Abstract Adam Smith made a persuasive case that “moral sentiments” are the foundation of ethical behaviors in his 1759 The Theory of Moral Sentiments. This view is still controversial as philosophers debate the extent of human morality. One type of moral behavior, assisting a stranger, has been shown by economists to be quite common in the laboratory and outside it. This paper presents the Empathy–Generosity–Punishment model that reveals the criticality of moral sentiments in producing prosocial behaviors. The model's predictions are tested causally in three neuroeconomics experiments that directly intervene in the human brain to “turn up” and “turn down” moral sentiments. This approach provides direct evidence on the brain mechanisms that produce prosociality using a brain circuit called HOME (human oxytocin-mediated empathy). By characterizing the HOME circuit, I identify situations in which moral sentiments will be engaged or disengaged. Using this information, applications to health and welfare policies, organizational and institutional design, economic development, and happiness are presented.