Altruism, and particularly costly altruism toward strangers, such as altruistic kidney donation, represents a puzzling phenomenon for many fields of science, including evolutionary biology, psychology, and economics. How can such behavior be explained? The propensity to engage in costly altruism varies widely and may be genetically mediated, but little is known about the neural mechanisms that support it. We used structural and functional brain imaging to compare extraordinary altruists, specifically altruistic kidney donors, and controls. Altruists exhibited variations in neural anatomy and functioning that represent the inverse of patterns previously observed in psychopaths, who are unusually callous and antisocial. These findings suggest extraordinary altruism represents one end of a caring continuum and is supported by neural mechanisms that underlie social and emotional responsiveness.

Abstract

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.