i28.
This is a highly condensed summary of the research reported at length by Alfie Kohn (1993), Punished by Rewards, especially chapters 4 and 5. The original coffee break study, later much elaborated and replicated, was by Edward L. Deci, “Effects of Externally Mediated Rewards on Intrinsic Motivation”, Journal of Personality and Social Psychology, 18, 1971, pp 105–115, cited in Kohn (1993), p 70.