Conditioning, in physiology, a behavioral process whereby a response becomes more frequent or more predictable in a given environment as a result of reinforcement, with reinforcement typically being a stimulus or reward for a desired response. Early in the 20th century, through the study of reflexes, physiologists in Russia, England, and the United States developed the procedures, observations, and definitions of conditioning. After the 1920s, psychologists turned their research to the nature and prerequisites of conditioning.
Stimulus-response (S-R) theories are central to the principles of conditioning. They are based on the assumption that human behaviour is learned. One of the early contributors to the field, American psychologist Edward L. Thorndike, postulated the Law of Effect, which stated that those behavioral responses (R) that were most closely followed by a satisfactory result were most likely to become established patterns and to reoccur in response to the same stimulus (S). This basic S-R scheme is referred to as unmediated. When an individual organism (O) affects the stimuli in any way—for example, by thinking about a response—the response is considered mediated. The S-O-R theories of behaviour are often drawn to explain social interaction between individuals or groups.
Conditioning is a form of learning in which either (1) a given stimulus (or signal) becomes increasingly effective in evoking a response or (2) a response occurs with increasing regularity in a well-specified and stable environment. The type of reinforcement used will determine the outcome. When two stimuli are presented in an appropriate time and intensity relationship, one of them will eventually induce a response resembling that of the other. The process can be described as one of stimulus substitution. This procedure is called classical (or respondent) conditioning.