ABSTRACT
In the everyday world, human behavior is changed by signs, symbols, gestures, and spoken words. Behavior is also regulated by sounds, smells, sights, and other sensory stimuli that do not depend on social conditioning. When social or nonsocial events precede operant behavior and affect its occurrence, they are called controlling stimuli. A controlling stimulus (S) is said to alter the probability of an operant, in the sense that the response is more (or less) likely to occur when the stimulus is present.1
