Reproductive history and biological aging: Evidence from the HRS and the UKHLS

This study investigates the relationship between reproductive history and biological aging (BA), estimated with DNA-methylation–based epigenetic clocks. In rapidly aging societies, reproduction may represent an important yet understudied pathway shaping biological aging. Reproductive history is conceptualized through two dimensions: fertility quantum – the total number of children ever born, and fertility tempo – life-course timing of the reproductive events.
The study addresses three research questions: (1) whether fertility quantum is associated with variation in BA pace; (2) whether the tempo of reproductive events relates to differences in BA; and (3) whether similar patterns emerge across longitudinal studies conducted in different contexts.
The empirical analysis first uses the Health and Retirement Study (HRS) in the United States. Initial unadjusted analyses of the association between the number of children ever born and epigenetic clocks reveal a U-shaped relationship between parity and BA. However, this relationship largely disappears once chronological age and birth cohort are considered, suggesting the pattern is mainly driven by cohort composition rather than parity itself. Gender-specific analyses show that women consistently display lower BA across clocks, independent of parity, while modest associations between parity and epigenetic age appear among men; however, the direction is inconsistent across the models and the clocks used. Across specifications, lifestyle and socio-demographic factors – especially smoking and educational attainment – emerge as stronger predictors of BA than fertility quantum.
The analysis is then extended to the UK Household Longitudinal Study. The dataset provides detailed reproductive histories, allowing the inclusion of fertility-timing indicators. Based on the HRS findings, we expect to replicate the relationship between parity and BA and observe faster aging among individuals experiencing earlier transitions to parenthood.
Future work will leverage the structure of Understanding Society to explore reproductive dynamics at couple-level, offering new insights into how shared reproductive trajectories may shape BA processes.