Partnership Status and the Human Sex Ratio at Birth

22 Pages Posted: 9 Dec 2004  

Karen Norberg

Boston University - Department of Psychiatry; National Bureau of Economic Research (NBER)

Date Written: November 2004

Abstract

If two-parent care has different consequences for the reproductive success of sons and daughters, then natural selection may favor adjustment of the sex ratio at birth according to circumstances that forecast later family structure. In humans, this partnership status hypothesis predicts fewer sons among extra-pair conceptions, but the rival "attractiveness" hypothesis predicts more sons among extra-pair conceptions, and the "fixed phenotype" hypothesis predicts a constant probability of having a son, regardless of partnership status. In a sample of 86,436 human births pooled from five US population-based surveys, I find 51.5% male births reported by respondents who were living with a spouse or partner before the child's conception or birth, and 49.9% male births reported by respondents who were not (X2=16.77, d.f. = 1, p<.0001). The effect was not explained by paternal bias against daughters, by parental age, education, income, ethnicity, or by year of observation, and was larger when comparisons were made between siblings. To my knowledge, this is the first direct evidence for conditional adjustment of the sex ratio at birth in humans, and could explain the recent decline in the sex ratio at birth in some developed countries.

Suggested Citation

Norberg, Karen, Partnership Status and the Human Sex Ratio at Birth (November 2004). NBER Working Paper Series, Vol. w10920, pp. -, 2004. Available at SSRN: https://ssrn.com/abstract=622634

Karen Norberg (Contact Author)

Boston University - Department of Psychiatry ( email )

Boston, MA
United States
617-414-7516 (Phone)

National Bureau of Economic Research (NBER) ( email )

1050 Massachusetts Avenue
Cambridge, MA 02138
United States

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