Unfairness commonly impacts human economic decision-making. However, whether inequity aversion impairs pro-social decisions and the corresponding neural processes, is poorly understood. Here, we conducted two experiments to investigate whether human gifting behavior and brain activity are affected by inequity aversion. In experiment 1, participants played as a responder in a joint donation game in which they were asked to decide whether or not to accept a donation proposal made by the proposer. In experiment 2, participants played a donation game similar to experiment 1, but the charity projects were classified as high-deservingness and lowdeservingness projects. The results in both of two experiments showed that the participants were more likely to reject an unfair donation proposal and the late positivity potential (LPP)/P300 elicited by fair offers was more positive than moderately unfair and highly unfair offers regardless of charity deservingness. Moreover, after principal component analysis, the differences in P300 amplitude between fair and highly unfair conditions were positively correlated with the acceptance rates in experiment 2. Taken together, our study revealed that late positivity (LPP/P300) reflected the evaluation of fairness of proposals, and could predict subsequent pro-social decisions. This study is the first to demonstrate that inequity aversion reduces pro-social motivation to help innocent third party.

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