In an echo of what Harlow discovered two decades earlier, Deci revealed that human motivation seemed to operate by laws that ran counter to what most scientists and citizens believed. From the office to the playing field, we knew what got people going. Rewards—esp;ecially cold, hard cash—intensified interest and enhanced performance. What Deci found ... was almost the opposite. “When money is used as an external reward for some activity, the subjects lose intrinsic interest....” Rewards can deliver a short-term boost—just as jolt of caffeine can keep you cranking for a few more hours. But the effect wears off—and, worse, can reduce a person’s long-term motivation to continue the project. [My emphases added.]
If you want to hear this straight from the horse's mouth, click here.