Reproductive Timing and Family Structure as Determinants of Early-Life Mortality in Historical Poland

This study examines how variations in parental age, birth spacing, sibling number and sex composition, and local socioeconomic and environmental conditions influenced early-life mortality in 19th-century urban Poland. Using reconstructed family histories from the Poznań Historical Population Database, we applied generalized additive models (GAMs) with binomial logistic regression to estimate risks of infant (under age 1) and child (aged 1–5) mortality. Separate models were estimated for firstborns (n = 1,022) and laterborn children (n = 2,572). Among firstborns, male infants faced significantly higher mortality in the first year of life, consistent with known patterns of male vulnerability. A larger parental age difference was associated with lower child mortality in this group, potentially reflecting selective mating or increased parental investment. Among laterborns, having a preceding brother increased infant mortality but decreased child mortality. Infant mortality decreased with advancing paternal age, likely reflecting enhanced social and economic resources. While birth order and interbirth interval had no significant direct effects, the impact of a preceding brother depended on birth order. Mortality risks varied greatly across parishes, underscoring the influence of local socioeconomic and environmental conditions. These findings highlight the importance of examining historical early-life mortality not as the result of isolated factors, but as the outcome of interacting biological, familial, and socioeconomic influences.