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Trivers-Willard at birth and one year: evidence from US natality data 1983-2001
1Department of Economics, Columbia University in the City of New York, New York, NY 10027-3308, USA.
Proceedings. Biological Sciences
|August 10, 2007
Summary
Evolutionary theory suggests parents in good condition favor sons, and those in poor condition favor daughters. This study on US births and infant deaths supports this, linking maternal status to offspring sex ratio and infant mortality patterns.
Area of Science:
- Evolutionary Biology
- Human Demography
- Reproductive Strategies
Background:
- The Trivers-Willard (TW) hypothesis posits that parental condition influences offspring sex ratio.
- Good parental condition is predicted to favor sons, while poor condition favors daughters.
- Empirical evidence for the TW hypothesis in humans remains debated.
Purpose of the Study:
- To test the Trivers-Willard hypothesis using a large dataset of US births and infant deaths.
- To investigate the relationship between maternal characteristics and offspring sex ratio.
- To examine the role of infant mortality in shaping sex ratio patterns.
Main Methods:
- Analysis of linked US birth and infant death records from 1983-2001.
- Inclusion of approximately 48 million births and 310,000 infant deaths.
- Statistical examination of correlations between maternal status indicators (marital status, education, age) and offspring sex.
Main Results:
- Mothers who were married, more educated, and younger had a higher proportion of sons.
- Infant deaths were disproportionately male when the mother was unmarried and young.
- These findings suggest maternal status influences offspring sex ratio at birth and via differential infant mortality.
Conclusions:
- The study provides evidence supporting the Trivers-Willard hypothesis in a large human population.
- Offspring sex ratio may serve as an indicator of maternal socioeconomic status.
- Infant mortality patterns contribute to the observed sex ratio deviations, aligning with evolutionary predictions.
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