the “free-rider problem.” In groups in which people make sacrifices for the common good, an individual who makes no such sacrifices—who in effect takes a free ride on the backs of the altruists—comes out ahead. In the cold logic of these computer simulations, whoever accumulates the most resources in one generation goes on to produce more children in the next, so selfishness is adaptive but altruism is not. The only solution to the free-rider problem is to make altruism pay,
