LAUSR

Significance “Do no harm” is a universal principle of human social life. But how do we learn which of our actions help or harm others? Learning theory suggests there are two different systems that govern how we link actions and outcomes: a model-free system that is efficient and a model-based system that is deliberative. Here we show that people rely more on model-free decision making when learning to avoid harming others compared to themselves. Model-free neural signals that distinguish self and other are observed in the thalamus/caudate, and reliance on model-free moral learning for others varies with individual differences in moral judgment. These findings suggest that moral decision making for others is more model-free and has a specific neural signature. Moral behavior requires learning how our actions help or harm others. Theoretical accounts of learning propose a key division between “model-free” algorithms that cache outcome values in actions and “model-based” algorithms that map actions to outcomes. Here, we tested the engagement of these mechanisms and their neural basis as participants learned to avoid painful electric shocks for themselves and a stranger. We found that model-free decision making was prioritized when learning to avoid harming others compared to oneself. Model-free prediction errors for others relative to self were tracked in the thalamus/caudate. At the time of choice, neural activity consistent with model-free moral learning was observed in subgenual anterior cingulate cortex (sgACC), and switching after harming others was associated with stronger connectivity between sgACC and dorsolateral prefrontal cortex. Finally, model-free moral learning varied with individual differences in moral judgment. Our findings suggest moral learning favors efficiency over flexibility and is underpinned by specific neural mechanisms.