The Secondary Sex Ratio and Male Mortality at Pre-Reproductive Ages: A Test of Selection In Utero.

Abstract

The secondary sex ratio (i.e., the ratio of male to female live births; hereafter referred to as the SSR) falls in populations encountering ambient stressors. Much theory and some empirical work indicates that males born to low SSR cohorts may be "positively selected" in that excess culling in utero may correspond with greater than expected survival among live-born males in that cohort. We extend prior work by testing, in historical Utah, whether the SSR varies positively with male mortality at pre-reproductive ages. This study uses detailed records from the Utah Population Database to focus on Utahns born 1850-1940. We use rigorous time-series methods, which control for strong secular declines in mortality as well as ambient perturbations shared equally among males and females, to investigate the male culling inference. We observe a positive relation between the SSR and male mortality during youth (i.e., 5 to < 20 years; p < 0.05) but not in infancy or early childhood. In this historical population, the SSR appears to gauge hardiness of surviving male cohorts. However, whether the high fertility and/or family structure context of Latter-day Saints in historical Utah explains the age-specific pattern of male mortality warrants further scrutiny.