-
son, 1988, pp. 140–142), they’ll be left out of the reproductive game altogether and
end up losing as a result. In other words, young men might lose if they are competitive,
Theft 165
but given that polygyny allows some men to monopolize all women, they will defi-
nitely lose if they aren’t competitive. So there’s little cost to being competitive even at
the risk of death or injury; the alternative—total reproductive failure—is even worse
in reproductive terms.
The cost of competition, however, rises dramatically with the birth of the first
child and subsequent children. True, men still benefit from competition (as Figure
9.1[A] shows), because such acts of competition might attract additional mates
and mating opportunities. However, men’s energies and resources are put to bet-
ter use by protecting and investing in their existing children. In other words, with
the birth of children, men should shift their reproductive effort away from mating
effort and toward parenting effort, in the equation: total reproductive effort = mating
effort + parenting effort. If men die or get injured in their acts of competition, their
existing children will suffer; without sufficient paternal investment and protection,
they might starve or fall victim to predation or exploitation by others. The costs of
competition therefore rapidly increase after the birth of the first child, which usually
happens several years after puberty because men need some time to accumulate suf-
ficient resources to attract their first mate. Nonetheless, in the absence of artificial
means of contraception, reproduction probably began at a much earlier age than
it does today. There is therefore a gap of several years between the rapid rise in the
benefits of competition and the similarly rapid rise in costs.
Figure 9.1(C) depicts a curve that represents the mathematical difference be-
tween the benefits and the costs of competition. The curve (in solid bold line) closely
resembles the typical age–crime curve. An evolutionary psychological perspective
suggests that male criminality varies as it does over the life course because it repre-
sents the difference between the benefits and the costs of competition. It is important
to note, however, that, unlike actors in decision theories in microeconomics (Grogger,
1998), men, from an evolutionary psychological perspective, do not make these cal-
culations consciously. The calculations have already been performed by natural and
sexual selection, so to speak, which then equip men’s brains with appropriate psy-
chological mechanisms to incline them to be increasingly competitive in their im-
mediate postpubertal years and to make them less competitive right after the birth of
the first child. Men simply don’t feel like acting violently or stealing, or they just want
to settle down, after the birth of the child, but they don’t necessarily know why.
Fluctuating levels of testosterone may provide the biochemical microfounda-
tion for this psychological mechanism. David Gubernick’s unpublished experiment
(discussed in Blum, 1997, p. 116) demonstrates that expectant fathers’ testosterone
levels precipitously fall immediately after the birth of their children. If high levels
of testosterone predispose men to be more competitive, then the sudden drop in tes-
tosterone after the birth of their children may provide the biochemical reason ex-
plaining why men’s psychological mechanisms to commit crime “turn off ” when
they become fathers. Mazur and Michalek’s (1998) finding that marriage decreases
and divorce increases testosterone levels in men provides a similar microfoundation
for the commonly observed negative effect of marriage on criminality (Kanazawa,
2003c; Laub, Nagin, & Sampson, 1998). Further consistent with this perspective,
McIntyre et al. (2006) show that married men who actively seek extrapair copula-
tions retain high levels of testosterone characteristic of single men.
Given that human society has always been mildly polygynous, there have been
many men who did not succeed at finding a mate and reproducing. These men had
everything to gain and nothing to lose by remaining competitive for their entire lives.
However, we are not descended from these men. As Buss (1994, p. 114) reminds us, all of
us are disproportionately descended from men and women who were very successful
at reproduction. Contemporary men therefore did not inherit a psychological mecha-
nism that forces them to stay competitive and keep trying to secure mates for their
entire lives. An evolutionary psychological perspective can thus explain why criminal
behavior, including theft, is largely represented by younger men, not older men.
An evolutionary psychological perspective on property crime in a sense under-
scores the nondistinctiveness of criminal behavior. Theft and robbery are among a
large repertoire of behavior that men engage in to attract mates in order to fulfill
their ultimate reproductive goals as biological organisms. In this sense, stealing is no
different from anything else men do, such as composing music, painting portraits,
writing books, and in fact producing scientific work (Kanazawa, 2003b, 2003c).
3. Why the Poor and Not the Rich?

Criminologists debate whether there is an inverse relationship between social class
and criminality. Shaw and McKay (1929) were among the first to show, using official
crime statistics, that the poor were more likely to commit crime than the rich. However,
later studies claimed that this observation was an artifact of a selection bias, whereby
lower-class criminals were more likely to be arrested, prosecuted, and convicted than
upper-class criminals and that there were no class differences in self-reported crimi-
nality (Short & Nye, 1957). Today some criminologists contend that the negative re-
lationship between social class and criminality is “a myth” (Johnson, 1980; Tittle &
Villemez, 1977; Tittle, Villemez, & Smith, 1978), while others claim that there is a
genuine relationship (Braithwaite, 1981; Clelland & Carter, 1980; Elliott & Huizinga,
1983). To make matters worse, the debate appears largely driven by ideological convic-
tion rather than empirical data; some scholars conclude that there is no relationship
between social class and criminality even when their own data show that the poor are
more likely to commit crime than the rich (Dunaway, Cullen, Burton, & Evans, 2000).
After nearly a century of debate, consensus on whether there is a negative relationship
between social class and criminality appears nowhere near sight, and the best crimi-
nologists can offer is that “it remains unclear whether and in what circumstances this
negative relationship exists” (Becker & Mehlkop, 2006, p. 194).
Because some criminologists claim that there are no theoretical reasons to expect
a negative association between social class and criminality (Tittle, 1983), perhaps a
new theoretical perspective may help clear the muddy debate. From an evolution-
ary psychological perspective, it is a straightforward prediction that lower-class men
Theft 167
will commit more crimes, particularly property crimes such as theft and robbery,
than upper-class men. If women are attracted to higher-status men with greater
resources, then lower-class men, who possess and have legitimate access to fewer
resources with which to attract mates, should be more motivated to acquire such re-
sources through illicit means than upper-class men. An evolutionary psychological
perspective would therefore predict a negative association between social class and
criminality. In this connection, it is important to note that some studies of juvenile
and adult men show that the social class of their family of origin does not affect
their criminality as strongly as their own social class (Stark, 1979; Thornberry &
Farnworth, 1982). This is perfectly consistent with an evolutionary psychological
perspective on social class and criminality.
An evolutionary psychological perspective on theft can also suggest new
hypotheses hitherto unexamined by criminologists. From this perspective, what
matters for men’s criminality is not social class per se or even resources per se but
reproductive opportunities, which highly correlate with their social class and re-
sources (Kanazawa, 2003a; Pérusse, 1993). For example, because women find taller
men more attractive as mates than shorter men (Gillis & Avis, 1980; Sheppard &
Strathman, 1989), shorter men are more delinquent and criminal than taller men
(Farrington, 1992, Table 11.2[g]; 1994, Table 2). Similarly, because women seek out
physically attractive men as short-term mates (Gangestad & Simpson, 2000), physi-
cally attractive men in general should be less criminal than physically unattractive
men. Further, physical attractiveness (or height) and social class should interact in
their effects on criminality. Social class should have a weaker negative effect on crim-
inality among physically attractive (taller) men than among physically unattractive
(shorter) men. Physically attractive (taller) men of lower class should be less criminal
than physically unattractive (shorter) men of lower class. Since social scientists in
general and criminologists in particular do not consider physical attractiveness or
height to be an important influence on human behavior, these hypotheses are un-
likely to be tested by traditional criminologists any time soon.
An evolutionary psychological perspective can also elucidate the mechanism
whereby social class influences men’s criminality. From this perspective, less intel-
ligent individuals are expected to commit more crime than more intelligent individu-
als (see later discussion). And social class is significantly negatively correlated with
intelligence (Herrnstein & Murray, 2004; Kanazawa, 2005b, pp. 254–255). Thus
lower-class men may commit more crime not necessarily or not only because they
are poor but because they are less intelligent. I would therefore predict that control-
ling for men’s general intelligence may attenuate or even eliminate the negative ef-
fect of social class on their criminality.
4. Why the Less Intelligent and Not the More Intelligent?

Criminologists have long known that criminals on average have lower intelligence
than the general population (Herrnstein & Murray, 1994; Hirschi & Hindelang, 1977;
Wilson & Herrnstein, 1985). Juvenile delinquents are less intelligent than nondelin-
quents (Wolfgang, Figlio, & Sellin, 1972; Yeudall, Fromm-Auch, & Davies, 1982),
and a significant difference in IQ between delinquents and nondelinquents appears
as early as ages 8 and 9 (Gibson & West, 1970). Chronic offenders are less intelligent
than one-time offenders (Wolfgang et al., 1972; Moffitt, 1990), and serious offenders
are less intelligent than less serious offenders (Lynam, Moffitt, & Stouthamer-Loeber,
1993; Moffitt, Gabrielli, Mednick, & Schulsinger, 1981). The negative correlation
between intelligence and criminality is not an artifact of a selection bias whereby
less intelligent criminals are more likely to be caught than more intelligent criminals
because the correlation exists even in self-report studies that do not rely on official
police statistics (Moffitt & Silva, 1988).
Why is this? Why do criminals have lower intelligence than the general popula-
tion? And why do more chronic and serious criminals have lower intelligence than
their less chronic and serious counterparts? A new hypothesis in evolutionary psy-
chology called the Savanna-IQ Interaction Hypothesis (Kanazawa, 2005b, 2006a,
2006b, 2007a) suggests one possible answer.
Relying on earlier observations made by pioneers of evolutionary psychology
(Crawford, 1993; Symons, 1990; Tooby & Cosmides, 1990), Kanazawa (2004a)
proposes what he calls the Savanna Principle, which states that the human brain has
difficulty comprehending and dealing with entities and situations that did not exist in the
ancestral environment. For example, individuals who watch certain types of TV shows
are more satisfied with their friendships, just as they are when they have more friends
or socialize with their friends more frequently (Kanazawa, 2002). This may be be-
cause realistic images of other humans, such as those portrayed in television, mov-
ies, videos, and photographs, did not exist in the ancestral environment, where all
realistic images of other humans were other humans. As a result, the human brain
may have implicit difficulty distinguishing “TV friends” (characters repeatedly seen
on TV shows) and real friends and may tend to respond similarly to both.
In an entirely separate line of research, Kanazawa (2004b) proposes an evolu-
tionary psychological theory of the evolution of general intelligence. In contrast to
views expressed by Cosmides and Tooby (2000, 2002) and Chiappe and MacDonald
(2005), Kanazawa (2004b) suggests that what is now known as general intelligence
may have originally evolved as a domain-specific adaptation to deal with evolution-
arily novel, nonrecurrent problems.4 The human brain consists of a large number
of domain-specific, evolved psychological mechanisms to solve recurrent adaptive
problems. In this sense, our ancestors did not really have to think in order to solve
such recurrent problems. Evolution has already done all of the thinking, so to speak,
and equipped the human brain with appropriate psychological mechanisms, which
engender preferences, desires, cognitions, and emotions and motivate adaptive be-
havior in the context of the ancestral environment.
Even in the extreme continuity and constancy of the ancestral environment,
however, there were occasional problems that were evolutionarily novel and non-
recurrent, which required our ancestors to think and reason in order to solve them.
Theft 169
To the extent that these evolutionarily novel, nonrecurrent problems happened fre-
quently enough in the ancestral environment (different problem each time) and had
serious enough consequences for survival and reproduction, any genetic mutation
that allowed its carriers to think and reason would have been selected for, and what
we now call “general intelligence” could have evolved as a domain-specific adapta-
tion for the domain of evolutionarily novel, nonrecurrent problems.
General intelligence may have become universally important in modern life
(Gottfredson, 1997; Herrnstein & Murray, 1994; Jensen, 1998) only because our
current environment is almost entirely evolutionarily novel. The new theory sug-
gests, and available empirical data confirm, that more intelligent individuals are
better than less intelligent individuals at solving problems only if the problems are
evolutionarily novel but that more intelligent individuals are not better than less
intelligent individuals at solving evolutionarily familiar problems, such as those
in the domains of mating, parenting, interpersonal relationships, and wayfinding
(Kanazawa, 2007b).
The logical conjunction of the Savanna Principle and the theory of the evolu-
tion of general intelligence suggests a qualification of the Savanna Principle. If gen-
eral intelligence evolved to deal with evolutionarily novel problems, then the human
brain’s difficulty in comprehending and dealing with entities and situations that did
not exist in the ancestral environment (proposed in the Savanna Principle) should
interact with general intelligence, such that the Savanna Principle holds stronger
among less intelligent individuals than among more intelligent individuals. More
intelligent individuals should be better able to comprehend and deal with evolution-
arily novel (but not evolutionarily familiar) entities and situations than less intelli-
gent individuals.
There has been accumulating evidence for this Savanna-IQ Interaction Hy-
pothesis. First, individuals’ tendency to respond to TV characters as if they were real
friends, first discovered by Kanazawa (2002), is limited to those with below-median
intelligence (Kanazawa, 2006a); individuals with above-median intelligence do not
become more satisfied with their friendships by watching more television.
Second, less intelligent individuals have more children than more intelligent in-
dividuals even though they do not want to, possibly because they have greater dif-
ficulty effectively employing evolutionarily novel means of modern contraception
(Kanazawa, 2005b). Another indication that less intelligent individuals may have
greater difficulty employing modern contraception effectively is the fact that the cor-
relation between the lifetime number of sex partners and the number of children
is positive among the less intelligent but negative among the more intelligent. The
more sex partners less intelligent individuals have, the more children they have; the
more sex partners more intelligent individuals have, the fewer children they have.
Third, more intelligent individuals stay healthier and live longer than less intelli-
gent individuals, possibly because they are better able to recognize and deal effectively
with evolutionarily novel threats and dangers to health in modern society (Deary,
Whiteman, Starr, Whalley, & Fox, 2004; Gottfredson & Deary, 2004; Kanazawa,
2006b). Consistent with the Hypothesis, however, general intelligence does not af-
fect health and longevity in sub-Saharan Africa, where many of the health threats
and dangers are more evolutionarily familiar than elsewhere in the world. For ex-
ample, relative to Western society, comparatively more people die of (evolutionarily
familiar) hunger and natural diseases and comparatively fewer from (evolutionarily
novel) automobile accidents or gunshot wounds in sub-Saharan Africa. Fourth,
more intelligent individuals are more likely to acquire and espouse evolutionarily
novel values, such as liberalism, atheism, and, for men, sexual exclusivity than less
intelligent individuals (Kanazawa, 2007a). However, consistent with the Hypothe-
sis, intelligence does not affect the acquisition and espousal of evolutionarily familiar
values for marriage, children, family, and friends.
Now what does the Savanna-IQ Interaction Hypothesis have to do with crime
in general, and theft and robbery in particular? How can it explain the empirical
observation that criminals tend to be less intelligent on average than the general
population?
From the perspective of the Hypothesis, there are two important points to note.
First, much of what we now call interpersonal crime, including theft and robbery,
comprised routine means of intrasexual competition and resource acquisition and
accumulation in the ancestral environment. This is most obvious from the fact that
our primate cousins engage in what we would call theft and robbery if perpetrated
by humans (de Waal, 1989, 1992; de Waal et al., 1993). More than likely, ancestral
men competed with one another for resources and mating opportunities by stealing
from one another if they could get away with it. In other words, most forms of crimi-
nal behavior are evolutionarily familiar.
Second, the institutions that deter, control, detect, and punish criminal behav-
ior today—CCTV cameras, police, courts, and prisons—are all evolutionarily novel;
there was no third-party enforcement of norms in the ancestral environment, only
second-party enforcement (by victims and their kin and allies). In other words, the
modern criminal justice system is an evolutionarily novel institution for dealing with
evolutionarily familiar criminal behavior.
Thus it makes perfect sense from the perspective of the Savanna-IQ Interaction
Hypothesis that men with lower intelligence are more likely to resort to evolution-
arily familiar means of competition for resources than to evolutionarily novel means
(e.g., theft rather than full-time employment in a capitalist economy). It also makes
perfect sense from the perspective of the Hypothesis that men with lower intelligence
fail fully to comprehend the consequences of their criminal behavior imposed by evo-
lutionarily novel entities of law enforcement and the criminal justice system. Hence
the Hypothesis can explain why less intelligent individuals are more likely to engage
in criminal behavior than more intelligent individuals.
The Savanna-IQ Interaction Hypothesis can also suggest a novel hypothesis
with regard to IQ and criminality. As mentioned previously, while third-party en-
forcement (by the police and the criminal justice system) is evolutionarily novel,
second-party enforcement (retaliation and vigilance by the victims and their kin and
Theft 171
allies) is not. Thus the Hypothesis would predict that the difference in intelligence
between criminals and noncriminals disappears in situations where third-party
enforcement of norms is weak or absent and where criminal behavior is controlled
largely via second-party enforcement, such as situations of prolonged anarchy and
statelessness—in fact, any situation that resembles the ancestral environment.
Conclusion
By focusing on the importance of material resources for survival and reproduc-

tive success and by underscoring the ultimate reproductive functions of all human
behavior, an evolutionary psychological perspective can shed new theoretical light
on theft and other property crimes. In particular, it can simultaneously explain
why theft and robbery (in fact, all interpersonal crimes) are an overwhelmingly
male enterprise; why young men are far more likely to engage in crime than older
men (the age–crime curve); why social class and criminality are negatively corre-
lated (the association being far from a “myth”); and why criminals in general tend
to be less intelligent than noncriminals. It can also elucidate the causal mechanism
behind why lower-class men are more likely to engage in crime than upper-class
men, and why less intelligent men are more likely to engage in crime than more
intelligent men.
At the same time, by focusing on individual characteristics that traditional
criminologists and social scientists tend to overlook, such as physical attractiveness,
height, and general intelligence, an evolutionary psychological perspective on crime
can suggest novel hypotheses. For example, lower-class men who are physically more
attractive should be less criminal than lower-class men who are physically less attrac-
tive, and the difference in intelligence between criminals and noncriminals should
weaken to the extent that third-party enforcement (characteristic of modern society
but not the ancestral environment) is absent. These and other novel hypotheses from
an evolutionary psychological perspective on crime await empirical tests.
ACKNOWLEDGMENTS
I thank Joshua D. Duntley for helpful comments on an earlier draft of this chapter. Direct all
correspondence to Satoshi Kanazawa, Interdisciplinary Institute of Management, London
School of Economics and Political Science, Houghton Street, London WC2A 2AE, United
Kingdom. E-mail: S.Kanazawa@lse.ac.uk.
Notes
1. Strangely, “theft” does not appear on Brown’s (1991) list of human universals, even
though “males more prone to theft” does. Given that Brown specifically excludes conditional
universals (“If theft occurs, then males are more prone to it”), one can safely infer that theft
itself is a human universal from the appearance of “males more prone to theft” on the list.
Similarly, one infers that murder is a human universal even though it is not on Brown’s list,
because “murder proscribed” is. Curiously, both “rape” and “rape proscribed” are on the list.
2. I have elsewhere explored the evolutionary psychological foundations of norms
(Kanazawa & Still, 2001).
3. In their defense, however, the thieves were constrained by the (in my opinion) insane
UK banking laws, which do not allow individuals to cash checks at all; personal checks must
be deposited directly into bank accounts.
4. I concur with Barrett and Kurzban (2006) and believe that the human brain is “mas-
sively modular.” Like them, I do not believe that any brain function is truly domain-general;
I believe even “general” intelligence is domain-specific (Kanazawa, 2004b). Unlike them, how-
ever, I do believe in a clear distinction between evolutionarily familiar and evolutionarily novel
problems, entities, and situations. For example, I believe that face recognition is a clearly evo-
lutionarily familiar problem (Kanazawa, 2004b, p. 513, Figure 1), despite the fact that faces
that we must recognize today never existed in the ancestral environment (Barrett & Kurzban,
2006, p. 635).
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10
In Cold Blood
The Evolution of Psychopathy
MARTIN L. LALUMIÈRE, SANDEEP MISHRA, AND GRANT T. HARRIS
Fool me once, shame on you. Fool me twice, shame on me.

—old (probably Chinese) proverb.
Of all the interesting topics in the field of forensic psychology, psychopathy probably
generates the most fascination. In university courses covering psychopathy, students
wake from their slumber and knock on professors’ doors to ask how they can get
involved in research on psychopaths. In crime fiction and historical biographies, psy-
chopathic characters are imbued with iconic qualities. It seems that our minds are
attuned to psychopathic characteristics in others, and probably for good reason: if
psychopaths have been a constant feature of the ancestral social environment of
Homo sapiens, they will have exerted significant selection pressure. Researchers have
not been immune to this fascination: despite the fact that psychopaths represent
a small proportion of criminal offenders, psychological research on psychopathy
seems to dominate the forensic literature.
The most important reason for the popularity of psychopathy among forensic re-
searchers is probably the empirical fact that measures of psychopathy are reliable and
robust predictors of future criminal behavior in both forensic and nonforensic popula-
tions (reviewed in Harris, Skilling, & Rice, 2001; Leistico, Salekin, DeCoster, & Rogers,
2007; Porter & Woodworth, 2006). In fact, one measure of psychopathy, the Psychop-
athy Checklist-Revised (Hare, 2003), might be the single best psychological predictor
of criminal recidivism. In actuarial assessments of dangerousness, scores on measures
of psychopathy have very large—often the largest—predictive weights (e.g., Hilton,
Harris, Rice, Houghton, & Eke, 2008; Quinsey, Harris, Rice, & Cormier, 2006).
Perhaps even more interesting, scores on measures of psychopathy reveal intrigu-
ing interactions in other research with offenders. Psychopathy and measures of sex-
ual deviance (or paraphilia) have been found to exhibit a multiplicative relationship
such that sex offenders who are both sexually deviant (e.g., pedophilic) and psycho-
pathic are much more likely to engage in sexually violent recidivism than all other
176
In Cold Blood: The Evolution of Psychopathy 177
group combinations (Rice & Harris, 1997; Seto, Harris, Rice, & Barbaree, 2004).
Psychotherapy effective in reducing the risk of violence among non-psychopaths has
been reported to have the opposite effect on psychopaths, increasing their risk of vio-
lence (Hare, Clarke, Grann, & Thornton, 2000; Harris, Rice, & Cormier, 1994; Rice,
Harris, & Cormier, 1992). Alcohol abuse is a good predictor of criminal recidivism
among schizophrenic offenders but not among psychopaths, even though psycho-
pathic offenders are more likely to abuse alcohol than schizophrenic offenders (Rice &
Harris, 1995). Even more intriguing are empirical reports that psychopaths rated by
therapists as having benefited from treatment are subsequently more dangerous than
psychopaths rated as not having benefited (Looman, Abracen, Serin, & Marquis, 2005;
Seto & Barbaree, 1999; but see Langton, Barbaree, Harkins, & Peacock, 2007).
Greatly facilitating this burgeoning research activity is a valid and reliable mea-
sure of male psychopathy, the Psychopathy Checklist, now revised (PCL-R; Hare,
2003). Researchers have also subsequently developed similar psychopathy measures
for nonforensic populations, teenagers, and even children. The PCL-R has provided
researchers with a common definition of psychopathy, greatly aiding communica-
tion and integration of results in the field. Other terms have sometimes (and mistak-
enly) been used to mean the same thing as psychopathy, such as sociopathy, antisocial
personality disorder, and Machiavellianism. Psychopathy now typically refers to “a life-
long persistent condition characterized, in males at least, by aggression beginning
in early childhood, impulsivity, resistance to punishment, general lack of emotional
attachment or concern for others, dishonesty and selfishness in social interaction, and
high levels of promiscuous and uncommitted sexual behavior” (Harris, Skilling, et al.,
2001, pp. 197–198). Psychopathy is more restrictive than antisocial personality dis-
order as defined in the Diagnostic and Statistical Manual of Mental Disorders because
the customary diagnostic cutoff for psychopathy is more stringent, but in fact the in-
dicators of psychopathy and antisocial personality disorder are highly correlated and
can identify essentially the same individuals (e.g., Skilling, Harris, Rice, & Quinsey,
2002)—contrary to the commonly accepted view (e.g., Livesley, 1998). Psychopathy
is mostly a male phenomenon, and in this chapter we focus on male psychopathy.
In sum, psychopathy is (perhaps naturally) fascinating, can be measured re-
liably, and is an important social phenomenon with significant practical implica-
tions. It is thus not surprising that it has generated a large amount of theoretical
interest. Where does psychopathy come from? Can evolutionary psychology help us
generate new hypotheses about the origins and causes of psychopathy? Before we
address these questions, let us examine more closely the construct of psychopathy.
The Construct of Psychopathy
Psychopathy as a Clinical Condition

A century and a half ago, the modern concept of psychopathy originated in the obser-
vation that a small minority of people seemed to engage in antisocial, irresponsible,
extremely selfish (and even apparently self-destructive) behavior without also dis-
playing any obvious signs of mental derangement. Beginning about seventy years
ago, Cleckley (1941) applied the term psychopathy and added clinical descriptions
of other, more affective aspects of this condition: superficial charm and good intel-
ligence, absence of nervousness, dishonesty, lack of remorse, incapacity for love, and
shallow emotional responses are examples. For the last four decades, Hare (1970,
1998, 2003) has elaborated on Cleckley’s clinical observations and brought the
study of psychopathy into the realm of scientific investigation. As mentioned, one
of Hare’s several contributions has been the development and validation of an ef-
fective way to measure the phenomenon. The PCL-R (Hare, 2003) comprises twenty
psychopathic characteristics to be assessed primarily based on an individual’s life-
long pattern of conduct as documented in official records and institutional files, but
the scoring of some traits (e.g., grandiose sense of self-worth, lack of remorse, lack
of realistic long-term goals, failure to accept responsibility for actions) may also be
inferred from a semistructured interview.
Twenty years ago, Hare (Harpur, Hackstian, & Hare, 1988) reported that scores
on the PCL-R consisted of two highly related (correlations greater than 0.50) but
conceptually and empirically distinct aspects. The first, usually called Factor 1, com-
prised interpersonal and affective characteristics (e.g., conning and manipulation, cal-
lousness and lack of empathy), while Factor 2 described a deviant, antisocial lifestyle
(e.g., proneness to boredom, poor behavioral controls, early behavior problems, im-
pulsivity, juvenile delinquency, parasitic lifestyle). A few characteristics (sexual pro-
miscuity, many short-term marital relationships, and criminal versatility) did not
appear to load on either factor. The names given to the factors did not strictly capture
their content, of course—boredom is an affective response; poor behavior controls
are about irritable, angry, hostile, violent emotional responses; conning and manip-
ulation are about overt antisocial conduct; and criminal versatility is certainly about
an antisocial lifestyle. Nevertheless, at the empirical level, this two-aspect nature of
psychopathy has generally held up ever since (Benning, Patrick, Hicks, Blonigen, &
Kreuger, 2003; Blackburn, 2007; Loney, Taylor, Butler, & Iacono, 2007; Patrick,
Edens, Poythress, Lilienfeld, & Benning, 2006; Skeem, Johansson, Andershed, Kerr,
& Louden, 2007).
It is evident that those who receive maximal scores on a measure of psychopathy
such as the PCL-R would, by definition, exhibit both aspects. Also, the well-established
empirical association between the two factors means that those who score highly on
one aspect have a high probability of also exhibiting the other (Skilling et al., 2002).
Nevertheless, some people who receive high scores on such a measure do so via a
maximal score on one aspect and perhaps only a moderate score on the other. In-
deed, these two aspects appear to be related in opposite directions to such emotions
as anxiety and depression (Hicks & Patrick, 2006). As well, many empirical findings
about psychopathic responding in the laboratory or in the natural environment seem
particularly characteristic of only one of the two aspects (e.g., Carlson, McLarnon,
& Iacono, 2007; Hare et al., 2000; Maccoon & Newman, 2006; Moltó, Poy, Segarra,
Pastor, & Montañés, 2007). Most relevant for forensic application, the second aspect
is more predictive of criminal recidivism, violent recidivism, substance abuse, and
suicidal behavior (Harris, Skilling, et al., 2001; Leistico et al., 2007; Salekin, Rogers,
& Sewell, 1996).
Thus, it appears that some violent offenders have such traits as remorselessness,
grandiosity, and insincerity and are presumably deliberately and premeditatedly
violent out of emotional detachment and indifference to others’ interests. Another
group of violent offenders seem to be impulsive and to experience considerable
anger, anxiety, and distress, and they are violent due to such negative emotions. This
distinction1 has long been noted in the psychopathy literature and the terms primary
and secondary psychopathy, respectively, are often applied. It is now evident that these
two aspects of the phenomenon are, at least partly, due to quite distinct underly-
ing basic processes. It has been assumed that the primary, affectively cold-hearted
version is the one that reflects psychopathy’s “core personality”and is more consti-
tutional and “biologically” based. On the other hand, the secondary, “behavioral”
version has been seen as acquired and contextually caused (Mealey, 1995; Skeem
et al., 2007). Current evidence appears, however, to make these etiological assump-
tions untenable.
Psychopathy in the Context of Development

The first relevant source of data comes from laboratory studies of adults. It is clear
that experimenters can arrange test conditions such that psychopaths obtain poorer
scores than other groups (e.g., Blair et al., 2006). But it is just as clear that some ex-
perimental conditions lead to equivalent or even better performance by psychopathic
participants (Book, Holden, Starzyk, Wasilkiw, & Edwards, 2006; Budhani, Richell,
& Blair, 2006). Indeed, the core affective psychopathic personality traits appear to be
so subtle that it is unclear how they can be characterized (Munro et al., 2007); for
example, psychopaths do not seem to exhibit deficits in detecting emotion in others
(Glass & Newman, 2006) and might even be better at it than non-psychopaths (Book,
Quinsey, & Langford, 2007).
Conversely, the more behavioral, antisocial lifestyle aspects of psychopathy ex-
hibit profound, inescapable (and utterly unsubtle) findings. As mentioned above,
these are the psychopathic traits most predictive of forensically relevant outcomes.
These so-called externalizing traits have been reported to exhibit a distinct, natural
class (Harris, Rice, Hilton, Lalumière, & Quinsey, 2007; Harris, Rice, & Quinsey, 1994;
also see Swogger & Kosson, 2007; but also Edens, Marcus, Lilienfeld, & Poythress,
2006), even in juveniles (Skilling, Quinsey, & Craig, 2001; Vasey, Kotov, Frick, &
Loney, 2005). Such externalizing traits, together with some callous and unemotional
traits, exhibit a distinct developmental trajectory detectable in individuals as young as
age 3 (Glenn, Raine, Venables, & Mednick, 2007; Moffitt & Caspi, 2001; Shaw, Bell, &
Gilliom, 2000; Shaw, Gilliom, Ingoldsby, & Nagin, 2003; Vizard, Hickey, & McCrory,
2007). There is also clear evidence that this pattern of externalizing traits represents
a stable, life-course phenomenon (Loney et al., 2007; Lynam, Caspi, Moffitt, Loeber,
& Stouthamer-Loeber, 2007). These externalizing aspects of psychopathy associated
with negative emotions are at least as heritable as the affectively coldhearted traits
(Burt, McGue, Carter, & Iacono, 2007; Hicks et al., 2007; Larsson et al., 2007; Lars-
son, Andershed, & Lichtenstein, 2006; Viding, Frick, & Plomin, 2007). Indeed, an
externalizing factor among elementary-school-aged children has been reported to
exhibit a heritability coefficient of 0.96 (Baker, Jacobson, Raine, Lozano, & Bezdjian,
2007). These externalizing traits seem to be more closely associated with and central
to the characteristic cognitive differences associated with psychopathy (Maccoon &
Newman, 2006). Finally, externalizing traits, as assessed by the PCL-R, for example,
appear to predict violent behavior even among adults who are unlikely to meet any
criteria for classification as psychopaths (Harris, Rice, & Camilleri, 2004; Hilton et al.,
2008; Rice & Harris, 1992).
The Two Factors Revisited

Recent empirical research on psychopathy has clarified many features of this foren-
sically important and fascinating condition. That same research, however, has also
raised new questions. For example, should the condition of psychopathy be concep-
tualized as a disorder of personality whose core features of callousness and affective
shallowness directly (but not inevitably) cause antisocial conduct (e.g., Cooke, Michie,
& Hart, 2006; Cooke, Michie, & Skeem, 2007; Widiger, 2006)? Is psychopathy better
conceived of as a collection of enduring characteristic behaviors and interpersonal
tactics (e.g., Hare & Neumann, 2006; Harris et al., 2007)? Is antisocial and criminal
conduct merely the rather obvious and expected consequence of theoretically more
interesting core psychopathic personality (e.g., Cooke et al., 2007)? Or are some ag-
gressive and violent behaviors actually so much at the core of psychopathy that such
antisocial behaviors are actually most diagnostic (e.g., Harris et al., 2007)? It is likely
that not all the phenotypic traits of psychopathy have so far been optimally identi-
fied. It also seems possible, even probable, that one aspect of the condition (and its
neurophysiological substrates) will ultimately be deemphasized in favor of the other.
Though it appears that the externalizing, behavioral horse has a small lead in this
race, it is not yet clear which path empirical and theoretical development will take
and what the final result will be. In the following section, we describe evolutionary
accounts of psychopathy and show that these can generate novel and testable hy-
potheses about the core features of psychopathy.
Explanations of Psychopathy
Traditional approaches to the study of antisocial behavior assume that the behavior
would not occur if appropriate genetic, prenatal, family, socialization, and economic
conditions were in place. There is, in fact, some support for these ideas; convincing
evidence suggests that antisocial behavior, and especially violent behavior, is some-
times associated with some rare genetic mutations, poor prenatal or perinatal con-
ditions (e.g., maternal malnutrition, birth complications), family instability, poor
parental monitoring during adolescence, and low socioeconomic status (for a re-
view see Quinsey, Skilling, Lalumière, & Craig, 2004). Because some of these putative
causes involve disruptions of otherwise normally functioning systems, it seems that
antisocial behavior can sometimes result from pathological causes.
Elsewhere, we have argued that the presence of pathological causes for a given
trait or behavior does not necessarily imply that the trait or behavior in question is
a pathological outcome (Lalumière, Harris, Quinsey, & Rice, 2005; Quinsey et al.,
2004; also see Chapter 8 of this volume). It is quite possible that antisocial behav-
ior is part of an adaptive response to specific and difficult conditions. For example,
pathological causes having to do with early development (e.g., early malnutrition,
the experience of physical abuse) may provide the child with information about
the likely features of his future environment. This information may divert the child
toward a developmental pathway that facilitates aggression, impulsivity, and high
mating effort, tendencies that might better allow him to reach fitness-relevant
goals. Alternatively, those early difficult conditions may reduce embodied capital
(i.e., intrinsic attributes, such as health, skills, or attractiveness), leading to reduced
ability to compete for resources, status, and mates, and forcing the adoption of al-
ternative tactics of social competition. These two scenarios imply that antisocial
behavior may be an adaptation (i.e., ancestrally selected) specific to “pathological”
circumstances (just as an immune response is an adaptation designed to respond to
infections). These ideas are discussed in more detail in Chapter 8.
What about psychopathy? Psychopaths are quite different from other offend-
ers, even other life-course-persistent violent offenders. They differ, for instance,
with regard to some aspects of their criminal behavior (e.g., more violent, more
goal-directed), how punishment and rewards affect their behavior in the laboratory,
how they process emotional information, their physiological responses to aversive
events, and their cerebral lateralization and cerebral activity while processing verbal
information (reviewed in Barr & Quinsey, 2004; Harris, Rice, et al., 2001; Lalumière
et al., 2005; Lalumière, Harris, & Rice, 2001; Quinsey et al., 2004). Also, signs of early
neurodevelopmental problems that are associated with persistent violent crimi-
nality are not associated (and perhaps are even negatively associated) with psychop-
athy (Harris, Rice, & Lalumière, 2001; Lalumière et al., 2001).
Can conditional (facultative) developmental accounts of the sort mentioned
above explain the psychopathic phenotype? Such an account would predict that
psychopaths have experienced difficult early conditions statistically predictive of an
inhospitable future biotic or social environment or that they have reduced embodied
capital and ability to compete. Evidence so far does not support such an account. As
mentioned, psychopathy is unrelated to early signs of neurodevelopmental problems
(e.g., obstetrical complications, low IQ, learning problems). Also, psychopaths show
lower fluctuating asymmetry—a measure of developmental instability and a possible
Figure 10.1. Fluctuating asymmetry values for nonoffenders (n = 31), non-psychopathic

offenders (n = 25), and psychopathic offenders (n = 15). Adapted from Lalumière, Harris,
& Rice, 2001.
indicator of low embodied capital—than other violent offenders (Figure 10.1;

Lalumière et al., 2001). In our study of fluctuating asymmetry, the psychopaths
with the most extreme PCL-R scores were just as physically symmetrical as the non-
offenders (who were members of the hospital staff), and much more symmetrical
than violent offenders with low PCL-R scores.
Psychopathy also seems to be unrelated to social factors generally associated
with delinquency and conduct problems. For instance, in a study of the link between
the quality of parenting and childhood conduct problems, ineffective parenting was
associated with a higher number of conduct problems exhibited by children (as
expected), but only among the children who did not display lack of empathy, ma-
nipulativeness, lack of guilt, or emotional constriction—all features of psychopathy
(Wootton, Frick, Shelton, & Silverthorne, 1997; see also Oxford, Cavell, & Hugues,
2003). Children displaying psychopathic features had more conduct problems than
other children, regardless of the quality of parenting. In fact, there was a tendency
for fewer conduct problems among “psychopathic” children who experienced inef-
fective parenting compared to “psychopathic” children who had experienced more
effective parenting. Although more research is needed, so far there is no evidence
that the origin of psychopathy involves the types of pathological causes implicated
for persistent violent offending more generally. This result should be surprising to
developmental psychologists because psychopathic children would be expected to at
least elicit parental behaviors and social responses that often lead to neural, devel-
opmental, and social problems (e.g., excessive physical punishment, withdrawal of
parental investment, peer rejection). If psychopathy is not the result of pathologi-
cal processes of the kind already identified for other life-course persistent offenders,
what might explain psychopathy?
Perhaps the most often discussed evolutionary explanation of psychopathy is
the frequency-dependent selection account. In the most common version of this ac-
count, psychopaths have evolved to take advantage of the fact that most people are
cooperators by defecting in social interactions. Thus, psychopathy represents an al-
ternative strategy (in the genetic sense) that is successful only at a particular low
relative frequency in the population. If there are too many cheaters (or defectors),
nonpsychopaths become very vigilant and cheating opportunities disappear. It is not
hard to imagine how the constellation of psychopathic characteristics (e.g., manipu-
lative, charming, lack of empathy, failure to learn from punishment, unresponsive
to cues of distress in others) would facilitate such a strategy. By this account, some
individuals are born with a propensity for psychopathy, and the phenotype manifests
itself early and perhaps without any environmental cues (e.g., Mealey, 1995). This
type of obligate strategy has been observed in other species (see Box 10.1), but it is
fairly rare compared to conditional (facultative) strategies.
Harpending and Sobus (1987) noted that an evolutionary explanation of psy-
chopathy based on nonreciprocation requires that the psychopath be not only dif-
ficult to detect and highly mobile but “especially skilful at persuading females to
copulate and at deceiving females about his control of resources and about the likeli-
hood of his provisioning future offspring” (p. 65S). Using contemporary terminol-
ogy, psychopaths should invest highly in mating effort (energy and resources devoted
to increasing mating access) and should advertise parenting effort without actually
engaging in it. High mating effort, however, is a hallmark of general criminal offend-
ing, not just psychopathy. Interestingly, of the three items that do not load onto one
of the PCL-R factors, two involve mating effort (many short-term marital relation-
ships and sexual promiscuity). Could it be that these two items do not capture the
type of mating effort that is required by an obligate account of psychopathy?
Harris et al. (2007) hypothesized that although high mating effort is associated
with persistent antisocial behavior, early and coercive mating effort should be par-
ticularly associated with psychopathy if it is an early-onset, obligate strategy. Harris
et al. suggested that the reason these two items are orphans on the PCL-R is because
they are diagnostic of antisocial behavior generally (and perhaps even other male
life history strategies), not psychopathy in particular. Under the obligate, frequency-
dependent selection explanation of the type discussed by Harpending and Sobus
(1987), psychopathy should emerge early, and the aspects of sexuality that are di-
agnostic of psychopathy should not be general features associated with high adult
mating effort but functional features that develop and are expressed early.
Box 10.1 Frequency-Dependent Selection

in the Animal World
In the bluegill sunfish, there appear to be three types of males, distinguishable both behav-
iorally and morphologically (Gross & Charnov, 1980). The largest males, called parental types,
invest heavily in growth in the first few years of life, delaying reproduction. These males even-
tually build nests and use their size to defend nesting territories. Satellite males mimic females
behaviorally and in physical appearance; they attempt to interrupt territorial males that are
courting and to intercept females in an attempt to fertilize them. Finally, sneaker males tend to
stay near the lake bottom and make quick attempts to enter and exit nests, releasing ejaculate
quickly. Sneaker males mature in two to three years, investing more in immediate reproduc-
tive capabilities than long-term growth. As sneaker males mature, they become satellite female
mimics, but never grow to the size of the parental males. The two smaller morphs do not incur
the same parental investment costs as the larger parental morphs—building a nest, defending
a territory, courting females, and caring for the eggs, a necessary condition for the hatching
and survival of young. Instead, the sneaker and satellite morphs parasitize the larger males by
attempting to gain fertilizations covertly (Neff, Fu, & Gross, 2003).
Sneaker males have a much larger testis-to-body mass ratio and also have greater sperm
counts in their ejaculates. These characteristics lead to increased success in sperm competition.
Fu, Neff, and Gross (2001) found that sneaker males fertilize more eggs than parental males
during sperm competition, with satellite males falling between the sneaker and parental types
in terms of fertilization success. Genetic analyses suggest that the mean paternity estimate for
parental males is 76.9% and for cuckolder males (sneakers and satellites) 23.1% (Neff, 2001).
Although cuckolder males are more successful in fertilizing females in the context of sperm
competition, the higher percentage of paternity in parental males is likely due to increased mat-
ing opportunities with females afforded by the defense and maintenance of a stable territory.
Previous studies have found that cuckolders form approximately 21% of the bluegill sunfish
population, suggesting that the parental and sneaker/satellite strategies have approximately the
same mean fitness outcome. Even though both parental and sneaker/satellite strategies appear
to offer the same fitness outcomes, the success of alternative cheater strategies is likely contin-
gent on their frequency in the population: modeling of bluegill populations suggests that cuck-
olders become less successful as their numbers increase (Gross, 1991). Bluegill sunfish are but
one of several species that appear to have undergone frequency-dependent selection for stable
alternative life history strategies. Alternative reproductive phenotypes have also been observed
in isopods, swordtails, and ruffs (Gross, 1996).
Harris et al. tested the predictions that a factor comprising early onset and co-
ercive sexuality items should positively correlate (in a sample of violent offenders)
with the traditional PCL-R factors, should show taxonicity (i.e., evidence that scores
on the factor identify types of offenders, psychopaths versus non-psychopaths, as
opposed to a dimensional trait), and should also show a pattern of correlation with
individual characteristics predicted by the account (e.g., negatively associated with
signs of early neurodevelopmental perturbations, positively associated with number
of victims of reproductive value). All of these predictions were confirmed. These re-
sults not only clarified the unique sexuality of psychopaths but also provided support
for the idea that evolutionarily informed research can improve the conceptualization
and measurement of the phenomenon.
At the moment, there is some evidence that psychopathy might be the product
of frequency-dependent selection. At the very least, there is evidence that psychopa-
thy is not the result of early pathological conditions, such as those associated with
general adult criminality. The particular structure of the definite evolutionary model
of psychopathy probably remains to be elucidated, but it is clear that studies designed
to test such models will continue to lead to further advances in our understanding
of psychopathy. In the remainder of the chapter we discuss the relevance of, and
some results from, three active lines of research for the study of the evolution of
psychopathy.
Computer Simulations and Experimental Games
Models of the evolution of cooperation can help to shed light on the evolution of
psychopathy. These models are also germane to the idea of different “types” of indi-
viduals (or strategies) interacting in a social environment. The Prisoner’s Dilemma,
a non-zero-sum game, has been used to model the evolution of cooperative behavior
in the face of defection (Axelrod & Hamilton, 1981; Axelrod, 1984).
In the Prisoner’s Dilemma, two players are in a hypothetical situation in which
both are imprisoned and accused of having colluded to commit a crime. If both play-
ers cooperate and do not implicate the other (mutual cooperation), they each receive
a minimum sentence. There is, however, a greater incentive for each player to impli-
cate the other (defection), thus earning his own complete freedom at the expense of
the other’s maximum sentence. If both defect and implicate the other (mutual defec-
tion), both remain imprisoned with a long sentence. In this game, there is a small re-
ward for mutual cooperation, a larger reward for the individual who defects (as long
as the other cooperates), and costs for mutual defection. If the Prisoner’s Dilemma is
played only once, the optimum strategy for each player is to defect. When the same
two players play repeatedly (the iterated Prisoner’s Dilemma, or IPD), mutual coop-
eration becomes optimal.
Axelrod and Hamilton (1981) invited game theorists to submit a computer
program designed to optimize success in a round-robin IPD tournament. The con-
ditions were as follows: two players (i.e., computer programs) interacted, simulta-
neous choices were made consisting of either cooperation or defection, the mag-
nitude of payoffs was fixed beforehand, and the history of choices made by the
other players was known to each player in the tournament. One simple strategy
triumphed over all others, known as tit-for-tat. In this strategy, cooperation is the
first move of the game, and the other player’s move is copied on all subsequent
moves. An ecological simulation comparing various strategies of cooperation also
showed that tit-for-tat quickly became the most common or evolutionarily stable
strategy in a population.
An evolutionarily stable strategy (ESS) is one that, if adopted, cannot be invaded

by alternative strategies. In the context of an IPD, Axelrod and Dion (1988) dem-
onstrated that if the chance of future interaction is high, no player in a population
can do better than to cooperate. Axelrod and Dion also suggested that even in an
already established population of defectors, a small cluster of players using coopera-
tion quickly takes over, and an established population of cooperators cannot be easily
invaded (replaced) by those using defection. More recent research, however, suggests
that there is no true ESS for the IPD (e.g., Marinoff, 1990). The strategy Axelrod and
colleagues (1981, 1984, 1988) described as an ESS is also subject to numerous re-
strictions, many of which are not ecologically tenable.
Of particular relevance to the study of psychopathy is the requirement of fu-
ture interaction. The tit-for-tat model of stable cooperation put forth by Axelrod and
colleagues specifically requires that players have multiple interactions and that all
interactions be remembered. Harpending and Sobus (1987) modeled a population
similarly to Axelrod and colleagues and found comparable results: If all players in a
simulation have perfect memories, cooperators (engaging in tit-for-tat) always do bet-
ter than individuals who always defect (cheaters). In a population where players are
fallible, however, with a 10% probability of any player forgetting all encounters, it was
observed that the relative frequencies of cooperators and defectors varied over time.
Harpending and Sobus argued that these findings suggest that a small population of
defectors could succeed if they were difficult to detect, mobile, and skilled at manipu-
lating others, with males successfully persuading females to mate. Thus psychopathy
is analogous to the strategy of repeated defection in social interactions (perhaps after
showing signs of cooperation) and so might persist at a low frequency in a population
if an absolute ESS for an “always cooperate” strategy cannot be strictly maintained.
More recent research by Kurzban and Houser (2005) is consistent with the idea
that there is not a single ESS for social exchanges but rather that there has been se-
lection for frequency-dependent cooperative “types” in humans, exhibiting a more
complex equilibrium, where the success of multiple strategies has been equal over
time. Three strategies or types were identified: reciprocity contingent on cooperation
by others (analogous to tit-for-tat), cooperation regardless of the actions of others,
and “free-riding”—consistent defection regardless of the actions of others. Although
groups comprising mostly cooperating or reciprocating types did better than groups
that included a free-rider, at the individual level all three types experienced equiva-
lent average earnings. These findings are consistent with the notion of a polymor-
phic equilibrium where payoffs for different types or strategies are equal.
Simulations and experimental games exploring how evolution by natural selec-
tion could have given rise to subpopulations of social cheaters can inform accounts
of psychopathy. It is certainly plausible that psychopathy evolved as a frequency-
dependent life history strategy of defection, whereby psychopathic characteristics
(manipulation, charm, dishonesty, callousness, aggression, irresponsibility, promis-
cuity, and a parasitic lifestyle) formed a suite of adaptive traits and behaviors that
exploited a social environment mostly characterized by cooperation.
The Genetics of Psychopathy
There are two general approaches to studying the genetics of a trait. The first is
quantitative genetics, the study of the relative contribution of genes and environ-
ment in explaining variance in the trait. The second is molecular genetics, the study
of the role of particular genes in producing the phenotypic characteristic. These
two approaches are often complementary (exceptions arise when a trait shows little
variance—e.g., number of fingers—but has a clear genetic basis). Although the con-
struct of psychopathy has a long history, its valid measurement is fairly recent, and
therefore there are very few genetic studies of psychopathy proper. In addition, psy-
chopaths are very socially mobile, so it might be difficult to include family members
in genetic studies. In the following, we briefly review the few studies available, but
first we discuss the relevance of such studies for testing the frequency-dependent
selection account of psychopathy.
The frequency-dependent selection explanation discussed above makes the clear
prediction that measures of psychopathy should show high heritability2 in quantita-
tive genetics studies and that gene variants unique to psychopathy would be identi-
fied in molecular genetics studies. Although these predictions are straightforward,
there are a few complications to consider. First, almost all psychological traits show
moderate to high heritability, but few evolutionary psychologists would suggest that
these traits (e.g., major personality dimensions) are the result of frequency-dependent
selection. High heritability can also result from weak ancestral selection pressure on
the trait in question, high mutation rates, sexual recombination, or a history of host–
parasite coevolution (see Tooby & Cosmides, 1990). At the very least, however, low
heritability of psychopathy would seriously question the validity of the frequency-
dependent selection account.
Second, gene variants might have low penetrance or require specific environ-
mental triggers for expression and so would be difficult to detect with simple genetic
linkage and association studies that do not examine contextual factors. For example,
Caspi et al. (2002) found an interaction between allelic variation in a gene coding
for a neurotransmitter enzyme and the experience of childhood maltreatment in
predicting adult antisocial tendencies. Although frequency-dependent models imply
genetic differences among individuals (or morphs), it is possible that gene expression
for a particular trait still require some kind of environmental cue. Perhaps psychopa-
thy remains “dormant” unless the relevant cues are present, increasing the chal-
lenge of genetic studies.
Third, how likely is it that just one or a few gene variants are contributing to
the development of something as complex as psychopathy? One might think that
a multitude of genes have to be involved, making the task of finding relevant genes
almost impossible because molecular genetics typically has low statistical power to
detect individual genes each with small effects. Recent studies in evolutionary devel-
opmental biology, however, suggest that the task might not be as hopeless as it looks.
These studies show that some master genes (e.g., hox genes) control the activity of
many other genes and thereby the development of complex phenotypic features. Per-
haps one or a few “psychopathy” master genes affect the expression of other genes,
leading to all characteristics of psychopathy. Psychopaths, after all, look very much
like exaggerated young males (except that they display risk-taking, high mating ef-
fort, antisocial behavior, etc. throughout their lifetimes). That is, the human genome
may already have the capacity to produce all or most of the characteristic pheno-
typic psychopathic traits. Perhaps all that evolution required was a master gene that
controls the expression of such existing traits (for an accessible and fascinating in-
troduction to the field of evolutionary developmental biology, see Carroll, 2005).
With these considerations in mind, we examine the few genetic studies of psychop-
athy. It should be noted that there have been dozens of studies of antisocial tendencies,
conduct problems, delinquency, and criminality, but these studies have not distin-
guished between psychopathy and general antisociality. These studies have typically
obtained fairly high heritability estimates (reviewed in Blonigen, Carlson, Krueger, &
Patrick, 2003; Lykken, 1995; Quinsey et al., 2004) but it is unclear whether, and by
how much, these estimates were influenced by psychopathy. The frequency-dependent
selection account would suggest that heritability estimates in these studies would be
positively affected by the number of psychopaths in the samples studied.
A study of children has revealed substantial heritability for callousness and un-
emotionality, traits that are strongly associated with psychopathy. Viding, Blair, Mof-
fitt, and Plomin (2005) examined a large sample of 7-year-olds rated by teachers
as extreme on a callous-unemotional scale. The monozygotic co-twins of these pro-
bands scored much more similarly to the probands on the same scale than dizygotic
co-twins, with an estimated heritability value of 0.67 and no effect of the shared en-
vironment. The heritability of a measure of antisocial conduct was also found to be
high, but only if the probands scored high on the measure of callous-unemotionality.
In a subsequent analysis, Viding et al. (2007) reported a substantial genetic influ-
ence overlap for callous-unemotional traits and antisocial conduct.
In a twin study of older male children (ages 10–12) and adolescents (16–18),
Taylor, Loney, Bobadilla, Iacono, and McGue (2003) examined the heritability of a
self-reported psychopathy measure comprising two related factors: antisocial and
impulsive behavior, and callousness and emotional detachment. The heritability es-
timates varied between 0.36 and 0.54 in the older age group and between 0.50 and
0.52 in the younger age group (our calculations). The univariate biometric analysis
revealed a significant additive genetic effect and an unshared environment effect for
both psychopathy factors.3 The bivariate model suggested that the correlation be-
tween the two psychopathy factors could be attributed to additive genetic and un-
shared environmental effects.
In another large study of preadolescent twin boys and girls (9 to 10 years old),
Baker et al. (2007) examined the heritability of a common factor underlying psycho-
pathic traits, aggression, and conduct problems. Most of the variation was accounted
for by additive genetic effects (0.96) and the remainder by unshared environment ef-
fects (0.04), in both sexes.
Blonigen et al. (2003) examined the heritability of a self-report measure of psy-

chopathic personality in a cohort of young adult male twins. Twin correlations sug-
gested perfect heritability, but the correlation for dizygotic twins was nonsignificantly
different from zero. Biometric modeling suggested moderate nonadditive genetic ef-
fects due to epistasis (interactions between genes at different loci) and a moderate
effect of unshared environment. Finally, in a large study of twins aged 16 to 17 using
a self-report questionnaire assessing three correlated psychopathic characteristics,
heritability estimates were 0.36 to 0.54 for boys and 0.52 to 0.70 for girls (our cal-
culations). All remaining variance could be attributed to unshared environmental
effects (Larsson et al., 2006). Additive genetic factors accounted for 63% of the vari-
ance in the latent psychopathy factor, with the remainder attributable to unshared
environmental effects (see also Larsson et al., 2007).
In sum, these five studies of psychopathic traits revealed moderate to high heri-
tability estimates and no effect of the shared environment (see also Waldman & Rhee,
2006). These results are consistent with the frequency-dependent selection account
of psychopathy and not with facultative (conditional) explanations. If any aspect of
the expression of psychopathy is conditional on environmental cues, those cues re-
main unidentified. More work remains to be done to elucidate the gene–environment
interactions that influence psychopathy. Although these studies have used different
measures specifically designed to assess psychopathy, three relied on self-report only,
all used different measures, and none used samples likely to contain a high propor-
tion of psychopaths (except perhaps for Viding et al., 2005). No study has yet exam-
ined the heritability of psychopathy using the PCL-R. We were unable to locate any
research on the molecular genetics of psychopathy.
The Brains of Psychopaths
Although differential brain functioning in psychopaths is not surprising—after all,

differences in behavior must be caused by some brain differences—studies that have
investigated differences between “normal” brains and psychopathic brains are useful
for understanding the mechanisms underlying psychopathy. It is a common mistake
in studies of psychopathy to assume evidence of brain differences (especially between
clinically identified subjects and unaffected controls) to be evidence of damage, dys-
function, defect, or pathology.
Kiehl (2006) reviewed several studies noting that damage to various components
of the paralimbic system results in some symptoms and cognitive impairment asso-
ciated with psychopathy. Changes in behavior associated with paralimbic damage,
however, are also associated with impaired decision making and greater reactive (but
not instrumental) aggression, both of which are strongly indicative of generalized
criminal behavior but not psychopathy. There is little evidence to suggest that the very
specific constellation of behaviors associated with psychopathy can be induced by
any specific brain injury. The frequency-dependent selection explanation would
suggest that the brains of psychopaths are necessarily different from the brains of
nonpsychopaths but that this difference is not due to pathological causes. Although
brain imaging studies cannot infer much about the pathological nature of brain differ-
ences, they can still shed light on patterns of brain functioning unique to psychopathy.
Imaging studies utilizing positron emission tomography (PET) and magnetic
resonance imaging (MRI) test whether the unique pattern of behavior observed
in psychopathy is due to differential anatomy or activation in the brain, as com-
pared to “normal” populations. Several imaging studies have been conducted on
violent offenders and those diagnosed with antisocial personality disorder (APD).
The problem with these studies, however, is that these samples contained proba-
ble life-course-persistent offenders, only some of whom were psychopaths. As we
mentioned already, an evolutionary view of psychopathy would expect important
differences between psychopathic and non-psychopathic life-course-persistent of-
fenders. Unless psychopathy is diagnosed using a validated measure, such as the
PCL-R, it is difficult to know whether these imaging studies reveal brain structures
or functions unique to psychopathy or structures or functions typical of violent
offenders or those with APD more generally. In this section, we report on the few
imaging studies conducted on psychopaths.
The paralimbic system has been identified as an important activating circuit pos-
sibly relevant to psychopathy. Damage to subcomponents of this system has been as-
sociated with an increase in some behaviors also symptomatic of psychopathy (e.g.,
extreme aggression, lack of empathy, general callousness), a phenomenon termed
“pseudopsychopathy” (Kiehl, 2006). Behaviors similar to these have been observed
in studies manipulating animal brains as well as case studies in humans where dam-
age has occurred as a result of head injury (for a comprehensive review, see Kiehl,
2006). Although these studies provide evidence that some clinical features that re-
semble psychopathic traits can be due to brain damage, it is far from clear that the
kind of damage studied so far can produce the full spectrum of psychopathic traits.
In the few studies that have investigated brain functioning in true psychopathic
samples, little convergent evidence has emerged to indicate whether particular brain
areas or systems are implicated in psychopathy. Two studies have found differences
in amygdala functioning, and one found a difference in the frontal cortex. The amyg-
dala plays a role in aversive conditioning, instrumental learning, and general process-
ing of emotion and fear (Blair, 2003; LeDoux, 2003). Thus, differences in amygdala
function could give rise to many of the characteristics of psychopathy, such as low
empathy, minimal response to aversive stimuli, and general absence of emotional re-
sponding. Using volumetric MRI techniques, Tiihonen et al. (2000) found that high
levels of psychopathy, as scored using the PCL-R, were associated with lower amyg-
dala volume. In another study, Kiehl et al. (2001) used an emotional memory task
in which participants processed words of neutral and negative valence. Participants
who scored high on the PCL-R showed reduced MRI-measured amygdala response
compared to lower-scoring individuals. These results are suggestive of potential dif-
ferential amygdala function in psychopathy.
The frontal cortex, encompassing both the orbitofrontal and prefrontal corti-
ces, is associated with conscious decision making and executive control. Thus, dif-
ferences in the functioning of this region could also result in some behaviors typical
of psychopaths, including social behavioral problems and high aggression (Blair,
2003). Damage to the frontal cortex, however, is inconsistent with other typical psy-
chopathic behaviors, especially instrumental violence and, thus, planning ability.
Raine et al. (2000) assessed individuals who scored high on the PCL-R using MRI and
found that they showed reduced prefrontal gray matter volume but not white mat-
ter volume. Another MRI study by Laakso et al. (2002) found no difference between
psychopathic and non-psychopathic populations in total prefrontal or prefrontal
white and cortical volumes. Together, these findings provide little evidence for gener-
alized frontal cortical dysfunction in psychopaths. Blair (2003) suggested, however,
that differences in one particular area of the frontal cortex—the orbitofrontal cortex
(OFC)—might be consistent with lower amygdala activity in psychopathy in that the
OFC shares several neural projections to and from the amygdala. Blair also points out
that parts of the OFC are associated with instrumental learning and response rever-
sal, functions said to be different in psychopaths. Nevertheless, few empirical data
exist to support the idea of impaired OFC functioning, and it remains unclear whether
any differences in frontal cortical structure are associated with psychopathy.
Conclusion
Further understanding of psychopathy will benefit from studies aimed at refining

its conceptualization and measurement. Much of the current debate focuses on the
distinction between core personality features and more easily observable antisocial
and externalizing traits, and on whether psychopathy is an aggregation of multiple
and distinct factors or one general constellation of traits. It is also possible that some
psychopathic features are missing or incorrectly conceptualized. One version of the
frequency-dependent selection account of psychopathy suggests that early-onset and
coercive mating effort might be a key feature of psychopathy, and this feature is not
properly captured in the current version of the PCL-R, the gold standard assessment
tool for offenders. Most of the theoretical work on psychopathy is traditionally bound
to medical and pathological notions and has suffered from evolutionary neglect (the
neglect of consideration of ultimate causes). So far, nonpathological (adaptation-
ist) accounts have survived empirical disconfirmations, and research in the area of
experimental games of cooperation, genetics, and brain imaging are likely to provide
further insights.
Many people, scientists included, find the contemplation of psychopathy espe-
cially fascinating. The idea that a minority of male criminals is engaged in a life his-
tory strategy distinct from that of most people can be hard to accept at first—that is,
most of us would like to believe that people are all generally alike and that the worst
antisocial conduct would disappear if everyone had free and equal access to the best
care, opportunities, and resources. Some selectionist accounts of psychopathy sug-

gest, however, that these beliefs might not be completely correct—perhaps most peo-
ple are alike and generally interested in mutual cooperation, and this has permitted a
niche for a minority, alternative, nonpathological life history strategy characterized
by social defection, emotional indifference to others, interpersonal exploitation, ag-
gression, and coercive mating effort. If so, free and equal access to resources and op-
portunities (and providing psychotherapy) might have little effect on the prevalence
of psychopathy, which might actually be altered only by the vigilance and contin-
gencies non-psychopaths can bring to bear. It is somewhat curious in this context
that few have noticed that phenotypes that include special skills (e.g., glibness and
charm, manipulation, parasitic lifestyle) are rarely the result of pathology.
ACKNOWLEDGMENTS
The authors would like to thank Annabree Fairweather, Vern Quinsey, Marnie Rice, Michael
Seto, Kelly Suschinsky, and, especially, Danny Krupp for commenting on a previous version of
this chapter, and the Social Sciences and Humanities Research Council for its financial support in
the form of a Standard Research Grant (M. L. L. and G. T. H.) and a Doctoral Fellowship (S. M.).
Notes
1. Readers might wonder how someone (with a maximal score on the PCL-R, for exam-
ple) could simultaneously possess the traits of shallow affect and strong negative emotionality.
In addition to the possibility of deliberate deception, the answer no doubt lies in the circum-
stances. Prototypical psychopaths are emotionally indifferent to the unhappiness and suffer-
ing of others (but not because they have any trouble perceiving it). They are, however, easily
angered and upset by threats to their own interests. When institutionalized, for example, they
worry about their fate (Cleckley, 1941), angrily guard their rights, and are prone to regard
themselves as “victims of the system,” their own misdeeds notwithstanding (Hare, 1998).
2. In quantitative genetics, heritability has a special technical meaning that is often mis-
interpreted. Heritability is the proportion of phenotypic variance that can be accounted for by
genetic variance.
3. Additive gene effects refers to the simple combination of gene effects at different loci (as
opposed to nonadditive effects, which refers to the interaction between different genes and gene
dominance, among other things). Unshared environmental effects refers to environmental fac-
tors that operate to make siblings different from one another.
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PART FIVE
VICTIMS OF CRIME
11
Victim Adaptations
JOSHUA D. DUNTLEY AND TODD K. SHACKELFORD
What Is a Victim?
There are victims of disease, victims of natural disasters, and victims of circum-
stance. People may even be victims of their own actions, hoisted by their own pe-
tards. For forensic psychologists who work within the legal system, victims represent
a more restricted class of individuals–people who have costs defined by legislators as
criminal inflicted on them by others.
An evolutionary exploration of victimization demands a more inclusive definition
of victimization. Specifically, we argue that the genetic relatives, romantic partners,
and close allies of the primary victims of exploitative or violent strategies also incur
costs and can be considered secondary victims. Primary victims of crime share genes
with all of their living genetic relatives. Because natural selection operates through
the differential replication of genes (Hamilton, 1963), costs to genetic fitness result-
ing from the victimization of a family member are shared across all of the person’s
genetic relatives. Because a victim’s closer genetic relatives share more copies of the
victim’s genes, the costs that they incur are greater than those endured by more dis-
tant genetic relatives. Spouses and close social allies can also be secondary victims, in-
curring costs as a result of loss of investment or protection, and perhaps by gaining a
reputation of being vulnerable to exploitation (Buss & Duntley, 2008; Duntley, 2005).
We hypothesize that selection fashioned adaptations in both primary and secondary
victims to prevent or stanch the costs of victimization.
Why Are Some Behaviors Considered Crimes?
Of all the human behaviors that inflict costs on others, only a subset are considered
to be criminal. Derogating competitors, for example, is not criminal but is a competitive
201
202 Victims of Crime
strategy that people use to inflict costs on intrasexual rivals (Buss & Dedden, 1990).
How do individuals and societies decide whether a behavior should be legally vili-
fied? Evolutionary psychologists propose that societal groups criminalize those be-
haviors that have the greatest negative consequences on reproductive fitness (Buss,
2007; Jones, 1997). Laws prohibiting cost-inflicting behaviors and the enforcement
of those laws are argued to be outcomes of evolved psychological mechanisms. Indi-
viduals with psychological predispositions to prevent being victimized and to punish
those who inflict costs would have had an evolutionary advantage over competitors
who lack such predispositions. As a result, the genetic foundation for the develop-
ment of mechanisms to punish exploitative behaviors would have been passed on
with greater frequency to subsequent generations than other strategies that were
less effective at stanching fitness losses from being victimized. Because all individuals
in a group would benefit from preventing others from victimizing them, it is likely
that selection favored cooperation among individuals in the same group, and espe-
cially the same family who have shared genetic interests, for the prevention and pun-
ishment of cost-inflicting behaviors against mutual allies.
Criminal Cost-inflicting Behaviors
The criminal cost-inflicting strategies that humans employ manifest in many differ-
ent guises, including robbery, assault, rape, and murder. Many hostile human activ-
ities have been proposed to be the result of psychological adaptations. Researchers
have found evidence for adaptations that contribute to the production of spousal
violence (Buss & Shackelford, 1997a), aggression (Buss & Shackelford, 1997b;
Campbell, 1993; Daly & Wilson, 1988), and rape (Thornhill & Palmer, 2000). At
the core of the selection pressures that shaped these adaptations is conflict between
individuals for limited resources. In this chapter, we will (a) discuss how natural
selection shaped strategies to inflict costs on other humans and (b) explain how
the recurrence of cost-inflicting strategies in predictable contexts of competition
selected for specific patterns of victim defenses. Because of the high fitness conse-
quences of homicide, we will focus on defenses against being murdered.
Contexts Selecting for the Infliction of Costs
To identify which individuals are in the greatest conflict over a limited resource, it is
necessary to explore the adaptive problems leading to conflict between individuals.
The conflict that exists between two individuals is tempered by genetic relatedness
(Hamilton, 1963). Because selection operates by differential replication of genes,
individuals should have evolved predispositions to favor genetic relatives who share
copies of their genes over nonrelatives. Thus, closer genetic relatives should experi-
ence less conflict over resources than more distant relatives or unrelated individuals.
Victim Adaptations 203
There can be dangers associated with adopting a strategy of cost-infliction against

competitors. Individuals who inflict costs on others may gain unfavorable reputa-
tions, become injured, or die as a result of carrying out their attacks. Because of
the potential dangers, the use of cost-inflicting strategies to best competitors should
be most likely when the contested resources are uncommon and the fitness pay-
offs are great. For example, men who hold high positions in status hierarchies have
greater success in attracting mates than do lower-status men (Buss et al., 1990).
High-status positions in social hierarchies are rare and are valuable for the repro-
ductive success of men, and would have created selection pressure for strategies ca-
pable of increasing status, including tactics of cost-infliction on rivals. In contrast,
there would not have been ancestral selection pressures to compete against others
for plentiful, easily obtainable resources or struggle to control items or entities that
contributed little or nothing to human reproductive success.
Conflict over Status

One broad context of conflict is over position in status hierarchies. All available
evidence indicates that high-status men have sexual access to a larger number of
women (Perusse, 1993). Men who are high in status also seek younger and more
fertile women (Grammer, 1992) and marry women who are more attractive (Taylor &
Glenn, 1976; Udry & Eckland, 1984) than their low-status rivals. Although no com-
prehensive evolutionary theory of the importance of status over our evolutionary
history has yet been proposed (Buss, 2007), the potential for large fitness gains as-
sociated with increases in status would have created selection pressure for cognitive
adaptations that produce desires and behaviors that lead to hierarchy ascension and
prevent large status falls.
Conflict over Material Resources

A second context of ancestrally recurrent conflict was conflict over material resources
that helped to solve recurrent adaptive problems. Such resources include territory,
food, weapons, and tools. There was also conflict over individuals who were the
suppliers of material resources, such as conflict between siblings for investment
from their parents and elder kin (Parker, Royle, & Hartley, 2002) and conflict be-
tween women for men with resources (Buss, Larsen, & Westen, 1996; Buss, Larsen,
Westen, & Semmelroth, 1992). The scarcer and more valuable the resource in terms
of its contribution to an individual’s reproductive success, the greater the conflict is
between individuals over access to the resource.
Conflict over Mating Resources

Whereas the minimum obligatory parental investment for women is nine months,
the minimum investment for men can be as little as a few minutes. Because women’s
minimum investment in reproduction is greater, the costs of a poor mate choice are
higher (Trivers, 1972). As a result, there is conflict between the sexes about the tim-
ing of sexual activity. Because sex is less costly for men, they desire sexual activity
much earlier in a relationship than do women (Werner-Wilson, 1998). Men also de-
sire a greater number of sexual partners than women (Schmitt, Shackelford, Dunt-
ley, Tooke, & Buss, 2002) and are more amenable to short-term, uncommitted sex
(Buss, 1996).
In sum, each context of conflict results from individuals pursuing evolved
strategies. Selection sculpted the adaptations that produce these strategies, in-
cluding those that exploit others, because of their benefits to the reproductive
success of the individuals who use them. It is important not to lose sight of the
fact that, over human evolutionary history, there were at least two sides to every
conflict.
The Coevolution of Cost-infliction and Victim Defenses
Coevolutionary arms races are part of the evolutionary history of all species. They
can occur between species, as with the fox and the hare, or within species between
different individuals’ competing adaptations in contexts of social conflict. Coevo-
lutionary arms races can create massive selection pressures capable of produc-
ing rapid evolutionary change (Phillips, Brown, & Shine, 2004). Any recurrent
context of conflict between individuals has the potential to be a hotbed for the co-
evolution of competing strategies to best a competitor or to defend against being
exploited.
The evolution of adaptations to inflict costs created selection pressures for the
coevolution of counter-adaptations in victims to decrease or prevent incurring
the costs. The strength of the selection pressure for victim adaptations is a func-
tion of the magnitude of costs inflicted, the frequency of such costs over evolu-
tionary time, and the certainty that the costs would be inflicted. Once adaptations
to prevent or minimize the costs of exploitation evolve in victims, they create new
selection pressures on those who inflict costs for refinements in their adaptations
capable of stanching the effectiveness of the victim adaptations. These refined
adaptations for cost-infliction in turn create new selection pressures for refined
victim adaptations capable of defending against the new cost-inflicting strategies.
This antagonistic, coevolutionary arms race between adaptations to inflict costs
and victim adaptations to defend against costs is hypothesized to have recurred
over human evolutionary history.
Victim adaptations to competitors’ cost-inflicting strategies can evolve only
when the strategies have been recurrent in predictable contexts over evolution-
ary time. Many evolved victim adaptations function by making a competitor’s
cost-inflicting behavior too costly to perform. This would create selection pres-
sure against the cost-inflicting strategy. If a cost-inflicting strategy persists over
evolutionary time despite its costs, then the cost-inflicting strategy may be func-
tional in producing a net benefit in a particular context. We propose that evidence
of such functionality is evidence of adaptation.
The Three Temporal Contexts of Victim Defenses
There are important differences between the form and function of victim defenses
depending on when they are enacted. Victims can defend themselves from the cost-
inflicting strategies of others (1) before the victimization occurs, (2) while the cost-
inflicting event is occurring, or (3) after being victimized. Each of these temporal
contexts of victim defenses was selected to minimize the impact of the outcomes of
victimization. We hypothesize that the strength of selection pressures operating to
design adaptations to address each temporal context varies as a function of the na-
ture of the costs inflicted. For example, there would be selection pressures on victim
adaptations against rape in all three temporal contexts. Women should have adapta-
tions to avoid victimization, to minimize costs during victimization, and to take steps
to prevent reputational damage and future victimization in the aftermath of rape.
However, there would not be selection pressures on all three temporal contexts of pri-
mary victims’ adaptations against being murdered. The primary victims of homicide
are incapable of directly influencing events after their deaths.
Adaptations to Prevent or Avoid Victimization

The best defense against being victimized is to never become a victim. To the extent
that strategies of cost-infliction were perpetrated by predictable conspecifics in pre-
dictable contexts, there would have been selection pressures for the evolution of
defensive adaptations to avoid them. Individuals with adaptations that led them to
recognize situational cues and individual characteristics associated with a higher
likelihood of incurring costs and to then avoid them would have had a large fitness
advantage over competitors without these abilities. Fear while walking through
dark alleys at night or of people who seem “shifty” and stranger anxiety in infants
are examples of the hypothesized outcomes of adaptations to avoid falling victim
to the cost-inflicting strategies of others.
Adaptations to Minimize Costs During Victimization

Selection also shaped adaptations to minimize the costs of victimization while it is
occurring. Defensive postures, verbal attempts at manipulation, and seeking or cre-
ating opportunities to flee an attacker are defensive strategies hypothesized to have
been selected because they decreased the costs of victimization. Curling into a fetal
position may help to deflect blows from an attacker away from a victim’s head and
internal organs. The use of language to activate sympathy or empathy in an attacker
or to frighten an attacker away may be effective in decreasing the duration or sever-

ity of the cost-infliction. Creating or waiting for an event that distracts an attacker or
temporarily incapacitating the attacker might give victims an opportunity to escape
or to hide and seek protection, decreasing the magnitude of costs they might other-
wise have incurred. Finally, Bracha (2004) has hypothesized that fainting may be an
evolved strategy to minimize the costs of being attacked or prevent victimization by
sending an honest signal to attackers that one is not a threat.
Post-victimization Adaptations
Finally, we hypothesize that selection shaped victim adaptations activated after the
occurrence of the cost-inflicting event that function to minimize the impact of the
victimization and to prevent it from being repeated. For example, acting as though
the injuries sustained during a fight are not as debilitating as they actually are or
using verbal assaults on an attacker that impugn the effectiveness of a person’s
attack, such as “You fight like my grandmother,” may decrease the status loss that
can be associated with losing a fight.
There are numerous avenues for the prevention of future occurrences of vic-
timization. One is learning cues to danger. By recognizing and subsequently avoid-
ing dangerous contexts and individuals, victims make themselves less likely to incur
costs in similar contexts in the future. A person victimized in a certain part of a city,
for example, may be motivated to avoid that part of the city. Similarly, a victim may
avoid future interactions with an attacker. Victims also may be proactive in avoiding
future conflicts by developing or acquiring defenses against future attacks by conspe-
cifics. For example, carrying a weapon for self-defense may decrease the likelihood of
incurring serious costs in future confrontations.
Another avenue for the prevention of future victimization is retaliation against
an attacker. Demonstrating an effective ability to retaliate may decrease the likeli-
hood of future victimization by sending a message to the perpetrator and others
that attacks or exploitation will be punished. Revenge has been suggested to be
wired into our psychology by natural selection (Buss & Duntley, 2006). Functional
magnetic resonance imaging (fMRI) research has demonstrated that pleasure cen-
ters of men’s brains become activated when they are successful in obtaining re-
venge against someone they perceive to have crossed them (Singer et al., 2006).
This suggests that the motivation for men to seek revenge may have evolutionary
underpinnings and supports the contention that maintaining status in social com-
petition was important for the reproductive success of ancestral men.
Selection pressures for each temporal category of victim adaptations are un-
likely to have been equal. Since avoiding victimization entirely was ancestrally
associated with the lowest costs, we hypothesize that there was proportionally
more selection pressure for the evolution of previctimization adaptations than
for victim adaptations that function during or after victims have incurred costs.
As a result, previctimization adaptations are hypothesized to be larger in number
and perhaps more elaborate in design than the other temporal categories of victim
adaptations.
In sum, it is useful to consider three temporal categories of victim adapta-
tions: those aimed at avoiding victimization, those that minimize the costs of
victimization while it is occurring, and those that function after victimization
to minimize its costs and prevent its recurrence. The nature of the victimization
will determine the degree of selection pressure for adaptations in each of these
contexts.
Adaptations to Damage Status
One strategy for inflicting costs on rivals in order to deprive them of reproductively
relevant resources is to damage their reputations. An individual in a group cannot
ascend in a status hierarchy without displacing someone above, bumping that per-
son to a lower position than he or she occupied previously and inflicting costs as-
sociated with status loss. Higher-status men have greater access to resources and
more mating opportunities than lower-status men (Betzig, 1993; Buss, 1996; Hill &
Hurtado, 1996; Perusse, 1993). Because additional mating opportunities enhances
the reproductive success of men more than it does that of women, it has been hy-
pothesized that there should be greater status striving among men (Buss, 2003a).
Research across the life span has found this to be the case, with men placing greater
importance on coming out ahead and women tending to be more focused on main-
taining social harmony (Maccoby, 1990; Pratto, 1996; Whiting & Edwards, 1988).
Defenses against Status Damage
A number of victim defenses may have evolved to combat the danger of status
loss caused by the cost-inflicting tactics of competitors. First, individuals should
be armed with the ability to constantly track their own position in a status hier-
archy while also keeping track of their closest competitors (Buss, 2004; 2007).
Individuals should be motivated to gather information about the strengths and
weaknesses of their closest status rivals to inform strategies of status defense
that may be required in the future. The strategic formation of alliances that will
strengthen one’s hold on a position in a status hierarchy can help defend against
status assaults from others. Offensive tactics such as competitor derogation (Buss
& Dedden, 1990) can assault the status of those most likely to challenge one’s po-
sition in the future, forestalling a status conflict. Competitor derogation may also
be an effective strategy after a status loss has occurred. Recouping status that has
been lost, however, can be a more formidable task than maintaining one’s position
in a status hierarchy and may require more drastic measures. Social status was so
important to the reproductive success of ancestral men that people may now
resort to violence and even murder in response to public humiliation or challenges

to status and social reputation. This made sense in the context of small-group liv-
ing in which we evolved (Tooby & Devore, 1987), where a loss of status could have
had devastating effects on survival and reproduction (Buss, 2007). The outcome
of selection for victim adaptations to defend status in the small-group living con-
ditions of our ancestors is evidenced today in research conducted on homicidal
ideation that finds the most frequent triggers of homicidal fantasies are status
related (Buss & Duntley, 1999) and in research on actual murders, which suggests
that experiencing reputational damage contributes to the activation of the moti-
vational mechanisms behind a substantial number of homicides (Daly & Wilson,
1988).
Adaptations for Theft and Cheating
A second strategy of cost-infliction that may be used to gain an advantage in com-

petition for resources is to steal those resources (Cohen & Machalek, 1988; see also
Chapter 9 of this volume) or or to cheat rivals out of their resources. A valuable
weapon can be stolen and used against its owner. Valuable territory can be en-
croached upon and its vegetation, water, shelter, and wildlife exploited (Chagnon,
1996). Mates can be poached from rivals (Buss, 2000, 2003a; Schmitt & Buss, 2001).
Public knowledge that an individual has been cheated or had valuables stolen also
can affect the person’s reputation. The person may gain a reputation as one who is
easy to exploit, perhaps increasing the likelihood that others will attempt to cheat or
steal from the person (Buss & Duntley, 2008). An easily exploitable person will likely
be less attractive to members of the opposite sex. Cheating and the theft of resources,
in short, can be effective strategies of cost-infliction for individual gain.
Defenses against Theft and Cheating
To prevent the threat of material resource theft, individuals are hypothesized to have
evolved adaptations to defend against theft and being cheated. These mechanisms are
hypothesized to motivate people to keep valuable items under protection, to conceal
them, or to make valuable commodities seem less desirable to rivals. Humans may
have also evolved adaptations to detect those who would cheat them. Deceiving ri-
vals about the location of a valuable resource, such as food, has been shown to occur
in other primates, like tufted capuchin monkeys (Cebus apella) (Fujita, Kuroshima,
& Masuda, 2002), as well as in pigs (Held, Mendl, Devereux, & Byrne, 2002) and in
ravens (Corvus corax) (Bugnyar & Kotrschal, 2004). The ability to detect cheaters in
contexts of social exchange is another strategy for preventing the loss of resources
to rivals. Sugiyama, Tooby, and Cosmides (2002) found evidence that the ability
to detect violations of conditional rules in contexts of social exchange (“cheater
detection”) is likely a cross-cultural universal. In their research, the Shiwiar hunter-

horticulturalists of the Ecuadorian Amazon performed similarly to Harvard under-
graduates in their ability to detect rule violations in contexts of social exchange. Both
groups, however, performed poorly when asked to detect violations of conditional
rules in contexts other than social exchange.
When the resource that is threatened is a mate rather than a material commod-
ity, Buss and Shackelford (1997a) found that men and women engage in tactics that
range from vigilance to violence to defend their relationships. Fueled by jealousy, an
emotion absent from contexts of material resource theft, men’s tactics of defending
against mate poachers were found to be different from women’s. Men are more likely
to conceal their partners, to display resources, and to resort to threats and violence,
especially against rivals. Women are more likely to enhance their appearance and to
induce jealousy in their partners, demonstrating their desirability by showing that
they have other mating options.
Adaptations for Violence
A third strategy for inflicting costs on rivals is to injure them physically. Individu-
als should disengage from competition for a contested resource when the inclusive
fitness costs of competing become greater than the benefits of controlling the re-
source. The direct infliction of costs on competitors in the form of violence can help
tip the outcome of competition in favor of the cost-inflicting individuals, increasing
the likelihood that they will gain control of contested resources. Healthy individu-
als can compete more effectively than their injured rivals. Rivals may be more likely
to avoid or to drop out of competitions with individuals who have injured them in
the past. Individuals capable of inflicting greater injuries on competitors than are
inflicted on them may gain a reputation as being difficult to exploit. This reputa-
tion is hypothesized to help protect those successful in the use of violence against
future violent confrontations and grant them easier access to resources with less
resistance from competitors. Some strategies employed to win competitions for re-
productively relevant resources offer a potential solution to a wider variety of prob-
lems than others. For example, violence can be used to help solve a broader range of
problems than theft. In a single instance, violence can be used as a strategy to aid in
theft, to demonstrate one’s ability to acquire resources to potential mates, to intimi-
date rivals, making them less likely to seek retribution, and to make future threats
of violence more credible.
Victim Defenses against Violence
The most effective strategy for dealing with violence capable of producing injuries
is to avoid it altogether. Adaptations for alliance formation may provide one form of
deterrence, as it is easier to attack an individual than a group. Adaptations that lead

to the avoidance of contexts likely to make one the target of violence may provide
another kind of protection against being injured in a violent confrontation. Humans
may also possess adaptations designed for attempting to reason with an attacker,
emphasizing the costs of their violent behavior or suggesting other resolutions to the
conflict. Finally, if an attack cannot be avoided, individuals may resort to violence or
even murder to defend themselves (Daly & Wilson, 1988).
Adaptations that Produce Rape
A fourth cost-inflicting strategy aimed directly at obtaining reproductive re-

sources is rape. A rapist may benefit from the behavior by siring offspring that he
may not have otherwise produced. Not only does rape inflict terrible emotional
(Block, 1990; Burgess & Holmstrom, 1974) and physical (Geist, 1988) costs on
women but it also inflicts fitness costs by bypassing female mechanisms of mate
choice (Buss, 2007). Although some scholars have concluded that there is not
enough evidence to determine whether men have adaptations for rape (Buss,
2003a, 2007; Symons, 1979), historical records and ethnographies suggest
that rape occurs cross-culturally and has been recurrent over deep time (Buss,
2003a).
Victim Defenses against Rape
A number of researchers have proposed the existence of anti-rape adaptations. The

formation of alliances with groups of other women and with men for protection has
been argued to be an evolved counterstrategy to rape (Smuts, 1992). The “body-
guard hypothesis” proposes that women’s preference for mates who are physically
formidable and high in social dominance is, in part, an adaptation to prevent rape
(Wilson & Mesnick, 1997). Specialized fears that motivate women to avoid situations
ancestrally predictive of an increased likelihood of being raped have been proposed
to help preemptively defend against rape. To prevent conception resulting from rape,
women may have evolved to avoid risky activities during ovulation (Chavanne &
Gallup, 1998). The psychological pain of rape has been argued to motivate women to
avoid being raped in the future (Thornhill & Palmer, 2000). In addition, women may
possess adaptations to minimize the costs of rape after it has occurred. To avoid the
reputational damage that can be associated with rape or to avoid losing their roman-
tic partner, women may feel motivated to keep their ordeal a secret. We hypothesize
that female rape victims’ common urge to bathe themselves after their victimization
functions to wash physical evidence of the forced encounter away so it cannot be de-
tected, especially by their romantic partners who may be more likely to abandon a sex-
ually exploited partner (Thornhill & Palmer, 2000). Finally, women may seek revenge
against their attacker by marshaling male relatives and allies to attack him, especially
if the rapist represents a persistent threat to the woman or her female relatives.
Adaptations that Produce Homicide
Buss and Duntley (1998, 1999, 2003, 2004) have proposed that humans possess
adaptations for murder. According to their homicide adaptation theory, over the long
expanse of human history there were recurrent sources of conflict between individ-
uals, such as conflict over reputation and social status, conflict over resources, and
conflict over romantic partners. Homicidal strategies are argued to be distinct from
nonlethal solutions to conflict in that they lead to an absolute end to the competition
between two individuals. Once dead, a person can no longer damage your reputa-
tion, steal your resources, prevent you from attracting a romantic partner, or have
sex with your spouse.
Homicide is hypothesized to be the designed output of evolved psychological
mechanisms. Killing conspecifics is argued to solve a variety of adaptive problems.
Specifically, the killing of a conspecific could have contributed to (1) preventing the
exploitation, injury, rape, or killing of self, kin, mates, and coalitional allies by conspe-
cifics in the present and future; (2) reputation management against being perceived
as easily exploited, injured, raped, or killed by conspecifics; (3) protecting resources,
territory, shelter, and food from competitors; (4) eliminating resource-absorbing or
costly individuals who are not genetically related (e.g., stepchildren); and (5) elimi-
nating genetic relatives who interfere with investment in other vehicles better able to
translate resource investment into genetic fitness (e.g., deformed infants, the chroni-
cally ill or infirm). Chapter 3 of this volume provides a more thorough exploration of
homicide adaptation theory.
Homicide as a By-product of Other Evolved Mechanisms
Adaptations for homicide need not be involved in the production of all homicidal
behavior. Another evolutionary explanation of conspecific killing was proposed by
Daly and Wilson in their book Homicide (1988). According to Daly and Wilson, ho-
micide may be considered an overreactive mistake, the by-product of adaptations
designed for nonlethal outcomes. They argue that homicide can be used “as a sort of
‘assay’ of the evolved psychology of interpersonal conflict” (Wilson, Daly, & Daniele,
1995). For example, if cognitive adaptations for parenting fail to engage, it may lead
a woman to abandon her infant in a dumpster. The resulting death, according to Daly
and Wilson, is not the result of maternal adaptations to kill; rather, it is a byproduct
of the activation failure of the woman’s parenting mechanisms. Similarly, male ad-
aptations for the experience of sexual jealousy and those that motivate men to coerce
and control their female partners may overreact, leading some men to mistakenly
use too much force when confronting an unfaithful partner, causing her death. In
this case, again, the homicide is a byproduct of the function of other mechanisms
that were designed by selection for their nonlethal consequences. In the case of a
husband who kills his wife for being sexually unfaithful, Daly and Wilson have ar-
gued that male mechanisms for sexual jealousy and the coercion and control of their
mates may mistakenly overreact, leading a man to kill his wife. Despite their conten-
tion that conspecific killing in humans is a maladaptive by-product of psychological
adaptations, Although Daly and Wilson (1988) think that adaptations for homicide
are very unlikely, they do emphasize that an evolutionary account of homicide is im-
portant: “[W]hat is needed is a Darwinian psychology that uses evolutionary ideas as
a metatheory for the postulation of cognitive/emotional/motivational mechanisms
and strategies” (pp. 108–109).
The Fitness Costs of Being Killed
Whether there are adaptations specifically for homicide or homicide is a byprod-

uct of adaptations that were designed to have nonlethal consequences, conspecific
killing was a recurrent feature of human evolutionary history (Chagnon, 1988;
Trinkhaus & Shipman, 1993). Examining the costs of homicide through an evolu-
tionary lens elucidates the nature and magnitude of the costs incurred by victims
of homicide and gives us a better understanding of how other humans were a sig-
nificant danger over our evolutionary history. A victim’s death has a much larger
impact on his or her inclusive fitness than just the loss of the genes housed in the
person’s body. The inclusive fitness costs of dying at the hands of another human
can cascade to the victim’s children, spouse, kin, and coalitional allies. The specific
costs include the following.
Loss of future reproduction. A victim of homicide cannot reproduce in the fu-
ture with a current mate or with other possible mates. On average, this cost
would have been greater for younger individuals than for older individuals.
Damage to existing children. The child of a murdered parent receives fewer
resources, is more susceptible to being exploited by others, and may have
more difficulty in ascending status hierarchies or negotiating mating rela-
tionships, which will likely lead to poorer fitness outcomes. Children of a
murdered parent may see their surviving parent’s investment diverted away
from them to a new mating relationship and to the children who are the
product of that relationship. A single parent can invest less than two and
might abandon his or her children in favor of better mating prospects in the
future. Finally, the children of a murdered parent risk becoming stepchil-
dren, a condition that brings with it physical abuse and homicide rates 40
to 100 times greater than those found among children who reside with two
genetic parents (Daly & Wilson, 1988).
Damage to extended kin group. A homicide victim cannot protect or invest in

kin. A victim’s entire kin network can gain the reputation of being vulner-
able to exploitation as a result of the person’s death. A dead victim cannot
influence the status trajectories or mating relationships of family members.
And the open position left by the victim in a kin network’s status hierarchy
could create a struggle for power among the surviving family members.
A homicide victim’s fitness losses can be a rival’s fitness gains. Killers can
benefit from the residual reproductive value and parenting value of the sur-
viving mate of their victim, sometimes at the expense of the victim’s children
with that mate. Killers can ascend into the vacancies in status hierarchies
left by their victims. The children of killers would thrive relative to the chil-
dren of homicide victims, who would be deprived of the investment, pro-
tection, and influence of two genetic parents. Many family members who
would have survived if the person were not killed will die before they can
reproduce, and many children who would have been born to members of
the family will never be born.
Defenses against Homicide
Of all the dangers created by other humans, homicide can be the most devastating in
terms of its effect on the inclusive fitness of its victims. If homicide recurred in pre-
dictable contexts over our evolutionary history, it would have created selection pres-
sures to avoid being killed in precisely those contexts. We propose that the selection
pressures created by the costs of being killed were powerful enough to shape distinct
adaptations to defend against homicide (Duntley & Buss, 1998, 2000, 2001, 2002;
Buss & Duntley, 2006, under review).
The strength of selection for any adaptation, including defenses against being
killed, is a function of the frequency of the event and the fitness costs of the event. Low
base-rate events that impose heavy fitness costs, such as homicide, can create intense
selection pressures for adaptations to prevent or avoid them. Ancestral homicides,
however, may not have been as infrequent as they are in many modern societies.
Homicide rates in hunter-gatherer societies, which more closely resemble the condi-
tions in which humans evolved, are far higher than those in modern nation-states
with organized judicial systems (Ghiglieri, 1999; Marshall & Block, 2004).
The Nature of Selection Pressures for Homicide

Defense Adaptations
Homicide defense adaptations would have been selected for only one function: to
avoid the massive fitness costs of being killed. This could have been accomplished
by (1) avoiding contexts that present a high risk of homicide, (2) manipulating con-
texts that have a high probability of prompting homicidal behavior in a conspecific
so they were less or no longer dangerous, (3) defending oneself against homicidal
conspecifics, and (4) stanching the costs of homicide among the genetic relatives of
the victim after it occurred. Because homicide has unique fitness consequences, we
hypothesize that the fear of being killed is a distinct emotional state. We propose that
it is accompanied by specific decision rules that function to help individuals defend
against being killed by a conspecific. Specifically, we propose that selection fashioned
homicide defense adaptations that lead to the avoidance of unfamiliar surroundings,
particularly those controlled by rivals; traveling through locations where one could
be ambushed; traveling at night; interacting with individuals who are more likely to
kill; and inflicting costs likely to motivate a conspecific to kill.
Avoiding Contexts Where Homicide Is Likely

One of the design features of homicide avoidance mechanisms is sensitivity to cues
of high-risk contexts. Cues to the presence of such contexts are hypothesized to in-
clude the following.
Who controls the territory one occupies. Who controls the territory an in-
dividual is occupying at a given moment is an important cue that is hy-
pothesized to have been reliably correlated with the ancestral likelihood
of being killed by hostile conspecifics. Individuals are more vulnerable to
attack when away from their home territory. Being in a rival’s territory
or even a neutral territory would be a cue to an increased risk of attack.
Chagnon (1996) reports that the Yanomamo sometimes lure members of
a rival group to their territory under the auspices of having a celebratory
feast. Away from their home territory, the rival group is at a strategic dis-
advantage. The Yanomamo attempt to lull their rivals into a false sense of
security only to ambush them. We hypothesize that individuals will experi-
ence more fear of being killed in the presence of cues indicative of being in
hostile territory.
Characteristics of the physical surroundings. We propose that characteris-
tics of the physical surroundings are another source of ancestrally relevant
cues to the likelihood of being killed. It is easier for a competitor to hide in
the shadows than in the light. Individuals are more likely to be ambushed in
areas where there are visual obstacles than in areas affording unobstructed
scanning of the surroundings. An individual is more vulnerable to attack
when his back is to an open room than against a wall. Individuals should
experience more fear of homicide and more ideation that their life may be in
danger in the presence of such cues to their vulnerability. Evidence support-
ing this hypothesis comes from investigations of the Savanna hypothesis.
Kaplan (1992) argued that the process of evaluating landscape involves in-
formation-gathering about places for surveillance, places for hiding, refuges
from predators, and possible routes of escape.
Characteristics of the rival. Certain personality and life history characteristics

of rivals are hypothesized to have been recurrently correlated over our evolu-
tionary history with the likelihood that a rival will kill: high levels of narcis-
sism, an antisocial personality, high impulsivity, low conscientiousness, high
levels of hostility, and a history of committing acts of violence or homicide
against others. A history of violent behavior is one of the strongest predic-
tors of future violence (Douglas & Webster, 1999). Ethnographic evidence
indicates that some men develop reputations as killers or thugs. The people
who live in the same communities as these men give them a wide berth, try-
ing to avoid doing anything that might antagonize them (Chagnon, 1996;
Ghiglieri, 1999). We hypothesize that a design feature of defenses against
homicide is the ability to recognize and track dangerous conspecifics, attrib-
uting to them states of mind that would assist in predicting and avoiding
their violent tendencies.
Features of the situation. Specific adaptations are hypothesized to have
evolved that lead people to be sensitive to circumstances ancestrally indica-
tive of an increased probability of being killed. Individuals who recognized
and avoided such situations would have had a survival advantage over those
who did not. Examples of these situations include the following:
1. injuring, raping, killing, or inflicting other serious costs on a rival, his
kin, his mates, or his coalitional allies;
2. damaging a rival’s reputation, leading others to perceive him or his ge-
netic relatives as easily exploited, injured, raped, or killed;
3. poaching the resources, mates, territory, shelter, or food that belongs to
a rival;
4. absorbing the resources of a nongenetic relative (e.g., stepchildren); and
5. interfering with parents’ or kin’s investment in vehicles who are less able
to translate resource investment into genetic fitness (e.g., deformed in-
fants, the chronically ill).
The experience of fear may be one adaptive mechanism that helps us to avoid
circumstances in which others may be threats to our lives. In his book The Gift of
Fear (1997), De Becker argues that fear, when applied appropriately, is a signal that
exists to aid in our survival, protecting us from violent situations. It is adaptive to
experience fear, he argues, when the fear is enabling—allowing people to effectively
address the danger they face. Real fear, according to De Becker, “occurs in the pres-
ence of danger and will always easily link to pain or death” (p. 285).
Marks (1987) has argued that fear and anxiety can be protective in four primary
ways. First, they can immobilize a person. This could help to conceal people from a
predator, allow them time to assess the situation, and perhaps decrease their likeli-
hood of being attacked. This is a valuable strategy when there is uncertainty about
whether one has been spotted by a predator or cannot determine a predator’s exact
location. Second, fear can motivate people to escape or avoid danger in the environment.
This can help to move out of harm’s way and find a location that provides protection
from future interactions with the source of the danger. Third, fear may lead people
to adopt a strategy of aggression in self-defense. A dangerous conspecific or predator
can be frightened away or killed through the successful employment of an aggressive
strategy. Finally, fear and anxiety can lead people to adopt a strategy of submission as
a way to appease a source of the hostility, usually another person.
Sometimes people do not detect or are unable (or unwilling) to avoid contexts in
which someone may try to kill them. We hypothesize that humans have evolved de-
fensive strategies to protect themselves from impending and actively occurring homi-
cidal attacks. Such strategies are hypothesized to take three primary forms:
1. Fleeing the potentially homicidal confrontation with the person. An
individual who is successful in fleeing from someone who tried to kill him may then
attempt to change the situation in ways that will decrease the likelihood of being
killed. One such strategy may be to leave the area he shares with the intended killer.
An explanation that has been proposed for human migration out of Africa, across
Europe and Asia, and into the Americas is that migrating groups were attempting to
avoid hostile confrontations with conspecifics (Diamond, 1997; Richerson & Boyd,
1998). Fleeing homicidal rivals can be an effective strategy if the intended victims
can move out of the attackers’ reach. But fleeing often represents only a temporary
solution: if nothing about the context of conflict between the killer and intended
victim changes, it is likely that a homicidal person will attempt to kill their intended
victim again.
2. Manipulating the situation to make killing less beneficial and more
costly. A person who believes he might be killed may be able to alter aspects of
the situation to increase the costs or decrease the benefits of a homicidal strategy,
making homicide less attractive to the killer than nonlethal alternatives. Examples
include forging alliances with powerful conspecifics; staying in the vicinity of coali-
tional allies who may serve as bodyguards; turning members of a group against the
person who may intend to kill you; resolving the conflict with the conspecific by of-
fering some form of benefit; helping the rival to salvage or restore a reputation that
the victim had a part in impugning; bargaining or begging for one’s life; threatening
retaliation by one’s kin and coalitional allies; and performing preemptive, perhaps
homicidal, attacks against the would-be killer, his kin, or his coalitional allies.
Many of these strategies may be implemented up to the moment of the victim’s
death. The implementation of these defensive strategies may not always be enough
to derail a homicidal strategy in favor of a nonlethal alternative. If not, the person
targeted by a killer would have no recourse but to defend against the attack.
3. Defending against homicidal attacks. At the point at which a rival is en-
gaging in behaviors capable of killing, it may be too late to flee or derail the homicidal
strategy. In such face-to-face confrontations with a killer, the options are to defend
oneself or to die. There are two strategies of self-defense: call for help by an indi-
vidual under violent attack or physically incapacitate the would-be killer so the in-
tended victim can flee. Screams for help by an individual under violent attack may be
uniquely identifiable from other calls for assistance. Selection could have fashioned
this kind of honest signal if fitness gains flowed to rescuers, such as kin or coalitional
allies who might benefit from reciprocal exchange with the intended victim. “Death
screams” (Buss, personal communication, 2004) may represent another category of
alarm: they do not function as a call for help but instead warn kin and mates of the
presence of a killer as the victim dies. References to “blood-curdling screams” and
“screaming bloody murder” may refer to such uniquely identifiable screams made by
people who are battling off an attacker’s attempts to kill them.
Physically incapacitating a would-be killer is another strategy a victim can use in
self-defense. Invariably, this strategy involves physically attacking the killer in some
way. At a minimum, the victim of a homicidal strategy must incapacitate the attacker
enough so that the victim can flee or buy enough time for help to arrive. In some
confrontations, the most practical strategy of physically incapacitating the killer may
be to kill the person in self-defense. Contexts leading victims to kill in self-defense are
hypothesized to include features such as a lack of kin or allies in close enough proxim-
ity to help, the failure of nonlethal strategies to incapacitate the attacker, and a lack
of other possible options.
One of the key differences between a would-be killer and a victim in confronta-
tions is that the killer is more often prepared to carry out his homicidal strategy than
the victim is to defend against being killed. The killer can select the time and place
best suited to carrying out homicidal plans. Selection would have favored psychologi-
cal adaptations that led killers to favor contexts in which they could catch victims
alone and by surprise, reducing the possible costs of killing (e.g., being injured or
killed by the victim or the victim’s kin). As a result, it is hypothesized that the major-
ity of face-to-face confrontations between a would-be killer and the intended victim
result in the death of the victim. Because the genetic relatives of a homicide victim
suffer fitness costs, we propose that adaptations to defend against being killed are
also found in victims’ kin.
4. Stanching the costs of homicide by genetic relatives after it has oc-
curred. At least two forces may have selected for adaptations in kin that function to
minimize the negative consequences of the killing of a family member by a conspe-
cific. First, damage to a homicide victim’s family reputation may be at least partially
repaired by inflicting roughly equivalent costs on the killer. A family that is capable
of striking back against the killer may be able to demonstrate that it is not or is no
longer exploitable. Second, the killer may be a persistent threat if he continues to
live. Avenging the death of a family member by killing the person’s killer may elimi-
nate a source of recurrent fitness costs.
All of the proposed adaptations for defending against homicide function by de-
railing or thwarting homicidal strategies or by inflicting heavy costs on killers. Homi-
cide defense adaptations are costly for killers. The evolution of adaptations to defend
against being killed is hypothesized to have created selection pressure for the evolution
of refined adaptations for homicide that were capable of circumventing the evolved
homicide defenses. The presence of refined homicide adaptations, in turn, would have
selected for refined homicide defenses, and so on, setting up an antagonistic coevolu-
tionary arms race between adaptations to kill and adaptations to defend against being
killed.
Evidence of Adaptations for Homicide and Homicide Defenses
Evidence for anti-homicide defenses has been documented across the lifespan (Duntley,
2005). In this section, we focus on early lifespan evidence for these defenses.
Homicide has the potential to occur wherever there are humans interacting
with other humans. This is as true of interactions between mother and child as it is
of those between enemy nations. It is even true of the relationship between a preg-
nant mother and her developing fetus. For a woman, the fetus she carries probably
does not represent her last opportunity to reproduce. Women were selected to invest
more in those offspring who are likely to yield the greatest reproductive benefit, even
in utero. If a fetus is not viable, for example, it would make more sense in terms of
inclusive fitness for a pregnant woman to forgo her investment in its development in
favor of investing in a subsequent pregnancy. Most fertilized eggs do not result in a
full-term pregnancy. Up to 78% fail to implant or are spontaneously aborted (Nesse
& Williams, 1994). Most often, these outcomes occur because the mother’s body
detects chromosomal abnormalities in the fetus. The body’s ability to detect such
abnormalities is the result of adaptations that function to prevent the mother from
investing in offspring that will likely die young. Most miscarriages occur during the
first twelve weeks of pregnancy (Haig, 1993), when the mother has not yet invested
heavily in a costly pregnancy and when the spontaneously aborted fetus is less likely
to lead to infection (Saraiya et al., 1999). The fetus, however, is not a passive pawn
in its mother’s evolved reproductive strategy. The fetus has only one chance to live.
Selection would have favored fetal genes that resist a mother’s attempt to abort the
pregnancy. The production and release of human chorionic gonadotropin (hCG) by
the fetus into the mother’s bloodstream, which is normally an honest signal of fetal
viability, has been hypothesized to be an adaptation fetuses have evolved to defend
against being spontaneously aborted. This hormone prevents the mother from men-
struating, allowing the fetus to remain implanted in its mother’s uterus. Maternal
physiology reacts to the production of hCG as a sign that the developing fetus is vi-
able (Haig, 1993). Children continue to face threats to their lives after they are born.
Infancy is a time in every person’s life when he or she is particularly vulnerable to the
homicidal strategies of others, especially when the attackers are those responsible for
the infant’s care.
There is conflict between parents and their offspring about the best allocation of
parental resources. Offspring have evolved to desire more investment than is optimal
for their parents to provide. Rather than investing the majority of resources in their
offspring, parents’ fitness benefits from also investing in other relationships, such as
mateships and friendships, and investing in their own survival. The optimal amount
of investment a parent can make in his or her offspring also varies as a function of
the parent’s likely reproductive success in the future, known as reproductive value.
(Trivers, 1974). The reproductive value of children is lowest at birth and increases as
they age, a function of the likelihood that they will survive to reproductive age.
A newborn infant has few options for defending itself from homicidal attacks
perpetrated by adults. To defend against maternal infanticide, a newborn’s best strat-
egy may be to display cues that it is a vehicle worthy of investment. Immediately after
birth, an infant should display cues to its health and vigor, cues capable of satisfy-
ing maternal adaptations that evolved to judge the probability of fitness payoffs for
investing in the infant (Soltis, 2004). Newborns who nurse in the first hour after
birth stimulate a surge in maternal oxytocin levels, strengthening the bond between
mother and newborn. Nursing mothers’ priorities become shifted. They become less
motivated to self-groom for the purposes of attracting a mate and more motivated to
groom their infants (Insel, 1992). By contrast, new mothers who do not nurse are
more likely to suffer from postpartum depression (Papinczak & Turner, 2000; Taveras
et al., 2003), a condition associated with higher rates of maternal infanticide (Hagen,
1999; Knopps, 1993; Spinelli, 2004) and maternal thoughts of harming their new-
borns (Jennings, Ross, Popper, & Elmore, 1999; Kendall-Tackett, 1994). More active
newborns, as evaluated by APGAR scores, are less likely to die (Chong & Karlberg,
2004; Morales & Vazquez, 1994), and, in terms of fitness, would be wiser objects of
maternal investment than newborns that are not active. Selection is hypothesized to
have favored early nursing, the production of loud cries, and robust movements in
newborns as defenses against maternal infanticide.
As they develop, infants are increasingly aware of their environment and able to
move about on their own. As a result, they are increasingly likely to encounter dan-
gers while outside the range of their parents’ and other genetic relatives’ protection.
Infants who possess some ability to recognize potential dangers in the environment
would have a significant advantage over infants with no such ability. Selection is pro-
posed to have favored knowledge in advance, in the form of specific fears, to steer in-
fants away from threats to their survival. The developmental timing of the emergence
of fears provides evidence that selection played a part in shaping them. Many fears
do not emerge in development until individuals first encounter adaptive problems.
For example, a fear of heights, if it emerges, does so when children begin to crawl.
The emergence of this fear corresponds with infants’ greater risk of falling. Fear of
strangers emerges at about the same time (Scarr & Salapatek, 1970), corresponding
with a greater risk of encountering hostile, unrelated conspecifics. Stranger anxiety
provides powerful protection against dangerous conspecifics. It prevents children
from approaching individuals they do not know and motivates them to seek parental
protection. Stranger anxiety has been documented in many different countries and
cultures, from Guatemala and Zambia, to the !Kung and the Hopi Indians (Smith,
1979). Infant deaths at the hands of unrelated conspecifics have been documented
in humans (Daly & Wilson, 1988; Hrdy, 2000) and among nonhuman primates (Ghi-
glieri, 1999; Hrdy, 1977, 2000; Wrangham & Peterson, 1996). Human children are
more fearful of male strangers than female strangers, corresponding to the greater
danger posed by unrelated males than unrelated females over human evolutionary
history (Heerwagen & Orians, 2002). Even though the majority of strangers may not
intend to inflict harm on children, if a fear of strangers prevented even a tiny fraction
of children from being killed over our evolutionary history, stranger anxiety would
have been favored by natural selection.
Strangers are not the only threat to the lives of children. Children raised with
a stepparent in the home are between 40 and 100 times more likely to be killed by
their stepparent or parent than children raised by two genetic parents (Daly & Wil-
son, 1988). Stepfamilies were likely a recurrent feature of ancestral environments.
Without modern medical treatments, disease killed many ancestral adults. Fathers
sometimes died in battles or on hunts. Mothers sometimes died during childbirth.
After their partner’s death, it was probably not uncommon for a surviving parent
to find a new mate. Along with the benefits that come from a new long-term rela-
tionship is the potential for significant costs to existing children. Because the risk
of being killed is so much greater for children with a stepparent in the home, one
risk that may have affected single parents’ mate choice was the risk their new mate
posed to their existing children. There would have been selection pressure for the
evolution of adaptations in single parents to prefer partners who presented little
risk to their existing children. Single parents’ evolved preferences for new partners
are hypothesized to be, at least in part, evolved defenses against homicide of their
existing children (Buss, 2005).
Stepchildren may also possess adaptations to help defend against potentially
homicidal stepparents. These adaptations are hypothesized to have been shaped to
recognize characteristics of potential stepparents that may be predictive of their like-
lihood of inflicting costs on the children, including killing them. Children’s evolved
intuitions about potential stepparents are proposed to lead them to influence their
surviving parent’s mate choice, providing some measure of defense against the pos-
sibility of being killed by a stepparent.
Selection also is hypothesized to have favored adaptations to guide the behav-
ior of children living with a stepparent. Stepchildren should take steps to minimize
their costliness to their stepparent, such as keeping a low profile and demanding
few resources. Stepchildren should also recognize opportunities to make them-
selves valuable to their stepparent, such as contributing to the care of children
that result from the relationship between their genetic parent and stepparent. The
best strategy of stepchildren who feel their life is in danger, however, may be to
sabotage the relationship between their genetic parent and stepparent. This may
involve stepchildren inflicting costs on their stepparents in an attempt to get the
stepparents to abandon the romantic relationship. It may also involve stepchildren
inflicting costs on themselves to compel their genetic parent to curb investment
away from a new mateship and toward their children. Engaging in delinquent and
self-injurious behaviors may be strategies that stepchildren use to inflict costs on
themselves. Living in a stepfamily, as compared to living with two genetic parents,
more than doubles a child’s risk of engaging in juvenile delinquent behavior

(Coughlin & Vuchinich, 1996).
The presence of a stepparent is a good example of a recurrent context of in-
creased risk of homicide that may have selected for anti-homicide defenses in step-
children and their kin. These adaptations are hypothesized to become activated in
stepchildren but remain dormant in children who reside with both of their genetic
parents. We propose that specialized adaptations to defend against homicide exist
for all contextual domains where there was a recurrent risk of being killed. Many
situations, however, do not provide complete information about the probability that
a person may fall victim to homicide. Because being killed is so costly, it is likely that
selection fashioned adaptively patterned biases that lead people to systematically
overestimate the likelihood that they will be killed in conditions of uncertainty.
Managing Errors to Avoid Being Killed
Goleman (1995) argued that most of what people worry about has a low probability
of happening, suggesting that people are wasting their time by ruminating on such
issues. However, a cognitive system that “irrationally” overestimated the likelihood
of violence, increasing the probability of avoiding attackers, would be favored by
selection over an unbiased, “rational” cognitive system that led an individual to be
more likely to incur the heavy costs of being victimized. Because many inferences
about whether one will be targeted by a killer are clouded by uncertainty, contexts
of homicide can be considered compatible with the logic of error management the-
ory (Haselton, 2003; Haselton & Buss, 2000). In situations involving uncertainty,
making an erroneous inference about the intentions of others can carry high fitness
costs. There are two types of errors one can make when inferring the intentions of
others: inferring an intention that is not present or inferring the absence of an inten-
tion that is present. In the case of avoiding homicide, selection pressure would have
shaped cognitive biases that lead people to overinfer homicidal intent in others. It
would be better, on average, to infer that someone might want to kill you when he
really does not rather than to infer that someone does not want to kill you when he
actually does. In this way, people would avoid making the more costly of the two
errors. In sum, a design feature of the psychology of evolved homicide defenses is a
cognitive bias that leads people to systematically overinfer homicidal intent in others
who occupy adaptive problem contexts historically solvable by homicide.
The amount of uncertainty surrounding a potentially high-cost situation is also
likely to have an effect. Imagine a man walking home from a bar late on a rainy
night. He decides to take a shortcut through a dark alley to shorten the distance he
must walk in the rain. As he is walking, he notices another man in the alley and im-
mediately identifies the man as his brother. Assuming the two had a good relation-
ship, there would be little reason for the man to infer that his brother might want to
kill him. Indeed, no fears of being killed should be triggered in this situation. Now
imagine that the same man takes a shortcut through an alley and sees another man
whom he does not know. Greater uncertainty about the intentions of the unknown
man, in addition to the other features of the context, may lead to an overinference of
the likelihood that this man might intend to harm or kill. In conditions of uncertainty
about the identity of another person, in vague situations, and in the absence of in-
formation to the contrary, the safer error would be to overinfer a conspecific’s hostile
intentions. In fact, the safest error would be to assume that the other person intended
to kill you. Selection is hypothesized to have shaped adaptations to defend against the
most costly possibility first. When the chaos of environmental cues creates uncer-
tainty, selection should mold psychological design to assume that the worst possible
fitness event is going to occur, facilitating the avoidance of fitness costs. The strate-
gies that people employ to defend against homicide (e.g., avoiding contexts solvable
by killing, fleeing from attackers, or killing one’s attacker) would also be effective in
defending against a number of nonlethal, cost-inflicting strategies, such as assault,
robbery, and rape, possibly providing additional selection pressure for the evolution
of victim defenses against the cost-inflicting strategies of others.
In sum, we propose that adaptations to minimize costly errors evolved in the form
of cognitive biases that overestimate the likelihood that another individual intends to
inflict costs proportional to the uncertainty surrounding the individual’s intentions
and the context. The bias toward inferring that another individual plans to inflict
costs should increase as uncertainty about the individual’s intentions and the con-
text increases. This is not to say that such an error management bias will be applied
equally to all, different individuals. The bias should be proportional to the ancestral
threat that different individuals posed. It should be especially strong for those who
posed the greatest threat, such as members of out-groups and young adult males,
and less strong or absent for others (e.g., infants, young children, the elderly).
There is evidence that people’s perceptions are biased in the direction predicted
by error management theory (Haselton & Buss, 2000). Experiments using schematic
facial stimuli demonstrate that different facial expressions are not processed the same
way (Öhman, Lundqvist, & Esteves, 2001). Participants in this research viewed stim-
uli of threatening and friendly faces that were constructed from identical physical
features. The threatening face was identified more quickly than the happy face from
among neutral distracters. Additionally, faces with V-shaped eyebrows of a schematic
angry facial display were more quickly and accurately identified than were faces
with inverted V-shaped eyebrows (friendly faces) among both neutral and emotional
distracters. These results are consistent with a perceptual bias as predicted by error
management theory that leads individuals to be especially sensitive to the presence
of potentially hostile conspecifics. Natural selection would have favored a greater sen-
sitivity to angry faces over friendly faces, as those with hostile intentions would have
posed an adaptive problem often requiring immediate action to avoid incurring the
potentially heavy costs resulting from being a victim of exploitation or murder.
Despite the proposed evolved defenses against being killed discussed previously,
many people still willingly enter into situations that could get them killed. People
have extramarital affairs. People derogate others to ascend status hierarchies. People
poach the material and mating resources of others. Why would people risk engaging
in such high risk strategies?
Secrecy
The answer may lie in the use of secrecy as a defense against being killed. People be-
come homicidal only if they are aware that someone else is inflicting heavy costs on
them or great benefits will flow to them as a result of the kill. Ignorance can provide
them bliss and provide those who sneak behind their backs some measure of pro-
tection from being killed. A sexual relationship carried on behind the back of one’s
partner, for example, has the potential to confer fitness benefits to men in the form
of more offspring. It can confer benefits to women as well, such as access to superior
or different genes and access to additional resources from an affair partner (Greiling
& Buss, 2000). Selection is hypothesized to have favored the use of secrecy to de-
fend against the costs of discovered infidelity, which includes being killed by a jealous
partner. This logic also applies to other behaviors that benefit one individual at a cost
to another. In the case of sexual infidelity, there is a clear pattern in the risks of being
killed. Men are more likely than women to kill their partner for a sexual infidelity. As
a result, selection pressures are proposed to have been stronger on women to adopt
clandestine tactics to conduct their affairs than it was on men. Women may have
evolved to be more motivated to hide, and better at hiding, their infidelities from their
partners than men. This may help to explain why men indicate a greater amount of
uncertainty about whether their romantic partner is having an affair than women
do (Buss, 2000): men encounter fewer cues to their partner’s infidelity. Clandes-
tine strategies, however, are not always successful. Sometimes men discover their
partner’s infidelity. As homicide statistics demonstrate (Buss, 2005; Daly & Wilson,
1988; Ghiglieri, 1999), perhaps the most dangerous human a woman will encoun-
ter in her lifetime is her romantic partner.
Killing in Self-defense: Preemptive Homicide

to Prevent Being Killed
In a review of 223 appellate opinions of the homicide cases of battered women

in Pennsylvania, 75% of the homicides occurred while the woman was being as-
saulted by her romantic partner (Maguigan, 1991). In a study of mate homicides
in North Carolina between 1991 and 1993, violence perpetrated by men preceded
75% of cases in which women killed their romantic partners. In contrast, there is no
evidence that violence perpetrated by women preceded any of the homicides com-
mitted by men (Smith, Moracco, & Butts, 1998). It can be argued that the majority
of women who kill their romantic partners do so in self-defense. The example pro-
vided by these female-perpetrated mate homicides is illustrative of the ultimate anti-
homicide defense: killing an attacker before the attacker kills you.
We propose that the costs of being murdered were substantial enough to select
for adaptations designed to eliminate the threat of homicidal conspecifics by killing
them. Selection for homicide defenses was unlike selection for the psychology of ho-
micide. Whereas adaptations for homicide are argued to have been selected to favor
killing only when available nonlethal alternatives delivering equivalent benefits were
exhausted, selection likely favored psychological design to prefer homicide as a strat-
egy of self-defense in some face-to-face confrontations with a would-be killer. Killing
someone to prevent him or her from killing you would have had distinct evolutionary
advantages over strategies of nonlethal violence. By killing a homicidal conspecific,
you eliminate any future threat the person may pose. Whereas an injured rival can
recuperate and attempt to kill you again, a dead rival cannot. By killing your would-
be killer, you also demonstrate a willingness and ability to kill, sending a powerful
signal to others that attempts on your life will be met with the ultimate cost.
Most legal systems do not treat homicides committed in self-defense the same as
other homicides. The law considers killing in self-defense to be a form of justifiable
homicide if the person who kills “reasonably believes that killing is a necessary re-
sponse to a physical attack that is likely to cause serious injury or death” (Costanzo,
2004, p. 83). In the evolutionary history of adaptations to produce preemptive homi-
cides, however, the management of errors in conditions of uncertainty would have
played a pivotal role in determining what a person reasonably believes. Individuals in
the past who erred on the side of preemptively killing those whom they perceived to be
a credible threat to their life or the lives of their genetic kin would have had an advan-
tage over individuals who erred in the opposite direction. The likely consequence is the
overestimation of the threat that some conspecifics pose and the preemptive killing of
some people who were not pursuing a strategy of lethal aggression. In the calculus of
natural selection, however, it is better to be in error and alive than risk being killed.
Conclusion
The evolution of adaptations to inflict costs created selection pressures for the coevo-
lution of victim adaptations to avoid or prevent incurring the costs. These coevolved
victim adaptations in turn created selection pressure for the evolution of refined ad-
aptations and new adaptations for cost-infliction, setting up antagonistic, coevolu-
tionary arms races between strategies to inflict costs and victim strategies to defend
against them. Coevolutionary arms races can be extremely powerful. They can exert
selection pressure on numerous physiological and psychological systems simulta-
neously, leading to rapid evolutionary change and great complexity of adaptive de-
sign. Adaptations for homicide and adaptations to defend against being killed are
hypothesized to be the results of such an antagonistic coevolutionary arms race. The
costs to genetic fitness of being killed are among the greatest an individual can en-
dure at the hands of a conspecific. These tremendous costs are proposed to have cre-
ated unique and powerful selection pressures for the evolution of victim adaptations
to defend against being killed. The available evidence is consistent with the theory
that coevolved adaptations for homicide and victim defenses against homicide guide
human behavior when we face contexts ancestrally solvable through the use of le-
thal aggression. We are likely the only species that possess psychological adaptations
that function specifically to kill humans.
ACKNOWLEDGMENT
Portions of this chapter are based, in part, on the following works: Duntley, J. D. (2005).
Adaptations to dangers from humans. In D. Buss (Ed.), The handbook of evolutionary psychology
(pp. 224–249). New York: Wiley; and Duntley, J. D., & Shackelford, T. K. (2008). Adaptations
to avoid victimization. Manuscript under editorial review.
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12
The Evolution of a Sense of Justice
DENNIS L. KREBS
Everyone possesses a sense of justice, however misguided it may be. How do

people acquire this sense? Where does it come from? In this chapter, I argue that
to account for the acquisition of a sense of justice, we must identify the mental
mechanisms that produce it and explain how they originated and became refined
in the course of human evolution. Explaining how a sense of justice originated in
the human species helps us understand what it is, what it is for, how it is designed,
what activates it, and why it sometimes fails to give rise to fair judgments and
behaviors.
A Working Definition of a “Sense of Justice”
A sense of justice consists of thoughts and feelings about what is fair and unfair
and what people deserve from and owe others (rights and duties). When we think
of justice, we think of balanced scales. In Nicomachean Ethics, Aristotle distin-
guished three forms of justice. The first pertains to how resources should be
distributed (distributive justice)—for example, in terms of principles of equality,
equity, desert, and merit. The second pertains to agreements between people—
promises, commitments, and other kinds of social contracts (commutative
justice). The final type pertains to the righting of wrongs (corrective justice). It
includes ideas such as forgiveness and a bunch of “r” words—revenge, repara-
tion, restitution, and retribution (“getting even”). Overriding all of these forms
of justice is procedural justice. To make fair decisions, people must use fair and im-
partial procedures such as the Golden Rule, balanced discussion, or democratic
decision making.
230
The Evolution of a Sense of Justice 231
Psychological Accounts of the Origin of a Sense of Justice
If you ask people how they acquired their sense of justice, most people, from the West-
ern world at least, would advance a social learning account. They would say that they
acquired a sense of justice from their parents and other mentors, who taught them
to behave fairly, to share, to take turns, to keep their promises, and so on. Although it
would be foolish to deny that social learning plays a role in the acquisition of a sense of
justice, more is involved. If children internalized their parents’ ideas about fairness, then
children would possess the same ideas their parents do, but they do not. Children argue
with their parents. They have minds of their own. They are able to think for themselves.
And the ways in which they think about fairness changes as they develop. Cognitive-
developmental theorists such as Kohlberg (1984) and Piaget (1932) have argued that
children derive their conceptions of justice from structures of moral reasoning.
Like social learning, reasoning plays a role in determining people’s sense of jus-
tice. However, like social learning, it does not account for all aspects of this sense. As
demonstrated by Haidt (2001), people sometimes simply feel that a behavior is fair
or unfair, right or wrong, without thinking about it or engaging in moral reasoning.
If someone cheats you or breaks a promise to you, you may experience an immedi-
ate sense of righteous indignation without engaging in rational deliberation.
The goal of virtually all psychological research on a sense of justice is to deci-
pher the design of the proximate mechanisms that produce it. Theoretical differences
arise with respect to the types of mechanisms responsible for producing it (e.g., social
learning versus reasoning versus affective mechanisms), the ways in which they are
designed (e.g., whether people possess one overriding structure of moral reasoning or
a bunch of different, domain-specific structures designed to deal with different aspects
of justice), and the ways in which they interact (e.g., whether reason structures affec-
tive reactions, or whether affective reactions structure reason). Although adherents
of different psychological approaches each tend to assume that their approach offers
a full account of the acquisition of a sense of justice, it is clear that each approach
accounts for only part of the process. A sense of justice stems from a system of mecha-
nisms. Sometimes people derive conceptions of justice from one mechanism, some-
times from another. Sometimes more than one mechanism is activated, and when this
occurs, the activated mechanisms may work in concert to support the same decision,
or they may engender internal conflict. What is needed is an overarching framework
that accounts for the origin of this system and integrates its components in meaning-
ful ways. The thesis of this chapter is that evolutionary theory fills this bill.
An Overview
The mechanisms that produce a sense of justice did not emerge in the human spe-
cies one sunny morning in full-blown glory. They emerged slowly over eons, through
the modification of more primitive mechanisms. Although this was a continuous
process, it is helpful heuristically to break it down into overlapping phases. I will sug-
gest that the first phase in the evolution of a sense of justice involved the evolution
of cooperative behavioral dispositions and the affective reactions that support them.
Precursors of this sense can be seen in chimpanzees and other primates. In the sec-
ond phase, this primitive sense became refined and elaborated in the context of stra-
tegic interactions among members of groups motivated to induce one another to
behave in cooperative ways. The acquisition of the capacity for symbolic language,
perspective-taking, and sophisticated forms of intelligence played important roles
in this process, which gave rise to moral judgments and moral norms. In the final
phase, humans acquired the capacity to imagine ideal social systems; to reflect on
moral issues; to figure out how, in principle, to solve complex moral problems; and to
develop ideal conceptions of justice.
The Evolution of Cooperation
From an evolutionary perspective, the key to understanding the origin of a sense of

justice lies in identifying the adaptive functions it evolved to serve. I will argue that
the overarching function of a sense of justice is to induce members of groups to up-
hold fitness-enhancing forms of cooperation. To understand the emergence of the
mechanisms that produce a sense of justice, we must first understand the emergence
of the mechanisms that induce animals to cooperate.
There is tremendous adaptive potential in cooperation. In conducive contexts,
two or more animals that work together and exchange goods and services can en-
hance their fitness much more effectively than they could by going it on their own.
This does not, however, guarantee the evolution of cooperative dispositions. All kinds
of traits and behaviors could enhance animals’ fitness better than those they already
possess. For cooperative dispositions to evolve, individuals must inherit genes that
guide the creation of mechanisms that dispose them to behave in cooperative ways,
and these mechanisms must pay off better genetically than competing mechanisms
such as those that dispose them to behave in selfish ways.
The Fundamental Social Dilemma

Assume that, originally, animals were disposed to help only themselves. It is relatively
easy to account for the evolution of mutualistic behaviors such as group hunting and
group defense, because the animals that engage in such behaviors could be coordi-
nating their efforts to maximize their own biological gains, helping others only in-
cidentally. In contrast, it is considerably more difficult to account for the selection of
mechanisms that dispose animals to engage in equitable exchanges. As expressed by
the philosopher Rawls (1999) in the opening pages of A Theory of Justice,
Although a society is a cooperative venture for mutual advantage, it is typically marked
by a conflict as well as by an identity of interests. There is an identity of interests since
social cooperation makes possible a better life for all than any would have if each were
to live solely by his own efforts. There is a conflict of interests since persons are not in-
different as to how the greater benefits produced by their collaboration are distributed,
for in order to pursue their ends they each prefer a larger to a lesser share. (Rawls,
1999, p. 4)
Modeled in evolutionary terms, assume that members of a group inherit genes that
dispose them to adopt one of two strategies—either to behave fairly (i.e., to cooper-
ate) or to behave selfishly (i.e., to cheat). If all members of a group inherited genes
that disposed them to cooperate, everyone could obtain more for himself or herself
though gains in trade than he or she could by failing to cooperate, and the group
could prevail in competitions against less cooperative groups. Cooperation could pro-
duce a utopia for all. The problem is, if other members of one’s group behave coop-
eratively, each individual can come out ahead by doing less than his or her share and
taking more. If those who are disposed to behave selfishly contribute more replicas of
their genes to future generations than those who are disposed to behave fairly, selfish
dispositions will be selected and evolve. Ironically, however, as the number of selfish
members of a group increases, there are fewer and fewer cooperative individuals to
exploit, jeopardizing the system of cooperation and forcing selfish individuals to in-
teract with one another, to their mutual detriment.
Theoretical Resolutions of the Fundamental Social Dilemma:

The Selection of Cooperative Strategies
Game theorists have created computerized simulations of evolution in which they
have pitted cooperative strategies against selfish strategies. These theorists have
found that certain conditionally cooperative strategies, such as various forms of
tit-for-tat (that is to say, strategies based on the decision rule, “make an initial co-
operative overture, then copy the response of your partner,”) and variations such
as tit-for-two-tats, are equipped to defeat unconditionally selfish strategies and
evolve in favorable conditions. The cooperative strategies gain their power either
by reducing the costs and increasing the benefits of behaving fairly or by increas-
ing the costs and reducing the benefits of behaving unfairly. The genetic costs of
contributing one’s share can be reduced by engaging in cooperative exchanges
with those who share one’s genes, by selectively engaging in exchanges with other
cooperators, and by reaping indirect benefits from acquiring a reputation as a co-
operator. The net genetic costs of failing to contribute one’s share can be increased
by diminishing the probability that recipients or others will interact with those
who behave selfishly and by increasing the probability that selfish individuals will
be punished—either by their interaction partners or by other members of their
group. Accounting for the evolution of dispositions to punish third parties is tricky,
because if we assume that it is costly to inflict punishments, those who refused to
accept responsibility for administering punishments would fare better than those
who accepted responsibility.
Reciprocity in Nonhuman Animals

The most appropriate place to look for precursors of the forms of cooperation prac-
ticed by humans is in other primates. Studies by de Waal and others (see Kappeler &
Schaik, 2006, for a review) have established that chimpanzees engage in calculated
forms of delayed reciprocity in which they remember who has helped them, track
credits and debts to particular partners, and repay them either in kind or in some
other currency. For example, chimpanzees are more likely to assist those who have
assisted them in agonistic exchanges with others (“one good turn deserves another”)
and to aggress against those who have sided with others against them (“an eye for an
eye”) (De Waal & Luttrell, 1988). In addition, chimpanzees are more likely to share
food with those who have groomed them earlier in the day. If we accept the idea that
chimpanzees inherit mechanisms that dispose them to reciprocate and engage in
other forms of cooperation, we can conclude that they possess mental mechanisms
that enable them to solve fundamental social dilemmas and induce them to engage
in primitive forms of fairness.
Games that Primates Play

The social lives of chimpanzees and members of other social species are dynamic.
Members of primate groups engage in ongoing contests in which they attempt to
induce one another to behave in ways that benefit them by invoking tactics such
as begging, offering, enticing, screaming, threatening, attacking, and shunning. De
Waal (1991) has suggested that the “active reinforcement of others” (p. 338) is re-
sponsible for the emergence of prescriptive rules in groups of chimpanzees.
In their essence, the games that humans play when they are in small groups are
the same as the games that other primates play. Like other primates, humans engage
in strategic interactions and attempt to press one another’s prosocial buttons. They
use physical, material, and social rewards and punishments to induce others to treat
them right. In Darwin’s (1874) words, “man [is] influenced in the highest degree by
the wishes, approbation, and blame of his fellow-men, as expressed by their gestures
and language” (p. 106).
The Origin of a Sense of Justice
Trivers (1985) suggested that “a sense of fairness has evolved in the human spe-
cies as the standard against which to measure the behavior of other people, so as to
guard against cheating in reciprocal relationships” (p. 388). According to Trivers
(2006), “such cheating is expected to generate strong emotional reactions, because
unfair arrangements, repeated often, may exact a very strong cost in inclusive fit-
ness” (p. 77). In a similar vein, de Waal and Brosnan (2006) have suggested that
“the squaring of accounts in the negative domain . . . may represent a precursor to
human justice, since justice can be viewed as a transformation of the urge for re-
venge, euphemized as retribution, in order to control and regulate behavior” (p. 88).
On the positive side, “the memory of a received service, such as grooming, induces a
positive attitude toward the same individual, a psychological mechanism described
as ‘gratitude’ by Trivers (1971)” (p. 93).
The affective precursors to a sense of justice discussed by Trivers and de Waal
stem primarily from the reactions of animals to the ways in which they are treated by
members of their groups. There is, however, more to humans’ sense of justice than
these reactions. Humans also experience emotional reactions to the ways in which
they and others treat third parties.
Affective Reactions to Third-Party Injustice

Although other primates display negative reactions to members of their troupes who
violate prosocial norms and take measures to punish them (Boehm, 2000), humans
may be the only species that is disposed to punish free riders and those who behave
unfairly toward third parties. Summarizing the findings from several studies, Gachter
and Herrmann (2006) conclude:
Overall, the results suggest that free riding causes negative emotions . . . [that are] con-
sistent with the hypothesis that emotions trigger punishment. . . . [T]he majority of pun-
ishments are executed by above-average contributors and imposed on below-average
contributors. . . . [P]unishment increases with the deviation of the free rider from other
members’ average contribution. . . . [E]vidence from neuroscientific experiments supports
the interpretation that emotions trigger punishment. (p. 297)
Although evolutionary theorists agree that humans are disposed to punish third-
party cheaters, they do not agree about how the mechanisms that give rise to these
dispositions evolved. On one side, mainstream evolutionary theorists argue that the
disposition to punish free riders evolved through standard forms of selection (kin se-
lection, reciprocal altruism, indirect reciprocity, and costly signaling). For example,
Trivers (2006) has suggested that because the groups formed by early humans con-
sisted mainly of kin, we would expect the mechanisms that dispose contemporary
humans to punish third parties to “misfire” by being activated by members of groups
who are not kin. Trivers’s account implies that “the human brain applies ancient
cooperative heuristics even in modern environments” (Gachter & Herrmann, 2005).
Other mainstream evolutionary theorists such as Alexander (1987) and Nowak and
Sigmund (1998) have argued that the disposition to punish third parties could have
been reinforced by the fitness-enhancing gains of an enhanced social image or a rep-
utation for cooperation. On the other side, theorists such as Fehr and Gächter (2002)
and Gintis, Bowles, Boyd, and Fehr (2003) have argued that biological evolution is
not, by itself, equipped to account for the disposition to punish free riders in one-shot
games among anonymous players and that this disposition could have evolved only
through gene-culture coevolution. The theoretical differences between theorists who
have advanced exclusively individual-level selection models and theorists who have
advanced coevolutionary models are significant psychologically mainly with respect
to their potential to produce hypotheses about how the mechanisms in question are
designed.
Affective Reactions to Treating Others Fairly and Unfairly

When we attribute a sense of justice to people, we imply that they possess stan-
dards of fairness that they apply to themselves as well as to others. If the function of
negative reactions to unfair behaviors committed by others is to motivate people to
uphold systems of cooperation by punishing cheaters, we might also expect people
to feel bad when they cheat and to be inclined to punish themselves. In fact, people
often do feel bad when they cheat others, but it is unclear whether such negative
reactions stem from the same mechanisms as their reactions to the transgressions
of others.
There is an important difference between inducing oneself to cooperate and in-
ducing others to cooperate. As discussed, in most contexts people are able to maxi-
mize their benefits by inducing others to do their share, or more than their share,
while doing less than their share themselves. From an adaptive perspective, we would
not expect people to be unconditionally motivated to behave fairly or to be naturally
inclined to pass judgment on themselves in an impartial way. Rather, we would ex-
pect the mechanisms that guide decisions about fairness to be calibrated in ways
that maximized the genetic benefits to early humans, inducing individuals to feel
inclined to behave only as fairly as they needed to maximize their benefits from social
exchanges. In support of these expectations, there is a great deal of evidence that
people are inclined to react more strongly to being treated unfairly by others than to
treating others unfairly, to hold others to higher standard of fairness than they hold
themselves, and to reckon costs and benefits for themselves and others in different
ways (Greenberg & Cohen, 1982). As expressed by Trivers (2006), “[A]n attachment
to fairness or justice is self-interested and we repeatedly see in life . . . that victims of
injustice feel the pain more strongly than do disinterested bystanders and far more
than do the perpetrators” (p. 77).
People’s negative reactions to others’ injustices usually involve anger, which
seems to emerge automatically. In contrast, people’s negative reactions to their own
injustices may be acquired more indirectly, through social learning. As emphasized
by Darwin (1874), humans are highly motivated to seek the approval and avoid the
disapproval of members of their groups. Contemporary learning theorists such as
Aronfreed (1968) have adduced evidence that children acquire negative reactions to
their own transgressions through classical and instrumental conditioning. Children
come to feel good about behaving fairly and bad about cheating because they are re-
warded when they behave fairly and punished when they behave unfairly. Although
such inputs structure children’s early conceptions of right and wrong, they do not
account for a fully developed sense of justice, as I will explain.
The Expansion and Refinement of Cooperative Systems

in the Human Species
Humans engage in concrete forms of reciprocity and feel angry when others cheat
them in much the same way as chimpanzees do. However, in addition, humans en-
gage in more complex forms of social exchange. They give to others over long peri-
ods of time before receiving any returns; they invest in long-term relationships; they
trade across widely diverse domains (often using money as a common medium); they
reckon equity in highly refined ways; they engage in indirect forms of reciprocity;
they create rules and formalize systems of sanctions that uphold cooperative sys-
tems; they coordinate their efforts on a massive scale to accomplish such tasks such
as constructing skyscrapers and building bridges.
The unique forms of cooperation practiced by modern humans became possible
when early humans acquired the intellectual and linguistic abilities necessary to
create them and uphold them. As expressed by Williams (1989), “the unparalleled
human capability for symbolic communication has an incidental consequence of
special importance for ethics. In biological usage, communication is nearly synony-
mous with attempted manipulation. It is a low-cost way of getting someone else to
behave in a way favorable to oneself ” (p. 211). Coupled with intelligence, symbolic
language would have enabled early humans to translate their affective reactions to
the behavior of members of their groups into words and communicate such reac-
tions to those who performed the behaviors and to third parties. Not only would it
have enabled them to express their immediate approval and disapproval with words
such as “good” and “bad,” but it would also have enabled them to pass judgment
on events that occurred in the past and to make judgments about events that could
occur in the future. It would have enabled them to transform primitive threats and
promises into long-term social contracts and commitments (Nesse, 2001) and to ver-
balize disapproval when others violated implicit social contracts such as those that
govern monogamous marriages. It would have enabled them to enhance or diminish
others’ reputations through gossip (Alexander, 1987; Dunbar, 1996) and to buttress
their judgments with reasons, explanations, and justifications designed to increase
their persuasive power.
The Expansion and Refinement of a Sense of Justice
Intelligence and language are two-edged swords. On the one hand, they enable hu-
mans to create and uphold significantly more complex forms of cooperation than
those practiced by any other species. On the other hand, they enable humans to en-
gage in significantly more complex forms of cheating. Although we would expect
people to be naturally inclined to make self-serving moral judgments, the process of
strategic interaction is equipped to counteract such biases. Because recipients are
unreceptive to judgments that exhort them to behave in ways that do not advance
their interests, blatantly self-serving judgments do not work, and because they do
not work, people are disinclined to make them.
Moral Argumentation
Language and intelligence endow humans with the capacity to resolve their conflicts
of interest through negotiation and discussion. Many theorists have focused on the
significance of moral argumentation in the production of standards of justice (e.g.,
Damon & Hart, 1992; Habermas, 1993; Piaget, 1932). When people engage in moral
argumentation, they may attempt to push one another’s emotional buttons (Haidt,
2001), or they may appeal to one another’s rational faculties (Saltzstein & Kasachkoff,
2004). As explained by the philosopher Singer (1981), publicly expressed rational ar-
guments tend to generate universal and impartial standards: “[I]f I claim that what
I do is right, while what you do is wrong, I must give some reason other than the fact
that my action benefits me (or my kin, or my village) while your action benefits you (or
your kin or your village)” (p. 118). When people use reason and logical consistency as
weapons in moral arguments, they often end up hoist on their own petards.
The Evolution of Rules and Justice Norms
The process of strategic interaction and the adaptive value of resolving conflicts of
interest through moral argumentation have implications for the evolution of rules of
conduct and universal norms of justice. Members of groups make rules to formalize
their agreements about how they should be treated by others, and they invoke sanc-
tions to induce others to uphold the rules. What goes around comes around (Alexander,
1987), such that the rules that members of groups invent to control the behavior of
others end up controlling their behavior. Inasmuch as recipients are more receptive to
some moral prescriptions than to others, recipients serve as agents of selection, deter-
mining which prescriptions succeed, get repeated, and develop into rules and moral
norms. We would expect people to be particularly receptive to moral judgments and
rules that prescribe fitness-enhancing forms of cooperation and to judgments that
enable them to resolve conflicts of interest in mutually beneficial ways. Consistent
with these expectations, there is evidence that judgments and rules that uphold fair,
balanced, and reversible solutions to social conflicts—such as those prescribed by the
norm of reciprocity and the Golden Rule—constitute universal moral norms (Brown,
1991; Gouldner, 1960; Sober & Wilson, 1998; Wright, 1994).
Clearly, however, not all moral rules and norms are fair or rational. Following
Aristotle, Darwin (1874) distinguished between two types of rules, akin to culturally
universal and culturally relative moral norms. He suggested the following:
The higher [moral rules] are founded on the social instincts, and relate to the welfare
of others. They are supported by the approbation of our fellowman and by reason. The
lower rules . . . arise from public opinion, matured by experience and cultivation . . . [and
may lead to] the strangest customs and superstitions, in complete opposition to the true
welfare and happiness of mankind. (p. 118)
Earlier I discussed differences between negative affective reactions to injustices com-

mitted by others and negative reactions to injustices committed by oneself. In the
same vein, there is a significant difference between believing that others should up-
hold standards of justice and believing that one is obliged to uphold them. People
could espouse norms of justice in order to manipulate others into behaving in coop-
erative ways without believing in the norms or incorporating them into their own
conceptions of justice. We would, however, expect the process of strategic interaction
to reduce the gap between conceptions of one’s own and others’ rights and duties.
To begin with, as emphasized by socialization theorists, people may be per-
suaded to accept as valid the standards preached by others. Evolutionary theory of-
fers a basis for predicting which, of the many ideas to which people are exposed, they
will be disposed to accept. It leads us to expect people to be most receptive to norms
and standards that have enhanced their fitness in the past and that they believe will
enhance their fitness in the future. Thus, for example, we would expect people to be
receptive to standards preached by those with a vested interest in their welfare and to
standards that are widely accepted by other members of their groups (see Richerson
& Boyd, 2005, for a more extended discussion of this issue).
In addition, preaching standards of justice to others may induce those who
preach them to accept them as their own. Believing in the validity of the prescrip-
tive judgments one makes may reap adaptive benefits by increasing their persuasive
power (Trivers, 1985). People may persuade themselves in the process of persuad-
ing others (Festinger, 1964). People may be inclined to believe moral judgments and
standards generated during moral negotiations because they actively participated
in generating them, because they are supported by others, because they are backed
up by reasons, and because they enable them to advance their own interests in op-
timal ways.
The Origin of Conscience

Most people locate their sense of justice in a mental mechanism they call their con-
science. The conditioned reactions to one’s transgressions discussed earlier may form
the core of conscience. Animals such as dogs appear to display affective reactions
akin to guilt when they anticipate punishment for their transgressions (Aronfreed,
1968). Humans differ from other animals, however, in their ability to construct
portable cognitive representations of others and store them in their minds, to view
events from others’ perspectives, and to imagine how others will respond to their be-
havior (Selman, 1980). In their imagination, people experience others as observing
them when they are in private and passing judgment on their behavior (Aronfreed,
1968; Higgins, 1987).
Ironically, perhaps, the mechanisms that enable people to take the perspective of
others may have evolved as tools designed to improve early humans’ ability to ma-
nipulate others in the context of strategic interactions. There is tremendous adaptive
potential in the ability to anticipate the moves of others in social games—what they
are thinking; what they intend to do; whether they will cooperate, pay one back,
detect one’s deception; and so on. To accomplish this, people internalize mental rep-
resentations of others and view events, including those which they themselves are
directly involved in, from their perspectives. After people internalize mental images
of others, they may experience these images as approving and disapproving of the
things they do in private, and this may be experienced as a “voice of conscience.”
As children’s perspective-taking abilities develop, their cognitive representations
of others become increasingly abstract, integrated, and general (Selman, 1980). As
expressed by Wilson (1993), “At first we judge others; we then begin to judge our-
selves as we think others judge us; finally we judge ourselves as an impartial, disinter-
ested third party might” (p. 33). We would expect highly developed perspective-taking
processes to give rise to fairer decisions than more primitive perspective-taking pro-
cesses.
To summarize, an evolutionary analysis suggests that conscience is a mental
mechanism that originated as a tool in strategic interaction. Conscience consists of
internalized images of others that enable people to predict how others will react to
their behaviors. In imagining the negative reactions of others, people experience an
anticipatory fear or embarrassment, which they experience as a sense of guilt or
shame. As people internalize an increasingly large number of cognitive representa-
tions and as they integrate them in their minds, the perspective from which they
judge themselves becomes increasingly abstract and impartial.
Reframing Traditional Psychological Accounts

of the Acquisition of a Sense of Justice
An evolutionary framework supplies a basis for reconceptualizing psychological

models of the acquisition of a sense of justice in ways that integrate their insights
and redress their limitations. The family contexts in which parents teach children
to behave fairly are microcosms of larger social groups. Members of families face
fundamental social dilemmas. Because parents and children need each other to
propagate their genes, it is in their genetic interest to help one another and uphold fa-
milial systems of cooperation. However, it may be in each member’s interest to favor
himself or herself and those with whom he or she shares the largest complement of
genes (Trivers, 1974). Conflicts of interest precipitate strategic interactions in which
members of families attempt to induce one another to behave in ways that maximize
their genetic benefits. The ways in which members of families resolve their conflicts
of interest affect the ways in which their conceptions of justice are structured and
calibrated.
Evolutionary theory leads us to expect the mechanisms that regulate strategic

interactions between parents and children to be designed in fitness-enhancing ways.
It follows that we would not expect children to conform to their parents’ injunctions
indiscriminately or docilely. We would expect children to resist injunctions that run
contrary to their interests and actively attempt to manipulate and control other mem-
bers of their families. Contemporary accounts of conscience that view the child “as
an agent in moral socialization who actively processes parental moral messages” and
engages in “discourse” with his or her parents (Kochanska & Aksan, 2004, p. 303)
fit comfortably in an evolutionary framework that emphasizes the role of strategic
interaction in the development of a sense of justice. From this perspective, the key to
instilling a balanced sense of justice in children lies in structuring their early interac-
tions in fair ways and inducing them to discover by their experience that it pays to
cooperate and treat others fairly.
An evolutionary analysis implies a different interpretation from that offered by
cognitive-developmental theorists of evidence that children acquire increasingly so-
phisticated structures of justice reasoning as they develop. The anthropologist Fiske
(1992) has amassed evidence that people from all cultures are innately disposed to
develop cognitive “schemata” that organize information about four types of social
relations—(1) affectionate relations among people who share social bonds, (2) hier-
archical relations among people who differ in social rank, (3) egalitarian exchanges
among equals, and (4) economic relations aimed at maximizing cost/benefit ratios
across different commodities. Chimpanzees possess the first three schemata; the
fourth appears to be unique to the human species (de Waal, 1996; Haslam, 1997).
Life history theory implies that the reason people are prone to invoke increas-
ingly sophisticated schemata and structures of moral reasoning as they develop is
that they need increasingly sophisticated schemata and standards of justice to solve
the increasingly complex and embedded social problems they encounter as they
progress through the life span. The reason young children view justice primarily
in terms of obedience to authority (Kohlberg, 1984) is that it is adaptive for young
children to subordinate themselves to older, wiser, and more powerful members of
their groups. The reason older children view justice primarily in terms of concrete
reciprocity is that reciprocity is a more adaptive strategy than obedience in egalitarian
relations among peers (Piaget, 1932). The reason young adults view justice primar-
ily in terms of principles that uphold long-term commitments, harmonious in-group
relations, and systems of indirect reciprocity is that these forms of cooperation are
best equipped to foster their interests (see Krebs, 2005a, 2005b, for elaborations
of these ideas). The sophisticated forms of justice reasoning that define Kohlberg’s
highest stages of moral development constitute creative ideas about how to resolve
conflicts of interest and reap the benefits of cooperation in optimal ways. From an
evolutionary perspective, cardinal moral principles such as “foster the greatest good
for the greatest number” equate to injunctions to foster one’s ultimate adaptive inter-
ests by upholding the standards, forms of conduct, and systems of cooperation that,
if adopted by everyone, would produce the greatest gains.
From a life history perspective, we would not expect new structures of justice
reasoning to “transform and displace” older structures, as Colby and Kohlberg
(1987) have hypothesized. We would expect people to acquire structures of justice
reasoning in an “additive-inclusive” way (Eisenberg, 1982; Levine, 1979), because
adults continue to experience the kinds of adaptive problems that early structures
evolved to solve. Adults may, for example, find themselves in subordinate positions in
which it would be adaptive for them to believe that they should show deference to au-
thority (Milgram, 1974). Viewed in this manner, the acquisition of a sense of justice
consists more in the acquisition of the flexibility necessary to solve social problems in
the most efficient, effective, and adaptive ways than in the ability to make highly so-
phisticated moral judgments in every context (Krebs & Denton, 2005). Although the
justifications that adults advance for obeying authority and engaging in tit-for-tat
exchanges may be more sophisticated than those advanced by children—for exam-
ple, because adults embed their justifications in principles that uphold more broadly
based systems of cooperation—their decisions may stem from essentially the same
affective and cognitive processes.
The Activation of Mechanisms that Produce a Sense of Justice
Given a suite of evolved mechanisms equipped to contribute to people’s sense of jus-

tice, the main task for those who seek to account for this phenomenon is to explain
how these mechanisms are activated and, if more than one is activated, how they
interact. Because complex forms of moral cognition are more costly than simpler
forms, we would expect people to be inclined to use simple, automatic forms as their
default (Gigerenzer, 2000; Gilovich, Griffin, & Kahneman, 2002). We would expect
affective reactions such as gratitude and righteous indignation to exert an immedi-
ate effect on people’s sense of justice (Haidt, 2001; Sunstein, 2005), and we would
not be surprised that people have difficulty justifying decisions derived in these ways
or, if called upon to justify them, that they offer plausible but invalid post hoc ratio-
nalizations (Haidt, 2001).
We also would expect people to invoke simple forms of justice reasoning to solve
simple, recurring social problems (Fiske, 1992), to make quick decisions in con-
texts in which the costs of deliberation are high, and to generate simple judgments
when such judgments constitute the most effective forms of persuasion and impres-
sion management (such as, for example, when they are directed toward children)
(Krebs & Janicki, 2004). We would expect people to adopt and to preach the moral
norms of their cultures without thinking much about them, as long as they worked
reasonably well, and to use mental shortcuts in contexts in which heuristics generate
acceptable moral decisions (Chaiken, 1987; Gigerenzer, 2000; Sunstein, 2005).
We would expect conceptions of justice to be customized to solve different kinds
of social problems and, therefore, for people to invoke different conceptions of jus-
tice in different domains, contexts, and conditions (Damon, 1980; Eisenberg, 1982;
Krebs & Denton, 2005; Krebs, Vermeulen, Carpendale, & Denton, 1991). We would
expect the cognitive apparatus that gives rise to conceptions of justice to be suscepti-
ble to framing, directional, motivational, self-serving, nepotistic, and group-serving
biases (Chaiken, Giner-Sorolla, & Chen, 1996; Krebs & Laird, 1998; Kunda, 2000;
Pyszczynski & Greenberg, 1987; Richerson & Boyd, 2005). And we would not be
surprised to find that people sometimes use justice reasoning for immoral purposes,
such as avoiding responsibility and justifying immoral acts (Bandura, 1991; Haidt,
2001).
There is nothing in this evolutionary analysis of the acquisition of a sense of
justice that is inconsistent with the idea that people have the capacity to derive con-
ceptions of justice from sophisticated forms of moral reasoning. As demonstrated
by cognitive-developmental theorists, most people do possess this capacity. How-
ever, an evolutionary framework induces us to ask how often, and in what contexts,
people invoke this tool rather than other tools in their moral-decision-making tool
boxes. We would expect people to invoke sophisticated forms of moral reasoning to
derive decisions about justice when they work better than alternative methods and
when the biological benefits from invoking them outweigh the costs. For example,
we would expect people to invoke sophisticated forms of moral reasoning to resolve
conflicts among moral intuitions and moral norms (Haidt, 2001) and the rights and
duties of people participating in embedded systems of cooperation (Kohlberg, 1984).
We would expect people to engage in reflective moral reasoning when they possess
ample processing capacity, when they are challenged (e.g., in moral argumentation),
when they have time to deliberate, when the costs of deliberation are low, when the
benefits of deliberation are high, when they are motivated to be accurate, when au-
diences are impressed by sophisticated moral judgments, and so on. Note that these
conditions are characteristic of those in which cognitive-developmental theorists as-
sess moral reasoning.
Conclusion
To understand how people acquire a sense of justice, we must understand why peo-
ple need one and what goals it helps them to achieve. The mechanisms that give rise
to a sense of justice evolved to help early humans maximize their gains from coopera-
tive social interactions. A sense of justice induces members of groups to distribute
resources in fair ways (distributive justice), to honor the commitments they make to
others (commutative justice), to punish cheaters (corrective justice), and to develop
effective ways of resolving conflicts of interest and making fair decisions (procedural
justice).
Contemporary humans inherit primitive predispositions to react positively to
being treated fairly and negatively to being treated unfairly, to pass judgment on those
who treat others fairly or unfairly, and to feel obliged to pay others back. This core is
refined and expanded during the process of strategic interaction in every generation
as people reward and punish one another for behaving in cooperative and uncooper-
ative ways, preach norms of fairness, negotiate mutually beneficial solutions to their
conflicts of interest, and attempt to create ever more effective systems of cooperation.
To achieve these goals, people use the tools with which they have been endowed by
natural selection, especially language, perspective-taking abilities, and social intel-
ligence. Although it is naïve to expect people to possess a universal sense of justice
that consistently disposes them to make fair and impartial decisions that jeopardize
their adaptive interests, it is realistic to expect people to be able to counteract one
another’s biases in ways that enable them to make fair decisions in contexts in which
such decisions advance everyone’s interests in optimal ways.
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PART SIX
APPLICATIONS AND FUTURE

DIRECTIONS
13
Reducing Crime Evolutionarily
LEE ELLIS
The evidence for genetic influences on criminality is no longer scientifically

questionable (reviewed by Anderson, 2007, pp. 95–124; Rose, 2000). This is not all
that surprising if one simply realizes that the brain—obviously a genetically influ-
enced biological organ—guides all behavior, including that which is socially defined
as criminal.
Even today, most criminologists only study social influences on criminal behav-
ior. That narrow-minded tradition should be in our past; the era of neurologically
specific theories has arrived (Rafter, 2006). These are theories that identify how the
brains of offenders differ on average from the brains of nonoffenders, whether the
causes are genetic or environmental. I have devised such a theory that I will briefly
present. Then, I will discuss a variety of ways in which it suggests criminal and delin-
quent behavior can be prevented and treated.
The Evolutionary Neuroandrogenic Theory
The theory is called the evolutionary neuroandrogenic (ENA) theory of criminal behav-
ior (for more details, see Ellis 2003, 2004, 2005). In a nutshell, ENA theory asserts
that genes have evolved ways of altering human brain functioning—particularly
among males—to exhibit increased criminality during their early reproductive
years. Theoretically, males as a whole have evolved greater tendencies than females
to victimize others. These male victimizing tendencies have been naturally selected
for because females have been favored for choosing mates who are reliable and ca-
pable provisioners of resources. Some details surrounding these basic arguments
appear below.
249
250 Applications and Future Directions
The Evolutionary Proposition

ENA theory asserts that both evolution and genetics are central to understanding
criminal/antisocial behavior, a view that is foreign to most criminologists. In par-
ticular, the theory asserts that natural selection has favored the evolution of a broad
class of behavioral tendencies known as competitive/victimizing behavior, one form
of which is behavior socially recognized as criminal.
The most fundamental premise of ENA theory derives from noting that among
mammals, only the females can gestate offspring. This simple fact drives the theory. As
the amount of time and energy required by females to produce an offspring increased—
which was certainly the case in our species—females came to exhibit a phenomenon
known as female choice, meaning that they exercise primary discretion in terms of mat-
ing decisions—becoming the mating gatekeepers, so to speak (Geary, 2000, p. 59).
According to ENA theory, female choice has had considerable influence on
the evolution of male behavior. In particular, it has led to the formation of social
hierarchies within which males compete for dominance (or status) throughout their
reproductive careers (Ellis, 2001). Males who exhibit at least modest success in sta-
tus attainment will pass more copies of their genes onto subsequent generations
than males who have little or no status attainment. The main reason for this is that
females use status (and status potential) as a criterion for choosing mates.
Females have been favored by natural selection for biasing their mate choice to-
ward males with status because doing so allows females to focus greater time and
energy on bearing and rearing offspring. Otherwise, a female will need to divert time
and energy toward provisioning resources on her own.
ENA theory asserts that female choice has imposed continuous natural selection
pressure on males to focus their time and energy on provisioning, and in most societies
this has entailed functioning within social hierarchies for status. Due to this natural se-
lection pressure, males throughout the world are found to be much more prone toward
overtly competitive activities than are females (Ellis et al., 2008, Table 6.2.3.13).
According to ENA theory, competitive/victimizing behavior exists along a con-
tinuum of crude to sophisticated expressions, and victimizing criminality is a crude
expression. Close to the sophisticated end of the same continuum are the sorts of
business and commercial activities that make life in complex societies possible. Near
the middle of the continuum are deceptive and shady business practices that may or
may not be considered criminal (Ellis, 2004, p. 147).
For natural selection to impinge upon a trait, the trait must be at least partially
influenced by genes (Pinker, 2002, p. 50). Thus, at the heart of ENA theory is the
assumption that genetic factors are contributing to people’s varying tendencies to
engage in crime (as well as other forms of competitive/victimizing behavior).
The Neuroandrogenic Proposition

As to how genes have made males more criminal than females, ENA theory as-
serts that testosterone and other so-called male hormones (collectively known as
Reducing Crime Evolutionarily 251
androgens) operate on the brain to facilitate the learning of competitive/victimizing

behavior, including its criminal forms. Theoretically, androgen-motivated inclina-
tions to learn competitive/victimizing behavior are partially wired into the brains
of males even before birth but begin to be much more fully expressed at the onset
of puberty. The most widespread manifestation of competitive/victimizing behavior
with the onset of puberty is juvenile delinquency, although general aggressiveness
and reduced compliance with authority are also common.
Eventually, these early postpubertal expressions of competitive/victimizing behav-
ior will be tempered with tendencies to compete within a hierarchical social system.
Rapid transition to the latter (sophisticated) forms of competitive/victimizing behavior
is facilitated by agile reasoning and learning abilities. Consequently, individuals who
have low intelligence or learning disabilities will be relatively slow to transition from
crude to sophisticated forms of competitive/victimizing behavior. These slow learners
are most likely to become what Moffitt (1997) has termed “life-course-persistent of-
fenders,” whereas males with average or above average learning ability are most likely
to be what she has termed “adolescence-limited offenders.”
It is important to mention that more detailed descriptions of the theory (e.g.,
Ellis, 2005) specify aspects of brain functioning that affect the probability of criminal
behavior, all of which are androgen influenced. They involve (a) the arousal control
process, mainly in the brain stem; (b) emotion control mechanisms, primarily in
the limbic system; (c) executive functioning processes, mainly in the prefrontal por-
tion of the neocortex; and (d) the relative strength of the left and right hemispheres.
I briefly allude to these brain functions as they pertain to specific crime prevention
and treatment approaches in the following section.
Using ENA Theory for Crime Prevention and Treatment
Numerous applications of ENA theory are possible regarding both the prevention
and treatment of criminality. These applications can be conceptualized within the
following three categories: social learning approaches, pharmacological/neurological
approaches, and eugenic approaches.
Social Learning Approaches

ENA theory suggests that several social learning approaches to crime prevention
and recidivism reduction should have their intended crime-reducing effects. These
approaches involve recognizing that while the motivation for criminal behavior is
largely unlearned, the behavior itself is learned. For example, largely unlearned de-
sires for creature comforts may lead to thefts, those for sexual satisfaction may cause
rape, and those for envy and revenge may induce many assaults and murders. Ac-
cordingly, any treatment programs that help offenders (or prospective offenders) to
identify and utilize alternatives to illegal means to satisfy innate desires should help
to reduce crime. Following are some of those programs and a review of the main
evidence pertaining to their effectiveness.
Mentoring Programs
Begun in the 1930s, the first mentoring type of crime prevention program came to
be known as Big Brother/Big Sister (Reymert, 1940). In such a program, a same-sex
adult is teamed with a delinquent or “at-risk” youth. The adult befriends, counsels,
and participates in recreational activities with the youth in order to guide him or her
toward becoming a responsible citizen. Do such programs have their intended effects?
The empirical evidence is limited but suggests that they have significant tendencies
to prevent delinquency, at least in terms of illegal drug use and truancy (Grossman &
Tierney, 1998).
Parenting Management Training
Anyone who has ever witnessed a mother yelling obscenities at her child in a grocery
store knows that some parents are atrocious when it comes to helping their children
learn to behave within acceptable social limits. Studies have shown that a lack of
parental competence is a significant predictor of offspring delinquency (Farrington,
1987, p. 32; Simons, Wu, Conger, & Lorenz, 1994). According to most research,
the most effective parenting for preventing delinquency involves firm but minimally
punitive discipline (Kandel, 1982; Patterson & Stouthamer-Loeber, 1984).
Assessments of programs designed to help parents acquire these types of child
management skills have suggested that most of them have modest success at reducing
delinquency (Klein et al., 1977; Woolfenden, Williams, & Peat, 2002; but for no signifi-
cant long-term effect see Bank, Marlowe, Reid, Patterson, & Weinrott, 1991). Theoreti-
cally, these programs are only modestly successful because the personalities of children
vary considerably independent of parental treatment. Most children have personali-
ties that allow them to rather quickly learn acceptable behavior even with relatively
poor parental guidance. On the other hand, children who are most likely to become
delinquent and criminal later in life demand much more skilled and patient parenting
to prevent antisocial behavior. ENA theory predicts that considerably greater parental
skills will be required to keep males from engaging in delinquency than females.
Language-Focused Programs
Research has repeatedly shown that persons with serious criminal histories exhibit
unusually high rates of deficits in language skills (e.g., Moffitt, Silva, Lynam, & Henry,
1994; Rodriguez, 1993). Reflecting these language-related deficits are studies show-
ing that offenders score distinctly lower on verbal aspects of standardized IQ tests
relative to their scores on nonverbal (performance) aspects of these tests, a phenom-
enon known as “intellectual imbalance” (e.g., Henry, Moffitt, & Silva, 1992; Lynam,
Moffitt, & Stouthamer-Loeber, 1993).
To explain such language deficiencies, ENA theory contends that testosterone

causes the brain to shift away from its normal left-dominated hemispheric brain-
functioning pattern toward a greater involvement of the right hemisphere (Ellis,
2005). Theoretically, this shift makes it difficult for individuals to maintain atten-
tion on and to intelligibly process linguistic stimuli. Not only do they perform poorly
in school as a result but they are also more likely to ignore laws and most other
language-based rules of socially acceptable conduct.
If ENA theory is correct regarding the left hemisphere being less dominant
among offenders than among nonoffenders, it may be possible to prevent offending
by immersing crime-prone individuals into language-oriented learning throughout
childhood. No specific programs directly bearing on this theoretical prediction were
located.
Self-Control and Moral Reasoning Training

According to ENA theory, the frontal lobes perform important “master control”
functions that help humans organize their daily activities into coherent themes (life’s
plans) and do so without routinely harming others (Ellis, 2005). Collectively known
as executive functioning, these higher thought processes make moral reasoning and
so-called self-control possible.
Can the frontal lobes be taught to improve in moral reasoning and self-control?
Without delving into the details of what “improve” means, one can think of the
brain in ways that might pertain to muscle tissue. Even though some people are nat-
urally much stronger than others, everyone can still enhance his or her muscular-
ity through exercise. Likewise, neurological processes can be substantially enhanced
through “exercise” by socially practicing moral reasoning and self-regulation.
Most moral issues boil down to weighing short-term versus long-term conse-
quences of one’s actions. The more individuals can be taught to focus on the long-
term consequences of their actions, the better their moral reasoning will be. Similarly,
self-control is usually achieved by being able to foresee the long-term advantages of
postponing immediate gratification.
Crime prevention programs having to do with promoting self-control are cur-
rently conceptualized mainly in terms of Gottfredson and Hirschi’s (1990) self-
control theory. Thinking about self-control in terms of ENA theory, on the other
hand, has at least two advantages. First, unlike control theory, ENA theory recognizes
that people do not simply vary in self-control due to upbringing. Instead, genetically
regulated brain processes are seen as having much to do with how quickly people
mature in their self-control. Second, ENA theory forces one to think of self-control,
moral reasoning, and long-term planning as interconnected phenomena. This com-
munality largely resides in executive functioning of the prefrontal lobes. Morgan and
Lilienfield (2000) reviewed considerable evidence that deficient executive function-
ing of the prefrontal lobes is a significant contributor to delinquent and criminal be-
havior. Beaver, Wright, and Delisi (2007) went on to argue that the well-established
link between poor self-control and antisocial behavior is the result of deficiencies in
prefrontal executive functioning.
Even though much of the variation in executive functioning appears to be under
genetic control, there are ways it can be enhanced through social training and rein-
forcement (Baumeister, Heatherton, & Tice, 1994; Strayhorn, 2002a). Accordingly,
at least one program for promoting self-control has shown promise for crime preven-
tion (Strayhorn, 2002a, 2002b).
Additional research supports the idea that even though executive functioning of
the prefrontal lobes has a major role to play in maintaining self-control, such control
can still be modified through social training. For example, clinical psychologists were
able to teach mothers how to instill more effective self-control strategies for resisting
temptation and being less impulsive in troublesome preschoolers (Mauro & Harris,
2000). Likewise, a set of reinforcement techniques have been developed that appears
to promote the learning of socially responsible behavior among conduct-disordered
children (reviewed by McMahon & Wells, 1998). The hope is that these children will
thereby avoid antisocial behavior later in life.
Pharmacological/Neurological Approaches
If ENA theory is true, there should be numerous ways to reduce crime through our
growing understanding of the brain. As discussed in the following sections, these
approaches would include the use of drugs for alleviating neurological symptoms
that are often precursors for criminal and antisocial behavior. Before exploring these
approaches, it should be emphasized that no neurochemical treatment program
should be employed as a first-line strategy but rather as a possible secondary or ter-
tiary approach when learning-based approaches prove to be unsatisfactory. Even
then, pharmacological therapies should only be used in conjunction with learning-
based approaches, not simply on their own (Harrington & Bailey, 2003, p. 27).
Stimulant Arousal Control Medication

Methylphenidate (Ritalin) has been used to treat symptoms of Attention-Deficit/
Hyperactivity Disorder (ADHD) for the past couple of decades (Swanson, McBurnett,
Christian, & Wigal, 1995). This and other stimulant drugs appear to have their main
effect by allowing the brains of ADHD sufferers to focus greater proportions of their
attention on ordinary incoming stimuli (as most people do) rather than attending only
to unusually intense and often socially disruptive stimuli (Polanczyk et al., 2007).
According to ENA theory, fairly high proportions of delinquents exhibit neuro-
logical underarousal, as do ADHD children (Ellis, 2005), a view supported by em-
pirical evidence (Sullivan & Rudnik-Levin, 2001). If so, stimulant medications may
help to suppress delinquent activities. One study did report that methylphenidate
reduced aggressive behavior among conduct-disordered children (Kaplan, Busner,
Kupietz, Wassermann, & Segal, 1990). However, too little research pertaining to
its postpubertal therapeutic effects is currently available to make a judgment with

confidence (see Garland, 1998). Nevertheless, one experimental study suggested
that methylphenidate helped reduce aggressive behavior among young adolescents
(Hinshaw & Erhardt, 1991), and another concluded that antisocial behavior gener-
ally was diminished with this medication (Klein, Alexander, & Parsons, 1997).
A stimulant drug that produces a more extended therapeutic response per dose
than methylphenidate is an amphetamine marketed under the brand name Adderall
(Pelham et al., 1999; Swanson et al., 1998). For this reason, it should be explored as
more appropriate for treating severe delinquency symptoms.
Anti-androgens
At the heart of ENA theory is the premise that testosterone and other androgens
operate on the brain in ways that promote criminal behavior. This leads one to ex-
pect drugs that reduce testosterone levels to help reduce the probability of offending.
Consequently, a class of drugs called anti-androgens—primarily cyproterone acetate
and medroxyprogesterone acetate (trade name Depo-Provera)—should reduce the
incidence of crime and delinquency.
So far, the best-documented effects of anti-androgens in treating criminality
have involved sex offenders. Provided that these offenders maintain their treatment
regimen, studies suggest that the commission of new sex offenses is substantially
diminished (reviewed by Grossman, Martis, & Fichtner, 1999; Maletzky, Tolan,
& McFarland, 2006; Rosler & Witztum, 2000).
According to ENA theory, anti-androgens should also help to reduce nearly all
types of offenses, not just those of a sexual nature. For example, administering anti-
androgens to young postpubertal males at high risk of offending, especially regard-
ing violent offenses, should help to suppress the dramatic surge in testosterone in
the years immediately following puberty. Males with the greatest difficulty learning
may need to be maintained on anti-androgen treatment for as much as a decade. No
specific evidence was located to assess the merits of this hypothesis.
Anti-androgens (the administration of which is also called chemical castration)
are often discussed in tandem with actual (surgical) castration. The main difference
between the two is that chemical castration is reversible and thereby less punitive
than surgical castration. Nevertheless, studies of the effects of surgical castration
have indicated that rapists, pedophiles, and exhibitionists who have undergone the
procedure have much lower sex offense recidivism rates than do comparable sex of-
fenders who are not surgically castrated (Hansen, 1991; Wille & Beier, 1989).
Antipsychotic Medication
Research indicates that criminality, especially of a violent nature, is more common
among schizophrenics (Tengstrom, Hodgins, Grann, Langstrom, & Kullgren, 2004;
Walsh, Buchanan, & Fahy, 2002) and manic depressives (Feldmann, 2001; Harrer
& Kofler-Westergren, 1986) than among persons in general. Various drugs known
as antipsychotics are often used in treating persons with these ailments. This has
raised the possibility that antipsychotics might also be helpful in preventing at least
some types of criminal behavior. The research bearing on this possibility is limited
but worth briefly exploring.
One study indicated that two drugs often used in treating schizophrenia—
chlorpromazine and thioridazine—were helpful in reducing assaultive behavior
among mentally retarded children (Campbell, Rapoport, & Simpson, 1999). Another
drug also used mainly to treat schizophrenia—risperidone—was deemed fairly effec-
tive in temporarily reducing aggression and other symptoms of childhood conduct
disorders (Findling et al., 2000).
A medication with a long history in treating symptoms of manic depression
is lithium carbonate (Baldessarini, Tondo, & Hennen, 1999). Actually the lightest
known metal, lithium in granulated form has helped for decades to manage ungov-
ernable tempers and acts of impulsive aggression among persons diagnosed with
manic depression (Shader, Jackson, & Dodes, 1974; Sheard, Marini, Bridges, & Wagner,
1976). It has also been found to alleviate explosive outbursts of aggression among
children and young adolescents with conduct disorders (Campbell et al., 1984;
Malone, Delaney, Luebbert, Cater, & Campbell, 2000).
ENA theory would attribute the success of these antipsychotic medications
to evidence that they all tend to temper the emotion control centers in the brain’s
limbic system (Ellis, 2005). From an evolutionary standpoint, the limbic system
houses many key survival instincts that help humans and other mammals to make
judgments about social relationships.
Atypical Antipsychotic Medication

In recent years, several antipsychotic medications have been developed that differ
substantially from more established antipsychotic medications in how they affect
the brain; thus they have been dubbed atypical. The main distinguishing feature of
atypical antipsychotics is that they target the type 2 neurotransmitter receptors for
both dopamine and serotonin in the limbic system much more directly than is true
for earlier developed antipsychotic drugs (Worrell & Marken, 2000).
One atypical antipsychotic medication, known as quetiapine, has been used in
treating conditions as diverse as schizophrenia (Kasper & Muller-Spahn, 2000) and
Parkinson’s disease (Targum & Abbott, 2000) with at least modest promise. How-
ever, in a limited clinical trial, it seemed to be very helpful in reducing impulsivity,
hostility, and aggression among four maximum-security inmates diagnosed with
antisocial personality disorder (Walker, Thomas, & Allen, 2003).
The Walker et al. study obviously needs to be extended before its findings can
be considered established. Nonetheless, it offers a ray of hope in terms of alleviating
behavior traits that are very common among persons with extremely high offending
rates—that is, psychopaths (Hare, 1993). ENA theory envisions the limbic system as
one of the key areas of the brain in which emotions conducive to criminality reside.
Anticonvulsant Medication
Anticonvulsant medications are primarily prescribed to persons with a history of ep-

ileptic seizures. Numerous studies have found a statistical link between epilepsy and
episodic bursts of aggression (e.g., Devinsky et al., 1994; Marsh & Krauss, 2000).
ENA theory can explain links between epilepsy and aggression in various neu-
rologically specific ways. One is to note that many types of epilepsy involved distur-
bances primarily in the limbic system and its connections with the prefrontal areas
of the frontal lobes (Dougherty et al., 2004; Raine & Yang, 2006; Trimble & Tebartz
van Elst, 2003). Since these are areas crucial to social emotions, long-term planning,
and moral reasoning, epilepsy may disturb the functioning of brain regions critical
to the control of emotionally charged aggression (Woermann et al., 2000). If this
line of reasoning is correct, it should be possible to treat impulsive types of criminal
behavior with anticonvulsant drugs.
Researchers have sought to determine whether anticonvulsant drugs can re-
duce the incidence of episodic aggression. Results have been positive, especially for
carbamazepine, which has been found to reduce the incidence of such aggression
in patients generally (reviewed by Young & Hillbrand, 1994), among conduct disor-
dered children (Kafantaris et al., 1992), and for epileptic and manic depressive adults
(Post, Rubinow, & Uhde, 1984). Other anticonvulsants that have shown promise in
suppressing violent outbursts among convicted offenders with and without a history
of seizures are Propranolol (Mattes, 1990; Sheard, 1984) and Valproate (Wilcox,
1995; Donovan, Susser, & Nunes, 1997).
Serotonin-altering Medications
Serotonin is an important neurotransmitter associated with feelings of calm and
contentment (Kalus, Asnis, & Van Praag, 1989; Plaznik, Kostowski, & Archer,
1989). Typically, low or unstable serotonin activity in the brain is linked to irritabil-
ity and impulsive violence (Matykiewicz, La Grange, Vance, Wang, & Reyes, 1997;
Virkkunen, Eggert, Rawlings, & Linnoila, 1996).
While the research is still limited and preliminary, a few studies suggest that
serotonin-altering drugs can be used to prevent recidivism among violent psycho-
paths (reviewed by Dolan, Deakin, Roberts, & Anderson, 2002) as well as deviant
sex offenders (Fedoroff, 2004). Serotonergic drugs may also help to reduce violence
among conduct-disordered children (Staller, 2007), marijuana use by ADHD ado-
lescents (Solhkhah et al., 2005), and cocaine addiction (Liu & Cunningham, 2005).
ENA theory predicts that serotonergic therapy would provide effective treatment
for criminal and antisocial behavior partly because serotonin pathways connect
the brain’s prefrontal areas with the emotion control centers in the limbic system
(Davidson, Purtnam, & Larson, 2000, p. 592). Theoretically, serotonin facilitates ex-
ecutive cognitive functioning that is required to restrain impulses that often originate
in the limbic system, especially those of rage and social frustration (Ellis, 2005). An-
other noteworthy point is that testosterone seems to fundamentally alter serotonergic
pathways in the brain (Birger et al., 2003; Fink, Sumner, & Rosie, 1999). Under-
standing the impact that testosterone has on serotonin could have a major impact on
pharmacologically regulating impulsive types of human aggression.
Reducing Lead Exposure

Lead was widely used in paint until the 1940s and is still peeling from the walls
of some old homes. It was also an additive in gasoline until the 1970s, thereby
contaminating the atmosphere of many large cities. Exposing the brain to lead
lowers intelligence-test scores (Bellinger et al., 2003; Canfield, Kreher, Cornwell, &
Henderson, 2003) and probably learning ability in general (Yuan et al., 2006).
ENA theory states that any suppression of learning ability or executive function-
ing will increase the likelihood of a sustained criminal career. Over the years, numer-
ous studies have indicated that bodily exposure to lead, even prenatally, is associated
with later delinquency and criminality, especially of a violent nature (e.g., Dietrich,
Ris, Succop, Berger, & Bornschein, 2001; Needleman, McFarland, Ness, Fienberg, &
Tobin, 2002; Nevin, 2000). Even when the detrimental effects of lead exposure on
IQ are controlled, links between this exposure and so-called externalizing behavior
problems remain (Chen, Cai, Detrich, Radcliffe, & Rogan, 2007). If ENA theory is
correct, this latter finding suggests that executive functioning is also compromised
by brain exposure to lead.
Criminologists should collaborate with other scientists to help reduce lead expo-
sure for persons of all ages. They should also seek to identify other neurotoxins that
may adversely affect learning ability and related aspects of human temperament. In
this regard, prenatal exposure to manganese appears to enhance the symptoms of
conduct-disordered behavior and ADHD (Ericson et al., 2007).
EEG Biofeedback
The brain is the most direct controller of behavior. Therefore, the brains of chronic
offenders must be functioning in ways that significantly differ from the brains of
nonoffenders. Some have proposed that it may be possible to use EEG biofeedback
techniques to divert the brains of chronic offenders away from functioning patterns
that are most conducive to offending (Raine, 1996, p. 56). So far, clinical research
suggests that ADHD symptoms can be at least partially suppressed with biofeedback
(reviewed by Monastra et al., 2005). Whether this can be extended into preventing
delinquent and criminal behavior remains to be determined.
Eugenic Approaches
The biggest concern that many have surrounding biosocial criminology is that it
could resurrect the twentieth-century eugenics movement and even Nazism. Despite
the controversy, this section will explore how two social/governmental policies may
in fact be having eugenic effects on criminality even though they were not specifically
instituted for this purpose. The two policies to be explored are the U.S. legalization of
abortion in the early 1970s and the dramatic increase in the rate of incarceration
beginning in the 1980s.
The context within which this exploration takes place involves noting that the
rate of crime, especially for violent crimes, began to decline in the United States in the
early to mid-1990s, and this decline can only be partially explained in terms of shifts
in population-age-related factors (Fox, 2000; Zimring, 2006). Similarly, the rates of
child abuse have also declined substantially in this same time period (Finkelhor &
Jones, 2006).
Because ENA theory is firmly embedded in evolutionary thinking, it inescapably
assumes that genetic factors contribute to criminality. Therefore, curtailing the re-
production rates of persons with “crime-prone genes” relative to persons with few
such genes should reduce a country’s crime rates. Is there any evidence that politi-
cal/governmental policies in the United States could have so altered people’s repro-
duction rates as to have significantly impacted crime rates?
Abortion Legalization
In 1972, a Supreme Court decision known as Roe v. Wade legalized abortions in
the United States. Since then, approximately one in six U.S. pregnancies have been
terminated, roughly 1.2 million per year (Spitz et al., 1996), with the greatest
proportion of terminations occurring in teenage pregnancies (Darroch, Singh,
& Frost, 2001).
Beginning in the twenty-first century, research began to implicate the decrimi-
nalization of abortion as a possible contributor to the U.S. declining crime rate that
became evident in the 1990s (Berk, Sorenson, Wiebe, & Upchurch, 2003; Donohue
& Levitt, 2001; Joyce, 2004; Sorenson, Wiebe, & Berk, 2002; for an exception see
Zimring, 2006). To explain how such a connection might occur, the main argument
has been that women who were unprepared for motherhood (at least at the time they
became pregnant) were most likely to obtain an abortion (Donohue & Levitt, 2001).
This could cause a decrease in criminality in the next generation of children, as un-
wanted pregnancies and out-of-wedlock births are predictive of offspring criminality
(Jonsson, 1967, p. 209; Kubicka et al., 1995; Walsh, 1990).
Eugenically speaking, an additional factor may be involved. Among both sexes,
out-of-wedlock pregnancies may be most common among persons who have genetic
propensities toward providing poor parental care to their offspring (Burt, Krueger,
McGue, & Iacono, 2003; Perusse, Neale, Heath, & Eaves, 1994). Diminished paren-
tal care may increase the likelihood of criminal and antisocial behavior in offspring
(Reiss et al., 1995). Another possibility is that many of the same genes that contrib-
ute to antisocial behavior also contribute to substandard parenting. In either case,
offending-prone parents may have been considerably more likely to have terminated
a pregnancy than parents in general, thereby lowering the proportion of children
being born with antisocial tendencies.
Incarceration Rates
Prior to 1970, the United States incarceration rate was “only” about twice as high
as other Western countries (Freeman, 1996), but in the late 1970s, imprisonment
rates began climbing so that by the 1990s they were over five times higher (Tonry,
1999; Uggen & Manza, 2002).
Many factors may have contributed to the high and growing rate of U.S. incar-
ceration. These include racism, poverty, out-of-wedlock parenthood, and increases in
the availability of hard street drugs such as heroin and crack cocaine, all of which are
arguably more prevalent in the United States than elsewhere in the Western world
(Blumstein & Rosenfeld, 1998; Bridges & Crutchfield, 1988; Levitt, 1996). Setting
aside the causes of high incarceration rates, the issue at hand is whether these rates
may be so high as to actually impact the reproduction rates of offenders relative to
the general population, thereby diminishing crime rates in subsequent generations.
Obviously, high incarceration rates may have deterrent and incapacitation
effects (Levitt, 1996), but these would occur fairly soon (i.e., within two or three
years) after an incarceration rate increase. The sort of effects at issue here would
not be apparent until a new generation began to enter its crime-prone years (i.e.,
approximately fifteen years following birth). This line of reasoning is obviously
not “politically correct,” but it is worth considering in the context of evolutionary
approaches to crime prevention.
So far, the evidence is difficult to assess, partly because crime statistics are influ-
enced by numerous factors, not the least of which are changes in the proportions
of offenses being reported to police (O’Brien, 1996). Also, if there are effects due to
legalized abortion (as discussed above), these may be interacting with the effects of
increased incarceration. Nevertheless, the decline in the U.S. crime rates through-
out the 1990s (although irregularly), especially for violent offenses (Blumstein &
Rosenfeld, 1998), is consistent with the view that it is at least partly the result of
removing increasing proportions of offenders from contributing to the nation’s gene
pool beginning in the late 1970s.
Conclusion
This chapter describes the evolutionary neuroandrogenic (ENA) theory of criminal

behavior, a theory that can be characterized in terms of two propositions, one evo-
lutionary and the other neurohormonal. The evolutionary proposition states that
criminality is part of a spectrum of largely male responses to female preferences
for mates who are capable provisioners of resources. This spectrum is in the form
of a neurologically programmed continuum of competitive/victimizing behavior
patterns that vary from very crude (usually criminal) to very sophisticated (rarely
criminal) behavior. Theoretically, at puberty, all males start near the crude end of
the continuum, and as they mature they move toward more and more sophisticated
expressions. Males who learn quickly will transition rapidly from crude to at least
moderately sophisticated expressions. Those with low intelligence, learning disabili-
ties, or few opportunities to practice competitive/victimizing behavior will transition
more slowly.
According to the theory’s second proposition, male brains on average are in-
clined more than female brains toward competitive/victimizing behavior as they are
exposed to higher levels of testosterone and other androgens. Theoretically, the most
permanent effects of androgens on brain functioning occur prior to birth, but the
most dramatic behavioral activation of these perinatal effects awaits the surge in
testosterone at puberty.
If the theory is true, three categories of treatment approaches to crime pre-
vention or treatment should all have some beneficial effects. These categories are
(1) social learning approaches, (2) pharmacological/neurological approaches,
and (3) eugenic approaches. Examples of each of these three approaches are dis-
cussed. Overall, the theme of this chapter is that Darwinian evolutionary thinking
is relevant not only to understanding criminal behavior but also to preventing and
treating such behavior. Especially promising in recent years have been a number of
pharmacological approaches to the prevention and treatment of antisocial precur-
sors of criminality.
ACKNOWLEDGMENTS
I thank Dr. Kevin Beaver and Dr. Anthony Walsh for providing helpful comments on drafts
of this chapter.
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14
Did the Victim Deserve to Die?

Darwin Goes to Court
The real voyage of discovery consists not in seeking new landscapes but in having
new eyes.
—Marcel Proust
Not long ago, at an annual meeting of trial attorneys, a famous criminal defense
lawyer who made his reputation by defending accused murderers spoke about de-
fendants’ rights. During the questioning phase, he was asked how he approached a
capital murder case. He paused and then said, “I approach a murder case by asking
myself two questions. Did the victim deserve to die? And, was the defendant the right
man for the job?”
The audience burst out laughing. But laughter quickly changed to nervous
chuckling when the group realized he was serious. He approached each murder case
with those questions firmly in mind. This attorney is not the originator of those ques-
tions, nor is he the only person to think that way. The Texas Constitution once had an
amendment nicknamed the “He needed killin’ ’’ clause. That someone “needed killin’ ”
was—and some say still is—a valid defense in a Texas courtroom.
The lawyer’s answer to the question of how he approached a capital murder
case reflects what we all now know: murder can be natural and understandable.
Through the lenses of evolutionary psychology, we can see how homicide may be
produced by design features of human minds, particularly men’s minds (Buss, 2005;
also see Chapter 3 of this volume). But I didn’t know that when I heard the famous
lawyer speak. I trained as a psychiatrist in the 1970s. My background is in psycho-
dynamics, psychoanalysis, general psychiatry, and traditional forensic psychiatry. In
the early days of my career, when someone committed murder, my colleagues and
I believed that psychopathology caused the violence. Murder meant madness. Who
in his “right mind” would kill? Now we know that there are parts of the mind that
originated much further back in deep time that can make killing quite natural, given
our species’ evolutionary history.
268
Did the Victim Deserve to Die? 269
Shortly after hearing the attorney’s speech, I was asked to evaluate Willie, a
19-year-old single man who, with his 17-year-old half-brother Steve, killed his fa-
ther. The father arrived home one afternoon with his fourth wife, the defendants’
stepmother, to be met by a hail of bullets from Willie and Steve. Willie fired the first
shots, which killed his father. Steve shot seconds later and struck the stepmother,
who survived but was made paraplegic, wheelchair bound for life by Steve’s bullets.
The young men gathered up their father’s gun collection and fled. Their father
was a relatively poor man, an auto mechanic, but he had built a gun collection of
considerable value relative to his lot in life. State police soon caught Willie and Steve
in North Carolina. They were charged with multiple firearm offenses and with capi-
tal murder. They faced the death penalty.
Willie’s capital-murder-certified attorneys asked my partner and me to evaluate
their client. I told them about the famous trial lawyer’s approach to murder cases,
and they decided to use it. Willie and Steve’s father truly was a monster. His modus
operandi remained consistent throughout his adult life. He raped a 13- or 14-year-old
girl and then married his victim. After having one or more children with his victim-
turned-wife, he discarded her and moved on to more fertile fields. Several of his former
wives came forth at the trial to testify about his monstrous nature. Auto mechanics
who worked with him testified that during work hours, even in the sweltering sum-
mers, he kept whoever was his current wife and their youngest children in his car in
the parking lot of whichever auto dealer then employed him. He physically and sexu-
ally abused all of his children, male and female alike, including both of the defendants.
Emergency room records surfaced on my defendant and confirmed one early episode
of sexual abuse, even though Willie did not remember that particular incident.
A psychiatric examination of Willie indicated that he suffered chronic depression
and post-traumatic stress disorder (PTSD). All of that was presented at trial. Willie’s
attorneys portrayed him as a victim who was protecting the family and ridding them
of a dangerous and potentially deadly man. They implied that Willie shot the father
to protect Steve. The father had sexually abused Steve several months before the kill-
ing. Willie knew this, and indeed, Steve still may have been in the father’s cross hairs.
Abuse caused the murders. Protection of the family was paramount. Revenge was
implied. Parent–offspring conflict and theft were ignored.
A Transitional Species
Before we continue to Willie’s fate, I offer thanks to the editors of this volume, Joshua
Duntley and Todd Shackelford, for inviting a practicing forensic psychiatrist to con-
tribute. Unbeknown to them, they asked a transitional species to weigh in as this
new field of evolutionary forensic psychology emerges. Thanks to a good friend and
gifted lawyer, Willis Spaulding, who gave me Robert Wright’s The Moral Animal ten
years ago, the lens of evolutionary psychology now influences my clinical and foren-
sic practices. Wright’s book ushered in a sea change in my understanding of human
behavior, psychology, psychiatry, psychopathology, and, in this instance, how I ap-

proach forensic cases. This opportunity gives me, as just one forensic practitioner,
the opportunity to share my views of the current impact and future directions of
evolutionary forensic psychology. In this chapter I illustrate my points with my own
cases, such as Willie’s capital murder case.
As Owen Jones notes, the law is about human behavior. To the degree that we
have an accurate understanding of human nature, the law will be more effective.
There is almost no area of the law where psychology, clinical psychology, and psychi-
atry fail to have impact. Clinicians are asked to determine competence to stand trial
(CST) and mental state at the time of the offense (MSO). Within our justice system,
every criminal defendant now has a constitutional right to a psychiatric evaluation
(Ake v. Oklahoma). Sex offenders are mandated by law to have psychiatric or psycho-
logical evaluations and treatment. In domestic relations courts, partner violence,
child abuse, divorce, and child custody and visitation require clinicians’ reports and
testimony. Parent–offspring conflict always surfaces in trust and estates litigation
and in the frequent challenges to wills.
There are few judges, civil litigators, criminal defense lawyers, prosecutors, and
clinicians familiar with evolutionary psychology. That will change as evolutionary
psychology becomes part of mainstream psychology. Future college graduates who
choose legal professions will know more about the discipline of evolutionary psy-
chology. As clinical programs incorporate more of an evolutionary perspective, fu-
ture forensic clinicians will both need and utilize it in ways we are beginning to see,
and which are discussed in the preceding chapters.
Willie’s Fate
Willie and Steve were convicted of second-degree murder, a victory for the defen-
dants and their attorneys. All of us involved in the case came away with the impres-
sion that had the stepmother not been permanently paralyzed, the young men would
have received an even lesser sentence.
At the time of the trial, I certainly believed, “to a reasonable degree of medi-
cal certainty,” that Willie’s depression, PTSD, and the horrendous abuse suffered at
the hands of his father caused the father’s murder and the stepmother’s malicious
wounding. Since learning evolutionary psychology, I have reason to reevaluate
that conclusion. The murder was premeditated. The defendants’ behavior and the
history of the case suggest that these impoverished boys, consciously or not, were
concerned about the distribution of sparse paternal assets. Their motives are better
understood through parent–offspring conflict, a concept unknown to me at the time.
Willie had lost his job shortly before the murder and needed money; this is a more
immediate motive, and one in keeping with what we now know. Willie had years of
ample opportunity and provocation to exact revenge or protect his siblings from the
father’s predation. Why did he kill when he did?
What evolutionary psychology helps one see in Willie’s case may be character-
ized as intentionality, which, for Willie, involved the pursuit of fundamental adaptive
goals. “Intent” in criminal law comprises concepts of mens rea, a mental state inferred
from conduct, and actus reus, a voluntary act. When there is a claim of “automatism”
(in cases of involuntary intoxication, temporal lobe epilepsy, dissociative reactions to
medication, etc.), the forensic evaluation usually focuses on the actus reus. Whether
mens rea was impaired is not an issue.
Many crimes, most notably homicide, are graded in terms of “intent,” with first-
degree murder requiring proof of the highest level of intent, “malice aforethought.”
Capital murder requires proof of other aggravating circumstances not directly
involving intent beyond malice aforethought. Capital murder defendants, however,
may offer by way of defense any evidence of impaired intent. By contrast, most non-
capital criminal defendants may not use expert testimony to mitigate or negate crim-
inal intent unless it is done in the form of an insanity defense.
An insanity defense in most states concedes that the requisite criminal intent
existed but that intent, in ways defined by the applicable law, was affected by mental
disorder to such an extent that the defendant is nonetheless not guilty “by reason of
insanity.” The insanity defense is thus called the NGRI (not guilty by reason of insan-
ity) or MSO (mental state at the time of the offense).
There is a long, rich history of insanity standards:
Wild beast test (Rex, B. Arnold, 1724). A man must be totally deprived of his
understanding and memory so as not to know what he is doing any more
than an infant, a brute, or a wild beast.
Irresistible impulse test (Regina v. Oxford, 1940). If some controlling disease was,
in truth, the acting power within him, which he could not resist, then the
defendant will not be held responsible.
McNaghten rule (McNaghten’s Case, 1743). A mental disease or defect at the time
of the act that caused the defendant not to know the nature and quality or
the wrongfulness of the act.
Durham rule (Durham v. United States, 1954). The accused is not criminally
responsible if his unlawful act is the product of a mental disease or defect.
Model Penal Code (American Law Institute, 1955). A person is not responsible for
criminal conduct if at the time of such conduct, as a result of mental disease
or defect, he lacked substantial capacity to appreciate the wrongfulness of
his conduct (cognitive arm) or to conform his conduct to the requirements
of the law (volitional arm). (Adapted from Simon & Gold, 2004.)
In my state, and in many others, the traditional McNaghten rule together with
some version of the irresistible impulse defense guides assessment. In conducting
an evaluation, a clinician has to assess the evidence of mental illness at the time
of the evaluation, at the time of the offense, and in the time prior to the offense.
Simple presence of a mental illness is not necessarily sufficient. A clinician then has
to determine evidence for impaired functioning within a few days of the offense and
at the time of the offense. The motive for the offense must be determined to the ex-
tent possible. The clinician attempts to gain a detailed understanding of the criminal
defendant’s thinking and behavior before, during, and after the offense. Also taken
into consideration is the prior legal history of the defendant.
The particular legal rules of the jurisdiction in which the evaluation is conducted
have two principal effects, one of which is more speculative than the other. The first
effect is to act as a guide to and sometimes a restraint on the testimony of a forensic
expert. Can the expert witness testify as to intent apart from an insanity defense?
Can the witness express an opinion on the “ultimate” issue of whether the defendant
is “insane”? Must the witness express the opinion in terms of “reasonable medical
certainty”? The answer to these questions will differ from jurisdiction to jurisdiction
and from court to court within those jurisdictions. Knowing the answers to these
questions—and the referring attorney may not always know the answers—is of
course important in conducting and reporting the results of an evaluation, but not
as important as the second effect, or lack of effect, of the legal rules.
The second effect of the rules comes in the form of jury instructions, which state
the law to the jury at the conclusion of the trial and by which they are supposed to
reach a decision. The jury, particularly on questions of credibility, on which point ex-
pert testimony is almost always prohibited, does what nature, not the law, dictates.
An understanding of evolutionary psychology better enables the forensic evalu-
ator to speak to the real concerns of the jury (or, in the absence of a jury, the judge).
Legal rules are institutionalized versions of our rules of thumb or heuristics, products
of human evolution no less than an upright gait or speech.
Litigation is an elaborate system of “cheater detection,” a basic condition of
human sociality, which enables us to determine, for example, whether to punish or
reward another who appears not to reciprocate our generosity.
One of the rules of thumb that we use in cheater detection, and indeed in re-
sponding to other threats of harm, is that “intentional” acts can be deterred by
punishment more than nonintentional acts. Another basic rule of thumb is to help
rather than punish someone whose lack of reciprocity is somehow related to his or
her sickness. Regardless of the legal rules, evolutionary psychology suggests that
these are the rules that juries struggle with.
But sickness and intent can overlap, putting these two rules of thumb in conflict.
It is the job of the evaluator to reconcile them. When I was working with Willie, it
was difficult for me to see anything other than the mental disorder. The adaptive,
albeit primitive, goals achieved by the murder are easier to spot from the long view of
evolution, and indeed some real mental disorders, such as depression or dissociation,
may themselves be ways of achieving adaptive goals. Of course, the mental disorder
may be feigned, but it’s not that simple to reconcile the two rules of thumb regarding
intentionality and sickness. Both can be present.
It is not simple because a very ill defendant can know both “nature and conse-
quence” and the difference between right and wrong. Todd, a young man with un-
treated paranoid schizophrenia, believed that two tourists who sat eating ice cream
by a university were sent by the Central Intelligence Agency to spy on him. He
assaulted both of them and fled. He knew the nature of what he was doing—that
is, striking them with his fists. He also knew the consequences: the police would be
called. He knew it was wrong to hit someone, but he believed he had no alternative
and acted in perceived self-defense. He was found not guilty by reason of insanity.
The crucial areas, once a severe mental illness is identified, are the defendant’s
ability not only to know the difference between right and wrong but also to refrain
from the action. The tangled web of mental disease or defect and impairment of the
knowledge of wrongfulness will certainly benefit from an evolutionary perspective.
Krebs’s (Chapter 12 in this volume) and others’ ideas on the evolution of morality
need to be applied to this prong of the insanity defense. A basic, ancient, evolved sense
of right and wrong is not easily overridden even by the severest of mental illnesses.
Individuals who most of us agree should fall under the protection of an insanity
defense can fail to secure its protection because of the way laws are written and in-
terpreted. Andrea Yates, the Texas mother who killed her children, was undoubtedly
psychotic at the time of the offense. All the experts on both sides of the case agreed
on the severity of her mental illness, but by law the focus needed to be on her ability
to tell right from wrong. The testifying forensic psychiatrists’ disagreements centered
on her ability to know the wrongfulness of her acts. Her insanity defense initially
failed in 2002, and she was convicted of capital murder. Due to the false testimony of
Park Dietz, one of the prosecution’s forensic psychiatrists, her conviction was over-
turned, and in 2006 she was finally acquitted by reason of insanity.1 Even in Texas,
even when psychosis was obvious, the jury struggled with the idea that young chil-
dren “needed killin’.” The teachings of evolutionary psychology about the adaptive
nature of infanticide might have made this act easier to understand.
Rape and Sexual Coercion
Evolutionary psychology, through the work of legal scholars such as Owen Jones
(1999) and investigators such as Randy Thornhill and Craig Palmer (2000), the
Gottschalls (2003), McKibbin et al. (Chapter 6 in this volume), and others, has ad-
vanced our understanding of rape. We can now discern the adaptive logic of rape
more than that of many other crimes.
One of my duties is to conduct the forensic evaluations at a local community
mental health center. Within the space of several months, two defendants were
charged with rape, and evaluations were ordered for both. Each case was unusual,
the likes of which I had never seen, and that there were two of them simultaneously
was especially surprising.
The facts of the cases were almost identical. Both defendants had followed a
lone young woman unknown to them to her apartment, broken in, and raped her.
Because the rapes occurred at night, neither woman was able to give a clear descrip-
tion of her assailant, identify him from pictures, or provide enough details to gener-
ate an artist’s approximation. However, that turned out to be unnecessary: within
several weeks of each attack, the perpetrators returned to the apartment of their
particular victim, knocked on the door, and politely asked the victim for a date. That
led to positive identification, confessions, and two convictions. Neither young man
had identifiable major psychopathology. They genuinely had no conception of the
harm caused by their attack. They truly believed their victims might have found the
experience pleasurable and might be interested in a relationship. Their self-deception
was breathtaking in its cruelty and stupidity.
At the time of those evaluations, my earlier training held sway. With both cases,
I concluded that the defendant did not suffer from any major psychopathology, and
neither case yielded a plausible explanation.
The traditional view—that rape has a contributing or causative psychopathology—
is so ingrained in the system that the following case was sent to me. A young man,
John, came to Charlottesville, Virginia, with no unusual developmental history. He
was born to an intact family and was educated. He dropped out of college after two
years. A long-term girlfriend broke up with him shortly before he settled in Charlot-
tesville, which is dominated by the University of Virginia. John worked below his
intellectual capacity at a pizza restaurant, where the workers partied hard. John fre-
quently drank to excess after closing the restaurant.
On one such work night, John planned to meet friends for late-night drinks. He
began to drink before he drove to rendezvous with them. When his friends failed to
show at the chosen bar, he set out to look for them. He first went to one friend’s apart-
ment, but the friend was not there. However, the friend’s girlfriend was there with
their infant child. She told the defendant that her partner, his drinking buddy, had
gone out to party. John was already intoxicated. He forced her upstairs and, while
she was bent over on the bed, lying over her child to protect the little boy, raped her;
he then fled. Later he told me that he had immediately realized what he had done
and felt intense shame and guilt. He fled in his car toward Richmond, sixty miles
east. Acute remorse made him drive onto the shoulder of the interstate and stop. He
decided to turn himself in, and he called the police.
At the preliminary hearing, the judge was stunned and insisted there must be
something terribly wrong with John. Only a very ill young man, he emphasized,
could rape a friend’s girlfriend, whom he barely knew and toward whom he alleg-
edly held no animosity, and do so in the presence of her child. The judge ordered the
psychiatric evaluation before the defense attorney moved to request it.
John suffered no major mental illness or personality disorder, and could be
given no Axis I or Axis II diagnosis. John remembered meeting the victim only once
before the rape and had only the vaguest recollection of that occasion. When he met
her, he remembered thinking she was attractive, but he maintained that she made
no large or lasting impression. The evaluation brought forth some evidence to sug-
gest that John might have been depressed at the time of the rape, feeling like his life
was on a downward trajectory. But by the time of the evaluation, he was depressed
by his charges and his life in jail. Determining his level of depression before the rape
was problematic. He knew he was drinking excessively, but that was all. There were
no long-standing psychiatric symptoms, personality disorder, hatred of women, or
any of the usual explanations for rape. Even though John pled guilty, the presence
of the child at the time of the offense contributed to his receiving a long prison
sentence.
One of my roles is staff psychiatrist at the university’s student health center.
Date rape occurs often. When the perpetrator is known to the mental health staff, a
consistent finding is no major psychopathology or even a consistent type of person-
ality disorder. How does one understand such behavior?
In the AEP years of my life (after evolutionary psychology), my reading of the lit-
erature and such rape cases convince me that rape is an adaptation, not a by-product
of male sexual aggressiveness. Owen Jones’s (1999) superlative law review article,
although neutral on the “adaptation versus by-product” debate, lays out the evidence
for rape as an adaptation. Randy Thornhill and Craig Palmer’s (2000) book con-
vinced me of Thornhill’s position that rape is produced by psychological adaptation.
The Gottschalls’ (2003) evidence that there are increased pregnancy rates with rape
relative to consensual sex adds additional evidence. Women’s evolved mechanisms
for avoiding rape suggest that rape lurked as an ever-present threat in ancestral envi-
ronments. If we look at the large disparity between the sexes in parental investment,
would we not be surprised if Homo sapiens males contained no rape adaptation? The
disparity in parental investment in our species provides fertile soil for the evolution
of a mechanism to override female choice.
What are the implications for the evaluation of a defendant charged with murder
or rape? The forensic clinician should now look at psychopathology as just one of a
multitude of factors that influence a defendant to deploy the adaptations for killing
or rape. Stupidity, personality disorders, depression, substance abuse, and even
psychosis may impair cost/benefit decision making, causing a young man like John
to deploy what was once adaptive behavior: ensuring survival of his DNA through a
forced coupling with a woman already shown to be fertile.
In short, and in any case involving both mental illness and adaptive behavior,
it would behoove those of us who practice forensic psychiatry to look at the illness
as something that triggers the behavior that is in our civilized society criminal but
which was once adaptive. We can consider it as a sort of mental “oncogene”—many
people have the genetic potential for various cancers, but most researchers now real-
ize that there may be some trigger that turns a normal gene into an oncogene and
begins the abnormal cell division. The relationship between psychopathology and
adaptive behavior may be the same.
Domestic Violence
Papa loved Mamma.

Mamma loved men.
Mamma’s in the graveyard.
Papa’s in the pen.
Male sexual jealousy, arguably another aspect of cheater detection, is the most
combustible element in domestic violence. Yet it is remarkable that such a relatively
simple concept has been ignored in teaching professionals how to conduct forensic
evaluations, although daytime talk shows that offer public revelations of DNA pater-
nity tests are oddly popular and can clearly show the doubt and anger that can arise
from a man’s concern that he has been deceived—or, in the sense of adaptation, that
the DNA carried in a child is not his.
No defendant has ever volunteered to me that he was unsure his children were
his own or that he believed his mate to be unfaithful. The most obvious and common
shift in my forensic assessments since learning of evolutionary psychology has been
to begin asking the relevant in-depth questions on fidelity and paternity. Does the
defendant think his partner has been unfaithful in the past, is unfaithful in the pres-
ent, and/or might be unfaithful in the future? Has he ever harbored doubts about the
paternity of his children? Such questions never fail to reveal information that would
remain hidden had the questions not been asked—information that invariably influ-
ences the psychiatric conclusions.
Bill, a 30-year-old manual laborer, was charged with burning down his home. He
did have a psychotic illness, probably a bipolar illness, though we debated that. Most
clinicians on the staff emphasized his bipolar illness as the explanation for his crime.
But on close questioning he revealed doubts about his wife’s faithfulness and the pa-
ternity of one of their children. In my opinion, that was the real spark for his arson.
Larry, a man who had no prior criminal history but did have a history of depres-
sion and suicidal threats in earlier years, incurred within the space of six months
numerous domestic charges centered around a turbulent second marriage. The
charges included assault, threatening to burn down a house, and intimidating a
witness. His first marriage had not been marked with violence, and he had a son
with his first wife. He claimed that the marriage ended because of his ex-wife’s “con-
trolling nature.” He could never go fishing, he said. He claimed that fidelity was not
an issue—he had never been unfaithful to his first wife, nor, in his opinion, had she
to him.
Within a year of the first marriage’s end, Larry met and married his second
wife, who also had been previously married. Larry was aware that her first hus-
band had strayed and that she had retaliated with an affair. When Larry and his
second wife came into conflict, he accused her of infidelity, frequently drove by her
workplace, and became criminally hostile. In my opinion, it was the infidelity in his
second wife’s past that cued his hostile behavior, and his suspicions, unreported
to prior examiners, led to the assaults, threats, and intimidation. A remarkable
impression I took away from his evaluation was of his otherwise mild and quiet
nature.
Lee Harvey Oswald’s murder of President Kennedy drew fuel from male sexual
jealousy. The evidence is considerable and includes Oswald’s wife Marina’s testimony
to the Warren Commission that she was “in love” with JFK and that the President
reminded her of a medical student she regretted not marrying. Oswald’s mar-
riage to Marina was characterized by domestic abuse. Several months before the
assassination, Oswald discovered that Marina had been unfaithful while they were
still in Russia, where they met and married. These may be crucial pieces of Oswald’s
motive for murder (Thomson, Boissevain, & Aukofer, 1997).
Slip-up versus Homicide Adaptation
One of the current debates in the field is noted in Chapter 4 of this volume. It is the
slipup explanation for homicide (Daly & Wilson, 1988) versus dedicated mechanisms
for murder (Buss, 2005; see also Chapter 3 of this volume). Chagnon’s (1997) work
with the Yanomamo provides an unambiguous view of the adaptive logic of mur-
der. Helene Valero’s memoir of her years with the Yanomamo counters the idea of
murder as a “slipup.” The Yanomamo men she described knew when they intended
to wound versus when they were attempting to murder (Biocca, 1996). Daly and
Wilson (1988) put to rest the idea that a face-off between two men is ever “trivial.”
My reports no longer mention such incidents with puzzlement or veiled condescen-
sion. Although I disagree with their “slipup” hypothesis, their focus on the dynamics
of male–male competition and its centrality to homicide should guide forensic clini-
cians’ formulation of murder cases.
Charles, a 50-year-old man, was charged with the capital murder of his 33-year-
old wife. Her body was discovered in a motel; she was on her back, in bed, nude,
with her legs pulled up and knees splayed open. She died of a single gunshot wound
that had entered her right chest and pierced major blood vessels. The husband was
apprehended in another state several weeks later. He had visited his extended family
and appeared normal.
Charles claimed he had been in a car with his estranged wife when he pulled out
a gun and threatened to kill himself. She had grabbed the gun barrel, he said, and the
gun discharged and fatally wounded her. Charles checked into a motel, carried her into
the room, and tried to tend to her wound. She died, and he fled. That was his story.
There were holes in his story and major inconsistencies in the forensic evidence.
She was his third wife and he was her first husband. He was seventeen years older
than she was. She had been morbidly obese but had undergone bariatric surgery,
and had shed over one hundred pounds. Photographs indicated that at the time of
her death she was attractive. Several weeks before the murder, she moved out of the
home she shared with the defendant and into an apartment. Unbeknownst to the
defendant, but confirmed through others, she started to date two new men.
However, she maintained contact with her husband, and on the morning of the
murder she went to the home she formerly shared with him. He claimed they had sex
that morning, and that subsequently he drove her around town in the car, begging
her to return to their marriage. Charles had a well-documented history of depression
and substance abuse. In a prior marriage, he had been tried and acquitted of mari-
tal rape. Those problems became the focus of the efforts to have psychiatric evidence
of mitigation introduced at trial or sentencing in the current case. But the evidence
suggested a deliberate murder. The way the body was left suggested a warning. His
behavior fits Buss and Duntley’s (1998, 1999) model of mate homicide (see also Buss,
2005; Duntley, 2005). The latent intent was the literal death of a mate, a desire to
send a warning to other men, and the need to deprive rivals of the wife’s attentions.
Substance Use
For the practicing forensic clinician, one of the most important developments will be
an evolutionary understanding of substance use. Most criminal defendants suffer
from substance abuse and are intoxicated at the time of their offense. An article in
Science by Nesse and Berridge (1997) and a special issue of Addiction (2002) devoted
to evolutionary psychology views of substance abuse light the way forward.
Our brains are not designed for supernormal substances like drugs. Our drugs
of abuse invariably act directly on limbic areas of the brain and activate old instincts
that suggest a fitness benefit is right around the corner. Drugs dissolve a sober as-
sessment of a situation, leading to an inaccurate analysis of the cost of deploying
an ancestrally adaptive, but currently criminal, behavioral solution such as rape or
murder. Abused substances override negative emotions, particularly the emotions
that might inhibit these dangerous actions.
A Savanna IQ Goes to Court
Kanazawa’s ideas on a Savanna-IQ interaction (Chapter 9 in this volume) reorient

any assessment for competence, particularly competence to stand trial (CST). A
courtroom is a novel situation. Any forensic clinician will tell you that evaluations
have been conducted in which the evidence suggests the defendant is incompetent,
even though in other areas of his life he functions relatively well. In the following case,
the defendant also demonstrated the ability to deceive. Findings of incompetence like
these are often challenged. The Savanna-IQ Interaction Hypothesis provides a test-
able way of understanding these cases.
In 1960, the U.S. Supreme Court detailed the current standard for competency
to stand trial (Dusky v. United States, 1960). The standard is whether a criminal
defendant “has sufficient ability to consult with his lawyer with a reasonable degree
of rational understanding, and whether he has a rational as well as factual under-
standing of the proceedings against him.” All states have a similar test for compe-
tence. A defendant is found incompetent if, because of a mental disorder, either of
the following is true:
1. The defendant is unable to understand the nature and objectives of the court
proceedings.
2. The defendant is unable to assist in the defense.
A problem in either part of this test can lead to a finding of incompetence. The
standard for proving incompetence is a preponderance of the evidence (Cooper v.
Oklahoma, 1996) by the defense. The standard in federal courts is similar to state
standards. Incompetence is proven when the defendant “is presently suffering from a
mental disease or defect rendering him mentally incompetent to the extent that he is
unable to understand the nature and consequences of the proceedings against him
or to assist properly in his defense” (19 U.S.C. §4241).
This is translated into assessing a defendant in the following areas:
1. Charges. Does the criminal defendant understand the nature of the charges?
Is he or she knowledgeable of the official name of the charge? But it is more
important that the person understand the nature of the act that he or she is
accused of committing.
2. Severity of the charge. A criminal defendant should understand the severity,
whether it is a misdemeanor or a felony, and the possible range of sentence if
he or she is convicted.
3. Pleas. A criminal defendant is assessed to see whether he or she understands
various pleas that are available, including guilty, not guilty, no contest, or not
guilty by reason of insanity.
4. Courtroom personnel roles. A criminal defendant is assessed regarding his or her
understanding of the roles of the defendant, the defense attorney, the judge,
the commonwealth’s attorney/prosecutor, the jury, witnesses, and the victim.
5. The adversarial nature of the courtroom. A criminal defendant has to under-
stand which court personnel oppose his or her interest. The person must
demonstrate some self-protective awareness.
With regard to ability to assist their attorney, defendants are assessed in the
following areas: ability to work with their attorney; understanding of their current
legal situation; comprehension of plea bargaining; ability to enter, if applicable, a
mental illness defense; capacity to appraise evidence and outcome; memory and con-
centration for trial decision making; awareness of appropriate courtroom behavior;
consistent and organized narrative of the offense; and the presence of self-defeating
behavior (Simon & Gold, 2004).
David, a 29-year-old single man who lived with his father and stepmother, alleg-
edly approached a woman at a bus stop, poked her face with his finger, and, when she
walked away, pushed her from behind. He was charged with assault. A competency
evaluation was ordered upon the motion of his defense attorney.
He was given the Wechsler Adult Intelligence test (WAIS-III). His verbal IQ was
71, which falls in the range of borderline intellectual functioning and indicated
verbal performance in the lowest 3% of the population his age. His performance IQ
of 60 and his full-scale IQ of 64 were in the markedly impaired range of intelligence
classification and indicated nonverbal and overall intellectual test performance in
the lowest 1% of the population his age. His current WAIS subtest performances were
deemed valid estimates of his current and recent levels of intellectual abilities. Five of
six verbal subtest performances fell in the impaired range. His elevated performance
on the digit span subtest indicated ability for passive attention and accounted for his
verbal IQ score being above the markedly impaired range.
The psychologist who tested David thought his level of skills was more consistent
with a diagnosis of mental retardation than with a diagnosis of borderline intellec-
tual functioning.
David’s Competency Exam

The defendant was brought to the community mental health center by his step-
mother and was left in the waiting room. He responded to his name being called and
accompanied the evaluator to the interview room without problem. He presented as
a young male dressed in blue jeans, shirt, and winter coat. He wore running shoes,
and the clothes appeared clean. His hygiene seemed without problem.
He could maintain eye contact with the examiner. His speech was generally
coherent. His thought processes tended to be linear and logical, though, as will be
noted, he made some nonsensical responses. There was no evidence of psychosis. He
denied psychotic symptoms.
He was asked first if he knew why he was at the interview. He said that he
didn’t know. When asked if he currently had legal charges against him, he said he
didn’t know. He then spontaneously said that, “C [stepmother] told me I have to go
to court on March 30.” When asked about what matters involving him are before
the court, he said he did not know. He then said it was “something about a girl.” He
was asked if he remembered anything about it. He said, “It has to do with speaking
to somebody.”
He was asked if he had been to court before. He said, “Plenty of times, but I
wouldn’t know what it is about.” This struck the examiner as a spontaneous claim
that he was not competent.
When asked what the judge’s job in a court is, he said, “He speaks to you.” When
asked what other jobs the judge had, he said, “That’s all I know.” When he was pressed
about other roles for the judge, he said, “He places your bail, he gives you counseling
and help or he’ll suspend your license.” When asked what bail meant, he said, “Money.”
When asked what the money was for, he said, “Anything. For going to a store.”
He was asked if he knew his attorney’s name. He said he had forgotten. When
pressed, he said, “Susan.” When told that his attorney was Valerie [last name], he
mimicked the last name with a word approximate to it.
When asked what his attorney’s job is, he said, “She gives you counsel.” When
asked what that meant, he said, “Help. She asks me questions. She tells me if I am
right or if I am wrong.” When pressed about other functions of his attorney, he said,
“Those were all.” He was again asked the name of his attorney, and he said, “Valerie
[correct last name].”
When asked the commonwealth attorney or prosecutor’s job, he said, “They ar-
rest you. They talk to you for a long time. They ask you how you like things around
the community. They tell you about different jobs that are going on. They talk to
the judge. They answer the questions for the judge.” When asked who presented the
evidence against him, he said, “Anybody.”
When asked if he knew the term plea bargain, he said he did not know what it
meant. Interestingly, he spelled plea, “p-l-e-a.” When asked what “plea” meant, he
said, “It’s when someone talks to you about money. It means not having authority
over me. It means talking to you about committing a crime.” When asked if he knew
what “to plead guilty” meant, he said, “Arrested.”
When asked if he had ever heard of the terms confidentiality or lawyer/client
privilege, he said, “I’ve heard of privacy.” When asked what that meant, he said,
“Getting along with the next human being.”
When asked if anyone could order his attorney to reveal what they have talked
about, he said, “I don’t think so, unless it is coming from a judge or an attorney. It is
confidential. It is secret.”
When asked what he would do if someone on the witness stand said something
that was not true, he said, “I’d ignore it.” When asked if he would say something or
stand up or yell at the witness or to the judge, he said, “Only if it is the right thing
to do.”
David’s Mental Status Examination

David knew the name of the month, that the day was the eighth, and that the year
was 2007. When asked who the president was, he said, “Clinton?” When told it was
not Clinton and again asked who it was, he said, “George Bush.” When asked who
the vice president was, he said he did not know. Similarly, he did not know the name
of the governor.
He said he lived on [street address in Charlottesville] and in [a neighboring rural
county]. When asked whom he lived with, he said, “A bunch of brothers, Chris and
R. J.” He said they lived in [the rural county] and were “out and about.”
The examiner said that he understood he lived with his stepmother, C, and her
sons. He said he did live with them and “her husband.” When asked who lived in the
home, he said, “People that are close kin to a lot of people.” When it was pointed out
that the man he referred to as C’s husband was his father, he said, “Well, that just
ran off my mind.”
When asked again about whom he lived with, he said that he lived in Charlot-
tesville with his mother, his stepfather, and his stepmother’s two sons, Junior and
Germaine.
When asked if he watched television, he said yes. When asked what was in the
news, he said, “The weather, I watch the weather report.”
The above is just a brief part of a several-hours-long interview, but I hope it
gives the reader a sense of David. In my report to the court, I concluded that David
was not competent to stand trial. Aspects of his competency exam suggested that
he attempted to deceive me, wanting me to see him as incompetent. He responded
to several questions with answers he knew were false and would portray him as less
intelligent. For example, he referred to his father as his stepmother’s husband. His
attempt to deceive me was detailed in the report. However, even if he was aware
enough to feign greater deficits than he suffered, his history and baseline intel-
lectual function indicated to me that David was incompetent and not restorable to
competence.
This conclusion was challenged in court by the prosecutor. “If he could fake it,
being worse than he is, he could make it,” captures the prosecutor’s argument. A
second exam has been ordered. My contention is that the ability to deceive is ancient
and can be present even in individuals with demonstrable IQ problems that render
them incompetent.
Daubert and Darwin
The U.S. Supreme Court’s 1993 decision in Daubert v. Merrell Dow Pharmaceuticals,
Inc. substituted a test grounded in Karl Popper’s conceptualization of falsifiability as
a hallmark of science. That test requires the trial judge to consider whether the tech-
nique or theory has been tested and whether it has been subjected to professional
scrutiny through peer review and publication, whether it yielded an acceptable rate
of error, and whether it has been accepted by the relevant scientific community. This
will only help evolutionary psychology, which is committed to empirical validation.
All too frequently in the psychiatric profession, theories have been promulgated and
used in courts of law when they have little or no empirical support.
Teaching Forensic Evolutionary Psychology
How does one teach forensic evolutionary psychology to clinicians and court person-
nel? Walsh and Beaver (Chapter 2 of this volume) captured my attention with their
succinct explanation of the well-known sex differences in criminal behavior. Why is
the greatest risk factor for crime maleness? With crime, we must confront the natu-
ralistic fallacy. Crime is an abridgment of/challenge to the rules of cooperation and
reciprocity. If life is mating effort and parenting effort, and mating effort dominates
men’s lives far more than it does women’s, crime covers the behaviors that promote
mating effort: deceitfulness, impulsiveness, sensation seeking, and aggression.
The crucial corrective idea to teach is the Darwinian bedrock of criminal behav-
ior. Psychopathology, personality disorders, substance abuse, and the other diagnos-
able problems identified by clinicians are incapable of generating organized behavior
by themselves. Organized behavior requires a foundation of functional mechanisms
capable of producing the behavior in appropriate circumstances. The cognitive struc-
tures for the production of criminal behaviors were shaped by Darwinian natural
selection and the selection pressure created by recurrent exposure to contexts

affecting reproductive success.
Policy Implications
How many judges know that theft is a mammalian trait (see Chapter 9 of this
volume)? If they knew its origin and presence in the other animals, might they view
the crime differently when certain individuals appeared before them? How would
they view it when committed by an individual who suffers from chronic schizophre-
nia? One of my tasks is to evaluate the criminal defendant who is one of the chroni-
cally mentally ill and who has incurred enough misdemeanor charges to qualify as a
felon, a “habitual offender.” Designed to identify repeat offenders who are antisocial,
habitual offender laws routinely ensnare the chronically mentally ill who shoplift
cigarettes or food. Their clumsiness secures their easy arrest, and then their attor-
neys often ask whether they qualify for the insanity defense. Even though they are
ill, they know the nature and consequences of their actions. They know they are
breaking the law and risking arrest and prosecution. Their illnesses leave them poor
and relatively helpless.
In my state, since 1978, the life-or-death decision in capital murder cases rests
in part on whether the Commonwealth can prove, beyond a reasonable doubt, that
there is a probability that in the future the defendant “would commit criminal acts of
violence that would constitute a continuing serious threat to society” (Va. Code Ann.
19.2–264[C]). Since Virginia abolished parole, the only society to which a life-sen-
tenced capital defendant can pose a “continuing serious threat” is prison society. But
juries are not allowed to hear evidence of security and actual rates of violence in
Virginia’s prisons. None of this information can enter through mental health ex-
perts, and they must assess the defendant’s future dangerousness with no regard to
the environment in which he or she will live.
Mental health experts must base their capital mitigation prediction on past
history and other indices of risk. Often they approach the assessment as if they
are measuring the heat level of a continuously boiling cauldron of homicidal fury.
There remains little comprehension that violence is context dependent, the probable
deployment of adaptations. The prison that any capital murderers inhabit defangs
their violence. In 2005, there were 26,581 inmates in Virginia’s prisons, including
385 convicted of capital murders and 2,000 convicted of first-degree murder. There
was one homicide, nineteen aggravated assaults on other prisoners (0.61 per 1,000),
and two aggravated assaults on prison guards (0.06 per 1,000).
Society now sees life imprisonment without hope of release as both retributive
and protective. Cruelty has been replaced with long-term imprisonment. Death sen-
tences might decrease if court personnel, mental health experts, and juries were to
understand that violence is context dependent, with the deployment of homicidal
adaptations occurring only in certain situations. Those contexts have been effectively
removed by the prison system (Bruck, 2007).
Conclusion
My daughter’s attribution for someone of considerable intelligence is MOFO, her ac-

ronym for “Master of the F—— Obvious.” That captures my inevitable reaction
as I delve into evolutionary psychology and learn its explanations for human nature
and criminal behavior. I was trained in a psychodynamic model that often led to tor-
tured and incomprehensible formulations. They utilized little or no empirical sup-
port, showed wide subjectivity, and offered no avenue for empirical verification.
As this chapter was being written, Seung-Hui Cho slaughtered thirty-two people
at Virginia Tech. Mental health experts and forensic clinicians have weighed in with
diagnostic assessments of him, which include psychotic illnesses such as paranoid
schizophrenia. An evolutionary psychology–informed view is that he was extremely
socially isolated and inept as a result of Asperger’s, pervasive developmental disorder,
or another nonpsychotic impairment of social functioning. This fits with Lee Kirk-
patrick’s research that found people low in social inclusion and high in feelings of
superiority are most likely to be aggressive (Kirkpatrick, Waugh, Valencia, & Webster,
2002). The descriptions of Cho’s father’s isolation are eerily similar to the younger
Cho’s, but without the violent edge. There are no reports to date of psychotic epi-
sodes or symptoms in Cho. Marginalized, rebuffed in his clumsy attempts to approach
women, and defeated socially, he would be more clearly motivated by a desire to get
revenge against those who represented the people who had excluded him than by psy-
chopathology. Revenge is an evolved aspect of the mind that functioned as deterrence
(Daly & Wilson, 1988). Men’s pleasure from revenge and absence of empathy for
those they dislike are now verified and supported by fMRI studies (Singer et al., 2006).
Cho took his revenge on those who he felt were responsible for his marginalized sta-
tus. Unlike our ancestors, he had access to far deadlier weapons and could slaughter
many before being stopped. As much as we hate to entertain the idea, Seung-Hui Cho
may have been less distant from all men than we might at first believe.
As Duntley and Shackelford argue in their introduction to this volume, if knowl-
edge is like a river, new knowledge can raise the level of the water until it overflows its
banks. The new streams it creates, the landscape it submerges, and the hills that are
carved away cannot be predicted. Evolutionary psychology will be like an overflow
of knowledge, cutting new rivers through once familiar lands. We can only guess
how it will change the shape of the legal landscape. What will happen when judges,
prosecutors, and defense attorneys have familiarity with an evolutionary psychol-
ogy perspective? What will happen when expert witnesses who are called to provide
testimony are familiar with forensic evolutionary psychology? What happens when
we have new and different understandings of the various behaviors that bring people
into court?
The MOFO reaction doubtlessly occurs among those who have discovered the
evolutionary paradigm and its application to human nature. Darwin, more than
Freud, gave us the tools to understand defendants and the juries who pass judgment
on them. My hope and hypothesis are that as more forensic clinicians learn evolu-
tionary psychology, the more its discoveries will be applied to the forensic setting.
That will lead us closer to assisting courts and juries in answering the questions
about the defendants that have been with us for all of human history.
ACKNOWLEDGMENTS
Clare Aukofer worked her magic on my prose. Willis Spaulding sharpened the discussion
of intent and contributed insights about expert testimony and legal rules as institutional-
ized rules of thumb. Owen Jones made helpful comments on a late draft. Joshua Duntley
and Todd Shackelford gave me this opportunity, editorial assistance, and several superb new
ideas. Over many years I have learned from Marilyn Minrath, my partner in private forensic
practice, and Richard Bonnie, Bruce Cohen, Dewey Cornell, John Monahan, Daniel Murrie,
Eileen Ryan, William Stejskal, and Janet Warren, my colleagues at University of Virginia’s
Institute of Law, Psychiatry and Public Policy, who provide unparalleled opportunities for
case consultation.
Notes
1. Fortunately, in my opinion, the case was retried. At the first trial in 2002, Dr. Park
Dietz, one of the prosecution psychiatrists, testified about an episode of Law & Order in which
a woman got away with drowning her children in a bathtub by pleading insanity. During the
first trial, prosecutors suggested that Ms. Yates watched the show and saw it as “a way out.”
But it was soon discovered that no such episode existed. The conviction was successfully over-
turned, and a retrial led to the 2006 finding of not guilty by reason of insanity.
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Author Index
Abbey, A., 85 Aronfreed, J., 236, 239

Abbott, J. L., 256 Ashton, J. R., 72
Abracen, J., 177 Asnis, G. M., 257
Abrahams, N., 69 Aukofer, C., 277
Adams, H., 107 Avakame, E. F., 51
Adler, R. S., 85 Averett, S. L., 151
Agnew, R., 20, 31, 149 Avis, W. E., 167
Agyei, Y., 127 Avner, J. I., 88
Aksan, N., 241 Axelrod, R., 185, 186
Alcock, J., 21, 23, 73, 127
Alexander, J. F., 255 Bachman, J. G., 139
Alexander, P., 125, 128 Bachman, R., 54
Alexander, R. D., 126, 235, 237, 238 Badcock, C., 26
Allan, E. A., 163 Bailey, J. M., 127
Allen, T., 256 Bailey, R. O., 71
Allgeier, E. R., 66 Bailey, S., 254
Altizer, S., 9 Baker, D. D., 81
Alvarez, A., 54 Baker, L. A., 180, 188
Amateau, S., 26 Baker, R. R., 71, 107
Amato Henderson, S., 127 Baldessarini, R. J., 256
Andershed, H., 178, 180 Bandura, A., 52, 243
Anderson, E., 32 Bank, L., 252
Anderson, G. S., 140, 249 Barash, D. P., 70, 71
Anderson, I., 257 Barbaree, H. E., 177
Anderson, J. E., 130 Barclay, L. A., 88
Anderson, J. L., 131 Bargh, J. A., 93
Anderson, K. G., 72 Barker, D. J. P., 148
Andersson, M., 70 Barkow, J. H., 20, 21, 23, 101
Andrews, P. W., 113 Baron, L., 102
Apt, C., 69 Barr, K. N., 150, 181
Archer, J., 26 Barrett, E. S., 147
Archer, T., 257 Bart, P. B., 112
Argys, L. M., 151 Basile, K. C., 69
287
288 Author Index
Bates, J. E., 45 Boudouris, J., 54

Bates, M. E., 140 Bourgeois, M. J., 91
Baumeister, R. F., 254 Bourhis, A. C., 88
Beaver, K. M., 253 Bowers, A. H., 87
Becker, D. V., 66 Bowles, S., 235
Becker, R., 166 Boyd, R., 215, 235, 239
Beckerman, S., 28 Boyle, S., 123, 127, 129
Beier, K. M., 255 Braithwaite, J., 166
Belcher, L., 106 Bravo, E., 88, 94
Bell, R. Q., 179 Brecklin, L. R., 104
Bellinger, D. C., 258 Breene, R. G., 56
Bellis, M. A., 71, 72, 107 Brenner, R. A., 41
Benenson, J. F., 96 Brewer, G., 131
Bennett, G., 128 Brezina, T., 149
Benning, S. D., 178 Bridges, C. I., 256
Berdahl, J. L., 82 Bridges, G. S., 260
Berendes, H. W., 41 Bristowe, W. S., 56
Bergen, R. K., 69, 104, 109 Bröder, A., 113
Berger, R. L., 258 Brooks, L., 88
Berk, R., 259 Brosnan, S. F., 234
Berkowitz, L., 52 Broude, G. J., 101
Berridge, K., 278 Brown, D. E., 161, 238
Best, C. L., 101, 105 Brown, G. P., 9, 204
Betzig, L. L., 5, 29, 66, 89, 126, 207 Browne, K. R., 81, 82, 87, 89, 94
Bezdjian, S., 180 Brownmiller, S., 69, 70, 101, 125
Bingham, C. R., 145 Bruck, D. I., 284
Biocca, E., 277 Bruggers, D. J., 70
Birger, M., 258 Buchanan, A., 255
Birkhead, T. R., 71, 103 Budhani, S., 179
Blackburn, R., 178 Bugnyar, T., 209
Blair, J. R., 188 Bukovec, P., 104
Blair, K. S., 179 Bullough, V. L., 122, 131
Blair, R. J. R., 190, 191 Burch, R. L., 107, 108
Block, A. P., 8, 209 Burgess, A. W., 8, 210
Block, C. R., 213 Burke, T., 103
Blonigen, D. M., 178, 188, 189 Burns, J. T., 70
Blum, D., 165 Burt, M., 90
Blumenthal, J. A., 88 Burt, S. A., 180, 259
Blumstein, A., 152, 163, 260 Burton, V. S., Jr., 166
Bobadilla, L., 188 Buschhaus, N., 103
Boehm, C., 235 Busner, J., 254
Bogin, B., 31 Buss, D. M., 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 16,
Boissevain, J., 277 26, 42, 43, 44, 47, 48, 49, 50, 51, 57, 65,
Book, A. S., 146, 179 67, 68, 86, 91, 92, 95, 97, 101, 102, 105,
Booth, A., 32 126, 128, 129, 161, 166, 201, 202, 203,
Borges, S. S., 69 206, 207, 209, 210, 211, 214, 216, 217,
Bornschein, R. L., 258 220, 221, 222, 223, 268, 277, 278
Author Index 289
Butler, M. A., 178 Cleveland, H., 24

Butts, J. D., 223 Clinebell, S., 97
Buunk, B. P., 126, 151 Clutton-Brock, T. H., 89, 127, 145
Buyske, S., 140 Coates, C. J., 69
Bygott, J. D., 57 Cohen, D., 51
Byrne, R. W., 209 Cohen, L. E., 7, 208
Cohen, R. L., 236
Cadoret, R. J., 144 Colarelli, S. M., 92
Cai, B., 258 Colby, A., 242
Camilleri, J. A., 70, 73, 180 Collins, R., 31, 32
Campbell, A., 25, 28, 29, 146, 162, 163, 202 Conger, R. D., 252
Campbell, M., 256 Cooke, D. J., 180
Canfield, M. E., 160 Cooper, V., 96
Canfield, R. L., 258 Cormier, C. A., 176, 177
Capaldi, D. M., 146 Cornwell, C., 258
Carlson, S. R., 178, 188 Corr, P. J., 85
Carpendale, J. E., 242 Cosby, L., 146
Carroll, S. B., 188 Cosmides, L., 43, 46, 50, 53, 59, 101, 168,
Carter, L. A., 180 187, 208
Carter, T. J., 166 Costa, F., 145
Cartwright, J., 26 Costanzo, M., 224
Cashdan, E., 27 Cote, G., 54
Caspi, A., 24, 149, 151, 152, 179, 180, 187 Coughlin, C., 219
Cassedy, E., 88 Cox, V., 148
Cater, J., 256 Craig, W. A., 140, 150, 179
Cavel, T. A., 182 Craig, W. M., 181
Chagnon, N. A., 7, 8, 14, 43, 55, 208, 212, Crawford, C. B., 126, 128, 130, 131, 168
214, 215, 277 Crean, C. S., 103
Chaiken, S., 242, 243 Cressey, D., 51
Chalmers, L. J., 106 Crippen, T., 20
Chavanne, T. J., 86, 112, 113, 211 Crockett, C. M., 57
Chen, A., 258 Crockett, L. J., 145
Chen, S., 243 Crutchfield, R. D., 260
Cheng, K. M., 70, 71 Cullen, F. T., 149, 166
Cherry, C., 106 Cunningham, K. A., 257
Chesney-Lind, M., 25
Chiappe, D., 168 Dabbs, J. M., 89
Choe, J., 126 Daly, K., 25
Chong, D. S., 219 Daly, M., 6, 7, 8, 9, 10, 16, 22, 27, 32, 42, 48,
Chow, K., 143 50, 51, 52, 53, 55, 65, 67, 68, 70, 111,
Christakis, N. A., 154 126, 139, 143, 144, 146, 147, 150, 163,
Christian, D. L., 254 164, 202, 207, 210, 212, 213, 220, 223,
Christopher, F. S., 109 277, 284
Clark, R. D., 127 Damon, W., 238, 242
Clarke, D., 177 Daniele, A., 55, 67, 212
Cleckly, H., 178 Dansky, B. S., 101
Clelland, D., 166 Darroch, J. E., 259
290 Author Index
Darwin, C., 3, 16, 65, 234, 236, 238 Dunn, D. W., 103
Davidson, R. J., 257 Duntley, J. D., 6, 9, 10, 13, 42, 43, 47, 51, 68,
Davies, P., 168 201, 206, 207, 211, 214, 278
Davis, J. A., 107, 111 Dutton, D. G., 67
De Becker, G., 215 Dworkin, A., 125
de Waal, F. B. M., 160, 162, 170, 234, 241
Deakin, W. J. F., 257 Eagly, A. H., 52
Dean, K. E., 104, 109 Eaves, L. J., 259
Deary, I. J., 169 Eckland, B. K., 5, 203
DeCoster, J., 176 Edens, J. F., 178, 179
Dedden, L. A., 202, 207 Edmunds, C., 101
Defeis, E. F., 82 Edwards, C. P., 5, 207
Delaney, M. A., 256 Edwards, M. J., 179
Delisi, M., 253 Eggert, M., 257
DeMaris, A., 69 Ehrenreich, N. S., 88
DeNardo, M., 82 Eisenberg, N., 242
Denton, K., 242 Eisikovits, Z., 87
Derrickson, S. R., 71, 103 Eke, A. W., 176
Detrich, K. N., 258 Elias, J. E., 131
Devereux, C., 208 Elias, V., 131
Devinsky, O., 257 Elliot, D., 166
Devitt, M. K., 127 Ellis, B. J., 127, 129
DeVore, I., 4, 207 Ellis, L., 20, 24, 26, 30, 31, 54, 101, 249, 250,
Diamond, J., 216 251, 253, 254, 256, 257
Dietrich, K. N., 258 Ellison, P. T., 147
Dijkstra, P., 126 Elmore, M., 219
Dion, D., 186 Ember, C., 24
Dixson, A. F., 71 Ember, M., 24
Dobash, R. E., 67 Enosh, G., 87
Dobash, R. P., 67 Erhardt, D., 255
Dodes, L. M., 256 Ericson, J., 258
Dodge, K. A., 45 Eriksson, J. G., 148
Dolan, M., 257 Esteves, F., 222
Donnelly, D. A., 69 Euler, H. A., 68
Donohue, J., 259 Evans, T. D., 166
Donovan, J. E., 145, 154
Donovan, S. J., 257 Fagot, B. I., 146
Dougherty, D. D., 257 Fahy, T., 255
Douglas, K. S., 215 Farley, L., 93
Douma, B., 81 Farley, R. D., 56
Dower, J. W., 42 Farnworth, M., 167
Draper, P., 24 Farrington, D. P., 69, 109, 167, 252
Drass, E., 65 Federoff, J. P., 257
Dugatkin, L., 27 Fehr, E., 235
Dunaway, R. G., 166 Feldmann, T. B., 255
Dunbar, R. I. M., 237 Felson, R., 31
Dunkle, K. L., 69 Ferracuti, F., 51
Author Index 291
Festinger, L., 239 Garofalo, J., 54

Fiasse, L., 151 Gartner, R., 45
Fichtner, C. G., 255 Garver, C. E., 68
Fienberg, S. E., 258 Garver-Apgar, C. E., 113
Figlio, R. M., 168 Gattiker, U. E., 91
Figueredo, A. J., 67 Gaulin, S., 127
Figueredo, J., 108 Geary, D. C., 26, 250
Findling, R. L., 256 Gebhard, P. H., 121
Fink, G., 258 Geist, R. F., 8, 210
Finkelhor, D., 69, 70, 72, 109, 259 Gelles, R. J., 51, 69
Firestone, P., 67 Genovese, E. D., 93
Fishbein, D., 20, 25 George, T. S., 42
Fisher, G. A., 42 Ghiglieri, M. P., 8, 9, 43, 54, 56, 57, 105, 214,
Fiske, A. P., 241, 242 215, 219, 223
Fiske, S. T., 109 Gibson, E., 54
Fitzgerald, L. F., 82, 92, 109 Gibson, H. B., 168
Fonseca, A. C., 149 Giedd, J., 31
Foote, W. E., 82 Gigerenzer, G., 242
Fossey, D., 57 Gilburn, A. S., 103
Foster, B. R., 121 Gilliom, M., 179
Fowler, J. H., 154 Gillis, A. R., 51
Fox, H. C., 169 Gillis, J. S., 167
Fox, J. A., 259 Gilovich, T., 242
Freeman, R. B., 260 Giner-Sorolla, R., 243
Frei, G., 103 Gintis, H., 235
French, D., 124 Gladue, B. A., 127
Frick, P. J., 151, 179, 180, 182 Glass, S. J., 179
Friedle, E., 9 Glenn, A. L., 179
Frieze, I. H., 67, 69, 72 Glenn, N. D., 5, 203
Fromm-Auch, D., 168 Glick, P., 109
Frost, J. J., 259 Gluckman, P. D., 148
Fu, P., 184 Godfrey, K., 148
Fujita, K., 208 Goetz, A. T., 67, 68, 69, 70, 71, 72, 73, 109,
Funder, D. C., 149 110
Furchtgott-Roth, D., 161 Golant, S. K., 67
Gold, L. H., 271
Gabrielli, W. F., 168 Goleman, D., 221
Gachter, S., 235 Gomendio, M., 71
Gächter, S., 235 Goodman-Delahunty, J., 82
Gage, A. J., 67, 69, 70, 72 Goodwin, D., 71
Gaither, G. A., 127 Gottfredson, L. S., 169
Gallup, G. G., 71, 108, 113 Gottfredson, M. R., 30, 55, 139, 140, 149,
Gallup, G. G., Jr., 86, 107, 111, 210 163, 253
Gangestad, S. W., 68, 106, 113, 126, 128, 129, Gottschall, J. A., 104, 107, 108, 273, 275
142, 167 Gottschall, T. A., 104, 273, 275
Garland, E. J., 255 Gould, S. J., 56
Garner, B. A., 121 Gouldner, A. W., 238
292 Author Index
Gove, W. R., 52 Hart, S. D., 180

Gowaty, P. A., 103 Hartley, I. R., 5, 203
Grammer, K., 5, 203 Harvell, D., 9
Grann, M., 177, 255 Harvey, P. H., 143
Gray, P. B., 147 Haselton, M. G., 49, 67, 86, 113, 126,
Greenberg, D. F., 163 220, 221
Greenberg, J., 236, 243 Haslam, N., 241
Greene, S. J., 101 Hatfield, E., 127
Greenfield, L., 105 Hausfater, G., 57
Greiling, H., 48, 129, 223 Haynie, D., 31
Griffin, D., 242 Heath, A. C., 259
Grogger, J., 165 Heatherton, T. F., 254
Gross, M. R., 184 Hebl, M. R., 91
Grossman, J. B., 252 Heerwagen, J. H., 220
Grossman, L. S., 255 Held, S., 208
Grumberg, F., 54 Henderson, C. R., 258
Grumet, B., 54 Hennen, J., 256
Guadagno, R. E., 66 Henry, B., 252
Gutek, B. A., 81, 85, 88, 89, 92, 93, 95 Herrmann, B., 235
Herrnstein, R. J., 167, 169
Haaland, S., 92 Hickey, N., 179
Habermas, J., 238 Hicks, B. M., 178, 180
Hackstian, A. R., 178 Higgins, E. T., 239
Hadi, A., 69, 109 Hill, E. M., 144
Hagan, J., 51 Hill, K., 5
Hagen, E. H., 219 Hill, M. H., 143, 144
Haidt, J., 231, 238, 242, 243 Hillbrand, M., 257
Haig, D., 218 Hilton, N. Z., 176, 179, 180
Haldane, J. B. S., 9 Hindelang, M. J., 54, 167
Hall, G. C. N., 107 Hinshaw, S. P., 255
Hamilton, W. D., 7, 185, 201, 202 Hirsch, D. J., 42
Hanneke, C. R., 72 Hirschi, T., 30, 139, 149, 163, 167, 253
Hansen, H., 255 Hirschman, R., 107
Hanson, M. A., 148 Hodgins, S., 54, 255
Harcourt, A. H., 71 Hödl, W., 103
Hare, R. D., 176, 177, 178, 180, 256 Hoek, H. W., 149
Harer, M. D., 163 Hoffer, C., 130
Harkins, L., 177 Hoffman, L., 97
Harpending, H. C., 24, 183, 186 Hoffman, M., 69
Harpur, T. J., 178 Hohmann, N., 113
Harrer, G., 255 Hoi, H., 71
Harrington, R. C., 254 Hoier, S., 68
Harris, A. R., 42 Holden, R. R., 179
Harris, G. T., 70, 105, 109, 140, 141, 148, 150, Hollabaugh, L. C., 87
176, 177, 179, 180, 181, 182, 183 Holmes, C., 31
Harris, Y. R., 254 Holmes, M. M., 105
Hart, D., 238 Holmstrom, L. L., 8, 210
Author Index 293
Hong, L. K., 107 Jones, L., 259

Horowitz, I., 20 Jones, O. D., 16, 202, 273, 275
Houghton, R. E., 176 Jonsson, G., 259
Houser, D., 186 Joyce, T., 259
Hoyenga, K. B., 52 Juliano, A., 88, 91
Hoyenga, K. T., 52
Hrdy, S. B., 58, 219 Kafantaris, V., 257
Hubbin, M., 105 Kahlenbreg, S. M., 147
Hughes, K., 72 Kahneman, D., 242
Hughes, S., 72 Kalichman, S. C., 106
Hugues, J. N., 182 Kalus, O., 257
Huizinga, D., 166 Kanazawa, S., 20, 22, 26, 161, 163, 164, 165,
Hulin, C. L., 82 166, 167, 168, 169, 170
Hull, N., 130 Kandel, D. B., 252
Hunter, F. M., 71 Kanin, E. J., 108
Hurlbert, D. F., 69 Kaplan, H., 142
Hurt, L. E., 88 Kaplan, S. L., 214, 254
Hurtado, A. M., 5 Kappeler, P. M., 234
Hutchinson, P. L., 67 Karlberg, J., 219
Hutsler, J., 130 Karnieli-Miller, O., 87
Karras, R. M., 122, 129
Iacono, W. G., 178, 180, 188, 259 Kasachkoff, T., 238
Ingoldsby, E. M., 179 Kasper, S., 256
Insel, T. R., 219 Kelly, J. M., 82
Isohanni, M., 54 Kendall-Tackett, K. A., 219
Kenrick, D. T., 16, 57
Jackson, A. H., 256 Keough, E., 69, 109
Jackson, C. J., 85 Kerr, M., 178
Jacobs, B., 29 Khoo, P. N., 85
Jacobson, K. C., 180 Kiehl, K. A., 189, 190
Jaffee, S., 24 Kilgallon, S. J., 71, 110
Janicki, M., 242 Kilpatrick, D. G., 101, 105, 109, 114
Jehle, R., 103 Kilpatrick, J., 97
Jennings, K. D., 219 Kimura, D., 26
Jensen, A. R., 169 King, E. G., 91
Jernigan, T., 31 Kirkpatrick, L. A., 284
Jessor, L., 145 Klein, N. C., 255
Jessor, R., 139, 145, 154 Klein, R. G., 252
Jewkes, R., 69 Klinger, B., 54
Johansson, P., 178 Knopps, G., 219
Johnson, H., 69 Koch, S., 66
Johnson, K. P., 89 Kochanska, G., 241
Johnson, L., 90 Kofler-Westergren, B., 255
Johnson, M. P., 69, 70 Kohlberg, L., 231, 241, 242, 243
Johnson, R. E., 166 Koiranen, M., 54
Johnston, L. D., 139 Koss, M. P., 69
Johnston, M. A., 128 Kosson, D. S., 179
294 Author Index
Kostowski, W., 257 Leistico, A. R., 176, 179

Kotler, M., 54 Leve, C. S., 146
Kotov, R., 179 Levine, C. G., 242
Kotrschal, K., 208 Levitt, S. D., 152, 259, 260
Koziol-McLain, J., 69 Lichtenstein, P., 180
Krauss, G. L., 257 Lilienfeld, S. O., 178, 179
Krebs, D. L., 241, 242, 243 Lilienfield, S. O., 253
Kreher, D. A., 258 Linder, J. E., 103
Kreuger, R. F., 259 Lindqvist, P., 54
Krill, A. L., 106 Linnoila, M., 257
Kristin, A., 71 Linton, M. A., 109
Krueger, M. M., 105 Lipson, S. F., 147
Krueger, R. F., 178, 188 Liu, S., 257
Kruger, D. J., 9 Livesley, W. J., 177
Kubicka, L., 259 Loeber, R., 180
Kulik, C. T., 88 Lohr, B., 107
Kullgren, G., 255 Loney, B. R., 178, 179, 180, 188
Kunda, Z., 243 Lonsway, K. A., 109
Kupietz, S., 254 Looman, J., 177
Kuroshima, H., 208 Lopreato, J., 20, 26
Kurzban, R., 186 Lorenz, F. O., 252
Kwaramba, R., 69, 109 Louden, J. E., 178
Low, B. S., 127, 144, 145
La Grange, L., 257 Lowenstein, S. R., 69
Laakso, M. P., 191 Lozano, D. I., 180
Lafontaine, E., 93 Luebbert, J. F., 256
Laird, P., 243 Lumey, L. H., 149
Lake, T. M., 106 Lummaa, V., 148
Lalumière, M. L., 70, 71, 72, 104, 105, 106, Lundqvist, D., 222
107, 108, 109, 139, 140, 141, 145, 146, Luttrell, L. M., 160, 234
147, 148, 150, 151, 152, 179, 181, 182 Lykken, D. T., 188
Langford, D., 179 Lynam, D. R., 168, 180, 252
Langkilde, T., 103
Langstrom, N., 255 Maccoby, E., 5
Langton, C. M., 177 Maccoon, D. G., 178, 180
Larsen, R. J., 65, 106 MacDonald, K., 168
Larsen, R. R., 5, 203 Machalek, R., 7, 208
Larson, C. L., 257 Madera, J. M., 91
Larsson, H., 180, 189 Magley, V. J., 82
Laub, J. H., 148, 165 Maguigan, H., 223
Laubscher, R., 69 Magurran, A. E., 103
LeBlanc, G. J., 65 Malamuth, N. M., 67, 104, 105, 106
LeDoux, J., 190 Maletzky, B. M., 255
Lee, K., 81 Malone, R. P., 256
Lee, L., 124 Manza, J., 260
Lee, R. B., 43 Marcovitz, H., 124
Leimar, O., 151 Marcus, D. K., 179
Author Index 295
Marini, J. L., 256 Moffitt, T. E., 24, 30, 140, 168, 179, 180, 188,
Marinoff, L., 186 251, 252
Marken, P. A., 256 Mokdad, A. H., 154
Marks, I. M., 216 Moltó, J., 178
Marlowe, J. H., 252 Monastra, V. J., 258
Marquis, P., 177 Montañés, S., 178
Marsh, L., 257 Moracco, K. E., 223
Marshall, I. H., 214 Morales, V. Z., 219
Martis, B., 255 Morgan, A. P., 253
Mason, D. A., 151 Moring, J., 54
Mason, R. T., 103 Morris, N. M., 113
Masuda, T., 208 Mouzos, J., 54
Mattes, J. A., 257 Muehlenhard, C. L., 87, 109
Matykiewicz, L., 257 Muller-Spahn, F., 256
Mauro, C. F., 254 Munro, G. E. S., 179
Maynard Smith, J., 7, 103 Murphy, R. O., 81
Mayr, E., 125 Murray, C., 169
Mazur, A., 29, 32, 165
McBurnett, K., 254 Nachimson, D., 106
McCarthy, M., 26 Nagin, D. S., 165, 179
McClosky, L. A., 67 Nath, J. K., 124
McCrae, R., 32 Nayer, V. R., 124
McCrory, E., 179 Ndlovu, M., 69, 109
McFarland, B., 255 Neale, M. C., 259
McFarland, C., 258 Needleman, H. L., 258
McGue, M., 180, 188, 259 Neff, B. D., 184
McIntyre, M., 166 Ness, R. B., 258
McKay, H. D., 166 Nesse, R. M., 9, 218, 237, 278
McKibbin, W. F., 68, 72, 110 Nettle, D., 67, 113, 150
McKinney, F., 70, 71, 103 Neumann, C. S., 180
McLarnon, M. E., 178 Nevin, R., 258
McMahon, R. J., 254 Newman, J. P., 178, 179
Mealey, L., 22, 87, 105, 179, 183 Nguyen, D.-H., 85
Mears, D., 25 Nicastle, L. D., 66
Mednick, S. A., 168, 179 Niehoff, D., 45
Mehlkop, G., 166 Nilsson, D. E., 4
Mendl, M., 208 Nisbett, R. E., 51
Mesnick, S. L., 111, 211 Noonan, K. M., 126
Messerschmidt, J., 29 Nowak, M. A., 235
Meyer, S., 69 Nuegebauer, R., 149
Michalek, J., 165 Nunes, E. V., 257
Michie, C., 180 Nye, F. I., 166
Milgram, S., 242
Millevoi, A., 66 O’Brien, R. M., 260
Mineau, P., 71, 103 O’Connell Davidson, J., 127
Mishra, S., 139, 152 O’Connor, M., 97
Mitani, J. C., 104 O’Donohue, W., 87
296 Author Index
O’Leary, K. D., 69 Piaget, J., 231, 238, 241

O’Malley, P. M., 139 Pinker, S., 43, 250
Öhman, A., 222 Pizzari, T., 103
Olsson, M., 103 Platek, S. M., 66, 106
Orians, G. H., 220 Plath, M., 103
Oros, C. J., 69 Plaud, J., 127
Osgood, D. W., 139 Plaznik, A., 257
Oshige, M., 88 Ploeger, M., 25
Osmond, C., 148 Plomin, R., 180, 188
Ouimet, M., 152 Podratz, K. E., 91
Overpeck, M. D., 41 Polanczyk, G., 254
Owens, L. A., 109 Polis, G. A., 56
Oxford, M., 182 Popper, S., 219
Porter, S., 176
Pagelow, M., 71 Post, R. M., 257
Painter, K., 69, 109 Pound, N., 71, 109, 110
Palmer, C. P., 101, 102, 103, 106, 107, 109, Poy, R., 178
112, 130, 273, 275 Poythress, N. G., 178, 179
Palmer, C. T., 72, 86, 89, 163, 202 Pratto, F., 5, 207
Papinczak, T. A., 219 Preston, K., 94
Parker, G. A., 5, 71, 89, 109, 127, 203 Prince, D. A., 123
Parsons, B. V., 255 Pryor, J. B., 93
Parzefall, J., 103 Purtnam, K. M., 257
Pastor, M. C., 178 Pyett, P., 129
Patrick, C. J., 27, 178, 188 Pyszczynski, T., 243
Patten, S. B., 153
Patterson, G. R., 52, 252 Quadango, D., 31
Paul, B., 105 Queen, C., 129
Peacock, E. J., 177 Quinsey, V. L., 32, 70, 105, 106, 107, 109, 140,
Pears, K. C., 146 141, 150, 176, 177, 179, 181, 188
Peat, J., 252
Peirce, E. R., 85 Radcliffe, J., 258
Pelger, S., 4 Rafter, N. H., 249
Pelham, W. E., 255 Raine, A., 27, 179, 180, 191, 257, 258
Pellatt, J. E., 71 Rapoport, J. L., 256
Perkins, J., 91 Rasanen, P., 54
Perkins, R., 128 Ratchnevsky, P., 89
Perot, A. R., 88 Rattner, A., 30
Perry, E. L., 88 Rawlings, R., 257
Pérusse, D., 5, 161, 167, 203, 207, 259 Rawls, J., 232, 233
Peters, J., 67 Raymond, P., 93
Peterson, D., 58, 103, 219 Ream, S. L., 86
Petralia, S. M., 112 Rees, D. I., 151
Pettit, G. S., 45 Reid, J. B., 252
Phillips, B., 9, 204 Reise, S. P., 127
Phillips, D. P., 143 Reiss, D., 259
Phillips, D. I. W., 148 Resnick, H. S., 101, 105, 114
Author Index 297
Restak, R., 31 Sanday, P. R., 101

Reyer, H.-U., 103 Sander van Doorn, G., 151
Reyes, E., 257 Sanders, T., 124, 127
Reymert, M. L., 252 Saraiya, M., 217
Rhee, S. H., 45, 189 Saroglou, V., 151
Rice, M. E., 70, 105, 109, 140, 141, 148, 176, Sauer, K., 103
177, 179, 180, 181, 182 Saunders, B. E., 101
Rice, W. R., 103 Scarpa, A., 27
Richell, R. A., 179 Scheib, J., 111
Richerson, P. J., 215, 239, 243 Scheppele, K. L., 112
Ringdal, N. J., 122, 123 Schinazi, J., 96
Ris, M. D., 258 Schlupp, I., 103
Robbins, M. M., 103 Schmitt, D. P., 6, 7, 67, 91, 128, 145, 208
Roberts, N., 257 Schneider, B. E., 93
Robinson, M., 20 Schulsinger, F., 168
Robinson, S., 148 Schützwohl, A., 66
Rodriquez, M., 252 Schwab, S. J., 88
Rogan, W. J., 258 Scrambler, G., 124
Rogers, A. R., 143 Scrimshaw, S., 130
Rogers, R., 176, 179 Seaman, J. A., 95
Roldán, E. R. S., 71 Segal, B., 254
Rose, N., 249 Segarra, P., 178
Rosenfeld, L. V., 87 Sekulic, R., 57
Rosenfeld, R., 260 Sellin, T., 168
Rosie, R., 258 Selman, R. L., 239, 240
Rosler, A., 255 Semmelroth, J., 5, 65, 203
Ross, L. T., 144 Semonsky, M. R., 87
Ross, S., 219 Senn, C. Y., 85
Rotundo, M., 85 Serbin, L. A., 146
Rowe, D. C., 20, 22, 23, 24, 27, 163 Serin, R., 177
Royle, M. J., 5, 203 Serran, G., 67
Rozée, P. D., 101 Seto, M. C., 106, 177
Rubinow, D. R., 257 Sewell, K. W., 179
Rudnik-Levin, F., 254 Seymour, A., 101
Ruff, J. R., 42 Seymour, N. R., 71
Ruffie, J., 21 Shackelford, T. K., 4, 6, 9, 32, 47, 65, 66, 67,
Rummel, R. J., 51 68, 71, 72, 106, 109, 110, 111, 126,
Russell, D. E. H., 67, 69, 70, 72, 109 202, 208
Ruth, T. E., 143 Shader, R. I., 256
Rutter, M., 122, 140 Shavit, Y., 30
Shaw, C. R., 166
Sackett, P. R., 85 Shaw, D. S., 179
Sagarin, B. J., 66 Sheard, M. H., 256, 257
Salekin, R. T., 176, 179 Sheets, V., 16, 58
Salmon, C., 126, 127, 128 Shelton, B., 122
Saltzstein, H. D., 237 Shelton, K. K., 182
Sampson, R. J., 165 Sheppard, J. A., 167
298 Author Index
Sherman, P. W., 73 Starratt, V. G., 68, 72, 110

Shields, L. M., 105 Starzyk, K. B., 179
Shields, N. M., 72 Stecker, H. L., 109
Shields, W. M., 105 Steel, P., 81
Shine, R., 9, 103, 204 Steem, J., 180
Shipman, P., 52, 212 Steffensmeier, D. J., 163
Shondrick, D. D., 107 Stein, Z. A., 149
Short, J. F., 166 Sterns, S. C., 142
Sigmund, K., 235 Stewart, G. R., 71
Silva, P. A., 168, 252 Stewart, M. A., 144
Silverthorne, P., 182 Stewart-Williams, S., 72
Simmons, L. W., 71, 110 Still, M. C., 161
Simon, R. I., 271, 279 Stockdale, M. S., 87
Simon, W., 124, 127 Stolba, C., 161
Simons, R. L., 252 Stolen, P., 71
Simpson, G. M., 256 Stouthamer-Loeber, M., 146, 168, 180, 252
Simpson, J. A., 51, 113, 128, 167 Strack, F., 93
Singer, P., 238 Strassman, B. I., 126
Singer, T., 206, 284 Strathman, A. J., 167
Singh, S., 259 Straus, M. A., 69, 70
Skeem, J., 178, 179 Strauss, M. A., 51
Skilling, T. A., 140, 150, 176, 177, 178, Strayhorn, J. M., 254
179, 181 Streifel, C., 163
Slaving, S., 122 Struckman-Johnson, C., 85
Slosnerick, M., 126 Struckman-Johnson, D., 85
Smith, D. A., 166 Studd, M. V., 91
Smith, J. M., 103 Succop, P. A., 258
Smith, P. H., 223 Sugiyama, L. S., 106, 208
Smith, P. K., 219 Sullivan, B., 123, 131
Smith, R. L., 71 Sullivan, E., 124, 127
Smuts, B. B., 103, 111, 211 Sullivan, M. A., 254
Smuts, R. W., 103 Sulloway, F. J., 150
Snyder, G., 124 Sumner, B., 258
Sober, E., 238 Sunstein, C. R., 242
Sobus, J., 183 Susser, E. S., 149, 257
Solhkhah, R., 257 Sutherland, E. H., 51
Soltis, J., 219 Swanson, J. M., 254, 255
Som, C., 103 Sweet, M. H., 56
Sorenson, S., 259 Swogger, M. T., 179
Sowell, E., 31 Symons, D., 8, 65, 111, 126, 127, 128, 129,
Spear, L., 31, 32 130, 131, 168, 210
Spencer, H. G., 148 Sztatecsny, M., 103
Spinelli, M. G., 219
Spitz, A. M., 259 Tang-Martinez, Z., 102
Staller, J. A., 257 Tangri, S. S., 90, 92
Stanley, K., 94 Targum, S. D., 256
Stark, R., 167 Tattersall, I., 52
Starr, J. M., 169 Taveras, E. M., 219
Author Index 299
Taylor, A., 24 Ullman, S. E., 104

Taylor, J., 178, 188 Upchurch, D., 259
Taylor, P. A., 5, 203
Tengstrom, A., 255 Vahed, K., 103
Terpstra, D. E., 81 Vaillant, G. E., 148
Thomas, J., 256 Valencia, A., 284
Thomas, S. H., 42 Valera, F., 71
Thompson, A. P., 126 Van den Berghe, P., 20
Thompson, P., 31 van den Oord, E., 24
Thomson, J. A., 277 van Elst, L. T., 257
Thornberry, T. P., 167 Van Praag, H. M., 257
Thornhill, N. W., 70, 105, 106, 111, 114 van Schaik, C. P., 233
Thornhill, R., 68, 70, 72, 86, 89, 92, 101, 102, Vance, P., 257
103, 104, 105, 106, 107, 108, 109, 111, Vasey, M. W., 179
112, 114, 129, 130, 163, 202, 211, Vazquez, C., 219
273, 275 Venables, P. H., 179
Thornton, D., 177 Vermeulen, S. C., 242
Tibbetts, S., 23 Viding, E., 180, 188, 189
Tice, D. M., 254 Vigil, J. D., 54
Tierney, J. P., 252 Villemez, W. J., 166
Tiger, L., 84 Vincent, A. C. J., 145
Tiihonen, J., 54, 190 Virkkunen, M., 257
Titman, R. D., 71 Vivian, D., 69
Tittle, C. R., 166 Vizard, E., 179
Tobin, M. J., 258 Vuchinich, S., 220
Toga, A., 31
Tolan, A., 255 Wagner, E., 256
Tondo, L., 256 Wagner, L. M., 143
Tonry, M., 260 Waldman, I. D., 45, 189
Tooby, J., 4, 43, 46, 50, 53, 101, 168, 187, Walker, C., 256
207, 208 Walker, E., 31
Tooke, W., 6 Walker, L. E., 67
Townsend, J. M., 127 Wallace, C., 54
Tracy, K., 126 Wallman, J., 152
Tredeau, L., 93 Walsh, A., 20, 23, 24, 25, 31, 33,
Trifiletti, L. B., 41 54, 259
Trimble, M. R., 257 Walsh, E., 255
Trinkhaus, E., 53, 211 Wang, M., 257
Trivers, R. L., 4, 6, 67, 102, 145, 203, 219, Wang, W., 71
234, 235, 236, 239, 240 Warr, M., 25, 30
Troughton, E., 144 Wasilkiw, L., 179
Trumble, A. C., 41 Wassermann, E., 254
Turner, C. T., 219 Watts, C., 69
Watts, R., 109
Udry, J. R., 20, 113 Waugh, C. E., 284
Udry, R. R., 5, 203 Webster, C. D., 215
Uggen, C., 260 Webster, G. D., 284
Uhde, T. W., 257 Weekes-Shackelford, V. A., 9, 68
300 Author Index
Weghorst, J., 66 Witoonchart, B., 151

Weghorst, S. J., 126 Witztum, E., 255
Wegner, D., 49 Woermann, F. G., 257
Wei, E. H., 146 Wolf, M., 151
Weinrott, M. R., 252 Wolfgang, M. E., 51, 168
Weis, K., 69 Woodworth, G., 144
Weiss, D. L., 126 Woodworth, M., 176
Weissing, F. J., 151 Woolfenden, S. R., 252
Weitzman, L. M., 92 Wootton, J. M., 182
Wells, K. C., 254 Workman, J. E., 89
Werner-Wilson, R. J., 6, 203 Worrell, J. A., 256
West, D. J., 168 Wrangham, R., 57, 103, 104, 219
Westen, D., 5, 65, 203 Wright, J. P., 149, 253
Whalley, L. J., 169 Wright, R., 29, 238
White, A., 30, 32 Wright, T. M., 127
White, G. L., 65 Wu, C. I., 71, 252
White, H. R., 140 Wyckoff, G. J., 71
Whiteman, M. C., 169
Whiting, B. B., 5, 207 Yagil, D., 87
Widiger, T. A., 180 Yang, Y., 257
Wiebe, D., 259 Yates, W. R., 144
Wiebe, R., 24 Yeo, R. A., 106
Wiederman, M. W., 66 Yeudall, L. T., 168
Wiener, R. L., 88 Yllo, K., 69, 70, 109
Wigal, T., 254 York, K. M., 88
Wilcox, J. A., 257 Young, J. L., 257
Wille, R., 255 Yuan, W., 258
Williams, E. A., 106 Yule, W., 149
Williams, G. C., 218, 237
Williams, K., 252 Zahavi, Amotz, 146
Willness, C. R., 81 Zahavi, Avishag, 146
Wilson, D. S., 238 Zajack, A. B., 88
Wilson, J. Q., 167, 239 Zammuto, R. M., 143
Wilson, M., 6, 9, 22, 42, 48, 51, 55, 66, 67, 70, Zerjal, T., 89
72, 111, 126, 144, 146, 147, 154, 163, 164, Zimring, F., 259
202, 211, 277 Zurbriggen, E. L., 93
Subject Index
Adaptations. See also homicide adaptation hormonal surges, 31, 33

theory risk-taking behavior, 140, 163
of adolescence, 32 transitional problems of, 30 – 31
Australia/U.S. environmental activation of Adolescence-limited delinquency, 140
adaptations, 54 Adulthood
coevolutionary theories of, 9 onset of delinquent behaviors, 140, 149
cognitive adaptations, 4, 5 responsibilities of, 30 – 31
compromises of, 11 risk-taking behavior, 140, 163
from cooperation, 232 African Americans
for cost-infliction, 15 hierarchy of prostitutes, 122
and creation of criminal behavior, 14 male homicide rates, 41 – 42
to damage status, 207 Against Our Will: Men, Women, and Rape
evolutionary psychology use of, 3 (Brownmiller), 101
against homicide, evidence of, 217 – 221 Age
homicide as by-product of, 55 – 58 age-specific risk rates, 143
leading to conflicts (between individuals), and crime, 24, 30 – 32
203 – 204 life-history analysis factor, 142 – 143
physiological/functional adaptations, 4 and likelihood of being killed, 41 – 42
productive of homicide, 210 – 212 and rape, 105, 106
productive of rape, 210 relation to antisocial tendencies, 146 – 147
by psychopaths, 5 relation to crime (age-crime curve),
and selective pressure creation, 13f 163 – 164, 171
for sexual coercion and rape, 104 – 111 and sexual harassment, 91
disadvantaged males, 106 and theft, younger vs. older men, 162 – 166
high mating-effort rapists, 108 – 109 “Age and Explanation of Crime” article (Hirschi
opportunistic rapists, 108 and Gottfredson), 163
partner rapists, 109 – 110 Aggression
specialized rapists, 106 – 108 childhood, and early pregnancy, 146
of stepchildren, 220 coalitional aggression, 14, 95
for theft and cheating, 208–209 in juvenile delinquents, 251
by victims, 10, 201 – 224 lethalness of, 42, 45, 50, 53 – 55, 56
for violence, 209 of life-course-persistent offenders, 140
Adaptationist research, implications of, 15 – 16 link with epilepsy, 257
Adolescence. See also juvenile delinquents male vs. female, 25 – 26, 29, 95
brain development during, 30 – 31 medication for, 254 – 255, 256
environmental influences on risk-taking and psychopathy, 177, 180 – 181, 186
behaviors, 30 and rape/sexual coercion, 27, 103, 108, 275
301
302 Subject Index
Aggression (continued ) “Because of sex” issue, sexual harassment, 94–96

as self-defense, 214 Behavior. See criminal behavior; human beings,
sex differences in, 26 behavior of
and spousal violence, 202 Biological sciences
and testosterone, 31, 258 impact of, 20 – 21
usefulness for mating efforts, 23 molecular genetic studies, 54
Alcohol use/abuse, 54 Bipolar disorder, 54, 276
Altruism, 7, 22, 65 Birth cohort (Northern Finland) studies, 54
reciprocal altruism, 23, 235 Black widow spiders, conspecific killings, 56
Ambulance-Homicide Theory, 42 Bluegill sunfish, frequency-dependent selection
American Old West, and prostitution, example, 184
122 – 123 Bodyguard hypothesis (of women) against rape,
Ancestral problem considerations of homicide, 111
44 – 45 Botswana, homicide rates, 43
Anti-androgen medication, 255 Brains. See also Savanna-IQ Interaction
Anticonvulsant medication, 257 Hypothesis
Antipsychotic medication, 255 – 256 of adolescents, 30 – 31
Antisocial personality disorder (APD), cognitive and emotional networks, 27
177, 190 development of, and natural selection, 102
Antisocial tendencies. See also life-course- hormonal influences on, 26
persistent offenders; psychopathy; informational incorporation capacity, 21
risk-taking behavior information-processing mechanisms of, 102
decisions for engagement, 143 – 144 injuries of, and antisocial behavior, 140
developmental pathways male vs. female, 26
adolescence-limited delinquency, 140 “massively modular” functioning of, 172
life-course-persistent offending, 140 – 141 prefrontal cortex (PFC), MRI studies, 31
genetic mutation associations, 181 of psychopaths, 189 – 191
substance use, 151, 278 Brothels, sanctioning of, 122 – 123
types of behaviors, 140
Anxiety, protective aspects of, 215, 216 “Cad” tactics of males, 27 – 28
Arms races, of species, 9 – 12, 12, 41, 45, 204, Canada
218, 224 homicide records, 7
Assault psychopathology statistics, 54
alliances as protections against, 111 punishment of prostitutes/clients, 123
as cost-inflicting strategy, 202 vs. U.S. rates of killing, 42
costs of, for potential victims, 14 Capital murder cases, 268
as evolved adaptation, 22 Charles, murder of wife, 277
medication for reduction of, 256 insanity defense, 271 – 273
and opportunistic rape, 108 life-or-death decisions (policy implications),
reasons for, 251 283
by romantic partners, 223, 276 Willie and Steve, murder of father, 269 – 271
sexual, and psychopathy, 109 Cheating
and sexual infidelity, 13 adaptations for, 208–209
use of insanity defense, 273 characteristics of, 59
A Theory of Justice (Rawls), 232 – 233 defenses against, 208
Attorneys, viewpoints of murder, 268 – 269 defined, 22 – 23
Atypical antipsychotic medication, 256 litigation as “cheater detection,” 272
Australia punishment for, 236, 243
environmental activation of adaptations, 54 sexual infidelity as cost of, 47
legalization of brothels/escort services, 123 social cheating, 186
psychopathology statistics, 54 strategy success, 184
Subject Index 303
victim reactions, 231 male vs. male, for access to females, 27,
vs. cooperation, 23 28 – 29, 104, 277
Children. See also stepchildren reproductive benefits for men, 163
costs of victimization, 10 and reproductive success, 163 – 164
development of life-course-persistent and risk-accepting behavior, 143 – 144
offending, 140 – 141, 149, 251 sperm competition hypothesis, 70, 71 – 73,
encouragement of violence in males, 53 106, 107, 109 – 110, 184
(see also honor subculture) Conflicts
killing of stepparent vs. genetic parent, 7 interindividual conflicts
of murdered parent, 212 over material resources, 5 – 6, 203
as psychopaths, 182 – 183 over mating resources, 6, 203 – 204
risks of being killed, 7, 150, 220 over status, 5, 203
and sense of justice, 231 and kin selection, 6 – 7
view of reciprocity, 241 Conscience, origins of, 239–240
Chimpanzees, conspecific killings, 58 Conspecific killing/theories, 49 – 50, 54
Chinese Americans, examination of death rates, insect (examples), 57
143 mammals (examples), 57 – 58
Civil Rights Act (1964), Title VII, 82 and molecular genetic studies, 54
Coalitional aggression, 14, 95 recurrence in human evolutionary history,
Coercion, sexual. See rape and sexual coercion 211 – 212
Coevolution. See also homicide adaptation Cooperation
theory; infectious diseases evolution of
arms races, of species, 9, 12, 204 fundamental social dilemma, 232–233
coevolutionary arms races, 9, 11 – 12, 41, games of primates, 234
204, 218, 224 reciprocity in nonhuman animals, 234
of cost-infliction, 204 resolutions of: cooperative strategies,
fitness cost of victimization, 9 – 10 233–234
time/opportunity, importance of, 12 – 15 systems for, expansion/refinement in humans,
victim defenses, 10 – 12, 204 237
Cognitive niche, of human beings, 4 Corrective justice, 230
Commutative justice, 230 Cost-inflicting strategies for outcompeting rivals
Comparative psychology of rape/sexual rape, 8
coercion, 103 – 104 theft, 7
male orangutans (example), 104 timing of, 14 – 15
male scorpionflies (example), 103 – 104 violence/homicide, 8 – 9
Competence to stand trial (CST), 270 Costs
Cooper v. Oklahoma, 279 of being cuckolded, by men, 48, 66, 126
David, assault case (example), 279 – 282 of being killed, 212 – 213, 224
Dusky v. United States, 278 advantage to rival, 212
and Savanna-IQ Interaction Hypothesis, damage to extended kin group, 213
278 – 282 future reproduction loss, 212
and Wechsler Adult Intelligence test of being victimized, 9 – 10, 14, 205 – 206, 209
(WAIS-III), 279 – 280 of committing rape, 108, 210
Competition of competition
for attracting mates, 4, 9, 28 – 29 after birth of first child, 165
costs of, 165 and age-crime curve, 164, 165
after birth of first child, 165 for resources, 4 – 5, 7, 9, 16, 170, 208
and age-crime curve, 164, 165 criminal cost-inflicting behaviors, 202
for resources, 4 – 5, 7, 9, 16, 170, 208 of nonreproduction, 142
group vs. group, 233 of partner sexual infidelity, 48, 67
homicide as end result, 44 of victim resistance, 47
304 Subject Index
Counter adaptations, 9, 11f insanity defense, 271 – 273

COYOTE (Call Off Your Old Tired Ethics) rights of, 268
organization, 124 Defense mechanisms, 10 – 12, 210
Criminal behavior. See also psychological against being killed, 41, 44 – 45, 212 – 218
mechanisms for homicide; against reproductive threats, 86
psychopathology theories of homicide; temporal contexts
psychopaths; Psychopathy Checklist, in flagrante adaptations, 205
revised (PCL-R) post-victim adaptations, 206
adaptations productive of, 11 pre-victimization adaptations, 205
age and, 30 – 32 against theft/cheating, 208–209
behavioral indications, 189 – 190 of women, against rape, 111 – 114
as biologically normal, 22 Depression
coevolutionary theories, 9 in Canada, decreased rates (1990s), 153
cognitive structures involved with, 282 – 283 impact on cost/benefit decision-making, 275
differential association theory, 51 – 52 and likelihood of homicide, 54
evolutionary neuroandrogenic (ENA) theory, manic depression, 256
249 – 251 postpartum depression, 219
evolutionary theories, 23 – 24, 166, 170 and post-traumatic stress disorder, 269 – 271
gender and, 25 and rape, 274
and homicide adaptation theory, 50 and sex workers, 129
intelligence factor, 167 – 171 treatment of, with lithium carbonate, 256
life-course-persistent offenders, 147 – 149 Developmental context of psychopathy, 179–180
poor vs. rich (social class), 166 – 167 Diagnostic and Statistical Manual of Mental
predictability of, 176 Disorders
reduced tendency of females, 28 – 29 psychopathy (defined), 177
related traits, evolution of, 21 – 23 Differential association theory (of criminal
resource acquisition via, 22 – 23 behavior), 51 – 52
Savanna-IQ Interaction Hypothesis, Dissociative identity disorder, 54
170 – 171 Distributive justice, 230
sex differences in, 282 Domain specificity premise (of evolutionary
social learning theory, 52 psychology), 102
time and opportunity, importance of, 12 – 15 Domestic violence, 275 – 277
traits for mating vs. traits for, 23 – 24 Dominance
triggering of, 58 and antisocial behavior, 147
Cuckoldry of gang leaders, 146
of men and high status, 28 – 29
costs of, 48, 66, 73, 126 homicide for maintenance of, 58 – 59
fertilization success of, 184 in honor subcultures, 31 – 32
and sexual coercion, 70, 72 and rape, 101, 108
of women, impossibility of, 73 sex-based language and, 94
Cultural theory of homicide, 50 – 51 sex differences in
Cultural variations in homicide rates, 42 – 43 females, 29, 70, 111, 210
males, 28 – 29, 32, 70, 84, 95 – 96, 106
Darwin, Charles. See evolution theory; natural and sexual harassment, 89
selection process striving for, riskiness of, 29
Date rape, 109, 275 and testosterone, 31, 89
Daubert v. Merrell Dow Pharmaceuticals, Inc., 282
Death penalty, for murder, 269 EEG biofeedback, for crime prevention/
Deception, and reproductive success, 22 treatment, 258
Defendants Ellison v. Brady (male vs. female, offensive
evaluation of, for rape, 275 conduct), 85
Subject Index 305
Embodied capital construct, of human behavior, teaching of, 282 – 283

147 – 150, 181 – 182 on theft, 160 – 161
Environmental cues/influences Evolutionary neuroandrogenic (ENA) theory,
on ancestral humans, 131 for crime prevention/treatment. See also
and antisocial behavior/risk-taking, 141, 144, testosterone
149, 150 eugenic approaches
and calibration of homicide adaptations, 51 abortion legalization, 259
and development of psychopathy, 183, 189 incarceration rates, 260
on life expectancy, time horizon for, 142 – 143 evolutionary proposition, 249 – 250
and rape mechanisms, 110 – 111 neuroandrogenic proposition, 250 – 251
on risk-taking behavior, 144 – 145 pharmacological/neurological approaches
adolescents, 30 antiandrogens, 255
males vs. females, 25 anticonvulsant medication, 257
as triggers of genetic traits, 187 antipsychotic medication, 255 – 256
uncertainty about, 49 atypical antipsychotic medication, 256
uncertainty from, 221 EEG biofeedback, 258
in U.S./Australia, for activation of adaptations, mentoring programs, 252
54 reducing lead exposure, 258
Error management theory, sexual harassment, serotonin-altering medication, 257 – 258
85 – 86 stimulant arousal control medication,
Escort services, 123 254 – 255
Ethnicity and likelihood of being killed, 41 – 42 social learning approaches
European Union, sexual harassment laws, 82 language-focused programs, 252 – 253
Evolution mentoring programs, 252
of cooperation parenting management training, 252
fundamental social dilemma, 232 – 233 self-control/moral reasoning training,
games of primates, 234 253 – 254
reciprocity in nonhuman animals, 234 Evolutionary pathways (homicide adaptation
resolutions of: cooperative strategies, theory), 45 – 48
233 – 234 social contracts, 47, 48
of criminal traits, 21 – 23 social exchange relationships, 47
of homicidal strategies, 44 Evolution theory (Darwin), 3, 16, 65, 261,
of homicide adaptation pathways, 45 – 49 282 – 283. See also natural selection
Evolutionary forensic psychology process
adaptationist approach of, 3 Evolved psychological mechanisms
adaptationist research, implications of, 15 – 16 (of evolutionary psychology), 102
contributions to understanding rape/sexual
coercion, 273 – 275 Families. See adolescence; children; parental
defined, 3 investment theory; parents, genetic;
environmental friendliness of, 21 stepchildren; stepparents vs. genetic
on intelligence factor of criminality, 168 parents
male criminality observations by, 161 Family violence, 65 – 73, 275 – 277. See also
misconceptions about: naturalistic fallacy, stepchildren; stepfamilies
102 – 103 forced in-pair copulation
perspective of Willie/Steve murder case, 271, in humans, 72 – 73
272 in nonhuman animals, 70 – 72
premises of, 102 sexual jealousy
promise of for criminology, 20 – 33 intimate partner violence, 67 – 69
prostitution perspective, 125 – 129 of males/partner uncertainty, 65 – 66
psychopathy explained by, 183 sexual violence in intimate relationships/
reasons for, 3 – 5 sexual jealousy, 69 – 70
306 Subject Index
Fantasies of homicide, 57 behavioral genetics, 3, 26, 144 – 145, 151

Fear, protective aspects of, 215, 216 and criminality, evidence for, 249
Females. See also sexual infidelity and ENA theory, 250
appearance-enhancing behaviors, 8 genetic determinism, 102 – 103
conflict over mating resources, 6 genetic relatives, favoring of, 6 – 7
conflicts over men, 5 and predisposition for risky behavior, 144
damage caused by sexual harassment, 81 and psychopathy, 180 – 181
defense against rape/sexual coercion, 111–114 of psychopathy, 187 – 189
bodyguard hypothesis, 111 studies of, 53– 54
menstrual cycle, 112 – 114 The Gift of Fear (De Becker), 215
psychological pain of victims, 111 – 112 Golden Rule, 230, 238
homicide statistics, 41 – 42 Gorillas, conspecific killings, 57
infanticide by, 43 Great Britain, homicide rates, 54. See also United
investment in offspring vs. mating effort, 102 Kingdom, sexual harassment laws
investment in reproduction, 6
male exploitation tactics of, 27 – 28 Heritability and life histories, 151 – 152
and parenting effort, 28 – 29 High mating effort
preference for dominant males, 111, 210 and antisocial behavior, 183
promiscuity in, 29 high mating-effort rapists, 108 – 109
reduced criminal tendencies of, 28 – 29 and Psychopathy Checklist, revised, 183
restrained reproductive strategies, 27 Hindu viewpoint of sex, 124
sexual harassment views of, 85 – 86 Homan v. Indiana sexual harassment case, 83
and theft, 162 Homicide. See also Ambulance-Homicide
Feminist theory Theory; capital murder cases
about evolutionary psychology, 102 and ascent of social group rivalry, 10
about prostitution, 125 as assay of family conflicts, 7
about rape, 101 avoidance contexts, 214 – 218, 221 – 223
Finland, criminalization of prostitution, 123 as by-product of adaptations, 55 – 58
Fitness costs as by-product of evolved mechanisms, 211–212
of being killed, 212 – 213 competition between explanations for, 50
of being victimized, 9 – 10 consequences for family members, 216 – 218
of homicide, 44 cultural theories, 51
of male cuckoldry, 48, 66, 73, 126 cultural variations, 42 – 43
of rape adaptations, 209 defenses against, 41, 44 – 45, 212 – 218
of rivals, 44 error management, for avoidance of, 221 – 223
Fleeing from homicidal confrontation, 216 evidence of adaptations, 217 – 221
Fluctuating asymmetry study, of psychopaths, evolutionary psychology understanding of, 9
182 as evolved adaptation, 22
Frequency-dependent selection, in animal fantasy vs. reality, 16
world, 183 – 184 fitness costs of, 44, 60, 211 – 212, 213–214
Future/present, survival of, 142 – 145 likelihood of, 41 – 43, 56, 215
male statistics, 41 – 42, 43
Games/game theory naturalness/understandability of, 172, 268
computer simulations/experimental games, preemptive homicide, 223–224
185 – 186 psychopathology theories, 54 – 58
on cooperative strategies, 233 risks of, for children, 7, 150, 219, 220
theorists on cooperative strategies, 233 social theories, 51 – 53
Gender. See females; males as strategy for outcompeting rivals, 8 – 9
Genetics. See also evolutionary neuroandrogenic U.S. statistics, 41 – 43
(ENA) theory, for crime prevention/ Homicide, psychological mechanisms
treatment; kin selection conspecific killing theories, 49 – 50
Subject Index 307
cultural theories, 50 – 51 evolutionary psychology perspectives, 65,

social theories, 50 – 53 125, 171
uncertainty, 49 evolved psychological mechanisms of, 102
Homicide adaptation theory, 43 – 50. See also influence of physical attractiveness, 167
conspecific killing/theories and law, 270
adaptationist vs. nonadaptationist perspective, natural selection and, 102
57 – 58 social learning theory explanation of, 52
ancestral problem considerations,
44 – 45 India, prostitution in, 122
evolutionary pathways, 45 – 48 Infanticide, 43
social contracts, 47, 48 Infectious diseases, influence on human
social exchange relationships, 47 evolutionary history, 9
intentionality, 48 – 49 Infidelity. See sexual infidelity
and intimate partner violence, 68 In-pair copulation, forced
purpose of, 50 in humans, 72 – 73
slip-up vs. homicide adaptation, 277 – 278 in nonhuman animals, 70 – 72
Homicide (Daly & Wilson), 55 Insanity defense (not guilty by reason of
Homicide rates insanity), 271 – 273
African American males, 41 – 42 Durham rule (Durham v. United States, 1954),
cultural variations in, 42 – 43 271
international rates (See individual countries) irresistible impulse test (Regina v. Oxford, 1940),
Honor subculture 271
defined, 31 – 32 model penal code (American Law Institute,
encouragement of violence in male children, 1955), 271
54 wild beast test (Rex, B. Arnold, 1724), 271
as explanation for homicide, 51 Insects, conspecific killings, 56
valorization of violence, 52 – 53 Intelligence factor, of criminal behavior,
Hormones. See also evolutionary 167 – 171
neuroandrogenic (ENA) theory, for crime evolutionary psychological theory, 168
prevention/treatment; testosterone juvenile delinquents, 168
adolescent surges, 31, 33 Savanna-IQ Interaction Hypothesis,
influence on behaviors, 26 168 – 171
of pregnant women (hCG), 218 International Criminal Statistics (Interpol), 160
Hostile-environment harassment, 83 – 84 Intimate partner violence
Human beings by-product/“slip-up” hypothesis, 67 – 68
antisocial/risk-taking behavior, engagement homicide adaptation theory, 68
in, 143 and male sexual jealousy, 67 – 69,
conflicts between, causes of, 5 – 6 275 – 276
cooperative systems, expansion/refinement
of, 237 Jealousy
cost-inflicting strategies of, 4 – 5 as fuel for violence, 8
evolutionary triggers, need for, 21 male sexual jealousy
in-pair forced copulation, 72 – 73 function of, and paternal uncertainty,
lifetime risk of being killed, 56 65 – 66
sensitivity to time horizons, 143 and intimate partner violence, 67 – 69,
specialized cognitive adaptations of, 4 275 – 276
theft’s influence on, 7 and Lee Harvey Oswald, 276 – 277
Human beings, behavior of. See also The Moral mechanisms of, 55, 126, 211, 212
Animal (Wright) Judeo-Christian viewpoint on sex, 124
biosocial theories of, 21 Justice (sense of)
embedded capital construct, 147 commutative justice, 230
308 Subject Index
Justice (sense of) (continued ) analysis of, 141 – 142

cooperation, evolution of 1990s crime drop (example), 152 – 155
fundamental social dilemma, 232 – 233 trade-off concept, 142, 145, 148, 154
games of primates, 234 and antisocial behavior, 149
reciprocity in nonhuman animals, 234 and decreased risk-taking, 144
resolutions of: cooperative strategies, and heritability, 151 – 152
233 – 234 and life-course-persistent offending, 151
corrective justice, 230 strategies as personalities, 149 – 151
definition (working), 230 Lions, conspecific killings, 56 – 57
distributive justice, 230
evolution of rules and norms, 238 – 240 Machiavellianism, 177
origins of conscience, 239 – 240 Magnetic resonance imaging (MRI) studies
expansion/refinement of, 237 – 238 for diagnosis of antisocial personality disorder,
moral argumentation, 238 190 – 191
mechanisms producing, activation of, of prefrontal cortex (PFC), 31
242 – 243 Maladaptive behavior. See criminal behavior
procedural justice, 230 Males
justice (sense of), origin of adaptation to female infidelity, 12
psychological accounts, 231 – 232 conflict over mating resources, 6
reframing of, 240 – 242 criminality bias of, 161
third-party injustice, affective reactions, and cuckoldry
235 – 236 costs of, 48, 66, 73, 126
treating others fairly/unfairly, affective fertilization success of, 184
reactions, 236 and sexual coercion, 70, 72
Juvenile delinquents dominant behavior, 28 – 29, 32, 70, 84,
and deviant antisocial lifestyle, 178 95 – 96, 106
intelligence of, vs. nondelinquents, 168 exploitation of females by, 27 – 28
and onset of puberty, 251 homicide statistics, 41 – 42
risks of living in stepfamily, 220 intrasexual mating competitions, 28 – 29
investment in mating efforts vs. offspring, 102
Kama Sutra, 122 lifetime risk of being killed, 56
Kin selection, and conflicts, 6 – 7 “mate retention”/“mate guarding” behavior,
68 – 69
Language-focused programs, 252 – 253 and mating effort, 25 – 28
Life-course-persistent offenders. See also parental investment of, 161
antisocial tendencies promiscuity in, 29, 126, 177
competitive disadvantages of, 147 – 148 prosecution for raping wives, 109
growth, reproduction, competitive reasons for theft by males, 161 – 162
disadvantage, 147 – 149 reproductive success limitations, 27
vs. adolescent-limited delinquents, sexual access and status, 5
146 – 147 sexual harassment views of, 85 – 86
vs. psychopaths, 141, 149, 181, 183 sexual jealousy mechanisms, 55
Life-course-persistent offending. See also striving for dominance by, 29
antisocial tendencies submission/self-abasement tactics of, 8
defined/described, 140 – 141 and testosterone, 26, 30
development factors, 140 – 141, and theft
148, 251 reasons for, 161 – 162
and embedded capital concept, 147 younger vs. older men, 162 – 166
and life histories, 151 Males and rape/sexual coercion
Life histories. See also time horizons, influence disadvantaged males, 106
on life history high mating-effort rapists, 108 – 109
Subject Index 309
opportunistic rapists, 108 Mental state at time of offense (MSO), 270

partner rapists, 109 – 110 Mentoring programs, 252
specialized rapists, 106 – 108 Meritor Savings Bank v. Vinson sexual harassment
Male sexual jealousy case, 83
function of, and paternal uncertainty, 65 – 66, Middle Ages, and prostitution, 122
68, 70 Misconceptions about evolutionary psychology,
and intimate partner violence, 67–69, 275–276 102 – 103
and Lee Harvey Oswald, 276 – 277 Misconceptions/miscommunication, about
mechanisms of, 55, 126, 211, 212 sexual harassment, 86 – 88
Mammals Monkeys, conspecific killings, 57
conspecific killings, 56 – 57 The Moral Animal (Wright), 270
Mania, unipolar, 54 Moral reasoning training, 253 – 254
“Mate retention”/“mate guarding” behavior, Motivations, for sex-based language, 94 – 95
of males, 68
Material resources Naturalistic fallacy (regarding evolutionary
conflicts over, 5 – 6, 203 psychology), 102 – 103
and jealousy, 143 Natural selection process
and theft, 160, 161, 171, 208–209 and criminal behavior, 22 – 23
Mating differential reproductive success, as central
of black widow spiders, 56 – 57 tenet, 141 – 142
and coevolutionary arms races, 12 and evolutionary psychology, 102
conflict over resources, 6 favoring of genetic kin, 7
and criminality bias of males, 161 influence on human psychology/behavior, 65
display of options by females, 8 refutation of denial attempts, 21
of higher-status gang members, 54 – 55 and sexual harassment, 84
humans vs. avian species, 70 Neanderthals, and evidence of violence, 52
interindividual conflict over, 6 Netherlands, regulation of prostitution, 124
intrasexual competitions by males, 28 – 29 Neuroscience, on gender-typical behavior, 26
investment in, males vs. females, 102 New York City, psychopathology statistics, 54
long-term efforts, 129 NGRI. See not guilty by reason of insanity
male behaviors (NGRI)
conditional strategies, 103 – 104, 105, 106 Nicomachean Ethics (Aristotle), 230
efforts of, 25 – 28 Not guilty by reason of insanity (NGRI), 271–273
high mating-effort rapists, 108 – 109 Durham rule (Durham v. United States, 1954), 271
male status and, 5 irresistible impulse test (Regina v. Oxford, 1940),
and parenting effort, 145 – 147 271
parenting effort and, 145 – 147 model penal code (American Law Institute,
polygynous mating systems, 6, 164, 166 1955), 271
rules of long-term relationships, 47, 48 wild beast test (Rex, B. Arnold, 1724), 271
short-term efforts, 85, 91, 128 Numerousness premise (of evolutionary
strategies for long-term success, 126 – 127 psychology), 102
traits for, vs. traits for criminal behavior, 23 – 24
Mechanisms for homicide. See psychological Oncale v. Sundowner Offshore Services, Inc. same-
mechanisms for homicide sex sexual harassment case, 95 – 96
Mechanisms of male sexual jealousy, 56, 126, Opportunistic rapists, 108
211, 212 Opportunity, importance of, 12 – 15
Media, portrayal of violence, 52
Medication. See pharmacological/neurological Parental investment theory, 101, 275
approaches, for crime prevention/ Parenting efforts
treatment ancestral parents, 22
Menstrual cycle and rape, 112 – 114 and childhood behavior, study of, 182 – 183
310 Subject Index
Parenting efforts (continued ) Procedural justice, 230

of females, 28 – 29 Promiscuity. See also sexual infidelity
of Homo sapiens, 23 in females, 29
increased violence by stepparents, 7, 150, in males, 29, 126, 177
220 – 221 and premarital sexual coercion, 109
love/nurturance, 22 as psychopathic characteristic, 186
of males, 161 as risk-taking behavior, 139, 140
mating and, 145 – 147 Prostitution. See also COYOTE (Call Off Your Old
Parenting management training, 252 Tired Ethics) organization
Parents, genetic decriminalization efforts, 124
vs. stepparents escort services, 123
and juvenile delinquency of children, 220 evolutionary psychology perspective, 125 – 129
and murder of children, 7, 212, 220 feminist viewpoint, 125
Partner rapists, 109 – 111 Hindu/Judeo-Christian viewpoint, 124
Partner violence. See intimate partner violence historical background, 121 – 123
Paternal uncertainty, 73 legal issues, 123 – 124, 129 – 131
adaptive problems of, 73 players, 124 – 125
of ancestral men, 126 public policy implications, 129 – 131
and male sexual jealousy, 65 – 66, 68, 70 reasons for existence, 126 – 127
Pathways of antisocial tendencies religious right viewpoint, 125
adolescence-limited delinquency, 140 teenage viewpoint, 124
life-course-persistent offending, 140 – 141 types of requests, 123
psychopathy, 140, 141 Psychological mechanisms for homicide
Pharmacological/neurological approaches, for cultural theories, 50 – 51
crime prevention/treatment homicide adaptation/conspecific killing
antiandrogens, 255 theories, 49– 50
anticonvulsant medication, 257 social theories, 50 – 53
antipsychotic medication, 255 – 256 uncertainty, 49
atypical antipsychotic medication, 256 Psychopathology theories of homicide, 54 – 58
EEG biofeedback, 258 Australia, disorder/trends study, 54
reducing lead exposure, 258 birth cohort (Northern Finland) studies, 54
serotonin-altering medication, 257 – 258 cross-cultural variations, 54 – 55
stimulant arousal control medication, schizophrenic study, 54
254 – 255 Psychopaths
Polygynous mating systems, 6, 164, 166 adaptations of, 5
Positron emission tomography (PET), 190 antisocial behavior development, 140 – 141
Post-traumatic stress disorder, 269 – 271 brains of, 189 – 191
Power vs. sex, 88 – 94 children as, 182 – 183
power/priming of sexual psychology, 93 – 94 life-course-persistent offenders vs., 141, 149,
rejection of “natural/biological” model, 181, 183
90 – 91 offenders, other types, vs., 181
status/unwelcomeness, conflicting predictions risk-taking behavior, 188
about, 91 – 92 selection pressure imposed by, 176
weakness of neglecting biology, 92 – 93 Psychopathy
Preemptive homicide, 223 – 224 Australian research study, 54
Prefrontal cortex (PFC), MRI studies, 31 and conditional (facultative) development,
Present/future, survival of, 142 – 145 181 – 182
Primates construct of
games played by, 234 as clinical condition, 177 – 179
reciprocity of cooperation in, 234 developmental context, 179 – 180
Problem-solving skills, and survival, 3 – 5 factors of (revisited), 180
Subject Index 311
defined, 140 high mating-effort rapists, 108 – 109

evolution of, 176 – 192 opportunistic rapists, 108
explanations of, 180 – 185 partner rapists, 109 – 111
externalizing aspects of, 179 – 180 specialized rapists, 106 – 108
as extreme example of personality type, 149 Reasonable person/reasonable woman issue,
frequency-dependent selection account of, 84 – 88
183 – 185 Ellison v. Brady case, 85
genetics of, 187 – 189 error management theory, 85 – 86
high mating efforts and, 108 misperceptions/miscommunication and sexual
men with, distinction from “normal,” 105 harassment, 86 – 88
as part of heritable/obligate life history, 151 Radtke v. Everett case, 85
as pattern of antisocial behavior, 141 Reciprocal altruism, 23, 235
related terms, 177 Reciprocity (of cooperation)
as subgroup of persistent offenders, 150 in humans, 23, 237, 238
testing for, 109 in primates, 234
Psychopathy Checklist, revised (PCL-R) as viewed by children, 241
aspects of Religious right, viewpoint on
Factor 2 (deviant, antisocial lifestyle), 178 prostitution, 125
Factor 1 (interpersonal/affective Reproductive strategies
characteristics), 178 female factors, 28
assessment of externalizing traits, 180 male success limitations, 27
and fluctuating asymmetry study, 182 male vs. female, 26 – 27
and high mating effort, 183 Reproductive success
and MRI testing correlations, 191 ancestral enjoyment of, 23
test for early onset/coercive sexuality, 184 and competition/death, 163 – 164
and conflicts over material resources, 203
Quid pro quo harassment, 82 – 83, 94 enhancement via romantic partnership, 47
and evolution by natural selection, 141 – 142,
Radtke v. Everett (female workplace 282 – 283
disenfranchisement case), 85 of male rapists, 104 – 105
Rape. See also Against Our Will: Men, Women, and male vs. female, 5, 26 – 27, 28, 127, 145 – 146,
Rape (Brownmiller) 207
as academic/public focus, 101 and polygynous mating systems, 6,
adaptations that produce, 210, 275 164, 166
date rape, 109, 275 and rape, jeopardization of for women,
as evolved adaptation, 22 111 – 112
feminist theory about, 101 and theft by males, 160, 161, 163
male prosecution for raping wives, 109 Risk-taking behavior. See also antisocial
as strategy for outcompeting rivals, 8 tendencies
and testing for psychopathy, 109 in adolescents/teenagers, 140, 143 – 146, 163
victim’s defenses against, 111 – 114, 210 in adulthood, 140, 143 – 144, 163
Rape and sexual coercion. See also sexual in college students, 144
harassment decisions for engagement, 143 – 144
comparative psychology of, 103 – 104 declines, since 1990, 154
male orangutans (example), 104 and economic hardship, 143
male scorpionflies (example), 103 – 104 environmental influence on, 144 – 145
contributions of evolutionary psychology, and high mating effort, 145
273 – 275 of males, 28, 162
evidence of human adaptations for, in psychopaths, 188
104 – 111 “taste for risk” construct, 139
disadvantaged males, 106 time horizons, influence on, 152 – 153
312 Subject Index
Rivals/rivalry power vs. sex, 88 – 94

damaging of, in competition for resources, 4 power/priming of sexual psychology, 93 – 94
deception of, 214 rejection of “natural/biological” model,
fitness costs produced by, 44 90 – 91
fitness gains of, 212 status/unwelcomeness, conflicting
intimidation of, against retribution, 210 predictions about, 91 – 92
and jealousy, 65 – 66 weakness of neglecting biology, 92 – 93
male/female differences, 209 reasonable person/’reasonable woman issue,
of male mountain gorillas, 57 84 – 88
outcompeting of/ cost-inflicting strategies, same-sex, 95 – 96
7–9 Sexual infidelity. See also promiscuity
reproductive rivalry, 32 cheating as cost of, 47
and sexual infidelity, 49 costs of, 48, 67
female adaptation to, 12
Same-sex sexual harassment, 95 – 96 male luring of females, 12
Savanna-IQ Interaction Hypothesis, 168 – 171 and risk of being killed, 223
competence to stand trial issue, 278 – 282 of women
Schizophrenic study, 54 discovery by partner, 12
Science article (Nesse and Berridge), 278 and male cuckoldry, 48
Self-control/moral reasoning training, motivations for, 72
253 – 254 murder by husbands, 48, 55, 211, 212
Self-defense, killing as result of, 13, 58, and rape/sexual coercion, 73, 109
223–224, 273 Sexual jealousy
Serotonin-altering medication, 257 – 258 intimate partner violence, 67 – 73, 275 – 277
Sex-based language, motivations for, 94 – 95 males mechanisms, 56, 126, 211, 212
Sex crimes. See rape; rape and sexual coercion of males/partner uncertainty, 65 – 66
Sex (physical act). See also escort services; Sexual role-reversal, 29
promiscuity; prostitution Single-parent families, 25, 140
competition for, 102 Social class factor, of criminal behavior, 166–167
costs of, male vs. female, 6 Social contracts
extortion of, for job benefits, 82 commutative justice (of Aristotle), 230
Hindu/Judeo-Christian viewpoint, 124 shaping of rules, 47
number of partners, and criminal violation of, and behavior adaptations, 48
behavior, 24 Social dilemma, fundamental, 232 – 233
with romantic partner vs. nonromantic Social exchange relationships, 47
partner, 47 Social learning approaches, for crime
sex-role reversal, 29 prevention/treatment
and violence, arousal from, 106 language-focused programs, 252 – 253
Sexual access, male status and, 5 mentoring programs, 252
Sexual coercion. See rape and sexual coercion parenting management training, 252
Sexual harassment, 6, 81 – 97. See also rape and self-control/moral reasoning training,
sexual coercion 253 – 254
“because of sex” issue, 94 – 96 Social learning theories, 52
damage caused to women, 81 Social parasitism, 23
definition/description, 82 – 84 Social role theory, 52
hostile environment harassment, 83 – 84 Social theories of homicide, 50 – 54
quid pro quo harassment, 82 – 83, 94 Sociopathy, 177
error management theory, 85 – 86 Specialized rapists, 106 – 108
legal cases, 83, 95 – 96 Sperm competition hypothesis, 70, 71 – 73
misconceptions/miscommunication about, Status, damage of
86 – 88 adaptations to, 207
Subject Index 313
defenses against, 207 poor vs. rich, 166 – 167

by violence, 8 – 9, 15 younger men vs. older men, 162 – 166
Stepchildren evolutionary psychological perspective,
adaptations of, 220–221 160 – 161
elimination of, 211 as evolved adaptation, 22
increased risks of violence for, 10, 16, 211, and females, 162
212 material resources and, 160, 161, 171,
vs. genetically related family members, 7, 43 208–209
Stepfamilies as strategy for outcompeting rivals, 7
increased conflict within, 7 Time horizons, influence on life history
increased risks within, 220 of humans, 143, 145
Stepparents vs. genetic parents and birth order, 150
and juvenile delinquency of children, 220 and life span, 154
and murder of children, 7, 150, 212, 220 and reproductive success, 144
Stimulant arousal control medication, and risky/antisocial behavior, 143, 152, 153
254 – 255 of organisms (other than human), 142 – 143,
Subculture of honor 145
defined, 31 – 32 Time/opportunity, importance of, 12 – 15
encouragement of violence in male Title VII, Civil Rights Act (1964), 82
children, 54 Trade-offs (in life history analysis), 142, 145,
as explanation for homicide, 51 148, 154
valorization of violence, 52 – 54 Triggering of criminal behavior, 58
Substance use, 151, 278 Twin study (British cohort study), 24
Sumeria, prostitution in, 128
Sweden Uncertainty, 49 – 50
criminalization of sexual services, 123 dangers of risk-averse strategy, 144
psychopathology statistics, 54 paternal uncertainty (See paternal
uncertainty)
“Taste for risk” construct, 139 value of fear and anxiety, 215, 216
Teenage mothers, abandonment of infant, 55 Unfaithfulness. See sexual infidelity
Temple prostitutes, 122, 124 Unipolar mania, 54
Testosterone. See also evolutionary United Kingdom, sexual harassment laws, 82–83.
neuroandrogenic (ENA) theory, for crime See also Great Britain, homicide rates
prevention/treatment United States
and dominance, 31, 89 chances of being murdered, 41
increases of, for extrapair copulation, 166 decreasing homicide rates, 42
links with aggression, 29, 258 encouragement of homicide, 51
lowering of, after onset of fatherhood, 147, environmental activation of adaptations, 54
165 escort services as prostitution, 123
and male brain organization, 26, 30, 31, homicide records, 7
250 – 251 killing statistics, 42
social effect of, 89 likelihood of murder, 41
Theft. See also Savanna-IQ Interaction prosecution for raping wives, 109
Hypothesis prostitution in Old West, 122 – 123
adaptations for, 208–209 punishment of prostitutes/clients, 123
defenses against, 208 sexual harassment laws, 82 – 83, 87, 88
defined, 160
empirical puzzles Venezuela, psychopathology statistics, 55
less intelligent vs. more intelligent, Victimization
167 – 171 adaptations to, 12
men vs. women, 161 – 162 anticipation of, as defense, 12
314 Subject Index
Victimization (continued ) Violence. See also family violence; intimate

assumptions made about, 49 partner violence
cost to children, 10 adaptations for, 209
crime-specific defenses against, 10 – 11 damage of status through, 8 – 9, 15
Victims as defense of reproductive resources, 47
co-evolved strategies against attacks, 47, 204 and reproductive success, 22
cost of rape, 8 by stepparents, 7, 150, 220–221
costs of being killed, 60 as strategy for outcompeting rivals, 8 – 9
counter adaptations in, 11f
definition of, 201 Willie and Steve, murder case
fitness costs of, 9 – 10 background of father (victim), 269
retribution by, 14, 15 CST/MSO evaluations, 270
Victims’ defenses, 10 – 12, 210 depression/PTSD of Willie, 269, 270 – 271
temporal contexts description of murder of father/stepmother, 269
in flagrante adaptations, 205 Women. See females
post-victim adaptations, 206 Workplace miscommunication, 86 – 88. See also
pre-victimization adaptations, 205 sexual harassment

Evolutionary Forensic Psychology

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EVOLUTIONARY FORENSIC PSYCHOLOGY

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Oxford New York

Copyright © 2008 by Oxford University Press, Inc.

Published by Oxford University Press, Inc.

Library of Congress Cataloging-in-Publication Data

Part One Introduction and Overview

2 The Promise of Evolutionary Psychology for

Part Two Adaptation and Violent Crimes

4 Intimate Partner Violence 65

Part Three Adaptation and Sex Crimes

6 Evolutionary Psychological Perspectives on Rape 101

7 The World’s Oldest Profession: Evolutionary

Part Four Adaptation and the Production of Criminal Behavior

10 In Cold Blood: The Evolution of Psychopathy 176

Part Five Victims of Crime

12 The Evolution of a Sense of Justice 230

Part Six Applications and Future Directions

14 Did the Victim Deserve to Die? Darwin Goes to Court 268

Author Index 287

KEVIN BEAVER SATOSHI KANAZAWA

TODD K. SHACKELFORD ANTHONY WALSH

INTRODUCTION AND OVERVIEW

Evolutionary Forensic Psychology

TODD K. SHACKELFORD AND JOSHUA D. DUNTLEY

Forensic psychology is a burgeoning ﬁeld in the social and behavioral sciences. It

Why Evolutionary Forensic Psychology?

Evolutionary psychology uses an adaptationist approach to explore the cognitive

characteristics contributing to better problem solving, the genes that contributed to

selection pressure for the purposeful inﬂiction of costs as a strategy to outcompete

Conﬂict over Status

Conﬂict over Material Resources

of its contribution to an individual’s reproductive success, the greater the conﬂict

Conﬂict over Mating Resources

Conﬂict and Kin Selection

Evolutionary researchers have documented that conﬂict is usually tempered by ge-

argued to be the evolutionary product of kin selection. According to kin selection

Speciﬁc Cost-Inﬂicting Strategies to Outcompete Rivals

Vigilance and Violence in Romantic Relationships

Violence and Homicide

From an evolutionary perspective, all crimes can be thought of as strategies that

The Fitness Costs of Being Victimized

The Importance of Time and Opportunity

such a disadvantage. It would be surprising if selection did not fashion adaptations

Implications of Adaptationist Research

Haldane, J. B. S. (1949b). Disease and evolution. La Ricerca Scientiﬁca, 19, 2–11.

The Promise of Evolutionary

shudders down the spines of traditionally trained criminologists, whose understand-

The Evolution of Traits Related to Criminal Behavior

The Basic Assumptions of Evolutionary Theories of Crime

All evolutionary theories of criminal and antisocial behavior focus on reproductive

Males and Mating Effort

Females and Parenting Effort

Age and Crime

deﬁned as “communities in which young men are hypersensitive to insult, rushing

commit antisocial acts only when forced to do so by “society.” Evolutionary psychol-

Mealey, L. (1995). The sociobiology of sociopathy: An integrated evolutionary model. Behav-

Walker, E. (2002). Adolescent neurodevelopment and psychopathology. Current Directions in

The Origins of Homicide

JOSHUA D. DUNTLEY AND DAVID M. BUSS