We’ve talked about how typically at conception, the sex ratio of males to females is even; but at birth, there are more males than females.
Now, let’s talk about what this looks like atypically: that is, in times of societal stress.
What? You might be thinking. Sex ratios change under societal stress?
Yes, they do. It’s called the “wartime effect.” We’ve seen it after tornados and earthquakes and terrorist attacks and other disasters too, not just wars. But it has mostly been studied in the context of how more males would be born toward and at the end of an armed conflict in the countries heavily involved in the conflict. To be clear, the proportion goes from 51.4% male to 51.7%, so it’s a small change, but definitely a strange one. And it’s one that’s statistically significant (see this article for the first comprehensive statistical analysis I could find of this phenomenon).
There are a lot of different theories for why this is, and there have been for quite some time.
One working theory for a while was that if one parent was healthier, the sex of the child would match the other parent. (This was the justification for one awful experiment in 1901 I came across that involved witholding food from a dog in order to test whether the sex of a pup matched the sex of whichever parent was in worse health.) If males had returned from war, where they had been injured or exhausted, then they were probably weaker than the females who had stayed home in presumed safety. Perhaps more male progeny at the point of conception was the result. However, while this was sort of borne out in the dog experiment (3 males to 2 females in the litter), this isn’t something I’ve seen investigated as a plausible theory recently.
Another scholar posited that females control the sex of a zygote, and females who were experiencing more testosterone (because they had to take on traditionally male roles and thus were temporarily becoming more “tough”) were more likely to produce male zygotes. There are a lot of assumptions that have to be right for this theory to work – like that toughness is connected to testosterone, for example. This theory is not very well received, as far as I can tell.
Remember how we learned about the theory that timing of intercourse and ovulation impacted the sex of the baby in our last post? That theory was referenced as the reason for the wartime sex ratio too. The thinking was that the hormone levels of males were affected by hormone-affecting weapons and stress AND that timing of intercourse was impacted by males taking short leaves to go home from the military, which would result in more frequent intercourse and thus more boys. One piece of this holds up: stress is actually really important. I’ll get back to that soon.
Another theory (that, full disclosure, kind of boggles my mind) is that taller male soldiers are more likely to survive wars and also more likely to father sons. Side note: the author claims this is more relevant to past wars than future wars. But this idea that genetics has something to do with it is in line with another scholar who argues that the ability to produce males is genetic; so there may be something that makes people who can produce males more likely to survive traumatic events such as wars.
(Content warning on what follows: loss.)
However, as research on the topic has improved, scientists have come to believe that sex ratio isn’t about the health of the parent pre-conception but rather about the post-conception loss of an embryo or fetus: that stressful events result in a higher societal proportional loss of females than males. (see last week’s post for more details.) And, recent research has found that parents who lose a child tend to try to have another child; so if a parent were genetically likely to have a male baby, and they had a child pass away, they’d be likely to have another male baby.
Stress and genetics are common denominators between smog, floods, earthquakes, and other disasters. But stress hasn’t actually been measured in many studies, so more research is needed in order to sort out the relationship between these two big potential factors, especially the role that maternal mental health may play in that relationship.
There are other factors that may matter too. In some cases, we’ve seen that ambient temperature might impact sex ratios, with hotter temperatures seeing more male babies. And we’ve seen in other studies that ambient temperature didn’t matter, but rather having more environmental pollutants was associated with having more boy babies. And as artificial reproduction becomes more influential in the world of sex ratios, societal preference for males may increase the prevalence of male babies (though sex selective abortions have played a role in this regard for a long time).
So could we be seeing more males born in the coming months than usual? Given that there’s been the massively stressful event of a global pandemic, that certainly seems plausible. As more research comes out in the coming months and years, we may get a better sense of how covid-19 has shaped sex ratios.