Deci, E. L. (1971). Effects of externally mediated rewards on intrinsic motivation. Journal of personality and Social Psychology, 18(1), 105.

Conducted 2 laboratory and 1 field experiment with 24, 24, and 8 undergraduates to investigate the effects of external rewards on intrinsic motivation to perform an activity. In each experiment, Ss performed an activity during 3 different periods, and observations relevant to their motivation were made. External rewards were given to the experimental Ss during the 2nd period only, while the control Ss received no rewards. Results indicate that (a) when money was used as an external reward, intrinsic motivation tended to decrease; whereas (b) when verbal reinforcement and positive feedback were used, intrinsic motivation tended to increase. Discrepant findings in the literature are reconciled using a new theoretical framework which employs a cognitive approach and concentrates on the nature of the external reward.

http://dx.doi.org/10.1037/h0030644