Demographic expertise has become a crucial perspective in debates around the development of modern welfare states. Since the 1960s, many reforms answer to demographic changes (e.g. ageing societies); and demographic calculations are now regularly used for the technical construction of social protection policies. In public debates, demographic statistics also serve as an objectifying rhetoric, legitimising welfare state reforms as necessary or inescapable. However, demography is not a black box: what exactly is counted in demographic assessments (age, sex, fertility, migration) varies a lot, between national contexts and historically over recent decades. This project examines from a historical and comparative perspective how demography became a core element of modern welfare state policies since the 1960s. By analysing three fields of the welfare state (old-age pensions programs, health care and family policies) in three representative European countries (France, Italy and Switzerland), SPADE aims to identify which actors and which institutions contributed to making the problem of the demographic sustainability of social protection systems a political priority, and which solutions were envisaged. The project thus broadens the historiography of the welfare states, traditionally marked by approaches from social and political history, with novel perspectives from the history of science. By combining transnational and comparative approaches and by looking at the origins of the discourse about the so-called “welfare state crisis”, SPADE cast a new light on the transformations that the welfare state as a whole has undergone since the 1970s.