Within sovereign states citizenship is arguably the most important political marker of in- and outsiders. As a result, questions about who gets to reap the benefits of citizenship often result in distributional conflict. This conflict becomes inflamed when a country goes through a period of significant inward migration. Given that citizenship is so important and so contentious, from where do the rules governing its acquisition come? Our starting point is the acknowledgment that migrants are mobile labor. From this perspective, countries in which elites benefit from an increased supply of productive labor—that is, those with high land/labor ratios—will be more likely to adopt policies that attract migrants, such as easier naturalization rules, including birthright citizenship. We illustrate the plausibility of our argument with some statistical evidence and suggest some avenues to further explore this crucial question.