Why has evolution produced a 50:50 sex ratio?

You would have expected people to observe that it was harder and harder to marry off young men (and thus get grandchildren) and stop aborting female fetuses before things got as extreme as they are now.

That assumes that human societies are able to view and judge their own cultures in a detached, objective way and then effect mass change to that culture based on those judgements. Even in our modern more enlightened times, that doesn't really seem to happen much.

People are a bit confused about how evolution works. The mechanism is important. How would some men dying younger cause a gene in other males to be passed on? I can't see how.

You can't see that?
But it is quite easy.

Let's say we have a village in Africa where 20 boys and 30 girls live to adulthood. Let's assume, to keep things simple, that they only marry within their own generation.

All 20 boys will find a wife and have children, some will be able to father children with more than one woman. If only 2 of those 20 boys have the same father, he will make double the contribution to the gene pool compared to all other men in the village. Parents of boys will have more grandchildren because one man can father children with two women, which is double the amount of children a girl could have.

If you have 30 boys and 20 girls, then either 10 boys will not have children at all, or, with polyandry, some males will not get to father as many children as others. In any case, the number of possible babies born is limited by the number of women.

So, if girls are rare, the parents who have daughters have an advantage in passing their genes to the next generation.

If you have 33 boys and 30 girls, but 3 boys die before reaching adulthood, then the other 30 can father a maximum number of children, the previous existence of three more boys doesn't change anything at all.

If 90% of us were female, the males would disproportionately get to reproduce their DNA. Therefore, natural selection would favour any mutation that led to having more sons, and there would be a trend towards increased number of sons until there was no advantage to having sons. If the ratio were reversed, natural selection would favour daughters.
Unless there are social reasons which favour an uneven sex ratio (viz, bees) most species tend to finish up with a roughly 50:50 ratio.

The mother already has 50% likelihood of having passed on any genetic predisposition contributing towards dying during childbirth though, eg a narrow pelvis, as will her daughter, so that won’t have an impact on evolution except that she might have only 1 child instead of 10! But the characteristic would never die out, hence female humans still struggle with childbirth. But modern medicine has ruled that out of evolution now, along with many other genetic things.

Might be because humanity has, up until rather recently, regularly waged wars in which young men who haven't fathered any children yet are killed.

How would that explain the same 50:50 balance in kangaroos? Or humpbacked whales? Or polar bears?

I think any explanation that involves some kind of exceptional human behaviour resulting in exactly the same outcome as other mammals should be treated with suspicion.

Men might be more likely to die in war/accident. But look at the rate of women dying in childbirth through history.

And im just hazarding a guess here. But males are more likely to die before and during prime reproducing age through accidents. Probably why the extra 2/100 are needed

They aren't needed, as such. They just aren't there at reproductive age to provide a disadvantage to the genes of parents who have too many sons.

What is interesting is how China managed to keep its cultural tendency to prefer sons for such a long time.
Culture isn't genetic, but it is just as subject to evolution as genes are.
You would have expected people to observe that it was harder and harder to marry off young men (and thus get grandchildren) and stop aborting female fetuses before things got as extreme as they are now.

I thought slightly more boys died in infancy and youth at least in part because of X linked recessive disorders such as haemophilia. So a slight excess at birth has evolved for the reasons explained in the Fisher links upthread, to yield around 50:50 after puberty.

As a species we could probably survive with much fewer men, in a co-operative society.

A ‘co-operative society’ in which people get to live based on whether we need them or not isn’t one I’d like to live in.

Nor one in which I’d get much say in the matter.

People are a bit confused about how evolution works. The mechanism is important. How would some men dying younger cause a gene in other males to be passed on? I can't see how.

Why it isn't exactly 50-50 I'm not so sure

Might be because humanity has, up until rather recently, regularly waged wars in which young men who haven't fathered any children yet are killed.

The more sons a man has, the more likely his Y-chromosome (and thus probably the tendency to father more sons) is to be inherited by future generations.

Once there's too many males, not every male can father offspring, and things even out again.

And im just hazarding a guess here. But males are more likely to die before and during prime reproducing age through accidents. Probably why the extra 2/100 are needed

Its 52:48 male : female

Particularly as we tend to have monogamous relationships it makes sense to have half and half

I wrote a long, boring post about this with lots of long words but what it boiled down to is that we haven't been monogamous long enough to produce an evolutionary effect. Chances are we aren't 'naturally' monogamous, certainly not for longer than about 2 years.

It is an interesting question why it is 50% males. It could be 60%. Or 30%.

It probably isn't 60% because men tend to kill women and children if allowed to have too much impact on society. It doesn't work with too many males.

Why not fewer males? Well, with slightly more than 50% males, I still have problems finding a good man to have children with.

More scientifically:
Surplus males are where selective pressure happens, especially under favourable conditions.
Some percentage of males are going to be subpar, they won't be selected by females.

If mammals had to rely just on harsh winters and predators to weed out disadvantageous genes, populations that had favourable conditions for a couple of years would be more vulnerable to being wiped out once the weather changes and the wolves can cross the frozen river, etc.

Evolution is about characteristics becoming more prevalent because they make the species more likely to reach sexual maturity and therefore reproduce. I’m not sure if the proportion of males/females is related to evolution or not, but if it is, then clearly the ideal scenario is 50/50 as that’s how it’s turned out! Particularly as we tend to have monogamous relationships it makes sense to have half and half, unlike lions which have one male and lots of females - anyone know if their birth rate is also 50/50?

Bees (and other insects) have a completely different sex determination system to mammals. en.m.wikipedia.org/wiki/Haplodiploidy

(Some reptiles are also different, sex depending on egg incubation temperature.)

Sex in bees is down to haplodiploidy, they effectively choose what sex to make an egg, though not a conscious choice obvs!

Sex ratios generally stay at 50-50 even in species where a small number of males impregnate all the females (leaving the other males not to reproduce at all). This is because if your species produced - say - 75% females, but you were an individual who had a gene that made you produce more males you'd have an evolutionary advantage (because males would on average have better odds on reproducing if there are fewer of them). So your male-producing genes would survive and go on doing so until the ratio reached roughly 50-50.

Why it isn't exactly 50-50 I'm not so sure.

The answer to this has been known to evolutionary biology for over 100 years. It's called Fisher's principle.

Bees aren't 50:50 though

But bees in a colony are all related to each other. So it's a completely different evolutionary pressure.

Because at conception it’s down to chance: XX or XY.
It may be that more boys are born due to higher female foetal mortality.

I think it's an interesting question. It's so efficient. It's nearly too efficient.

Bees aren't 50:50 though, that must be evolutionary. IIRC only about 30%, perhaps even less, are male.

Surely it just shows there's somewhere near a 50/50 chance of a fetus being male or female and so the resulting numbers bear this out? As you need one of each sex there's no way for evolution to work on producing more males or females - ie, if you came from a family of all female children you couldn't breed with someone from a family of all female children as they'd need to be male so no way for natural selection to get going. (I know some regions have bigger or smaller percentages but it all comes down to somewhere around 50/50ish)