The Human Sex Ratio at Conception

A recent study has attempted to estimate the primary sex ratio i.e. the sex ratio at conception, using a variety of studies in the extant literature. Starting with data at birth, the authors projected back in time, estimating foetal loss at each gestational stage. Their overall conclusion was that there are equal numbers of males and females formed at conception. This paper contradicts these estimates using two very large samples of accurately recorded 19th century births. These datasets come from a time when contraception and sex selection were not widely practised. The conclusions are that at the time of conception, there is a substantial excess of males, the excess probably being determined by the hormone levels of both parents in accord with the hormonal hypothesis, and if conditions during pregnancy are stressful, then frail male foetuses will preferentially be culled. In short, more males than females are conceived, and that more males are miscarried, and that more males still survive to birth.