Introduction The viewpoint adopted for experimental purposes in this study was that the behavior of an organism is generated and maintained chiefly by its consequences on the environment. Over a wide range of conditions, a hungry rat, placed in a so-called "Skinner box," will persist in pressing a bar if, as a consequence of this response, a pellet of food is delivered to it at least some of the time. This exemplifies the principle of reinforcement which is at the core of current behavior theory. Conditioning of this type is termed operant (instrumental) to distinguish it from respondant (classical Pavlovian) conditioning. The probability of a given response in relation to the conditions of reinforcement of that response has been the subject of much study of Skinner,4 Ferster and Skinner,2 an others. Recently, controlled experiments conducted with human subjects using cubicles analogous to the