Fairness concerns predict medial frontal negativity amplitude in ultimatum bargaining

Despite evidence that anterior cingulate cortex (ACC) activity is related to social exclusion, rejection and unfairness, evidence that medial frontal negativity (MFN), which has its source in the ACC, reflects these constructs remains unforthcoming. In the present study, subjects participated as recipients in an ultimatum game, while we recorded their electro-encephalogram (EEG). Participants responded to fair and unfair offers from different human proposers. In addition, participants rated themselves on a measure evaluating their concern for fairness. Results showed that MFN amplitude was more pronounced for unfair offers compared to fair offers. Moreover, this effect was shown to be most pronounced for subjects with high concerns for fairness. Our findings suggest that the MFN not only reflects whether outcomes match expectations, but also reflects whether the process by which these outcomes came about matches a social or even a moral norm. In addition, the present results suggest that people in bargaining situations are not only concerned with their own outcomes, but are also concerned with the fairness of the process that resulted in these outcomes.