Kathleen Vohs in The Michigan Daily
Why might we be happy to do things for free but not when we are paid to do them? If your neighbor asks you to watch their dog for a night, you will be happy to do it. But if they pay you after, you may not only be offended by the amount, you might expect to be paid even more next time. Humans are social animals and we are happy to help out others. But when we are paid for our work and market norms enter into the scene, social norms run out the door.
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