Stable polymorphism of cooperators and punishers in a public goods game.

The role of punishment in the maintenance of cooperation has been emphasized recently. However, the maintenance of punishment is not an obvious consequence because punishment itself is also a public good; it is costly to perform and hence vulnerable to exploitation. For example, cooperative punishers, who help others and punish free riders, are disadvantageous in competition against pure cooperators, who cooperate but do not punish free riders.