The overjustification effect is a phenomenon in psychology in which providing expected external incentive, such as money or prizes, for an already intrinsically rewarding activity can reduce a person's intrinsic motivation to perform that activity. It is commonly understood in terms of the interaction between intrinsic and extrinsic motivation, where external rewards may undermine pre-existing intrinsic interest, a process often described as motivational "crowding out". The effect has been widely discussed in research by Edward Deci and Mark Lepper. Once rewards are no longer offered, individuals may show reduced interest in the activity, namely, cognitive re-evaluation, although the extent of this effect can vary depending on contexts.