This week two interesting papers appeared who had unusual design to study fairness by using ultimatum games. In the Proceedings of the National Academy of Sciences USA of October 2, Björn Wallace and his colleagues performed large series of ultimatum games with monozygotic twins and dizygotic twins using a Swedish Twin Registry. A significant correlation of acceptence threshold was found for the twins with the same set of genes (monozygotic twins), while this was not the case for twins whoms genes are not the same (dizgotic twins). Additional statistical analysis shows that at least 40% of the decisions can be explained by genetic information.

In Science of October 5, Keith Jensen and colleages report on mini-ultimatum games with chimpanzees. In the experiment the proposer has to make an initial decision which of two sliding trayes with raisins. The dishes of raisins have different distributions of raisins. The other chimpanzee can pull the trayes further so that both chimpanzees will get raisins. With a mini-ultimatum game is meant that predefined sets of distributions of raisins are given. Interestingly, the responder almost never refused to help with getting the raisins, even when the distribution is not fair. Looking more closely we see that refusal correlates with unfair distributions. When the responder gets nothing, the rejection rate is 44%, while it is 0% when teh responder gets 80%. The authors concluder that chimpanzees are rational maximizers. I would not agree, since chimpanzees do not play an anonymous one-shot game, like mimiced with experiments with human subjects. Hence rejecting to help, may affect the options of getting help in future events.

In sum, both papers show some interesting material that suggest that there is genetic influence, although inherent recently in evolutionary time.