Common Actions Normalize Inequality in Third-party Morality

Inequality aversion has motivated demands for social change throughout history, yet stark inequalities remain widely tolerated. We propose that this can arise because the common-is-moral heuristic (the tendency to judge frequent actions as more acceptable than rare ones) suppresses sensitivity to inequality when the unfair behavior producing it is common. Across three experiments (N = 904), we examined this interplay by systematically manipulating an action’s commonness and the inequality it generated. In Study 1, we evaluated two minimal tasks which separately captured inequality aversion and the common-is-moral heuristic. To investigate the interaction between both moral motivations, we combined the two tasks into a novel paradigm in which we manipulated inequality aversion and the common-is-moral heuristic. Study 2 revealed simultaneous effects of inequality aversion and the common-is-moral heuristic, and, crucially, demonstrated an interaction between the two. Common actions reduced sensitivity to inequality, such that identical payoff differences were judged less harshly when they resulted from frequent rather than rare behaviors. Study 3 replicated this pattern and showed that punishment intentions followed the same trajectory as moral judgments, highlighting that widespread actions dampen the moral condemnation of unfairness. Finally, we employed agent-based simulations to investigate the broader social consequences of this interaction. While inequality aversion promoted collective welfare and the common-is-moral effect sometimes stabilized cooperation, their interaction frequently undermined welfare by normalizing unequal outcomes. Together, these findings identify a mechanism through which the common-is-moral heuristic can suppress fairness concerns and entrench societal inequality.