Timothy D. Wilson:

When economists think about how to solve a problem such as closing the achievement gap in education, or reducing teenage pregnancy, their inclination is to use incentives. What if we pay people to do well in school, give kids money to study and to get good grades? Or what if we take girls who are at-risk for becoming pregnant and pay them a dollar for each day they are not pregnant?

To a social psychologist, it is a little naïve to think that adding external incentives is all you have to do. Not to say that incentives can't work, but they can sometimes backfire if you look at it through the eyes of the person who is getting that incentive. There's some research in social psychology suggesting that external incentives can undermine intrinsic interest in an activity because people begin to think that the only reason they're doing it is for the money. That erodes any interest in that activity there was to start with.