In a social dilemma game, a period of discussion among subjects substantially increases the incidence of cooperative choices. We conducted two experiments in an effort to explain this effect. Experiment 1 tested and rejected the hypothesis that discussion of the dilemma problem promotes generalized norms in favor of cooperation. Content analysis of discussion sessions in Experiment 1 suggested that promises to cooperate are important in an explanation of discussion's effect. Experiment 2 showed that promises to cooperate substantially increased cooperation rates, but only when everyone in the discussing group promised. We discuss one model in which discussion promotes group identity (as indicated by consensual promising) and therefore cooperation, and another in which discussion provides an opportunity for promise making, which-at least when it is universal-explains discussion's effect without any involvement of group identity.At least two broad conclusions from social dilemma experimental research are now generally accepted. First, many subjects do not defect. Despite the fact that defection returns greater private payoffs whatever others do (viz., cooperation is dominated) and despite often substantial dollar temptations in that direction, it is normal for experimentalists to observe a substantial base of cooperation across experimental conditions (e.g., Marwell & Ames, 1979, 1980Orbell, Schwartz-Shea, & Simmons, 1984). This base is not, generally, sufficient to ensure the social optimum, but it is sufficient to avoid the worst social outcome predicted by the economists' model of selfish rational players. This cooperation occurs without any help from side payments (Olson, 1965), iteration of the game (Axelrod, 1984;Taylor, 1976), designated contributing sets (Amnon Rapoport, 1985;van de Kragt, Orbell, & Dawes, 1983), or other devices that provide self-interested reasons for cooperation.The second conclusion is that a period of group discussion greatly increases the incidence of cooperation. Substantial improvements in cooperation rates with discussion have been reported by, among others, Caldwell (1976), Dawes, McTavish,

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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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