Tipping the balance between fairness and efficiency through temporoparietal stimulation.

Maximizing the welfare of society requires distributing goods between groups of people with different preferences. Such decisions are difficult because different moral principles impose irreconcilable solutions. For example, utilitarian efficiency (maximize overall outcome across individuals) may need trade-off against Rawlsian fairness norms (maximize the outcome for the worst-off individual). We identify a brain mechanism enabling decision-makers to solve such trade-offs between efficiency and fairness using separate neuroimaging and sham-controlled brain stimulation experiments. As activity in the temporoparietal junction (TPJ) increases, people are more likely to implement the Rawlsian fairness criterion rather than efficiency or inequality concerns. Strikingly, reducing TPJ excitability with brain stimulation reduces the concern for fairness in fairness-efficiency trade-offs. Moreover, the reduced fairness concerns statistically relate to stimulation-induced reductions in perspective-taking skills as measured in a separate task. Together, our findings not only reveal the neural underpinning of efficiency-fairness trade-offs but also recast the role of TPJ in social decision-making by showing that its perspective-taking function serves to promote fairness for the worst-off rather than efficiency or equality.