More generosity, less inequity aversion? Neural correlates of fairness perception under social distance and of its relation to generosity.

Humans instinctively react negatively to inequity, while generosity counters this tendency. Previous studies show that both fairness perception and generosity involve balancing behaviors and motivations in social interactions. However, their relationship remains underexplored, limiting our understanding of the complex psychological processes underlying social behavior. Using a social discounting task, we assessed individual generosity, while an Ultimatum Game task with concurrent electroencephalogram recording allowed us to quantify inequity aversion and fairness perception by manipulating social distance and inequity levels. We found that both generosity and fairness perception decrease with increasing social distance, whereas inequity aversion increases. Modeling the decay of generosity across social distances, we found that decayed generosity was positively associated with inequity aversion in the friend condition and negatively correlated with the attenuation of fairness perception. These results suggest that the decay of generosity with social distance is linked to reduced sensitivity to inequity toward friends and heightened neural differences in fairness perception across social relationships. Our study provides electrophysiological evidence of individual variability in generosity and inequity aversion influenced by social distance, expanding inequity aversion theory.