The stats I found range from a 0.5 to 3% difference between the male and female population. What is the science behind this? How come there’s not significantly more of one or the other?
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Women having the XX chromosome and men having the XY chromosome. Both give either one of these to their child. So women always give an X, men give a Y half the time. So roughly half the children born are male, the other female.
Now there are more cases to look at (XXY etc), it’s not actually truly 50-50, but it’s close enough.
XX and XY chromosomes are part of the story, but not the whole story – the fact is, you get this kind of sex ratio across the animal kingdom, even though there are all kinds of ways sex could be determined, not just via specific chromosomes.
Basically, it comes down to the fact that each child gets half their DNA from the mother, and half from the father.
So imagine you’re a gene who has some influence on whether the kids will be male or female. You’re competing with other genes that also influence that. And suppose that currently, the species produces kids who are 80% boys, and 20% girls. The genes in the 20% are almost certainly going to survive to the next generation, but the genes in the 80% have to be pretty lucky. The 20%, though, includes genes that make kids likely to be girls, so the next generation will have more girls than before. The sex ration is pushed towards 50/50 by natural selection.
The same logic applies if the sex ratio was 20% boys and 80% girls – the next generation will have taken a step closer to 50/50.
So natural selection pushes the sex ratio to close to 50/50 in every species where mum and dad contribute equally to the DNA of the kids.
In species like bees and ants, mum and dad *don’t* contribute equally to the kids’ DNA, and you can do the same kind of maths to calculate what the ratio should be between boy bees and girl bees, and when you work it out, you get the answer observed in nature.
Humans are one of species that obeys the Fisher’s Principle. For us and many other related species evolution has sort of achieve an almost even gender split.
If anyone had a mutation where they produced more of one gender than another. The gender that was suddenly rarer would have an evolutionary advantage when it comes to pairing of and having offspring, thus the way humans reproduce evolution will always end up bringing things back into balance.
That being said, it is not a perfect 50/50 split.
In humans naturally slightly more male babies are born than females.
However for cultural and biological reason human males don’t live as long as females.
As children grow older more boys die than girls and by the time they are old enough to have children themselves they are close to parity. Then in old age men die years before women. And among the older age groups women significantly outnumber men.
The gender ratio at birth appears to be not set in stone, but affected by outside factors. In times of crisis like a mother’s body that has undergone significant stress and malnutrition may be more likely to produce baby girls. The whys and how are not rally well understood.
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