Inequality-induced emotions might promote cooperation in evolutionary games

Two types of emotions, envy and guilt, could be induced when people encounter inequality; especially, persons who are disadvantaged may feel envy toward the ones at advantage, whereas those in superior positions may feel guilty. These two emotions can affect subjective evaluations of their own utilities. In most previous studies of evolutionary games, the payoffs gained from games are directly used for payoff comparison when individuals consider to imitate others’ strategies, regardless of the impacts caused by the inner emotions. Here, we introduce two types of inequality aversion in evolutionary games and assume that the inequality aversion can induce two different emotions and subsequently affect the utilities of individuals. We investigate how the inequality aversion affects the evolution of cooperation in spatial evolutionary games. The numerical simulation results show that envy makes the evolution of cooperation more difficult, while guilt can effectively promote cooperation. Moreover, we have provided some intuitive explanations by scrutinizing the microscopic evolutions of the strategy patterns for different types of players, which are categorized according to strategy and emotion. We find that, to cooperators and defectors, the two emotions have different influences on strategy transmission in different evolution stages, respectively. Roughly speaking, envy hinders cooperation by weakening cooperators, while guilt promotes cooperation by weakening defectors. Besides, we also study the effects of inequality aversion on payoff distribution and find that envy influences fairness negatively, whereas guilt plays a positive role in fairness. Graphic abstract