Sex ratios at birth vary with environmental harshness but not maternal condition
Schacht, Ryan; Tharp, Douglas; Smith, Ken R.
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.
Schacht, Ryan, & Tharp, Douglas, & Smith, Ken R.. (June 2019). Sex ratios at birth vary with environmental harshness but not maternal condition. Scientific Reports, (9:1), p.1-7. Retrieved from http://hdl.handle.net/10342/8336
Schacht, Ryan, and Tharp, Douglas, and Smith, Ken R.. "Sex ratios at birth vary with environmental harshness but not maternal condition". Scientific Reports. 9:1. (1-7.), June 2019. July 12, 2020. http://hdl.handle.net/10342/8336.
Schacht, Ryan and Tharp, Douglas and Smith, Ken R., "Sex ratios at birth vary with environmental harshness but not maternal condition," Scientific Reports 9, no. 1 (June 2019), http://hdl.handle.net/10342/8336 (accessed July 12, 2020).
Schacht, Ryan, Tharp, Douglas, Smith, Ken R.. Sex ratios at birth vary with environmental harshness but not maternal condition. Scientific Reports. June 2019; 9(1) 1-7. http://hdl.handle.net/10342/8336. Accessed July 12, 2020.