Reinforced variation and selection

Long-Evans rats were reinforced for generating variable sequences of four left (L) and right (R) leverpress responses. If the current sequence, for example, LLRR, differed from each of the preceding five sequences, then a food pellet was provided. Otherwise, there was a brief time-out. After the rats were generating a variety of sequences, one sequence was concurrently reinforced whenever it occurred, that is, an “always-reinforced” contingency was superimposed. The frequency of that sequence increased significantly from the variable baseline (Experiment 1). The “difficulty” of the always-reinforced sequence influenced the extent to which its frequency increased—“easy” sequences increased greatly, but “difficult” ones increased not at all (Experiment 2, and a replication with pigeons in Experiment 3). However, when reinforcement for baseline variability was systematically decreased while a difficult sequence was always reinforced, that sequence slowly emerged and attained relatively high levels (Experiments 4 and 5). Thus, a technique for training even highly improbable, or difficult-to-learn, behaviors is provided by concurrent reinforcement of variation and selection.