Previous research indicated that fairness consideration can be influenced by social distance. However, it is not clear whether social distance and anonymity have an interactive impact on fairness evaluation during asset distribution and whether these processes can be documented in brain activity. Using a modified ultimatum game combined with measures of event related potential (ERP), we examined how social distance and anonymity modulate brain response to inequality. At the behavior level, we found that acceptance rate and reaction time can be substantially modified by social distance and anonymity. Feedback-related negativity, an ERP component associated with conflict between cognitive and emotion motives, was more negative in response to unfairness than fairness from strangers; however, it showed an opposite trend for unfair offers provided by friends, suggesting that the influence of social distance on fairness perception is relatively fast. The P300 in response to fair offers was more positive when the proposers made offers when uncertain about partner identity than when certain about partner identity. These results suggest that unfairness is evaluated in a fast conflict detection stage and a slower stage that integrates more complex social contextual factors such as anonymity.
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