Parallel Ageing: The Synchronised Postponement of Fertility and Mortality

As births and deaths occur at progressively older ages, further delays must increasingly encounter resistance from existing physiological constraints. For female fertility, these constraints ultimately manifest as menopause. Whether an analogous limit exists for survival, however, remains debated. Evidence indicates that reproductive and actuarial ageing share underlying physiological constraints. We hypothesize that these common constraints limit fertility and mortality postponement in similar ways, becoming particularly apparent as ages at childbearing and death shift upwards and physiological factors gain in importance. Consequently, we propose leveraging the known limit to female fertility to shed light on the unknown limit to survival. Utilizing percentile-based aggregate measures of mortality and fertility schedules to compare their trends since the 1940s in various low-mortality and low-fertility female populations, we show that for the last 35 years, the pace of mortality and fertility postponement has been surprisingly consistent, forming two parallel lines. This pace persists throughout almost the entire period of fertility postponement and therefore appears independent of the proximity to an upper age-limit. These results support the hypothesis of common constraints governing fertility and mortality delay, offering deep implications for the nature of human ageing. Moreover, we highlight shortcomings in previous approaches to locate a limit to human lifespan using aggregate measures, thereby refining future research on the subject.