Occasional reflections on Life, the World, and Mathematics

Prenatal sex ratio


A paper that I’ve been involved with for a dozen years already has finally been published. We bring together multiple data sets to show that the primary sex ratio — the ratio of boys to girls conceived — is 1, or very close to 1. Consequently, the fact that more boys than girls are born — the ratio is about 1.06 pretty universally, except where selective abortion is involved — implies that there must be a period in the first trimester when female embryos are more likely to miscarry than male.

This is one of those things that is unsurprising if you’re not an expert. The experts had developed something close to a consensus, based on very little evidence, that the sex ratio at conception was much higher, some saying it’s has high as 2 (so that 2/3 of the conceptuses would be male), with excess female mortality throughout gestation. (We know that male mortality is higher in the second half of pregnancy, and after that… forever.)

The paper has its problems, but I think it’s a useful contribution. It’s also the first time I’ve been involved in research that is of any interest to the general public. Several publications have expressed interest, and an article has already appeared in two German magazines online, including the general news magazine Der Spiegel.

Update: Guardian too. This makes it interesting, in retrospect, that we had such a hard time getting a journal even to be willing to review it. One said it was too specialised.

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