The status of the hypothesis that the human sex ratio at birth is associated with the cycle day of conception

Abstract

Dear Sir, Data have been published (Gray et al., 1998) suggesting that the sexes of human offspring are not related to their cycle day of conception. These authors acknowledged that with their data, they had only around 80% power to detect (at the 5% level of significance) a difference of 20% in the sex ratios (proportions male) of conceptions on the most fertile days and on other days. But it is possible that if there were a difference, it would be less than this. So it seemed worth pooling these data with all the further recent data known to me. They are given in Table I. For the pooled data, χ2 = 9.9, P < 0.002. So direct data support the hypothesis that cycle day of conception is (weakly) associated with sex ratio of offspring. Conceptions on the most fertile days have an overall sex ratio of .492, while those on other days have a sex ratio of .536.

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