If your parent or teacher…is sitting in judgement of what you do, and if that judgement will determine whether good things or bad things happen to you, this cannot help but warp your relationship with that person.  You will not be working collaboratively in order to learn or grow; you will be trying to get him or her to approve of what you are doing so you can get the goodies.

-Alfie Kohn, from Punished by Rewards: The Trouble with Gold Stars, Incentive Plans, A’s, Praise, and Other Bribes