Analyses of the costs and benefits of immigration have not considered the dependence of an ethny’s reproductive fitness on its monopoly of a demarcated territory. Global assays of human genetic variation allow estimation of the genetic losses incurred by a member of a population when random fellow ethnics are replaced by immigrants from different ethnies. This potential loss defines an individual’s ethnic genetic interest as a quantity that varies with the genetic distance of potential immigrants. W. D. Hamilton showed that self-sacrificial altruism is adaptive when it preserves the genetic interests of a population of genetically similar individuals. Ethnic genetic interest can be so large that altruism on behalf of one’s ethny—’ethnic nepotism’—can be adaptive when it prevents replacement. It follows that ethnies usually have an interest in securing and maintaining a monopoly over a demarcated territory, an idea consonant with the universal nationalism of Bismarck and Woodrow Wilson.