Some people are more generous in social situations—offering to cover the dinner bill for example. Others are more individualistic, preferring to keep resources for themselves. A recent study in Nature Neuroscience finds that perceptions of fairness vary from person-to-person based on highly-individual reference points, and that the neuropeptide oxytocin can selectively alter these perceptions by changing brain activity. “What we are arguing is everyone has their personal sweet spot,” says cognitive neuroscientist Yina Ma, who coauthored the study from Beijing Normal University in China.

To investigate social decision making, Ma’s team brought pairs of strangers into the lab, and introduced them briefly. Then the participants took turns in an fMRI machine, where they looked up at a screen. Money-sharing scenarios flashed across the monitor above them. Looking up at the screen, each participant saw what they received for that scenario—$6, for example—and what their partner received– perhaps $3. Using a handheld button, each person ranked their satisfaction with the allocation of money on a scale of one to four. Then the next scenario flashed across the screen to be ranked.

Throughout the experiment, some people were consistently individualistic, strongly preferring to take much more money than their partner. Others were more prosocial, preferring a 50/50 split, or even preferring to have the other person do better. Using each participant’s responses, the researchers created a computational model to explain and predict how participants made decisions; they assessed their satisfaction with the distribution of money based on how far it deviated from their fairness setpoint. Establishing this new social reference model, Ma says, is “the biggest contribution of this work.”

Physiology seemed to support that model. While the participants clicked away, the fMRI scans showed that activity spiked in response to “unfair” scenarios— those far from a person’s sweet spot—providing further evidence that the brain tracks distance from a highly individual fairness set point, Ma says.

The fMRI scans suggest that prosocials and individualists favor two different regions to track distance from their sweet spot in unfavorable scenarios: the bilateral amygdala in prosocials and the right lateral orbitofrontal cortex in individualists. The finding is consistent with previous research, notes biologist and psychologist Carolyn Declerck of the University of Antwerp in Belgium, and she says this work nicely brings together multiple threads.

Previous research identified the amygdala as a prime location to evaluate unfair scenarios, because it tracks reward, punishment, and social preferences. Prosocials are known to react emotionally to unfairness, so it makes sense that their response favors the amygdala, an area of the brain associated with threat and alertness, Declerck says. Individualists react to unfairness more strongly from their forebrain, which also makes sense, she adds, because it’s associated with computation and conflict resolution. Individualists, Declerck says, “see unfairness more as a conflict of interest, whereas prosocials see it more as a threat, something emotionally not right.”

But fairness set points are not set in stone. A nasal spray of oxytocin changed brain activity selectively for participants. Best known as a neuropeptide associated with love and social bonds, oxytocin can also do the opposite, increasing envy for example.

Previous studies suggested oxytocin could have different effects on the prosocials and the individualists. It did, with selective impacts on the individualistic group. They began to prefer outcomes benefitting other people, based on their choices made in the scenarios in the fMRI. The scans confirmed amplified activity in their amygdalas. Oxytocin’s effects on naturally prosocial participants were not as straightforward. “The most exciting aspect of this new finding is that oxytocin seems to selectively enhance prosocial behavior in people with individualistic social reference points by influencing this reference point,” says Yale neuroscientist Steve Chang. “And that this change is reflected in brain activations.”

One direction of future research, Ma says, is to probe the origins of these social reference points, which could be inherited genetically or learned. And oxytocin could have related clinical applications, she says, for example in people with autism spectrum disorders who are disinterested in social interactions.

Declerck isn’t so sure though—the effects of oxytocin may hinge on individual differences, and she thinks it’s too early to say which ones. As for personalized oxytocin nasal sprays to make colleagues, friends, and family more fair or generous, Ma just laughs. “We can’t say more prosocial or more independent is better,” she says wryly, “but being extreme, that’s the problem.”