Men's Biological Clock? Paternal age matters in an era of late parenthood

Yen-hsin Alice Cheng , Academia Sinica

As childbearing is increasingly postponed in developed countries over the past decades, the “biological clock” is typically framed as a constraint on women, while men are often assumed to face minimal reproductive limits. This study challenges that assumption by examining whether paternal age contributes to birth risks and whether such risks can be offset by social and demographic changes. Using population-wide birth registration data from Taiwan, we compare two periods, 1998–2000 and 2018–2020, to assess how paternal and maternal age are associated with preterm birth and low birth weight. Advanced paternal age is associated with elevated risks of adverse birth outcomes, although these effects are smaller than those of maternal age and have attenuated over time. Analyses distinguishing low birth weight among all births and term births indicate that paternal age effects operate primarily through increased risks of prematurity. Contrary to the assumption that men can compensate for delayed fatherhood by partnering with younger women, we find little evidence that partner age differences substantially reduce birth risks. Counterfactual analyses further show that improvements in both paternal and maternal education reduce predicted risks, but do not fully offset the risks associated with delayed parenthood. These findings call into question the assumption that men can indefinitely delay parenthood without consequence and highlight the need to incorporate paternal age into frameworks of reproductive timing in low-fertility societies.

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 Presented in Session 1. Flash Session Fertility and Health