At the beginning of the twentieth century the sex ratio for South Africans differed markedly according to racial group. Those for white South Africans remained almost invariable, with more boys than girls, while black South Africans had a clear majority of girls, a situation that the literature has almost completely overlooked. This high proportion of black girls was also present in most sub-Saharan countries. The reasons are still not completely clear. Sex ratios at birth show more births of boys than girls. Boys’ mortality was higher than girls’ mortality. But that does not explain why the twentieth-century black sex ratio was much lower than the sex ratios of pre-industrial European countries. We test several possible complementary explanations. The anomaly was caused, we argue, by a combination of higher mortality of boys and a preference for girls.