Here's the number of boys to every 100 girls born in France during the 20th century.
There's two major spikes during and after each World War.
It was the same in England and Wales...
Germany...
Actually pretty much every country and every major war since records began.
And no one knows why.
*Theme Music*
More male babies are born even in peacetime, most countries float around 105 boys to every 100 girls level.
But China and India bring the global average up to 109.
If you create a system where having a girl puts families at a disadvantage, then large numbers of infant girls will die.
The natural ratio may be slightly in favor of boys,
but they're more susceptible to disease in early childhood, and much more likely to die from accident or homicide.
By the time of 30 the ratio levels out to about one to one, and once when our eighties women tend to outnumber men by two to one.
The variation in the sex ratio as we get older is mostly due to human activity.
Yet we tend to assume the ratio at birth is free from influence. Each individual pregnancy being fairly random.
And it is.
But on a global scale trends do start to emerge.
Like more male babies being born immediately after or during major conflicts, dubbed the "returning soldier effect".
It's been observed many times in many countries.
It was most noticeable during the First and Second World War, probably due to the sheer number of casualties involved. But why?
It's certainly an evolutionary advantage,
nature's way of replacing the fallen men. But the mechanism of how it happens is still a mystery.
We know fathers determine the sex of a child.
Sperm either carries an X or Y chromosome and on average men have a fairly even distribution of X and Y sperm.
Yet the sex of a child
can be affected by all sorts of things; age of parents, height of parents, birth order, number of aunts and uncles, stress, and even latitude.
The question is, which of these are influenced by war?
Astudy of babies born in Denmark between 1980 and 1993
showed that younger fathers are slightly more likely to produce boys. War kills a lot of young men,
But those that do survive tend to "celebrate" more than usual. In the US
an entire generation was named after the post war sexual activity of their parents.
Unfortunately, not enough data was collected about the age of fathers during World War one and two, to draw any concrete links to the returning
soldier effect. But it's likely to have had some contribution.
Many studies have actually linked stress to a decrease of male births. The sex ratio fell in Japan after their 1995 earthquake, East
Germany following economic collapse in '91, and during the Great London Smog of '52. This implies
there's something unique to war. Despite being one of the biggest causes of national stress the sex ratio increases.
Some have theorised that the jubilation of a war ending, or loved ones returning could have the opposite effect.
The relief of stress causing a surge in male births.
This would explain the distinction between war and other disasters. Earthquakes
and market crashes don't have a definitive end that calls for celebration.
But this theory still doesn't address the increase of male births during war, the biological mechanisms are still mostly unknown, and
national relief is almost impossible to quantify.
Taller men have more boys. Here's the height distribution of British soldiers that died during the First World War.
The mean height was 166.26 centimeters, for surviving soldiers
it was 168.63 centimeters, a difference of 2.37 - or just under an inch. Based on height alone
surviving soldiers were 5% more likely to father sons.
7 times the actual increase of Britain sex ratio following World War One. In order to account for the change
1/7th of Britain's male population would have had to have seen combat.
In reality 1/3rd did. If anything the mystery is why more male babies weren't born.
This is probably the most accepted theory, but studies with bigger samples
and another countries are required before we can say the height difference is the definitive cause.
And still debate continues as to why taller men are more likely to survive war, or why they're more likely to have sons.
Essentially a huge amount of research into sex determination is still waiting to be done.
For example,
here's the sex ratio at birth of every country in the world against the distance from the equator to their capital city.
On average the closer you are to the equator, the more likely you are to have girls. And whether this is due to temperature,
economics, or genetics is completely unknown.
All we do know is that sex determination may not be as random as we once thought. Our actions
(deliberate or not) have an impact on the global sex ratio. The long-term effects of which are still to be seen. As we enter an
age of designer babies and sex selective engineering, our influence and
responsibility over the human sex ratio becomes greater than ever.
And now a message from top patreon supporter Aleksei, "A summary isn't everything - truth is inherently nuanced.
Investigation that gives life meaning is worth doing. And in 2018, the greatest library is in your hand."
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