The strong reciprocity model of the evolution of human cooperation has gained some acceptance, partly on the basis of support from experimental findings. The observation that unfair offers in the ultimatum game are frequently rejected constitutes an important piece of the experimental evidence for strong reciprocity. In the present study, we have challenged the idea that the rejection response in the ultimatum game provides evidence of the assumption held by strong reciprocity theorists that negative reciprocity observed in the ultimatum game is inseparably related to positive reciprocity as the two sides of a preference for fairness. The prediction of an inseparable relationship between positive and negative reciprocity was rejected on the basis of the results of a series of experiments that we conducted using the ultimatum game, the dictator game, the trust game, and the prisoner’s dilemma game. We did not find any correlation between the participants’ tendencies to reject unfair offers in the ultimatum game and their tendencies to exhibit various prosocial behaviors in the other games, including their inclinations to positively reciprocate in the trust game. The participants’ responses to postexperimental questions add support to the view that the rejection of unfair offers in the ultimatum game is a tacit strategy for avoiding the imposition of an inferior status.

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Two explanations of why shared group membership promotes cooperation in social dilemmas were compared. According to the fear-greed model of social identity proposed by Simpson (2006), shared group membership reduces greed but not fear and, thus, should promote altruistic behavior toward in-group members in the absence of fear. According to the group heuristic model proposed by Yamagishi and colleagues, altruistic behavior toward in-group members is a 'ticket' to enter a generalized exchange system; people are not predicted to behave altruistically when it is made salient that no system of generalized exchange operates in the group. We tested these models in a dictator game experiment with two conditions. In the common knowledge condition, either model predicts greater altruism toward in-group recipients. In the unilateral knowledge condition -when the dictator knows the group membership of the recipient, but the recipient does not know the membership of the dictator -the fear-greed model predicts greater altruism toward ingroup recipients. In contrast, the group heuristic model does not predict such in-group bias in altruism. The results of the experiment support the group heuristic model.

To test the hypothesis that sensitivity to monitoring drives people to act altruistically toward members of their own community, two experiments investigated whether an eye-like painting promotes altruism toward in-group members, but not toward out-group members. Participants played the role of dictator in a dictator game with another participant (a recipient) who was from the minimal in-group or out-group. Participants knew if their recipient was an in-group member or an out-group member, but were informed that their recipient did not know the group membership of the proposer. In-group favoritism occurred only when participants were facing a computer desktop which displayed a painting of eyes, but did not occur in the absence of eyes. These findings demonstrate that the eye painting displayed on the participant's computer screen worked as a cue for monitoring, and thus enhanced the participant's altruistic behavior

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