Traditional economics assumes that humans are rational. However, it is known that humans behave fairly in the ultimatum game (UG). There are various explanations for this apparent paradox, such as the “inequity aversion.” However, the role preference (proposer or responder) of humans in the UG is obscure. I conducted a UG scenario experiment where subjects were asked their role preference in addition to their decision in the game. The results showed that the subjects prefer to be proposers rather than responders. In particular, it was found that rational subjects had a high preference for the proposer role. On the basis of these results, I conducted evolutionary simulations of the UG, where each individual has role preference intensity. A role is allocated to the individual proportional to the preference intensity. The results showed coevolution of role preference and fairness. The preference for the proposer role evolved when rational strategy evolved, whereas this preference weakened as rationality decreased. This indicates that fairness has a strong link with role preference; in other words, human fairness is always threatened by the “power and position” of some particular individuals. Hence, its equal distribution among individuals may be effective in maintaining a high level of fairness.
Coevolution of role preference and fairness in the ultimatum game