This article puts forward a theoretical explanation for why norms of international behavior change over time. It argues that the mainstream neorealist and neoliberal arguments on the static nature of state interests are implausible, as the recent empirical work of the growing constructivist school has convincingly shown. But the constructivists have not yet provided a theoretical basis for understanding why one norm rather than another becomes institutionalized, nor has learning theory yet provided an adequate explanation. An evolutionary approach that draws its hypotheses from an analogy to population genetics offers a promising alternative. This article briefly outlines the constructivist critique of neorealism and neoliberalism. It develops the evolutionary analogy, illustrating the model with a case study on the emergence of a norm of transparency in international security and briefly discussing how the model might apply in several other issue areas.