Dads and Cads: Parental Cohabitation and the Human Sex Ratio at Birth

Karen Norberg, National Bureau of Economic Research

Evolutionary theory predicts that parents may bias the sex ratio of their offspring according to environmental conditions. Here, I test the prediction that parents may tend to produce males under conditions forecasting two-parent care, and females under conditions forecasting one-parent care. Using individual-level longitudinal data pooled from four public-use US surveys, I find that parents who were living with an opposite-sex partner or spouse were more likely to have a male child than parents who were living apart. The effect is small, but statistically significant (p < .0001). It is discernable when the parents' household is observed before the child's conception, and it is discernable when comparisons are made among siblings within the same family (OR 1.17, p<.001). This "partnership status" effect may be the result of modern reproductive exposures, but a parental investment effect would also fit closely with the predictions of adaptive sex allocation theory.