This article reports small group experiments in which subjects may choose to contribute a fixed amount of money toward a monetary public good, and in which the good itself is supplied only if a specified number of contributions (or more) are made. Given the opportunity to communicate, our subjects organized themselves by specifying precisely the number of required contributors and who they would be. This organization, which we call designation of a minimal contributing set, always resulted in provision of the public good, and provision in a nearly optimal manner. In contrast, groups presented with the identical problem but not allowed to communicate failed to generate a sufficient number of contributions 35 percent of the time, and in slightly over half of the successful groups, overprovision produced inefficiency.We present hypotheses about why designating a minimal contributing set works, and data indicating that the mechanism results in reduced normative conflict and felt risk, as well as increased efficiency. The essential property of the minimal contributing set, we hypothesize, is criticalness: the contributions of the members of the minimal contributing set are each critical to obtaining the public good the members desire, and they know it. It is reasonable (albeit not a dominant strategy) to contribute because reasonable behavior can be expected from other minimal contributing set members who are in the same situation. Unreasonableness is a problem that increases with the size of groups, but adaptations exist that, we argue, can reduce its seriousness.

Social dilemmas occur when the pursuit of self-interest by individuals in a group leads to less than optimal collective outcomes for everyone in the group. A critical assumption in the human sciences is that people's choices in such dilemmas are individualistic, selfish, and rational. Hence, cooperation in the support of group welfare will only occur if there are selfish incentives that convert the social dilemma into a nondilemma. In recent years, inclusive fitness theories have lent weight to such traditional views of rational selfishness on Darwinian grounds. To show that cooperation is based on selfish incentives, however, one must provide evidence that people do not cooperate without such incentives. In a series of experimental social dilemmas, subjects were instructed to make single, anonymous choices about whether or not to contribute money for a shared “bonus” that would be provided only if enough other people in the group also contributed their money. Noncontributors cited selfish reasons for their choices; contributors did not. If people are allowed to engage in discussion, they will contribute resources at high rates, frequently on irrational grounds, to promote group welfare. These findings are consistent with previous research on ingroup biasing effects that cannot be explained by “economic man” or “selfish gene” theories. An alternative explanation is that sociality was a primary factor shaping the evolution of Homo sapiens. The cognitive and affective mechanisms underlying such choices evolved under selection pressures on small groups for developing and maintaining group membership and for predicting and controlling the behavior of other group members. This sociality hypothesis organizes previously inexplicable and disparate phenomena in a Darwinian framework and makes novel predictions about human choice.

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How can the beneficiaries of collective action be persuaded to contribute the resources (time, energy, money) necessary for the effort to succeed? Rational and selfish players will recognize they can free ride on the successful contributions of others. If the effort is not successful, they will lose a contribution—and be “suckered.” Other than relying on altruism, organizers of the group effort can modify incentives so that players are more prepared to contribute. Laboratory experiments offer one way of assessing the effectiveness of various such modifications; we conducted such tests to see how well contributing is promoted by (1) assuring contributors that they will not lose if the group effort fails (a “money-back guarantee”) and (2) enforcing contributions if it succeeds (“fair share”). We expect the latter to be more successful because it is “stable,” unlike the former, whose success can be undermined by expectations of that success. Three experimental replications demonstrate that the money-back guarantee is no more successful than a standard dilemma, but fair-share requirements increase contributing significantly over that base. Analysis of subjects' expectations about others' behavior offers some support to the hypothesized process undermining the money-back guarantee, but motivational factors must also be taken into account for a full explanation.

How can the beneficiaries of collective action be persuaded to contribute the resources (time, energy, money) necessary for the effort to succeed? Rational and selfish players will recognize they can free ride on the successful contributions of others. If the effort is not successful, they will lose a contribution—and be “suckered.” Other than relying on altruism, organizers of the group effort can modify incentives so that players are more prepared to contribute. Laboratory experiments offer one way of assessing the effectiveness of various such modifications; we conducted such tests to see how well contributing is promoted by (1) assuring contributors that they will not lose if the group effort fails (a “money-back guarantee”) and (2) enforcing contributions if it succeeds (“fair share”). We expect the latter to be more successful because it is “stable,” unlike the former, whose success can be undermined by expectations of that success. Three experimental replications demonstrate that the money-back guarantee is no more successful than a standard dilemma, but fair-share requirements increase contributing significantly over that base. Analysis of subjects' expectations about others' behavior offers some support to the hypothesized process undermining the money-back guarantee, but motivational factors must also be taken into account for a full explanation.