["Sociology of Health &Illness, Volume 48, Issue 7, September 2026. ", "\nABSTRACT\nLongitudinal birth cohort studies are an under‐examined context for considering the dynamics of healthism. This article presents a comparative perspective from the ‘Biosocial Lives of Birth Cohorts’ study, which takes birth cohorts as an ethnographic object of knowledge‐making, social practice and participation in the Netherlands, Brazil, Portugal and the United Kingdom. Drawing from ethnographic research with participants and professionals involved in cohort data collection, we examine how, in the routine monitoring and measuring of bodies and lives in cohort studies, ‘healthism’, as an individualised form of moral self‐responsibility for health, is entangled with medicalisation. Healthism is also at stake in the way that individual cohort test results are communicated and received. Yet the relational dynamics of cohort studies, which encompass but also exceed the scope of healthism, are also shaped by the precarity of public health infrastructures and closely entangled with the motivational desire of cohort participants, as birth cohorts can act as a stopgap for public health failures. In this way, we demonstrate how birth cohorts are an important context for considering the politics of health and how medicalisation is entangled with healthism in cohort data collection, even as this unfolds in tension with efforts to collectively intervene in public health.\n"]