A sibling to all: Reversed neural pattern of social affiliation in extraordinary altruists

Humans typically prioritize their closest social connections. Prior neuroimaging work found that approximately 80% of the differential brain processing across social network members favors individuals in one's innermost social circle. But is this neural pattern inherent, or influenced by individual differences in social characteristics? To explore this, we studied a unique group of people: extraordinary altruists, who had donated a vital organ to an anonymous recipient, and compared them with demographically closely-matched controls. We measured brain responses using functional MRI as participants processed individuals from various circles of their social networks. In control participants, 75% of social-circle-sensitive brain volume responded most strongly to the closest circle, replicating previous findings in young adults. In contrast, extraordinary altruists showed the opposite pattern: only 30% of this brain volume preferred the closest circle, while 70% showed stronger responses to more distant social circles. This difference was particularly pronounced in core social-cognition regions, especially the temporoparietal junction, which is typically tuned to close others. Within the temporoparietal junction, the closest vs. distant response contrast was significantly smaller in extraordinary altruists than in controls, indicating reduced selectivity for the innermost circle. Entropy analysis also suggested more uniform response profiles in extraordinary altruists in several regions, including the temporoparietal junction. Collectively, these findings suggest that extraordinary altruists process social networks uniquely at the neuroanatomical level, irrespective of demographic background, displaying a broader pattern of affiliation that may underlie their remarkable capacity to extend care beyond conventional social boundaries. SIGNIFICANCE STATEMENT: Extraordinary altruists, such as kidney donors to strangers, perform acts of care that defy conventional social boundaries. Can such rare behavior be traced to differences in brain processing? Using fMRI, we found that while most people's brains respond most strongly to their closest social ties, extraordinary altruists display a reversed pattern - allocating greater neural resources to more distant social circles. This shift is especially pronounced in key social-cognitive regions, including the temporoparietal junction. Such a broadened, less selective neural response may support care that transcends conventional social boundaries. These findings offer a neurobiological account of selfless behavior and reveal brain mechanisms underlying extraordinary altruism.