The Overjustification Effect
Pay someone to do a thing they already enjoyed and they will do less of it once you stop paying. The external reward supplies a reason, which crowds out the reason they already had. The finding is real and it is conditional, and the two meta-analytic traditions that have argued about it for thirty years agree on the facts and disagree over whether to call the effect pervasive or limited. Neither side conceded in their 2001 exchange, and any one-sentence version of this concept is wrong.
What it is
The founding experiment is Mark Lepper, David Greene and Richard Nisbett's, published in 1973. Preschool children who already loved drawing were offered a good player award. The children promised the award in advance drew less in free play a week later than children who received a surprise award and than children who received none at all. The reward had turned play into work.
The meta-analysis that made the case is Edward Deci, Richard Koestner and Richard Ryan's, in Psychological Bulletin in 1999, covering a hundred and twenty-eight studies. Its shape is conditional rather than blanket, and the conditions are the finding.
What undermines: expected tangible rewards reduce free-choice persistence on an already-interesting task, with engagement-contingent and completion-contingent rewards doing it most strongly and performance-contingent rewards less so. What does not: task-noncontingent rewards are null, unexpected tangible rewards are flat at essentially zero, and verbal praise and positive feedback enhance intrinsic motivation on both outcome measures. The self-report effects are tiny across the board. The free-choice column is where the finding lives, and the stated-interest column is close to nothing.
One design decision defines the result and almost nobody prints it. The authors excluded uninteresting and boring tasks from the primary meta-analyses by design, on the stated ground that where there is no intrinsic motivation there is nothing to undermine. Thirteen studies manipulating task interest were analysed separately. So the headline is a finding about interesting tasks, and it is a finding about free-choice behaviour, and a sentence that drops either clause is wrong. Two further qualifications come from the paper itself: tangible rewards are more detrimental for children than for college students while verbal rewards are less enhancing for children, and the performance-contingent category is coded four ways by control group and feedback valence, because the competence a performance-contingent reward affirms can offset its controlling aspect, so the sign of that effect depends on which control group it is measured against.