  Psychological behaviorism is a research program within psychology.  It purports to explain human and animal behavior in terms of external physical stimuli, responses, learning histories, and (for certain types of behavior) reinforcements. Psychological behaviorism is present in the work of Ivan Pavlov (1849–1936), Edward Thorndike (1874–1949), as well as Watson. Its fullest and most influential expression is B. F.  Skinner’s work on schedules of reinforcement.