Rewards Undermine Interest

From How Not to Teach Values, by Alfie Kohn:

In general terms, what the evidence suggests is this: the more we reward people for doing something, the more likely they are to lose interest in whatever they had to do to get the reward. Extrinsic motivation, in other words, is not only quite different from intrinsic motivation but actually tends to erode it. This effect has been demonstrated under many different circumstances and with respect to many different attitudes and behaviors.

Alfie Kohn has an entire book on this subject, Punished by Rewards.

One of the reviewers writes: "Love is its own reward. Meaningful debate/discussion is its own reward. Generosity is its own reward." And, I would add, learning is its own reward.