The Overjustification Effect

Psychologists Mark Lepper, David Greene, and Richard Nisbett took children who loved drawing and promised one group a Good Player Award; another group just drew. Later, during free time, the rewarded children drew far less than the children who had never been promised anything.

This is the overjustification effect: adding an external reward recategorizes an activity from "I love this" to "I do this for the reward." Intrinsic motivation gets crowded out.

The implications are enormous. Paying a bonus for creative work can reduce its quality. Praising a child for reading can make them read less once the praise stops.