Document Type



Data have been reviewed on the variation of the human sex ratio at birth. It is clear that it varies (if only slightly) with a large number of variables, among which may be mentioned race, season, wartime, legit­imacy, birth order and paternal age. In addition one may suspect variation also with the psychological status of parents, the smoking status of the mother, the handedness of the parents, the location of an anomalously implanted pregnancy, occupation and maternal age. None of this variation is substantial: however, more recent research stemming from medical (rather than demographic) sources seems to have identified a few conditions with which sex ratio varies substantially, e.g. time of insemination within the cycle, some forms of parental disease at the time of conception, hor­monal treatment of the parents for subfertility or other condition, and inadvertant exposure of parents to deleterious chemicals. These latter forms of research occupy only a tiny fraction of the existing literature on sex ratio—but they seem to comprise the most promising lead in the search for causes of variation (and for means of control) of sex ratio. In a second paper (to be published in the next issue of Human Biology) a hypothesis will be offered to account for all this variation; its adequacy will be discussed and a research program suggested.