Altruistic behavior improves the welfare of another individual while reducing the altruist's welfare. Humans' tendency to engage in altruistic behaviors is unevenly distributed across the population, and individual variation in altruistic tendencies may be genetically mediated. Although neural endophenotypes of heightened or extreme antisocial behavior tendencies have been identified in, for example, studies of psychopaths, little is known about the neural mechanisms that support heightened or extreme prosocial or altruistic tendencies. In this study, we used structural and functional magnetic resonance imaging to assess a population of extraordinary altruists: altruistic kidney donors who volunteered to donate a kidney to a stranger. Such donations meet the most stringent definitions of altruism in that they represent an intentional behavior that incurs significant costs to the donor to benefit an anonymous, nonkin other. Functional imaging and behavioral tasks included face-emotion processing paradigms that reliably distinguish psychopathic individuals from controls. Here we show that extraordinary altruists can be distinguished from controls by their enhanced volume in right amygdala and enhanced responsiveness of this structure to fearful facial expressions, an effect that predicts superior perceptual sensitivity to these expressions. These results mirror the reduced amygdala volume and reduced responsiveness to fearful facial expressions observed in psychopathic individuals. Our results support the possibility of a neural basis for extraordinary altruism. We anticipate that these findings will expand the scope of research on biological mechanisms that promote altruistic behaviors to include neural mechanisms that support affective and social responsiveness.psychopathy | organ donation | prosocial behavior

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In the Ultimatum Game (UG), incurring a cost to punish inequity is commonly termed altruistic punishment. This behaviour is thought to benefit others if the defector becomes more equitable in future interactions. However, clear connections between punishment in the UG and altruistic behaviours outside the laboratory are lacking. We tested the altruistic punishment hypothesis in a sample of extraordinarily altruistic adults, predicting that if punishing inequity is predictive of altruism more broadly, extraordinary altruists should punish more frequently. Results showed that punishment was not more prevalent in extraordinary altruists than controls. However, a self-reported altruism measure previously linked to peer evaluations but not behaviour, and on which extraordinary altruists and controls did not differ, did predict punishment. These findings support suggestions that altruistic punishment in the UG is better termed costly punishment and may be motivated by social, but not necessarily prosocial, concerns. Results also support prior suggestions that self-reported altruism may not reliably predict altruistic behaviour.

Shared neural representations during experienced and observed distress are hypothesized to reflect empathic neural simulation, which may support altruism. But the correspondence between real-world altruism and shared neural representations has not been directly tested, and empathy's role in promoting altruism toward strangers has been questioned. Here, we show that individuals who have performed costly altruism (donating a kidney to a stranger; n = 25) exhibit greater self-other overlap than matched control participants ( n = 27) in neural representations of pain and threat (fearful anticipation) in anterior insula (AI) during an empathic-pain paradigm. Altruists exhibited greater self-other correspondence in pain-related activation in left AI, highlighting that group-level overlap was supported by individual-level associations between empathic pain and firsthand pain. Altruists exhibited enhanced functional coupling of left AI with left midinsula during empathic pain and threat. Results show that heightened neural instantiations of empathy correspond to real-world altruism and highlight limitations of self-report.

Altruistic kidney donation is a form of extraordinary altruism, the antecedents of which are poorly understood. Although well-being is known to increase the incidence of prosocial behaviors and there is significant geographical variation in both well-being and altruistic kidney donation in the United States, it is unknown whether geographical variation in well-being predicts the prevalence of this form of extraordinary altruism. We calculated per capita rates of altruistic kidney donation across the United States and found that an index of subjective well-being predicted altruistic donation, even after we controlled for relevant sociodemographic variables. This relationship persisted at the state level and at the larger geographic regional level. Consistent with hypotheses about the relationship between objective and subjective well-being, results showed that subjective well-being mediated the relationship between increases in objective well-being metrics, such as income, and altruism. These results suggest that extraordinary altruism may be promoted by societal factors that increase subjective well-being.