Two parallel norms mandate an international duty to hold state leaders individually accountable for serious corruption and human rights crimes. The development of these new norms is poorly explained by realist and neoliberal perspectives, but there are also weaknesses in recent constructivist explanations of norm diffusion that emphasize agency at the expense of structure. Such approaches have difficulty explaining the source of and similarities between new norms, and treat norm entrepreneurs as prior to and separate from their environment. In contrast, drawing on sociological institutionalism, we present a more structural explanation of individual accountability norms. The norms derive from an overarching modernist world culture privileging individual rights and responsibilities, as well as rational-legal authority. This culture is more generative of norm entrepreneurs than generated by them. The specific norms are instantiated through a process of “theorization” within permissive post–Cold War conditions, and diffused via mimicry, professionalization, and coercive isomorphism.