Paternal-age and birth-order effect on the human secondary sex ratio.

Because of conflicting results in previous analyses of possible maternal and paternal effects on the variation in sex ratio at birth, records of United States live births in 1975 were sorted by offspring sex, live birth order (based on maternal parity), parental races, and, unlike prior studies, ungrouped parental ages. Linear regression and logistic analysis showed significant effects of birth order and paternal age on sex ratio in the white race data (1.67 million births; 10,219 different combinations of independent variables). Contrary to previous reported results, the paternal-age effect cannot be ascribed wholly to the high correlation between paternal age and birth order as maternal age, even more highly correlated with birth order, does not account for a significant additional reduction in sex-ratio variation over that accounted for by birth order alone.