Table 1.

Commonly used behavioral economic games to study intergroup prosocial behavior.

Name	Abbreviation	Reference	Brief description	Preferences and beliefs
Common Pool Dilemmas	CPD	Hardin (1968), Messick et al. (1983)	A typical CPD might have four players and a common pool consisting of a certain number of points (e.g., 200 points). In a round, each player can take up to 50 points. This amount is then halved and earned by the player; the remaining money gets split equally among all players.	In a CPD, prosocial behavior can be attributed to some combination of preferences (e.g., a motivation to help the other players) or beliefs (e.g., believing that prosocial behavior will be reciprocated in future).
Dictator Game	DG	Kahneman et al. (1986), Forsythe et al. (1994)	One player—the dictator—makes a unilateral decision about how to divide an amount of money with a second player—the recipient. The dictator is able to allocate any amount of money to the recipient—from nothing to the entire amount—and the recipient must accept this amount.	In principle, the DG excludes any role of beliefs, since the experimental set-up is described in a way that makes it clear that there can be no reciprocity or interdependence of outcomes; the dictator has complete power over the situation and the recipient must accept whatever amount the dictator decides. Therefore, behavior in the DG can be interpreted as resulting primarily from social preferences.
Intergroup Prisoner’s Dilemma	IPD	Bornstein and Ben-Yossef (1994)	In this game there are two groups, with three members in each group. Each player receives an endowment of two monetary units, and is informed that they can either keep this unit or contribute it to a common pool. For every contribution, each ingroup member (including the contributor) gains one unit and each outgroup member loses one unit. Three key strategies can be discerned. First, individuals can play according to the individual strategy, where individually the best strategy is to not contribute anything because the individual’s return from contributing 2 units is only 1 unit. Second, individuals can play according to the ingroup strategy, where the dominant group strategy is for all group members to contribute; this is because group-wide contribution generates a total of 3 units for the ingroup while costing it only 2 units. Third, individuals can play according to the collectively optimal strategy, where because the ingroup’s gain from contribution is exactly offset by the outgroup’s loss, contribution is a net waste of units from the collective point of view, and so the collectively optimal strategy—the one that maximizes the payoff of both groups and all players—is for all players to defect.	Prosocial behavior can be attributed to some combination of preferences (e.g., ingroup love or outgroup derogation) and beliefs (e.g., adherence to social norms of cooperation; expectations of future reciprocity).
Intergroup Prisoner’s Dilemma—Maximizing Difference	IPD-MD	Halevy et al. (2008)	In the IPD-MD, group members are able to direct their contributions to one of two pools: a between-group pool, or a within-group pool.The between-group pool parallels the original IPD whereby an increase in the payoffs to each ingroup member by 1 unit decreases the payoff to each outgroup member by 1 unit. In contrast, the within-group pool increases the payoffs to each ingroup member by 1 unit but has no effect on the outgroup. Contribution to the within-group pool indicates ingroup love—the cooperative motivation to increase the ingroup’s payoff. In contrast, contribution to the between-group pool indicates outgroup derogation—the aggressive motivation to hurt the outgroup (or the competitive motivation to increase the ingroup’s relative payoff).	Prosocial behavior can be attributed to some combination of preferences (e.g., ingroup love or outgroup derogation) and beliefs (e.g., adherence to social norms of cooperation; expectations of future reciprocity).
Minimal Group Paradigm Matrices	Tajfel Matrices	Tajfel (1970)	The Tajfel Matrices require individuals to distribute points between other participants who are identifiable only by code number and their group membership (e.g., “participant number 34 from Group A”). Participants are informed that after the task was finished, they will receive the total number of points that had been allocated to them by the other participants. Participants tend to allocate points according to three main strategies: aiming for maximum joint profit of all players, for maximum profit for the ingroup, or for maximum difference in points between the ingroup and outgroup.	Participants cannot allocate points to themselves, which was intended to eliminate direct reciprocity. Therefore, in theory the Tajfel matrices allow the researcher to isolate the contribution of social preferences to ingroup favoritism in economic games.
Prisoner’s Dilemma	PD	Rapoport and Chammah (1965), Axelrod (1980)	In this game, players (the “prisoners”) can choose to cooperate or defect. If players both cooperate, they achieve a good outcome. However, if one player defects while the other cooperates, the defector gets the highest payoff, and the cooperator gets the lowest payoff—giving both players an incentive to defect. If both defect, both do poorly, and so the PD demonstrates the tension that lies between individual rationality (reflected in the incentive of both sides to be selfish) and group rationality (reflected in the higher payoff to both sides for mutual cooperation over mutual defection)	Cooperation can arise from a genuine desire to cooperate with the other player (preferences), the expectation that the other person is likely to cooperate and so it makes sense for you to also cooperate (beliefs), or some combination of the two.
Public Goods Dilemmas	PGD	Hardin (1968)	A typical experimental PGD has four participants who choose how many points to contribute to a common project. The points that are contributed are then multiplied by some amount from 0.25 to 1 and then redistributed equally to each player.	As with other social dilemmas, in a PGD prosocial behavior can be seen to arise from preferences and beliefs.
Trust Game	TG	Berg et al. (1995)	This game has two participants: an investor and a trustee. The investor is given some money and told that they must send a proportion (from zero to the full amount) of this money to the trustee, and that the experimenter will multiply the money by some amount. Once the trustee receives the money, they are told that they must send back a portion of it to the investor, again ranging from zero to the full amount.	With its focus on trust, for the first mover in a one-shot game, behavior is likely to be driven by both preferences and beliefs. In particular, behavior is likely to be motivated by expectations about whether the second player will return any money, and whether this probabilistic outcome justifies the potential gain in winnings. For the second mover in a one-shot trust game, however, behavior is likely to be driven primarily by preferences: does the person want to return some money back to the first player? The trust game, then, taps a mixture of preferences and beliefs.
Ultimatum Game	UG	Güth et al. (1982)	One player (the proposer) receives an amount of money and makes a proposal to the other player (the responder) regarding how to divide the money between them. If the responder accepts the proposed split, both players receive the allocated money. However, unlike the DG, the responder has the option to reject the proposed split, leading both players to receive nothing.	Prosocial behavior by the proposer in the UG can be attributed to a mixture of social preferences and beliefs, as they gauge both how much they would like to offer, but also the likelihood that such an offer would be accepted. Meanwhile, prosocial behavior by the responder, who decides whether to accept or reject the offer, can be interpreted as resulting from preferences alone.