Reproductive suppression follows threats to child survival

Abstract

Natural selection presumably conserved mechanisms that allow females to block or terminate gestation when environmental circumstances threaten the survival of offspring. One example of this adaptive reproductive suppression, the Bruce effect, has been identified in several species, both in the laboratory and in the wild. Although descriptive epidemiology reports low fertility among women experiencing stressful circumstances, attempts to detect a Bruce effect in humans have been rare and limited. We contribute to this limited work by examining the relationship between the odds of child death and the sex ratio at birth in Sweden for the years 1751–1840. We find evidence of a generalized Bruce effect in humans in that unexpected changes in child mortality predict opposite unexpected changes in the secondary sex ratio in the following year, even after adjusting for period life expectancy. Our analysis broadens the scope of the Bruce effect literature to include humans, suggesting that women, through noncognitive decisional biology, adjust reproductive strategies and investments in response to changing environmental conditions.