Humans are the most cooperative species on the planet, and the most punitive. This is no coincidence. When I promise to take a friend to the airport to catch an early-morning flight, I have to overcome the temptation to sleep an extra couple of hours. My motivation to follow through on my promise is influenced by my awareness of the consequences: my friend will miss her plane, and I will feel guilty. My friend will probably be angry with me, and she may refuse to help me the next time I ask. Worse yet, she may tell other people what I did, and they may share her outrage and anger.
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