The ratio of the sexes at birth is remarkably constant for various countries over long periods of time for populations as a whole. Data are presented to show that the sex-ratio at conception is significantly than sex-ratio at birth. The lesser percentage of males at birth than at conception is accounted for by a differential prenatal mortality bearing more heavily upon the male fetus than upon the female fetus. Thus the sex-ratio at birth is a function of the sex-ratio at conception and the sex-ratio of intrauterine mortality. Various lines of approach are utilized to prove that social factors affect the amount of prenatal mortality and finally the sex-ratio at birth. A group of 5,466 completed families, forming a higher social class, are utilized to test the general hypothesis. They show a significantly sex-ratio at birth than does the general population. The conclusion is reached that more desirable social environments are important factors tending to raise the sex-ratio at birth. As suc...