 It is clear in the “one-shot” Prisoner’s Dilemma that defection is strictly dominant: whether the other player cooperates or defects, one gains more points by defecting. But if defection always gives a higher payoff, what sense does it make to cooperate? In a Hobbesian population of egoists, with payoffs as in the Prisoner’s Dilemma, it would seem that we should expect mutual defection as both a matter of course and the rational outcome—Hobbes’ “war of all against all”. How could a population of egoists come to cooperate? How could the ethical desideratum of cooperation arise and persist?