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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In an ultimatum game, the responder must decide between pursuing self‐interest and insisting on fairness, and these choices are affected by the intentions of the proposer. However, the time course of this social decision‐making process is unclear. Representational similarity analysis (RSA) is a useful technique for linking brain activity with rich behavioral data sets. In this study, electroencephalography (EEG) was used to measure the time course of neural responses to proposed allocation schemes with different intentions. Twenty‐eight participants played an ultimatum game as responders. They had to choose between accepting and rejecting the fair or unfair money allocation schemes of proposers. The schemes were offered based on the proposer's selfish intention (monetary gain), altruistic intention (donation to charity), or ambiguous intention (unknown to the responder). We used a spatiotemporal RSA and inter‐subject RSA (IS‐RSA) to explore the connections between event‐related potentials (ERPs) after offer presentation and intention presentation with four types of behavioral data (acceptance, response time, fairness ratings, and pleasantness ratings). The spatiotemporal RSA results revealed that only response time variation was linked with the difference in ERPs at 432–592 ms after offer presentation on the posterior parietal and prefrontal regions. Meanwhile, the IS‐RSA results found a significant association between inter‐individual differences in response time and differences in ERP activity at 596–812 ms after the presentation of ambiguous intention, particularly in the prefrontal region. This study expands the intention‐based reciprocal model to the third‐party context and demonstrates that brain activity can represent response time differences in social decision‐making.

Social information has substantial influences on prosocial behavior. In this study, we performed an event-related potential (ERP) experiment to examine the effect of social influence on giving. The participants were allowed to form an initial decision on how much money to donate to a charity provided the program’s average donation amount and to make a second donation decision. Social influence varied in different directions (upward, downward, and equal) by altering the relative donation amount between the average donation amount and the participants’ first donation amount. The behavioral results showed that participants increased their donation amount in the upward condition and decreased it in the downward condition. The ERP results revealed that upward social information evoked larger feedback-related negativity (FRN) amplitudes and smaller P3 amplitudes than in the downward and equal conditions. Furthermore, the pressure ratings, rather than the happiness ratings, were associated with the FRN patterns across the three conditions. We argue that people in social situations are more likely to increase their donations owing to pressure than voluntary altruism. Our study provides the first ERP evidence that different directions of social information evoke different neural responses in time course processing.

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