At the end of the last chapter, I showed that giving someone a negligible reward (or no reward) for performing an unpleasant act makes the act seem less disagreeable than it really is. One can also ask what is the effect of a large reward on the perception of a pleasant task. The answer is unequivocal: it devalues the task — in the eyes of those performing it. Nursery school children were provided in their playtime with brightly coloured Magic Markers and attractive drawing paper. Those who showed an interest in drawing were subsequently given the same apparatus in the classroom and encouraged to draw. One group was promised a glossy certificate for good drawing, while another was given no reward. Two weeks later the material was again provided and the children were told it was up to them whether they wanted to draw or not. The group previously given the certificate showed a marked decline in interest, while the other group drew as much as they had done in the previous two sessions. Presumably the children thought that drawing could not be of much interest in its own right if a reward was needed to make them engage in it.
Excerpt from: Irrationality: The enemy within by Stuart Sutherland