Inequality Aversion and the Common-is-Moral Heuristic shape Third-party Morality

• DOI: 10.31234/osf.io/navkd_v1
• Publication Date: 2025-03-10T14:07:27Z
• Authors: David Schultner; Philip Pärnamets; Ekaterina Yarmolenko; Björn Lindström

Moral norms regulate social life in ways that can promote prosocial behavior. Not only directly involved individuals, but also uninvolved third parties judge and punish norm violators. The principles underlying third-party judgments however remain unclear, particularly since moral decisions often involve the interplay of multiple psychological processes. Here, we investigate the combined effects of two key moral motivations: Inequality aversion (equal outcomes are seen as more moral than unequal ones) and the common-is-moral heuristic (frequent actions are seen as more moral than rare ones). Study 1 participants provided third-party judgments of other players’ behavior in two economic games designed to engage each motivation separately. Results reveal distinct contributions of both motivations to moral judgments. In Study 2, participants completed a combined game designed to reveal the interplay between inequality aversion and the commonness effect. By manipulating both factors independently, we find clear evidence for their joint contribution. However, their weak association highlights how a combination of separable cognitive processes shapes moral norms. Lastly, we used agent-based simulations to show how the joint effect of the common-is-moral heuristic and inequality aversion can lead to the emergence of prosocial behavior in social groups. Results demonstrate that inequality aversion helps social systems overcome a selfish status quo by spurring the formation of prosocial states; the common-is-moral heuristic instead reinforced the selfish default state. Together, these findings disentangle the roles of fairness preferences and the common-is-moral heuristic, informing the principles underlying third-party moral judgments and prosocial behavior in social groups.