Reproductive suppression follows threats to child survival

Area of Science:

• Evolutionary biology
• Reproductive science
• Human demography

Background:

• Females in many species possess mechanisms to terminate gestation during adverse environmental conditions, a phenomenon known as reproductive suppression.
• The Bruce effect, a specific form of reproductive suppression, has been documented in various animal species, but its presence in humans remains largely unconfirmed.
• Previous epidemiological studies suggest a correlation between stressful circumstances and reduced fertility in women, hinting at potential human reproductive adjustments.

Observation:

• This study investigated the relationship between child mortality rates and the sex ratio at birth in Sweden between 1751 and 1840.
• The research analyzed historical demographic data to identify patterns in reproductive outcomes in response to environmental pressures.
• The analysis focused on unexpected fluctuations in child mortality and their subsequent impact on the secondary sex ratio.

Findings:

• Evidence suggests a generalized Bruce effect in humans, indicating adaptive reproductive suppression.
• Unexpected increases in child mortality were followed by a decrease in the proportion of male births in the subsequent year.
• These findings remained significant even after accounting for variations in period life expectancy.

Implications:

• This research expands the understanding of the Bruce effect by demonstrating its presence in human populations.
• It suggests that women's reproductive strategies are sensitive to environmental conditions, operating through non-cognitive biological mechanisms.
• The findings highlight the adaptive capacity of human reproduction in response to perceived threats to offspring survival.