LAUSR

Extraordinary acts of altruism towards strangers represent puzzling phenomena not easily explained by dominant biological models of altruism, such as kin selection and reciprocity1–3. These theories stipulate that genetically or socially close others should be the beneficiaries of costly generosity4,5. Extraordinary altruists exhibit increased empathic sensitivity and a fast, intuitive decision-making style6,7, but no clear explanation yet exists for the most perplexing feature of these altruists, which is that they incur significant risks to benefit strangers5. Here, we considered two related proximal mechanisms—social discounting (valuational) and social distancing (perceptual)—that have been proposed to explain why costly help is preferentially given to close others. We hypothesized that variations in one or both mechanisms drive costly altruism towards distant others. We show that extraordinary altruists exhibit reduced social discounting, with altruists discounting the subjective value of outcomes for socially distant others less than controls. Group differences in social discounting were associated with self-reported other-oriented preferences and could not be accounted for by variation in social distancing. These results suggest a psychological mechanism by which costly helping behaviour towards genetically and socially close others might be extended to unrelated others.