Fair and unfair punishers coexist in the Ultimatum Game

Fairness norms are crucial in understanding the emergence and enforcement of large-scale cooperation in human societies. The most widely applied framework in the study of human fairness is the Ultimatum Game (UG). In the UG, a proposer suggests how to split a sum of money with a responder. If the responder rejects the proposer’s offer, both players get nothing. Rejection of unfair offers is considered to be a form of punishment implemented by fair-minded individuals, who are willing to sacrifice their own resources in order to impose the fairness norm. However, an alternative interpretation is equally plausible: punishers might actually be using rejections in a competitive, spiteful fashion as a means to increase their relative standing. This hypothesis is in line with recent evidence demonstrating that “prosocial” and “antisocial” punishers coexist in other experimental games. Using two large-scale experiments, we explore the nature of UG punishers by analyzing their behavior in a Dictator Game. In both studies, we confirm the coexistence of two entirely different sub-populations: prosocial punishers, who behave fairly as dictators, and spiteful (antisocial) punishers, who are totally unfair. Such a result is fundamental for research on the foundations of punishment behavior employing the UG. We discuss how focusing only on the fairness-oriented part of human behavior might give rise to misleading conclusions regarding the evolution of cooperation and the behavioral underpinnings of stable social systems.