This article puts forward a theoretical explanation for why norms of i
nternational behavior change over time. It argues that the mainstream
neorealist and neoliberal arguments on the static nature of state inte
rests are implausible, as the recent empirical work of the growing con
structivist school has convincingly shown. But the constructivists hav
e 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 d
raws its hypotheses from an analogy to population genetics offers a pr
omising 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 discussi
ng how the model might apply in several other issue areas.