Analysis of individual differences in prosociality from evolutionary psychology and behavior genetics

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We examined the origin of individual differences in human prosociality from evolutionary and genetic point of views. We conducted a dictator game and ultimatum game with a Japanese adult twin sample. It was shown that the amount of offer on the two games were not greatly affected by genetics since the within pair correlations were not different between the identical twins and the fraternal twins. On the other hand, the within-pair correlations of minimal acceptable amount on the ultimatum game were greater for the identical twin pairs, suggesting genetic influences. We also had two experiments in which undergraduates participated in both of the dictator game and ultimatum games. It was shown that the difference between the offers on the two games was related to manipulative traits, such as Machiavellianism. Also, it was shown that those who changed the offer between the two games were more likely to choose the dictator game when they could choose which game to play.