Season of Birth and Survival to 105 Years: Demographic Evidence

This study provides the first comprehensive demographic analysis of Belgian semi-supercentenarians (SSCs, individuals aged 105+), with a particular focus on the role of season of birth in shaping survival to extreme ages. Using exhaustive, rigorously validated data covering 1471 SSCs born between 1870 and 1919, supplemented with data from population censuses or extracted from the Belgian National Population Registry, we examine long-term trends in SSC numbers, sex ratios, and seasonal birth distributions. The probability of surviving to 105 increased exponentially across cohorts, from fewer than 2 per 100,000 for the 1870 cohort to more than 50 for those born in 1915–1919. Although women continue to largely outnumber men at these ages, the female-to-male ratio narrowed from nearly 20:1 at the turn of the 20th century to 8:1 across successive cohorts, reflecting broader demographic and social changes. Our findings also reveal pronounced seasonal patterns: SSCs are disproportionately born in autumn and spring, with the autumn advantage strengthening across advancing ages and a lateemerging spring effect appearing beyond age 100. These dual seasonal effects suggest that favorable early-life conditions continue to influence survival even at the limits of the human lifespan, while highlighting the importance of selective processes acting at different age thresholds. By situating Belgian SSCs within broader international comparisons, this study underscores both the universality and context-specificity of seasonal birth effects on longevity. The results emphasize the lasting role of early-life exposures in shaping survival trajectories, while also pointing to the need for future interdisciplinary and comparative research to disentangle biological, environmental, and societal mechanisms underlying exceptional longevity.