Whether others were treated equally affects neural responses to unfairness in the Ultimatum Game.

People expect to be treated equivalently as others in like circumstances. The present study investigated that whether and how equal or unequal treatments of others in like circumstances affected individuals' responses to unfairness through justifying their reference points for fairness considerations. Twenty-five participants were scanned while they were playing an adapted version of the Ultimatum Game as responders. During the experiment, the participant was not only informed of the offer given by her/his proposer but also informed of the division scheme of another proposer-responder pair. It turned out that participants were more likely to accept unequal offers and reported higher fairness ratings when other responders received unequal offers compared with equal offers. Stronger bilateral anterior insula and dorsal anterior cingulate gyrus activities were observed when only participants (but not other responders) received equal offers, whereas greater right dorsolateral prefrontal cortex activity was found when both of them received unequal offers, especially when participants accepted the unequal offers. Taken together, the results demonstrated that whether others in like circumstances were offered equally also plays an important role in responders' fairness-related social decision making.