large numbers of genetically unrelated strangers, and punish those who cheat by free-riding on others’ good will. The challenge to any cooperative enterprise is to avoid getting suckered by free-riders who cheat and add little or no help. As group size grows, the opportunity to cheat and get away with it grows as well given the challenges of storing information about reputation. How do individuals and groups avoid this sucker problem? When nonhuman animals cooperate with members of the same species, they typically target kin. Helping kin provides a buffer against the sucker’s costs because investing in kin means investing in genetic posterity. Helping relatives, even at a cost, translates to helping ones genes move on into the next generation — an insight developed by the British evolutionary biologist William D. Hamilton. When cooperation involves unrelated others, nonhuman animals attempt to circumvent the sucker’s problem by working with a small number of familiar others whose reputation is known, targeting contexts where all participants benefit more from working cooperatively than working alone. These mutual benefits help offset the costs of cooperation. By restricting cooperation to relatives or small numbers of unrelated but familiar group members, animal societies have buffered themselves from extreme cheating. This is significant because cheaters arise in a variety of contexts where there are rules of engagement, including both cooperative and non- cooperative situations. For example, both lions and chimpanzees cooperate in group defense against dangerous neighbors. Some individuals cheat by lagging behind, or failing to join in altogether. In societies organized around hierarchies, low ranking animals sometimes cheat by attempting to eat more than their fare share or by reproducing when their societal norms explicitly forbid it. Interestingly, cheaters in cooperative situations such as those in lions and chimpanzees, never suffer any adverse consequence