Heritability of ultimatum game responder behavior

Abstract

Experimental evidence suggests that many people are willing to deviate from materially maximizing strategies to punish unfair behavior. Even though little is known about the origins of such fairness preferences, it has been suggested that they have deep evolutionary roots and that they are crucial for maintaining and understanding cooperation among non-kin. Here we report the results of an ultimatum game, played for real monetary stakes, using twins recruited from the population-based Swedish Twin Registry as our subject pool. Employing standard structural equation modeling techniques, we estimate that >40% of the variation in subjects' rejection behavior is explained by additive genetic effects. Our estimates also suggest a very modest role for common environment as a source of phenotypic variation. Based on these findings, we argue that any attempt to explain observed ultimatum bargaining game behavior that ignores this genetic influence is incomplete.cooperation ͉ experimental economics I t is frequently pointed out that humans exhibit unusually high rates of cooperation among non-kin (1), and it has further been suggested that one important factor for enhancing cooperation is that humans appear willing to forego material payoffs to punish unfair behavior (2-4). Such fairness preferences have been widely studied by using experimental games, in particular the ultimatum game (5-7).In the ultimatum game, two subjects are assigned the role of either proposer or receiver, and then they bargain over a sum of money (the ''cake''). The proposer makes an offer on how to divide the cake. If the receiver accepts the proposer's offer, the players are paid accordingly, whereas if the offer is rejected, both players receive a zero payoff. In a one-shot game, rational and moneymaximizing responders should accept any positive offer because the alternative is a zero payoff. Two stylized facts about responder behavior emerge from the ultimatum game literature: first, that unfair offers are often rejected and second, that the acceptance threshold varies substantially between individuals (5, 6). The average responder behavior has been shown to be relatively stable across Western cultures (8), whereas more variation has been observed among non-Western small-scale societies (9).Although there is a voluminous literature discussing the cultural and evolutionary origins of observed fairness preferences, the relative social and genetic contributions have hitherto been left unexplored. In this work, we use the classical twin design to estimate the heritability of the propensity to reject unfair offers in the ultimatum bargaining game. In doing so, we not only provide the first decomposition of the social and genetic contributions to ultimatum game rejection behavior but also to behavior in experimental games in general. The virtue of the twin design is that by comparing monozygotic (MZ) twins, who share the same set of genes, and dizygotic (DZ) twins, whose genes are imperfectly correlated, we can estimate the proportion of variance in...

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