Except that “always cooperate” is not a stable strategy. As soon as everybody cooperates, an always-defect strategy can invade, just like a hawk among the doves, and clean up. So you start with all defect, go to tit for tat, then generous tit for tat, then all cooperate, then all defect. “And this,” said Nowak, “is the theory of war and peace in human history.”17


Nevertheless, humans do cooperate. If indirect reciprocity isn’t responsible for that cooperation, what is? Lately, one popular view seems to be that cooperation thrives because it is enforced by the threat of punishment. And game theory shows how that can work.

Among the advocates of this view are the economists Samuel Bowles and Herbert Gintis and the anthropologist Robert Boyd. They call this idea “strong reciprocity.” A strong reciprocator rewards cooperators but punishes defectors. In this case, a more complicated game illustrates the interaction. Rather than playing the Prisoner’s Dilemma game—a series of one-on-one encounters— strong reciprocity researchers conduct experiments with various versions of public goods games.

These are just the sorts of games, described in Chapter 3, that show how different individuals adopt different strategies—some are selfish, some are cooperators, some are reciprocators. In a typical public goods game, players are given “points” at the outset (redeemable for real money later). In each round, players may contribute some of their points to a community fund and keep the rest. Then each player receives a fraction of the community fund. A greedy player will donate nothing, assuring a maximum personal payoff, although the group as a whole would then be worse off. Altruistic players will share some of their points to increase the payoff to the whole group. Reciprocators base their contributions on what others are contributing, thereby punishing the “free riders” who would donate little but reap the benefits of the group (but in so doing punish the rest of the group, including themselves, as well). As we’ve seen, humankind comprises all three sorts

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