An unfair situation is enough to get anyone’s hackles up. But is our aversion to inequity innate or the product of our social mores? A new study published in Nature suggests that biology does play a role: the brain’s reward centers respond more strongly to situations in which people are treated equally as opposed to unfairly, even when fairness comes at a personal cost.

Researchers gave pairs of young men $30 each and then randomly picked one of them to receive a $50 bonus. Using functional MRI, they scanned each of the men’s brain activity while asking them to judge how they would feel if an ad­ditional one-time gift of more cash went to themselves or to the other person in the pair. As expected, when the man who had not received the bonus imagined getting the gift, his ventral striatum and ventromedial prefrontal cortex—brain areas associated with reward—became active. But surpris­ingly, when the man who had received the initial bonus imagined the other subject getting the gift, his reward centers lit up, too. In other words, his brain responded favorably to an act that reduced inequality but was not in his best interest.

Although the data suggest that an appreciation for fairness is at least partly biological, no one yet knows whether it is innate or learned, because both genetics and experience can affect brain pro­cesses, explains study co-author Elizabeth Tricomi, a psychologist at Rutgers Uni­versity. “It is not unreasonable, however, to think that there could be an evolutionary benefit to a preference for fairness. Fair­ness helps us work together, which can benefit everyone,” she says.