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Why IQ is not a covariate in cognitive studies of neurodevelopmental disorders

Published online by Cambridge University Press:  01 May 2009

MAUREEN DENNIS ,
DAVID J. FRANCIS ,
PAUL T. CIRINO ,
RUSSELL SCHACHAR ,
MARCIA A. BARNES  and
JACK M. FLETCHER
MAUREEN DENNIS*
Affiliation:
Program in Neurosciences and Mental Health, Department of Psychology, The Hospital for Sick Children, Toronto, Ontario, Canada Department of Surgery, The Hospital for Sick Children, Toronto, Ontario, Canada
DAVID J. FRANCIS
Affiliation:
Department of Psychology, University of Houston, Houston, Texas Texas Institute for Measurement, Evaluation, and Statistics, University of Houston, Houston, Texas
PAUL T. CIRINO
Affiliation:
Department of Psychology, University of Houston, Houston, Texas Texas Institute for Measurement, Evaluation, and Statistics, University of Houston, Houston, Texas
RUSSELL SCHACHAR
Affiliation:
Program in Neurosciences and Mental Health, Department of Psychology, The Hospital for Sick Children, Toronto, Ontario, Canada Department of Psychiatry, University of Toronto, Toronto, Ontario, Canada
MARCIA A. BARNES
Affiliation:
Children’s Learning Institute, Department of Pediatrics, University of Texas Houston, Houston, Texas
JACK M. FLETCHER
Affiliation:
Department of Psychology, University of Houston, Houston, Texas
*
*Correspondence and reprint requests to: Maureen Dennis, Program in Neurosciences and Mental Health, Department of Psychology, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8. E-mail: maureen.dennis@sickkids.ca
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Abstract

IQ scores are volatile indices of global functional outcome, the final common path of an individual’s genes, biology, cognition, education, and experiences. In studying neurocognitive outcomes in children with neurodevelopmental disorders, it is commonly assumed that IQ can and should be partialed out of statistical relations or used as a covariate for specific measures of cognitive outcome. We propose that it is misguided and generally unjustified to attempt to control for IQ differences by matching procedures or, more commonly, by using IQ scores as covariates. We offer logical, statistical, and methodological arguments, with examples from three neurodevelopmental disorders (spina bifida meningomyelocele, learning disabilities, and attention deficit hyperactivity disorder) that: (1) a historical reification of general intelligence, g, as a causal construct that measures aptitude and potential rather than achievement and performance has fostered the idea that IQ has special status and that in studying neurocognitive function in neurodevelopmental disorders; (2) IQ does not meet the requirements for a covariate; and (3) using IQ as a matching variable or covariate has produced overcorrected, anomalous, and counterintuitive findings about neurocognitive function. (JINS, 2009, 15, 331–343.)

Keywords

IntelligenceHistoryIntelligence testsMeningomyeloceleAnalysis of varianceLearning disabilityAttention deficit disorder with hyperactivity
Type
Critical Review
Information
Journal of the International Neuropsychological Society , Volume 15 , Issue 3 , May 2009 , pp. 331 - 343
Copyright
Copyright © The International Neuropsychological Society 2009

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