Neural signatures of harm aversion predict later willingness to exert effort for others' rewards.

Prosocial behaviours, central to human social life, are actions that benefit others. Two key domains are harm aversion, where individuals forgo personal gains to prevent harming others, and prosocial effort, which involves exerting effort to benefit others. Although previous studies suggest a relationship between these behaviours, it remains unclear whether neural responses in one domain can predict prosocial motivation in another. Here, we tested whether neural sensitivity to morally salient information in harm aversion could predict prosocial effort later. Participants (n = 66) completed two tasks: a harm aversion task during fMRI, in which they traded off monetary profit against delivering electric shocks to another person; and, one week later, a prosocial effort task outside the scanner, in which they decided whether rewards for others were worth the required physical effort. We focused on three regions implicated in cost-benefit decision-making and social cognition: the anterior cingulate cortex (ACC), anterior insula (AI), and temporoparietal junction (TPJ). Behaviourally, greater harm aversion was associated with increased prosocial effort. Neurally, AI responses to others' harm predicted sensitivity to others' rewards in the effort task, consistent with a role in representing others' outcomes across positive and negative valences. By contrast, TPJ responses to profit from harming others predicted decreased sensitivity to others' rewards, suggesting a role in context-dependent valuation that may constrain prosocial behaviour. These findings reveal that neural responses to morally salient information in one context correlate with prosocial motivation in another, suggesting mechanisms that bridge moral sensitivity and effortful prosociality.