The sex ratios of black births.

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.