In human societies, cooperative behaviour in joint enterprises is often enforced through institutions that impose sanctions on defectors. Many experiments on so-called public goods games have shown that in the absence of such institutions, individuals are willing to punish defectors, even at a cost to themselves. Theoretical models confirm that social norms prescribing the punishment of uncooperative behaviour are stable: once established, they prevent dissident minorities from spreading. But how can such costly punishing behaviour gain a foothold in the population? A surprisingly simple model shows that if individuals have the option to stand aside and abstain from the joint endeavour, this paves the way for the emergence and establishment of cooperative behaviour based on the punishment of defectors. Paradoxically, the freedom to withdraw from the common enterprise leads to enforcement of social norms. Joint enterprises which are compulsory rather than voluntary are less likely to lead to cooperation. Keywordsevolutionary game theory; public goods games; cooperation; altruistic punishment; voluntary interactions An impressive body of evidence shows that many humans are willing to pay a personal cost in order to punish wrong-doers (1-8). In particular, punishment is an effective mechanism to ensure cooperation in public goods interactions (9-11). All human populations seem willing to use costly punishment to varying degrees, and their willingness to punish correlates with the propensity for altruistic contributions (12). This raises an evolutionary problem: in joint enterprises, free-riding individuals who do not contribute, but exploit the efforts of others, fare better than those who pay the cost of contributing. If successful behaviour spreads, for instance through imitation, these defectors will eventually take over. Punishment reduces the defectors' payoff, and thus may solve the social dilemma. But since punishment is costly, it also reduces the punishers' payoff. This raises a 'second order social dilemma': costly punishment seems to be an altruistic act, since individuals who contribute, but do not punish, are better off than the punishers.