Behaviorism is a school of thought in psychology defined by the belief that behavior of organisms can be described in terms of stimuli and reactions to them (without considering cognition or internal psychological experiences). This was later expanded to include the characteristics of the organism into S-O-R (stimulus-organism-reaction).The most prominent behaviorist theories of this period include the concept of classical conditioning developed by Ivan Pavlov and John Watson, the concept of operant conditioning developed by B.F. Skinner, and the Drive Reduction Theory of Clark Hull and others (Radonjić, 1992).