For more about how extrinsic incentives can undermine intrinsic motivation, see the classic experimental demonstration (1) and a more recent review of research that has been conducted subsequently (2):

1. Lepper, M. R., Greene, D., & Nisbett, R. E. (1973). Undermining children’s intrinsic interest with extrinsic reward: A test of the ”overjustification” hypothesis. Journal of Personality and Social Psychology, 28, 129-137.