Extrinsic motivation is subject to the overjustification effect. People who receive a reward for good behavior often come to believe that they exhibited the behavior for the reward, rather than due to internal motivation. This understanding in turn undermines intrinsic motivations. This study examines the overjustification effect in the realm of prosocial sharing, and finds similar results.
Two studies investigated the influence of external rewards and social praise in young children's fairness-related behavior. The motivation of ninety-six 3-year-olds' to equalize unfair resource allocations was measured in three scenarios (collaboration, windfall, and dictator game) following three different treatments (material reward, verbal praise, and neutral response). In all scenarios, children's willingness to engage in costly sharing was negatively influenced when they had received a reward for equal sharing during treatment than when they had received praise or no reward. The negative effect of material rewards was not due to subjects responding in kind to their partner's termination of rewards. These results provide new evidence for the intrinsic motivation of prosociality—in this case, costly sharing behavior—in preschool children. (Source)
See also: Rewarding Creativity