Abstract
The sex ratio at birth (SRB) may be patterned by maternal condition and/or environmental stressors. However, despite decades of research, empirical results from across the social and biological sciences are equivocal on this topic. Using longitudinal individual-level data from a US population during the interwar period (1918–1939), inclusive of three distinct eras (Spanish Flu, Roaring ‘20 s, and the Great Depression), we evaluate predictions from two theoretical frameworks used to study patterning in SRB – (1) ‘frail males’ and (2) adaptive sex-biased investment theory (Trivers-Willard). The first approach centers on greater male susceptibility to exogenous stressors and argues that offspring survival should be expected to differ between ‘good’ and ‘bad’ times. The second approach contends that mothers themselves play a direct role in manipulating offspring SRB, and that those in better condition should invest more in sons. In-line with ‘frail male’ predictions, we find that boys are less likely to be born during the environmentally challenging times of the Spanish Flu and Great Depression. However, we find no evidence that maternal condition is associated with sex ratios at birth, a result inconsistent with the Trivers-Willard hypothesis.
Highlights
It is well-documented that sex ratios at birth (SRB; presented here as the proportion of boys to girls born at a given point in time) vary within and across species
We model the relationship of our predictors for ‘bad’ times (Spanish Flu, Great Depression) with whether or not a birth results in a male to evaluate ‘frail male’ expectations
We focus our analysis on data from the first half of the 20th century in the US, where the population was experiencing extreme swings in ecological and economic stress
Summary
It is well-documented that sex ratios at birth (SRB; presented here as the proportion of boys to girls born at a given point in time) vary within and across species. A second framework, which does not contest sex differences in exogenous sources of mortality, argues that mothers themselves play a direct role in altering offspring SRB This theoretical framework, known as Trivers-Willard (TW), makes an evolutionary argument that maternal condition (e.g., social status) ought to play an important role in the amount mothers are willing to invest in sons versus daughters[11]. Other work analyzing data from over three years in China during the Great Leap Forward, known as the Great Chinese Famine (三年大饥荒 in simplified Chinese; 1959–61), finds a precipitous drop in male births during the period of mass starvation[4] ( alternative findings have been reported[6]) While these results seemingly conflict, one interpretation of differing findings is that shorter periods of deprivation are not enough to trigger male-biased fetal wastage, and instead these events need to be protracted, as in the case of the famine in China. Humans too show a pattern of mixed support with women of high socioeconomic status either producing more sons[17] or displaying no bias[18]
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