John B. Watson, an animal experimentalist who argued that all human action is merely a conditioned response to external environmental stimuli, first popularized stimulus-response psychology. Watson's theory became known as behaviorism in recognition of its narrow focus on isolated human behaviors.

Behaviorists rejected psychology's widely held assumption that higher mental processes (that is, conscious thought or reflection) ordinarily control human action. In contrast to such “mentalist” views, behaviorists argued that the only purpose served by consciousness was to rationalize behaviors after they are triggered by external stimuli.

Behaviorists attempted to purge all mentalist terms from their theories and to deal strictly with observable variables - environmental stimuli on the one hand and behaviors on the other. By studying the associations that existed between specific stimuli and specific behaviors, behaviorists hoped to discover previously unknown causes for action. One of the central notions in behaviorism was the idea of conditioning.

Behaviorists argued that most human behavior is the result of conditioning by the external environment. We are conditioned to act in certain ways by positive and negative stimuli - we act to gain rewards or avoid punishments.