People often feel more altruistic when they have without receiving payment or implied social pressure than when offered a reward for doing so. this is best explained by

This is best explained by the overjustification effect. This effect occurs when the person loses the intrinsic motivation to perform a task when they receive an external incentive, such as a payment. The person has different motivations that help him sustain the activity he performs, for example, altruistic motivations, but by losing these motivations the person loses interest in carrying out the activity.
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