ABSTRACT
Radical behaviorism eschewed theory that involves intervening variables or states of the organism. Nevertheless, Skinner’s idea of the three-term (stimulus–response–reinforcer) contingency provided the nucleus of a behavioral unit. By the 1970s, the idea that operant learning is best understood as a process of Darwinian behavioral variation guided by selection provided by reinforcement, the Darwinian metaphor, had become generally accepted. The emphasis was on the dramatic and highly visible effects of selection, but soon the importance of processes of variation, the origin of emitted behavior, came to the fore.
