The impact of unfairness experience on cooperative behavior revealed by ultimatum game-public goods game integration:...

Fairness critically shapes cooperative behavior in social dilemmas, yet the neurocognitive mechanisms linking unfairness experiences to cooperation remain underexplored. Twenty-four participants (M _age = 19.50 years, SD = 1.06) completed the Ultimatum Game (UG) with three proposal types (fair, moderately unfair, and extremely unfair) while event-related potentials (ERPs) were recorded, followed by the Public Goods Game (PGG) to assess cooperation. Behavioral results revealed that participants exhibited robust inequity aversion, rejecting moderately and extremely unfair UG proposals at significantly higher rates than fair one. Exposure to unfairness reduced subsequent PGG contributions, underscoring fairness as a priority over material gains. ERPs results showed that unfair proposals elicited stronger medial frontal negativity (MFN), reflecting norm violation detection, while fair proposals evoked larger P300 amplitudes, indexing reward valuation. Exploratory analyses revealed that P300 amplitudes positively related to cooperative behavior, suggesting reward-related neural activity facilitates post-inequity cooperation. These findings elucidate behavioral patterns of inequity aversion in interactive games and their neurophysiological correlates, advancing the understanding of how fairness preferences regulate cooperative decision-making.