Life-history tradeoffs in a historical population (1896-1939) undergoing rapid fertility decline: Costs of reproduction?

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.

Keywords: Demographic transition; Evolutionary demography; Historical demography; Life-history theory; Quantity-quality tradeoff.

Graphical abstract

Figure 1.

No effect of parity on offspring outcomes. (a) The main effect of parity, (b) the residual correlation between parity and offspring outcomes within mothers and (c) the residual correlation between mothers. Plotted are the posterior distributions with 90% credible interval, and numbers give the proportion of the posterior that supports the prediction (e.g. the proportion of the posterior <0, or P_<0). LB, Probability of live birth (expressed as an odds ratio); GA, gestational age; PW, placenta weight; BW, birth weight; BL, birth length; HC, head circumference.

Figure 2.

Interactions between parity and maternal condition on offspring outcomes, namely (a) probability of live birth, (b) gestational age, (c) placenta weight, (d) birth weight, (e) birth length and (f) head circumference. Maternal condition was proxied by marital status (MS, indicating being married vs. not married), height (HT), age of menarche (AM), socio-economic position (SEP) and year. Plotted are the posterior distributions with 90% credible interval, and numbers give the proportion of the posterior that supports the prediction (e.g. the proportion of the posterior <0, or P_<0). Estimates for the interaction between parity and the year spline included three parameters, which are not shown here owing to a lack of clear interpretability.

Figure 3.

Effects of maternal condition on parity. Maternal condition was proxied by marital status (MS), height (HT), age of menarche (AM), and socio-economic position (SEP). Plotted are the posterior distributions with 90% credible interval, and numbers give the proportion of the posterior that supports the prediction (e.g. the proportion of the posterior <0, or P_<0)

Figure 4.

Effects of (a) twin births (twins vs. non-twins) and (b) offspring sex (male vs. female) on offspring condition. Plotted are the posterior distributions with 90% credible interval and numbers give the proportion of the posterior that supports the prediction (e.g. the proportion of the posterior <0, or P_<0). LB, Probability of live birth (expressed as an odds ratio); GA, gestational age; PW, placenta weight; BW, birth weight; BL, birth length; HC, head circumference.