Human Cooperation, Evolution of

Human cooperation is a novel evolutionary puzzle because we cooperate with genetically unrelated individuals in groups that comprise millions of people. Direct reciprocity, especially when considering errors in behavior, has shed light on pairwise cooperation among well-known but genetically unrelated people. Three mechanisms have been identified to explain large-scale cooperation in humans. Cooperation can evolve by indirect reciprocity in groups as long as people can observe each other's behavior, or can garner honest information via gossip on who cooperates and defects in their interactions. Humans also have a disposition to cooperate and to punish those who do not, even at a cost to oneself, and such costly punishment can sustain cooperation in even larger groups of people who do not know much about each other. Cultural group selection explains the scale of human cooperation, why it is variable, and why norms enforced by sanctions are group-beneficial. Support for these theories has come from laboratory experiments using a variety of behavioral economic games, and from field studies in small-scale societies. Key open questions include understanding what characterizes goodness in indirect reciprocity, why gossip is sufficiently accurate, and why people are motivated to engage in costly punishment.