Self-determination claims have abounded in the international system since the end of World War II. But these claims have not emerged everywhere. About half of the states in the international system face some challenge related to self-determination today. Why do some states face these demands while others do not? We argue that ethno-national self-determination is one of many identities with which individuals can find affinity. While an international norm related to self-determination has developed globally, its use as a basis for political claims has diffused regionally. Diffusion of self-determination occurs through observation of others using self-determination as a basis of organization, generating a sense of legitimacy, sensitivity to related grievance, and perceptions of tangible benefits related to self-determination identification. We test this empirically on global data on self-determination claims from 1960 to 2005 and find evidence of spatial diffusion, suggesting that self-determination is, to some extent, contagious.