Behaviorism, a theory that limits psychological investigation to objective events. As formulated by United States psychologist John B. Watson in 1913, it opposes the study of mental processes as reported by the person experiencing them, and instead emphasizes laboratory experiments concerned with observable responses to stimuli. Behaviorist doctrine has been modified by such psychologists as E. C. Tolman, C. L. Hull, B. F. Skinner, and N. E. Miller. Behaviorism has greatly influenced American psychology.