Time of fertilisation and sex of infants.

The sex of a human zygote may depend on the time during the mother's menstrual cycle that it is formed. Direct data to support this hypothesis suggest that male zygotes tend to be formed both early and late in the fertile period, and female zygotes in the middle. Criticisms of the direct data seem to be only that measurements of the timing of events around conception are inexact. There are several kinds of indirect data: (1) the variation of sex ratio with war and with duration of marriage, and (2) the non-binomial distribution of the combination of the sexes in human dizygotic twins and other mammalian polyzygotic litters. The sex of a zygote may be determined by maternal gonadotrophin levels when it is formed. This elaboration of the hypothesis could explain the low sex ratio in Negroes.