Why has evolution produced a 50:50 sex ratio?

I don't think it has. Historically more boys were born but they tended to die young, not just because if wasr but because they were less robust babies

No, we'd branched off into a separate discussion about the Chinese cultural preference for male children.

If you are arguing that cultural factors can cause an imbalance that isn’t unreasonable.

If 50:50 wasn’t efficient we wouldn’t have it. Nor would other mammals.

We have a 50-50 ratio because it's an evolutionary stable strategy, not because it's efficient. It's in fact less efficient than a female-skewed sex ratio, which is what farmers maintain as candidpeel pointed out.

I’m not sure what you mean by ‘cheats’ either.

I was indulging in a bit of genetic anthropomorphism. The 'cheaters' in a female-skewed population are those who have more male offspring - increasing their own reproductive fitness, but decreasing the reproductive effectiveness of the group as a whole.

There's some snake species that procreate by parthenogenesis, I would assume their sex ratio would have a lot more females.

Sex ratios at birth are the same.

Do we really want to go down the assigned sex at birth route? I was hoping this might be the one thread we can avoid it..,

Seriously, do people really believe that we have a 50:50 balance because that’s what the cultural elites want?

No, we'd branched off into a separate discussion about the Chinese cultural preference for male children.

Whether or not we're naturally monogamous has nothing to do with the average number of children males and females have, so has no impact on the stable sex ratio.

I had a look into why the ratio is about 52% male (rather than 50-50 as it ought to be). The most convincing theory appears to be that humans do conceive exactly 50% males, but that female feutuses are slightly more likely to miscarry (during the second trimester, mostly before we'd know whether a feutus was male or female).

So there is no evolutionary advantage at all to being prone to conceiving males or females. They just face slightly different odds of dying at different stages in life.

Why look for a cultural explanation for the balance in humans when snakes reach the same balance?

Do they, really?

There's some snake species that procreate by parthenogenesis, I would assume their sex ratio would have a lot more females.

It has been mentioned twice and linked to- why are you all arguing about something that by all accounts appears to be a fairly well settled principle?

This^. From the link upthread:

1. Suppose male births are less common than female.
2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
4. Therefore the genes for male-producing tendencies spread, and male births become more common.
5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
6. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.

Seriously, do people really believe that we have a 50:50 balance because that’s what the cultural elites want?

ScienceRoar thanks, I think your answer makes the most sense.

To whose advantage, though? Most men do marry, so there are many people benefiting from the current system through having both the prestige of a son and grandchildren. And these people are going to be over-represented amongst the kind of social and economic elites that influence culture. What's in it for them to try to change things?

This looks like a cultural explanation.

I don’t see the need for a cultural explanation to explain the 50:50 balance in humans when most other vertibrates seem to reach the same balance without ‘culture’ as such.

Why look for a cultural explanation for the balance in humans when snakes reach the same balance?

If you come up with a cultural explanation you are implying that without it humans wouldn’t have a 50:50 balance, making is virtually unique.

Humans really aren’t that exceptional.

There are cultural factors that can make it unbalanced - as in China - but we don’t need a cultural explanation for when it is balanced.

A 50/50 sex ratio is highly inefficient. From the point of view of maximizing the reproduction of the species, it would be must more effective to have (say) 65% female, 35% male or even more extreme. But 50/50 persists because an unbalanced sex ratio (either way) is unstable. Even though 65-35 is better overall, it can't be sustained because 'cheats' (who have more male offspring) would prosper at the expense of everyone else.

If 50:50 wasn’t efficient we wouldn’t have it. Nor would other mammals.

I’m not sure what you mean by ‘cheats’ either.

It has been mentioned twice and linked to- why are you all arguing about something that by all accounts appears to be a fairly well settled principle?

It only assumes that they are able to see their own advantage, really, but that's probably expecting too much, still.

To whose advantage, though? Most men do marry, so there are many people benefiting from the current system through having both the prestige of a son and grandchildren. And these people are going to be over-represented amongst the kind of social and economic elites that influence culture. What's in it for them to try to change things?

Surely natural selection would have little effect on division of the sexes as how would you select a mate based on their past children? How would you know? Other characteristics like strength etc can be demonstrated, shown via physical characteristics or communicated through pheromones etc but how would you select a mate who's more likely to sire males/females?

But @HestiaParthenos the example the PP had was of men in their middle age dying, not men who hadn't reproduced yet.

Which post do you mean? I couldn't find a reference to middle aged men.
But the principle that other men's genes profit if one man is taken out of the race early still applies right up until that man would have been out of the dating pool of women of reproductive age, anyway.

...scienceroar's explanation is much clearer and simpler than mine!

We still (especially men it seems to me) have strong mate guarding instincts

It all boils down to egoism. For a man, it is evolutionary advantageous to have as many women as possible who bear his children year after year. If they get pregnant by someone else, he misses a chance to spread his genes.

For a woman, on the other hand, it is advantageous to not put all her eggs in one basket, so to speak, and have a different father for every child, so a disease is unlikely to kill all her children. But it also promotes the children's survival if there's another person who feeds and protects them, not just the mother.

Monogamy is a compromise.

In humans, it is hard to say whether monogamy evolved genetically or culturally, but seeing as there are many patriarchal cultures that don't expect men to be monogamous, it may well be cultural.

clearly the ideal scenario is 50/50 as that’s how it’s turned out

A 50/50 sex ratio is highly inefficient. From the point of view of maximizing the reproduction of the species, it would be must more effective to have (say) 65% female, 35% male or even more extreme. But 50/50 persists because an unbalanced sex ratio (either way) is unstable. Even though 65-35 is better overall, it can't be sustained because 'cheats' (who have more male offspring) would prosper at the expense of everyone else.

China together with other paternalistic societies favour sons as inheritance of land and money are usually through the male line and this continues despite the cultural revolution. The lack of social welfare means parents would need a son to look after them in their old age. Daughters would be married off and help to look after her in laws.

gently it is a really good question: not how we have a 50:50 ratio (e.g half the sperm are X half are Y etc....) but why ? why did evolution lead to this ratio?

Farmers don't keep 50: 50 bulls and cows in their breeding stock because you don't need all those bulls to sire the next generation, so why waste all that food on them. So why does evolution end up with 50:50? (or not quite 51:49 or whatever...)

As Mrs Terry Prachet the answer is what benefits the species doesn't 'win' the evolutionary race. But its also not "what's what benefits the individual" either ....its what benefits the individual gene (i.e. leads to the gene being replicated more often in the next generations) . This is what "the selfish gene" means. It is a totally mind blowing idea in biology.

So you have to think of it from a genes-eye view. Being a gene that is carried in a female organism represents quite a different risk/return prospect than being a gene that is carried in a male . Female humans can have something like between 0-10 live offspring in a lifetime with probably the most likely number being a couple surviving (pre antibiotics etc.). Males can have between 0 and thousands of offspring, with the most likely number probably being 0 (many young men die in battle before having a child etc...but some become rich men with many wives, concubines, mistresses etc...). So for the genes being carried in a female organism is like betting on black at roulette, you may well win, but not big, whereas being carried in a male is like putting everything on a single number. The gene will probably loose (end up not being passed on), but occasionally it will win big by being passed into a lot of grandchildren.

So you can think about different gene options competing with each other over rounds and rounds of reproduction. A gene mutation that leads to a man producing more viable Y sperm is a higher risk prospect (for the gene) and can easily get wiped out (there are no grandchildren to carry the gene in one generation), whereas a gene mutation that leads to a man producing mainly viable X sperm (mainly daughters) is at less risk of being wiped out but will tend to get overtaken in the evolutionary race by the gene which results in more equal X/Y sperm -- so the evolutionary equilibrium tends toward.

....I'm not sure if i've explained that well!

But @HestiaParthenos the example the PP had was of men in their middle age dying, not men who hadn't reproduced yet.

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.

It only assumes that they are able to see their own advantage, really, but that's probably expecting too much, still.

Apparently, by now, a young man in China must have his own car and own flat to even dream of getting married, whereas women can have their pick.
That could have been predicted, obviously.

Fascinating question. I love this sort of stuff but must say I am not qualified or anything.

Some percentage of males are going to be subpar, they won't be selected by females.

This is far too simplistic, suggesting as it does, that the only reason a females genes would not be selected is due to her pickiness. Or that the above par males all chose other females.

Chances are we aren't 'naturally' monogamous, certainly not for longer than about 2 years.

I don't agree with this and think that most (all?) Of our attitudes towards sex are evolutionary. I don't see how a male who has dozens of children to different partners would be a good bet (safe sex being a recent invention). We still (especially men it seems to me) have strong mate guarding instincts resulting in jealousy and "heartbreak" undignified pleading etc. Why does love even exist if we are not supposed to be monogamous ?

Interesting thread.

... I just re-read my post and noticed I said Africa. I think I wanted to include sickle-cell anemia as example for selective pressure.

But that'd get too long.