Reinforcement learning theory powerfully characterizes how we learn to benefit ourselves. In this theory, prediction errors-the difference between a predicted and actual outcome of a choice-drive learning. However, we do not operate in a social vacuum. To behave prosocially we must learn the consequences of our actions for other people. Empathy, the ability to vicariously experience and understand the affect of others, is hypothesized to be a critical facilitator of prosocial behaviors, but the link between empathy and prosocial behavior is still unclear. During functional magnetic resonance imaging (fMRI) participants chose between different stimuli that were probabilistically associated with rewards for themselves (self), another person (prosocial), or no one (control). Using computational modeling, we show that people can learn to obtain rewards for others but do so more slowly than when learning to obtain rewards for themselves. fMRI revealed that activity in a posterior portion of the subgenual anterior cingulate cortex/basal forebrain (sgACC) drives learning only when we are acting in a prosocial context and signals a prosocial prediction error conforming to classical principles of reinforcement learning theory. However, there is also substantial variability in the neural and behavioral efficiency of prosocial learning, which is predicted by trait empathy. More empathic people learn more quickly when benefitting others, and their sgACC response is the most selective for prosocial learning. We thus reveal a computational mechanism driving prosocial learning in humans. This framework could provide insights into atypical prosocial behavior in those with disorders of social cognition.reinforcement learning theory | prosocial behavior | empathy | reward | subgenual anterior cingulate cortex P rosocial behaviors, namely, social behaviors or actions intended to benefit others, are a fundamental but poorly understood aspect of social interaction (1). To behave prosocially, animals need to learn about the consequences that their actions can have for others. In reinforcement learning theory (RLT), prediction errors (PEs)-differences between expected and actual outcomesdrive learning (2). RLT provides a powerful framework for understanding how animals learn to obtain rewards for themselves (3). However, the processes by which animals learn to make choices that benefit others are unknown. Here we use RLT to characterize prosocial learning, combining functional magnetic resonance imaging (fMRI) and detailed computational modeling of behavior.Studies using economic games, moral judgments, or charity donation tasks have consistently reported activity in the ventral striatum, posterior regions of the subgenual cingulate cortex/basal forebrain (hereinafter referred to as sgACC), dorsal anterior cingulate cortex (dACC), and dorsolateral prefrontal cortex (DLPFC) during prosocial behavior (4-7). Each of these regions receives input from midbrain dopaminergic neurons (8), and these cortical regions all project to the ventral ...