Abstract
Evolutionary demographers often invoke tradeoffs between reproduction and survival to explain reductions in fertility during demographic transitions. The evidence for such tradeoffs in humans has been mixed, partly because tradeoffs may be masked by individual differences in quality or access to resources. Unmasking tradeoffs despite such phenotypic correlations requires sophisticated statistical analyses that account for endogeneity among variables and individual differences in access to resources. Here we tested for costs of reproduction using N=13,663 birth records from the maternity hospital in Basel, Switzerland, 1896-1939, a period characterized by rapid fertility declines. We predicted that higher parity is associated with worse maternal and offspring condition at the time of birth, adjusting for age and a variety of covariates. We used Bayesian multivariate, multilevel models to simultaneously analyze multiple related outcomes while accounting for endogeneity, appropriately modeling non-linear effects, dealing with hierarchical data structures, and effectively imputing missing data. Despite all these efforts, we found virtually no evidence for costs of reproduction. Instead, women with better access to resources had fewer children. Barring limitations of the data, these results are consistent with demographic transitions reflecting women's investment in their own embodied capital and/or the adoption of maladaptive low-fertility norms by elites.
Highlights
Evolutionary demographers are interested in explaining variation in fertility, mortality and parental investment across populations (Lawson & Borgerhoff Mulder, 2016; Mace, 2000; Sear, Lawson, Kaplan, & Shenk, 2016)
Most evolutionary demography has focused on pre- or post-demographic transition societies, such as hunter-gatherers (Blurton Jones, 2016; Hill & Hurtado, 1996) or present-day industrialized societies (Nettle et al, 2013; Uggla & Mace, 2015), as well as populations in developing countries experiencing the early stages of the demographic transition (Alvergne & Lummaa, 2014; Lawson, Alvergne, & Gibson, 2012; Sear, Mace, & McGregor, 2003)
Trade-offs between fertility and survival have played an important role in adaptive explanations of demographic transitions, as women may reduce their fertility when having more children is costly for themselves or their offspring (Gibson & Lawson, 2011; Lawson et al, 2012; Lawson & Borgerhoff Mulder, 2016; Ruth Mace, 2000; Penn & Smith, 2007)
Summary
Evolutionary demographers are interested in explaining variation in fertility, mortality and parental investment across populations (Lawson & Borgerhoff Mulder, 2016; Mace, 2000; Sear, Lawson, Kaplan, & Shenk, 2016). Some studies have shown a cost of reproduction for mothers (Hruschka & Hagaman, 2015; Ruth Mace, 2000; Penn & Smith, 2007; Stieglitz et al, 2019; Tracer, 2002), the most detailed, longitudinal study to date found little evidence for worsened nutritional or health status with increasing parity (Gurven et al, 2016) Such deviations from theoretical predictions may highlight other crucial socio-ecological inputs into human life history such as social support (see below), or point to “hidden” costs such as reduced bone mineral density (Stieglitz et al, 2014) and fractures (Stieglitz et al, 2019). Higher parity is expected to result in worse maternal condition (Prediction 1)
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