Over Justification Effect (Greene)

The overjustification effect occurs when an external incentive such as money or prizes decreases a person's intrinsic motivation to perform a task. According to self-perception theory, people pay more attention to the incentive, and less attention to the enjoyment and satisfaction that they receive from performing the activity. The overall effect is a shift in motivation to extrinsic factors and the undermining of pre-existing intrinsic motivation.

In one of the earliest demonstrations of this effect, researchers promised a group of 3-5 year old children that they would receive a "good player" ribbon for drawing with felt-tipped pens. A second group of children played with the pens and received an unexpected reward (the same ribbon), and a third group was not given a reward. All of the children played with the pens, a typically enjoyable activity for preschoolers. Later, when observed in a free-play setting, the children who received a reward that had been promised to them played significantly less with the felt-tipped pens. The researchers concluded that expected rewards undermine intrinsic motivation in previously enjoyable activities

Lepper, M. R., Greene, D., & Nisbett, R. E. (1973). Undermining children's intrinsic interest with extrinsic reward: A test of the "overjustification" hypothesis. Journal of Personality and Social Psychology, 28, 129-137.

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