Some researchers claim that variability is an operant dimension of behavior. The present paper reviews the concept of operant behavior and emphasizes that differentiation is the behavioral process that demonstrates an operant relation. Differentiation is conceived as change in the overlap between two probability distributions: the distribution of…

Stimuli that precede aversive events are typically less preferred than stimuli that precede nonaversive events. It has recently been demonstrated that stimuli that "follow" less preferred events may become favored more than stimuli that follow more preferred events. This phenomenon has been investigated under a variety of names, most commonly,…