It is hypothesized that maternal gonadotrophin levels at the time of conception somehow partially control the sex of the zygote, high levels being associated with female offspring. Black populations in general have high dizygotic twinning rates: and dizygotic twinning rates are also thought to be partially controlled by maternal gonadotrophin levels. So if the hypothesis were true, then Black populations should have low sex ratios. In general they do, but recently claims have been made to the contrary in respect of large samples of Yoruba and Hausa births. In the present report it is argued that these claims should be treated with reserve. Moreover, new data are presented which show that Black births in England and Wales have a remarkably low sex ratio.

It is suggested that the human sex ratio at birth is stabilized only to a minor extent by the direct processes of natural selection. Instead the major factors stabilizing sex ratio seem to be behavioural (coital rates) and psychological (parental perceptions of adult sex ratios). It is suggested that parental hormone levels are (a) a consequence of perceived adult sex ratios, and (b) a cause of sex ratio in the next generation, thus providing the basis for a negative feedback process stabilizing the sex ratio.

It has been reported that the regression of human sex ratio (number of males per 100 females at birth) on cycle day of insemination is U-shaped. The point is not yet established, so a search was made for possible effects of this phenomenon on the regression of sex ratio on duration of gestation (from last menstrual period to delivery). A previously unremarked feature of this latter regression is that it, too, is U-shaped. It is accordingly suggested that the one regression is responsible for the other, and a simple mathematical model indicates that the magnitudes of the two regressions are compatible with this conclusion.

An attempt is made to isolate and quantify a positive correlation of parental coital rate with dizygotic twinning probability, by considering the variations of DZ twinning with marital stage and status. It is concluded that if the coital rate of young women is doubled, their DZ twinning probability is increased by a factor of very roughly 25-30%.