Behaviorism (or behaviourism) is a systematic approach to the understanding of human and animal behavior. It assumes that the behavior of a human or animal is a consequence of that individual's history, including especially reinforcement and punishment, together with the individual's current motivational state and controlling stimuli. Thus, although behaviorists generally accept the important role of inheritance in determining behavior, they focus primarily on environmental factors.

Behaviorism combines elements of philosophy, methodology, and psychological theory. It emerged in the early twentieth century as a reaction to depth psychology and other traditional forms of psychology, which often had difficulty making predictions that could be tested experimentally. Its early influences were Ivan Pavlov, who investigated reflexes andclassical conditioning, and Edward Thorndike, one of the first to study operant (or instrumental) behavior. Together with John B. Watson and others, these investigators rejected introspective methods and sought to understand behavior by measuring observable behaviors and events. Behaviorist philosophies shifted somewhat during the 1940s and 1950s and again since the 1980s. Radical behaviorism is a conceptual variant proposed by B. F. Skinner which acknowledges the presence of private events—including cognition and emotions—and suggests that they are subject to the same controlling variables as observable behaviors.