The last three centuries have witnessed a moral and political transformation. Groups previously denied equivalent moral standing—including propertyless men, women, ethnic and religious minorities, homosexuals, and slaves—became moral equals deserving of similar legal treatment. Here we argue that this process was driven by the reputational benefits of demonstrating commitment to individualist moral principles. These principles flourished in “fluid” social ecologies with high relational mobility and weak kinship institutions, both as individuals aimed to signal impersonal prosociality and as they strove to be governed under institutions that protected substitutable individuals unbound by formal obligations. As long as parties benefited from appearing committed to these principles, and denying rights to certain groups appeared inconsistent with these principles, then those parties were incentivized to grant those rights. Given the universalist nature of these principles, people signaling commitment were also incentivized to sanction rights-based violations in other countries, helping expand rights beyond their original context. We use this account to explain both expansions and contractions of the moral circle and reconcile the roles of ideas, markets, reasoning, reputation, the Catholic Church, argumentation, moral intuition, social organization, individual strategizing, and large-scale cultural evolution.