Strangers to Ourselves: Discovering the Adaptive Unconscious. The discounting principle describes the human tendency to discount our judgement about the causal role of one factor (for instance intrinsic motivation for playing the piano) when there are other plausible explanations (for instance being rewarded with candy for playing the piano) (Wilson, 2002). In other words: when you 'reward' someone for doing something this may well undermine his intrinsic motivation for the task and hurt his performance too. If this sounds intruiging to you and you want to know more, reader, please check out Punished By Rewards: The Trouble with Gold Stars, Incentive Plans, A's, Praise, and Other Bribes, in which you can read much more about this. You may not have time to read a book. Is there convincing research to be found on the Internet which confirms the existence of this motivation and performance-harming effect of rewarding performance? Sure, here is one example which I came across today.
Also read: The unexpected role of adaptation