The puzzle that is being altruistic and cooperative when it does not serve our self-interest. Why do humans over-tip to a waiter they’ll never see again? Why are people nice to strangers?
Because life is about succeeding in the “repeated games” that are interactions with friends, family, and co-workers. In those games, altruism pays. It pays to be generous, to do favors, to go out of our way–we will see those people again, and the altruism may come back to help us. So, when we are in a “single shot game” — for example, deciding on how much to tip the waiter at the diner on the side of the road in a city far away from home — this cooperative instinct spills over. Our moral intuitions spring from the repeated games that matter most and we inadvertently channel them to all games/situations. We forget when we’re in a one shot game; we forget we could get away with leaving no tip and it not harming us in the slightest. We forget, that is, until we start to think hard about what the tip should be. In one study, people who start reflecting actively on an appropriate level of altruism (say, the size of the tip) tend to end up less altruistic in single shot games, because they take the time to realize their self-interest calls for them to be…selfish.
This is the argument advanced by David G. Rand, who helped conduct the studies, in this excellent Bloggingheads episode with Joshua Knobe. They cover why humans are selfish or cooperative, among other topics in the annals of human psychology and evolution.