A lot of people I know are having babies lately, and it got me thinking back to something I read a long time ago, about how after wartime a lot of male babies are born, almost as if there was a coordinated effort to replace lost sons on the battlefield.

Now, I hate Elsevier and the researcher whose papers I’m most interested in doesn’t seem to have published into any open access journals. So I’m left to just reading abstracts and summary articles from interviews – this post is just going to summarize the summary.

Here’s the basic idea: empirically it seems that sex ratio is heritable through males. This points to the idea that there is likely (but not proven) to be a gene in the Y-Chromosome that controls the sex ratio at birth.

And through basic deduction it is enough to enough to explain higher male birth rates post-war. It is also useful for predicting your male:female ratio if you’re planning on having N kids, assuming you know your family tree.

How many males vs females will we have?

Since sex ratio is heritable through males, you should look at the baby-daddy’s siblings. If daddy had more males in his cohort, then you are more likely to have males. If daddy had more females, you are more likely to have females.


Anecdotally this phenomenon seems to apply to my family tree, my wife’s, and most of our friends.

  • My mother has 4 sisters, and her 1 brother has two daughters.
  • My wife has two male siblings, and her brother is expecting his second child. Both children are male.
  • My father has 1 brother and 1 sister, so his output should be male biased. What do you know: I have a brother and a sister.

One of my coworkers just had her third son, which makes me think her husband probably has a brother. And his father probably had brothers too.

How does this explain post-war male births?

If sons are more likely to go to war, then families with more sons are more likely to have at least one son come back alive than a family with fewer sons. Which means the returning pool of men are probably going to produce more male offspring than female offspring.



I’m not sure there are large enough family tree studies to show what the likelihood bias is, but my personal takeaways are:

  1. If you’re trying to select for a specific sex, it’s not actionable. Even if the split isn’t 50/50, the chances of having either sex is still high enough that you can’t really make use of this insight. My father inherited male-leaning genes and his first child was a daughter. My wife’s family has the same phenomenon. My mom’s family had sister-leaning genes, and resulted in 4 females and 1 male.
  2. If you’re trying to decide when to stop trying to have kids, it could be useful. Like say the father has male-leaning genes, and your first child is a male, and you slightly prefer not having two males. Then you should probably not roll the dice again. But if you have female-leaning genes, and your first child is a male, and again you slightly prefer not having two males, it might still be reasonable to roll the dice. After all, your preference is slight :)

On the ethicality of sex preferences for babies: I don’t think I’ve thought it all the way through and it is a complex issue. One thing to note however is that global preference for male babies is higher, and infanticide rates on female babies is high. Smart folks probably agree that this is a problem. The other thing to note is that the steady state has more male babies produced than female babies. That might worsen the problem.