Volunteering as a Red Queen Mechanism for Cooperation in Public Goods Games Cooperation among unrelated individuals is a fundamental challenge in biology and social sciences. Reciprocal altruism fails when interactions are infrequent or groups are large. Punishment and reward are effective but require identifying defectors. This study presents a simple mechanism under full anonymity: optional participation allows individuals to avoid exploitation and overcome social dilemmas. In voluntary public goods interactions, co-operators and defectors coexist. This result holds under diverse assumptions about population structure and adaptation mechanisms. Voluntary participation offers an escape from social traps. Cooperation can persist in large groups even without repeated interactions, with anonymous defectors and no memory, and random assortment. Public goods are essential to all societies. Collective efforts to shelter, protect, and nourish the group form the backbone of human evolution. Individuals face the temptation to defect, taking advantage of the public good without contributing. This is known as the 'Tragedy of the Commons', 'Free Rider Problem', 'Social Dilemma', or 'Multi-person Prisoner's Dilemma'. Public goods games are used to study this issue. In these games, groups of co-operators do better than groups of defectors, but defectors always outperform co-operators in their group. With a return rate r smaller than the group size, this is a social dilemma: every individual is better off defecting than cooperating. Groups would consist of defectors only, and forego the public good. In two-player groups, this is the prisoner's dilemma game. Co-operation based on direct or indirect reciprocation can be established if the probability of another round is sufficiently high. But retaliation does not work if many players are engaged in the game, because players intending to punish a defector can do so only by refraining from co-operation in subsequent rounds, thereby also punishing the co-operators in the group. If players are offered the possibility to fine specific co-players after each round, co-operation gets firmly established. This happens even if punishment is costly to the punisher and if players believe they will never meet again. But such fining or rewarding requires that players can discriminate individual defectors. While reward and punishment are major factors in human cooperation, we want to draw attention to a simpler mechanism. It consists in allowing players not to participate and to fall back on a safe 'side income' which does not depend on others. Such risk-averse optional participation can foil exploiters and relax the social dilemma, even if players have no way of discriminating against defectors. We consider three strategic types: co-operators and defectors, both willing to engage in the public goods game and speculate on the success of a joint enterprise, and 'loners' who rely on some autark way of life. Co-operators will not stably dominate the population in such a voluntary public goods game, but neither will exploiters. Their frequencies oscillate,Volunteering as a Red Queen Mechanism for Cooperation in Public Goods Games Cooperation among unrelated individuals is a fundamental challenge in biology and social sciences. Reciprocal altruism fails when interactions are infrequent or groups are large. Punishment and reward are effective but require identifying defectors. This study presents a simple mechanism under full anonymity: optional participation allows individuals to avoid exploitation and overcome social dilemmas. In voluntary public goods interactions, co-operators and defectors coexist. This result holds under diverse assumptions about population structure and adaptation mechanisms. Voluntary participation offers an escape from social traps. Cooperation can persist in large groups even without repeated interactions, with anonymous defectors and no memory, and random assortment. Public goods are essential to all societies. Collective efforts to shelter, protect, and nourish the group form the backbone of human evolution. Individuals face the temptation to defect, taking advantage of the public good without contributing. This is known as the 'Tragedy of the Commons', 'Free Rider Problem', 'Social Dilemma', or 'Multi-person Prisoner's Dilemma'. Public goods games are used to study this issue. In these games, groups of co-operators do better than groups of defectors, but defectors always outperform co-operators in their group. With a return rate r smaller than the group size, this is a social dilemma: every individual is better off defecting than cooperating. Groups would consist of defectors only, and forego the public good. In two-player groups, this is the prisoner's dilemma game. Co-operation based on direct or indirect reciprocation can be established if the probability of another round is sufficiently high. But retaliation does not work if many players are engaged in the game, because players intending to punish a defector can do so only by refraining from co-operation in subsequent rounds, thereby also punishing the co-operators in the group. If players are offered the possibility to fine specific co-players after each round, co-operation gets firmly established. This happens even if punishment is costly to the punisher and if players believe they will never meet again. But such fining or rewarding requires that players can discriminate individual defectors. While reward and punishment are major factors in human cooperation, we want to draw attention to a simpler mechanism. It consists in allowing players not to participate and to fall back on a safe 'side income' which does not depend on others. Such risk-averse optional participation can foil exploiters and relax the social dilemma, even if players have no way of discriminating against defectors. We consider three strategic types: co-operators and defectors, both willing to engage in the public goods game and speculate on the success of a joint enterprise, and 'loners' who rely on some autark way of life. Co-operators will not stably dominate the population in such a voluntary public goods game, but neither will exploiters. Their frequencies oscillate,