Paolo Terni sent me a video of a TED lecture by Barry Schwartz which contains an example of the discounting principle. The discounting principle describes the human tendency to discount our judgment 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 (also read: Demotivating effects of incentive pay and Praise can demotivate).
Here is that example: "In Switzerland, about 15 years ago, they were trying to decide where to site nuclear waste dumps. There was going to be a national referendum and some psychologist went around and polled citizens who were very well informed and they said would you be willing to have a nuclear waste dump in your community. Astonishingly 50 % of the citizens said 'yes'. They knew or they thought it was dangerous. They thought it would reduce their property values but …. it had to go somewhere and they had responsibilities as citizens. The psychologist asked other people a slightly different question. They said: if we paid you six weeks' salary every year, would you be willing to have a nuclear waste dump in your community? Two reasons: it’s my responsibility and I’m getting paid. Instead of 50% saying 'yes', 25% said 'yes'."