Examining non-reinforced lever pressing as intrinsically-motivated behavior.

Athletes practice, artists craft, and academics read; often for no nominal outcome. According to the overjustification hypothesis, these intrinsically motivated behaviors can be reduced if followed by a reinforcer. Those listed above are uniquely human behaviors, but there are little data evaluating this effect in other animals. Two experiments evaluated the overjustification effect in rats. In Experiment 1, rats lever pressed for no reinforcer (initial baseline), followed by a manipulation period where lever presses resulted in an expected or unexpected reinforcer (food), or no reinforcer, and lastly, rats again lever pressed for no reinforcer (final baseline period). Increased lever pressing was observed initially for groups that received a reinforcer, but it quickly returned to baseline and no evidence for the overjustification effect was found. In Experiment 2, sensory effects of lever pressing were added to increase the rate of behavior. The baseline rate of lever pressing increased for all groups, and lever pressing after unexpected reinforcement occurred at a higher rate than after expected reinforcement, but this group did not differ from the group with no reinforcement. These findings underscore the challenge of predicting overjustification effects and point to additional factors that may be critical for identifying intrinsically motivated behavior in human and non-human animals.