In the modern nation‐state, birthright citizenship laws – jus soli and jus sanguinis – are the two main gateways to sociopolitical membership. The vast majority of the world’s population (97 percent) obtains their citizenship as a matter of birthright. Yet because comparative research has focused on measuring and explaining the multiple components of citizenship and immigration policies, a systematic analysis of birthright citizenship is lacking. The article bridge this gap by analyzing the birthright component in prominent databases on citizenship policies and complementing them with original data and measures. This allows the authors to systematically test institutional and electoral explanations for contemporary and over‐time variation in birthright citizenship. Institutional explanations – legal codes and colonial history – are consistently associated with limitations on birthright law. As for electoral explanations, specific electoral powers – Nationalist, Socialist and Social‐Democratic parties – rather than the traditional left/right‐wing divide, are linked with reforms in birthright regimes.