Economic games that probe of human behavior (including games that allow punishment of cheaters and non-reciprocators), together with research by biologists, zoologists, and primatologists have delivered strong evidence that traditional assumptions of universally strictly egoistic (rationally self-interested) behavior are at least partially wrong: People repay gifts and punish cheaters, even at a cost to themselves, even among strangers in one-shot games where there is not possibility of reaping future repayment. This practice of "strong reciprocity" has been explained evolutionarily as a maladaptation. The authors of this survey marshal evidence that theories of kin selection (altruism on behalf of genetic relatives that provides reproductive advantage to those who share the altruist's genes), reciprocal altruism (gifts that are made with expectation of eventual repayment by the giftee), indirect reciprocity (gaining a reputation that could pay off in future encounters with other members of the group) costly signaling (acts that cost the actor, but which signal desirability of the signaler as a potential ally or mate) do not sufficiently explain strong reciprocity – and evidence that contradicts these theories as explanatory mechanisms. A cultural evolution hypothesis is proposed: groups that are not closely genetically related can gain survival advantage in competition with other groups if a disproportionate number of strong reciprocators are present – and the presence of strong reciprocators is only possible when cheaters are punished. At the same time, other selection pressures drive the presence of purely selfish humans. Both types coexist because they have coevolved in human cultural practice. The authors offer a beginning, not an ultimate answer, to questions about strong reciprocity, suggesting further research.

Cooperation through indirect reciprocity, can be captured by the phrase "I help you, someone else helps me". Indirect reciprocity helps explain how cooperation is possible at all when economic transactions move beyond small villages where one can easily keep track of one's interactions with everyone else. The success of strategies of indirect reciprocity in empirical studies might be attributable to the fact that humans care so deeply not only about how they are treated, but about the results of interactions between third parties. This concern and the desire to communicate concerns, or gossip, might in turn be explained by evolutionary psychology and the benefits of cooperation in large groups, surpluses resulting from division of labor. To test strategies of indirect reciprocity no two players can interact more than once and the scores of players (the portion of times they have cooperated with others) must be visible. A player choosing a simple version of indirect reciprocity will only cooperate with those whose score is above a certain threshold. However, this player might be punishing another player using indirect reciprocity who has only interacted with defectors. "Effectively, discriminating players pay a cost for punishing bad co-players. Such a form of altruistic punishment can promote cooperation in the community, but at a cost to the punisher, and thus can be viewed as a social dilemma." A more sophisticated strategy would have a player discriminate between justified defection (defecting to punish someone who always defects) and unjustified defection (defecting regardless of the recipients reputation). This strategy avoids the case where a group of players who always cooperate is invaded by a group of players who always defect, but it requires the cognitive abilities to keep track of interactions that are far removed from one's own.

The evolutionary origins of human cooperation pose a puzzle - why do people so frequently cooperate with non-relatives, including people they are not likely to meet again? Existing theories for explaining the evolution of cooperation in a competitive environment include kin selection, reciprocal altruism, and costly signaling. Kin selection, performing altruistic acts at a cost to oneself but at the benefit of one's genes, does not explain non-familial cooperation. Reciprocal altruism does not explain generalized reciprocity, in which one performs altruistic acts for a member of a group, but not limited to actors who have specifically performed altruistic acts one one's behalf in the past. Signalling theory, which holds that altruistic acts enhance one's reputation and increase the chances of mating or useful alliances, does not explain human cooperation when reputation enhancement is not a factor. Using economic games like Prisoner's Dilemma, in which players were given the opportunity to punish free-riders from previous rounds at a cost to themselves, Fehr and Gachter show that cooperation flourishes when free-riders are punished, and that negative emotions toward free-riders "are the proximate mechanism behind altruistic punishment." These results suggest that future study of the evolution of human cooperation should include a strong focus on explaining altruistic punishment.

The seven games explored here, ultimatum, public goods, dictator, prisoner's dilemma, trust, gift-exchange, and third party punishment, can be used both as metaphor to describe prototypical situations in the social world and as a tool to predict the behavior of players in the context of other players' likely actions. Data on the responses of real players can help guide the formation of successful and sustainable institutions for collective action. In a public goods game, for instance, contributions to the public good declined over repeated periods as cooperative players eventually became frustrated with an instigating group of free-riders. Once the structure of the game is altered to allow for punishment of free riders, the average contribution rises steeply to over 95 percent of the endowment. The actual rate of punishment does not have to be that high to generate this increase either; "the mere threat of punishment, and the memory of its sting from past punishments, is enough to induce potential free riders to cooperate."