Extrinsic Reward does not Decrease Intrinsic Motivation to Lever Press in Rats

Abstract

The overjustification effect can be described as the effect that occurs when an extrinsic reward is briefly given for a behavior that already occurs in the absence of external rewards, and once the external reward is no longer given for a certain behavior, the behavior will decrease to a level below what it was before the external reward was given. This effect applies broadly to everyday life. For example, this effect applies to the process of encouraging students to finish schoolwork. The current study investigated whether the overjustification effect can be observed in rats using lever pressing as a measure of intrinsic motivation. In this experiment, rats were split into three groups: Group No reward (NR) control, Group Unexpected (UE) reward control, and Group Extrinsic reward (ER). In the first phase, all groups pressed the lever without a reward to measure intrinsic motivation. In the second phase, Group ER was reinforced for lever pressing, Group UE was reinforced intermittently for lever pressing, and Group NR was placed in the experimental chamber without the presence of a lever. Finally, in the third phase, lever pressing was observed across the groups in the absence of reinforcement (as in Phase 1). Evidence for the overjustification effect would be a decrease in lever pressing during Phase 3 in Group ER that fell below the level of both Group UE and Group NR. In Phase 1, lever pressing decreased across Days 1-3 in all groups. In Phase 2, lever pressing by Group UE and Group ER increased across Days 6-8. In Phase 3, lever pressing by Group ER decreased rapidly, lever pressing by Group UE decreased gradually, and lever pressing by Group NR started low and remained at a low level throughout the phase. By the end of Phase 3, all rats were pressing the lever at a similarly low level. These data do not support the idea that the overjustification effect can be observed in lever pressing in rats.