The reward that removed the interest

Children who enjoyed drawing were given a certificate each time they drew. When the certificates stopped, they drew less than children who had never been rewarded at all.

The finding, repeated many times since, is one of the most counter-intuitive in the field. A reward adds a reason to do something, and adding a reason should not subtract one. What appears to happen is that the reward replaces rather than adds: the activity is reinterpreted as work done for a certificate, and when the certificate disappears, so does the reason.

The effect is not universal, and the conditions matter more than the headline. It appears most strongly when the activity was already enjoyable, when the reward is expected in advance, and when it is tangible and contingent on simply doing the task. It is much weaker or absent when the activity was tedious to begin with, when the reward is unexpected praise after the event, or when it carries information about competence rather than compliance. 'You solved the hard one' behaves quite differently from 'here is your sticker'.

The practical implications are narrower than either enthusiasts or critics usually claim. Rewards are a reasonable tool for getting a difficult routine started, which is why they work for flossing and for early reading practice. They are a poor tool for something a learner already loves, where the main risk is not too little effort but the loss of the reason they were doing it.

The design question is therefore when to stop. A reward that is withdrawn as soon as the habit holds does its job and leaves. One that continues indefinitely has become the point.