Partnership Status and the Human Sex Ratio at Birth

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.

Possible constraints on adaptive variation in sex ratio at birth in humans and other primates

“…It is noteworthy in this context that (consistent with the hypothesis) dominant women are reported to produce an excess of sons (Grant, 1994) and that apparently non-dominant women produce excesses of daughters (Johns, 2004;Norberg, 2004) (though see Kanazawa, 2004).…”

Estimating a Dynamic Model of Sex Selection in China

“…Recent reports that Chinese gangs are beginning to 30 Note that the Trivers and Willard hypothesis (1973) also predicted a positive correlation between status and education if a species can vary the male fraction of births in response to anticipated success in mating (because males have more variable mating outcomes). This hypothesis is not thought to apply among human populations in a matter that would affect the male fraction of births by more than a few percentage points (Norberg 2004). 28 Fertility surveys in Taiwan indicate that higher-educated women are less likely to report having a gender preference for births (Taiwan's KAP Survey 2003).…”

Where have all the young men gone? Using sex ratios to measure fetal death rates

“…These are one-time extreme shocks, while the CAAA was a large permanent improvement in ambient air quality. As an example of a more permanent program, Almond and Hoynes (2011) find the Food Stamp Program (FSP) rollout increased the fraction of births that are male among whites by 0.09% and blacks by 0.32%, while Norberg (2004) finds parents living together at time of conception were 14% more likely to conceive males. We are unaware of any conversion of health impacts from subclinical levels of radiation, nutritional deficiencies, or family makeup to the reduction in air pollution levels we examine, but we note that findings for other in utero health and stressor shocks are qualitatively similar to those we find for air pollution and the CAAA.…”

“…Relationships between the sex of live births and socioeconomic status make cross sectional analysis of pollution and sex ratios difficult. , Currie (2011), andNorberg (2004) showed a higher male/female sex ratio in two-parent households, suggesting a link between family structure and sex of live births-such effects, combined with how selection into pregnancy might change with factors correlated with pollution (Dehejia and Lleras-Muney, 2004) suggest potential bias in cross-sectional analysis of sex ratios in response to treatments.…”