What is the overjustification effect?
It is the effect that occurs in a situation where one is promised a reward for doing what one already likes to do and then the person sees the rewards, rather than having an intrinsic interest, as the motivation for performing the task. Zimbardo originated this concept. He gave the example of Nunzi, a New York shopkeeper who, having been insulted by kids, started to pay them to do so. When he cut their pay, they went away.