The fragile male

doi:10.1136/bmj.322.7286.617/a

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Male zygotes are often formed at suboptimal times in fertile cycle

William H James, honorary research fellow

Galton Laboratory, University College London, London NW1 2HE
Men's Health Forum, London WC1H 9JP

EDITOR—Kraemer adduced large quantities of data to substantiate the proposition that, from conception, males are more vulnerable than females.1 Postnatal vulnerability has some social causes, which are known, but the causes of the prenatal vulnerability of males are not established, so I wish to suggest one.

During the menstrual cycle women have a fertile window lasting several days.2 There is strong direct3 and indirect4 evidence that the regression of the sex ratio of the offspring (proportion male) on the time of fertilisation is …