Separating the Bruce and Trivers-Willard effects in theory and in human data

Area of Science:

• Evolutionary biology
• Human reproductive ecology
• Demography

Background:

• Reproductive suppression theories posit natural selection favors mechanisms that terminate pregnancies of offspring unlikely to survive.
• Human reproductive suppression may involve the Trivers-Willard Effect (maternal stress) and the Bruce Effect (social stress).
• Previous research has not compared these mechanisms or quantified their impact on human birth cohorts.

Observation:

• This study compares the Trivers-Willard and Bruce Effects using historical Swedish data.
• Trivers-Willard Effect exposure was measured by female reproductive-age mortality rates.
• Bruce Effect exposure was measured by child mortality rates.

Findings:

• Time-series regression analysis indicates the Bruce Effect is a stronger predictor of the secondary sex ratio (male-to-female ratio at birth) than the Trivers-Willard Effect.
• Temporal variations in historical Swedish birth cohort sex ratios are better explained by child mortality rates.

Implications:

• The Bruce Effect should be considered a significant factor in reproductive suppression research.
• Future studies on reproductive suppression mechanisms and birth cohort characteristics should include the Bruce Effect.
• Understanding these mechanisms offers insights into human adaptation and population dynamics.