Coordinated Punishment of Defectors Sustains Cooperation and Can Proliferate When Rare

Learning to Work Together In group endeavors, there is often a tension between working for the greater good of the group as a whole versus working for one's own benefit. Sometimes these paths coincide and sometimes they do not; furthermore, the choices made by other group members can influence the calculation of which path to take. A pair of studies now approaches this challenge from experimental and theoretical points of view. In a forest or fishery, harvesting of wood or food needs to take into account the renewable character of the resource, as well as spatial heterogeneity. Janssen et al. (p. 613; see the Perspective by Putterman) show that communication among the group members is key, both to establishing a maintainable rate of harvesting, as well as enforcement via punishment of noncompliers. Boyd et al. (p. 617; see the Perspective by Putterman) develop a model showing that punishment, which is a costly activity, is most effectively levied when implemented with the approval of group members; that is, coordinated punishment works to the benefit of the whole, whereas individual actions do not. Communication and coordination are essential components in cooperative endeavors. Because mutually beneficial cooperation may unravel unless most members of a group contribute, people often gang up on free-riders, punishing them when this is cost-effective in sustaining cooperation. In contrast, current models of the evolution of cooperation assume that punishment is uncoordinated and unconditional. These models have difficulty explaining the evolutionary emergence of punishment because rare unconditional punishers bear substantial costs and hence are eliminated. Moreover, in human behavioral experiments in which punishment is uncoordinated, the sum of costs to punishers and their targets often exceeds the benefits of the increased cooperation that results from the punishment of free-riders. As a result, cooperation sustained by punishment may actually reduce the average payoffs of group members in comparison with groups in which punishment of free-riders is not an option. Here, we present a model of coordinated punishment that is calibrated for ancestral human conditions and captures a further aspect of reality missing from both models and experiments: The total cost of punishing a free-rider declines as the number of punishers increases. We show that punishment can proliferate when rare, and when it does, it enhances group-average payoffs.