The reason a reward too large kills the habit

Children who drew freely were offered a certificate for drawing. Drawing increased while the certificate was available and fell below the original level when it was withdrawn. The reward did not add to the existing motivation; it replaced it.

What everyone sees

Rewards are assumed to add a second reason on top of the first: intrinsic interest plus external incentive equals more motivation. The possibility that the external incentive crowds out the intrinsic one is counterintuitive, because adding something should not subtract something else.

What is actually happening

Lepper, Greene and Nisbett demonstrated the overjustification effect: children who expected a reward for an already enjoyed activity showed less subsequent interest than those who received no reward or an unexpected one. Deci’s cognitive evaluation theory explains the mechanism: an external reward shifts the perceived locus of causality from internal to external, so the activity is now understood as something done for the reward rather than for itself. When the reward is removed, the internal reason does not automatically return, because the activity has been recategorised.

Why it stays hidden

The replacement hides because both motivations feel like they are operating at the same time during the reward period. The child draws and receives a certificate and enjoys both. The shift in perceived causality is not felt; it is revealed only when the reward is withdrawn and the drawing stops, at which point the internal motivation has already been overwritten.