Why has evolution produced a 50:50 sex ratio?

[Gentlygently]

I presume there is some evolutionary reason to why humans (all mammals?) have a 50:50 sex ratio. But what is it? As a species we could probably survive with much fewer men, in a co-operative society. But maybe a race with more men (hence physically stronger) would fight the female dominated race and win?

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.

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.

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.

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.

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..,

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.

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.

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

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.

It depends on what you mean by efficient. The artificial imbalance maintained by farmers is efficient for humans, not for cattle. It’s not efficient for cows bearing male calves that will be slaughtered at birth.

FISHERS PRINCIPLE!

Current best guess is exactly what notdavidtenant says upthread - fishers principle.

You can model it mathematically - the pressure drives it back to the middle from either extreme.

I’ve always wondered if there is a slight driver because the simplest way of creating sex cells in an XY/XX type system is just division and recombination.

Some animals can skew their sex ratio in response to environmental pressures. The red deer on the Ilse of rum have been studied irt this (basically bad good years = more females because even low quality females mate, good food year = more males because a male needs to be in good nick to breed in a harem arrangement.)

There’s loads of work on parental resource allocation and tons of mathematical modelling of it if you’re into that.

It depends on what you mean by efficient.

I mean maximizing the long-term number of individuals of the species with the minimum usage of resources.

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

The point is that there clearly is a measurable imbalance in China. We were discussing why the Chinese hadn't done anything about it.

It's in fact less efficient than a female-skewed sex ratio, which is what farmers maintain as candidpeel pointed out

Farmers are not trying to get the biggest possible herd though. The efficiency can I let be judged in terms of the end game which is multiplication.

I let ? ...only

I was told in A level biology that more males are conceived because the sperm swim faster because they are slightly lighter due to the Y not an X, but then as a PP says they are more likely to die due to Y related genetic disorders. No idea if it's actually true though it sounds plausible!

I think more males are born - I remember that in IVF there are slightly more male babies born. Then around the age of 28-30 the ratio of males / females skews back the other way, because males are more likely to die as babies, more likely to die from male stupidity (wars / driving too fast), or from male suicide (patriarchal pressures) and then they just tend to get sick more than women (though I suspect again patriarchal pressures / male pride puts them off going to the doctors when they are sick). The ratio then continues to increase throughout each decade. If you don't mind doing the maths you can look on the census pages for males / females in each age group. It makes for interesting / sobering thought.

Because our young are born defenceless and needing care for many years, so a two adult unit works better.

There's a brilliant book about evolution of sexual strategies called "The Red Queen" by Matt Ridley. Worth a read.

Hestia on the face of it, it sounds plausible that it would be advantageous in evolutionary terms for women to have children with many men, but the same is equally true for men surely ? It would be disastrous if they chose a woman who could only have a single child or was infertile or passed on a disease etc etc. It doesn't seem compelling that men would be the sex given this handicap when reproduction costs them a lot less. This also fits with the narrative about men sowing their wild oats and all that shit.

Also, I still don't understand how a woman who's partner cheats on her feels heartbreak/jealousy/pain at such a visceral level if it's just socialised. Same could be said of "love".

Thank you - I hadn't heard of Fisher's principle and that seems pretty clear. I was musing because of the farming aspect mentioned above - I know that there is plenty of research into how to get a higher female:male sex ratio in farm animals (techniques such as sperm spinning, washing, heating etc) and I know that men can produce sperm with a sex ratio different from 50:50 in certain situations (astronauts and deep sea divers I think).

I suppose it boils down to the fact that genetically, your own genes want you to reproduce.

I know that men can produce sperm with a sex ratio different from 50:50 in certain situations (astronauts and deep sea divers I think).

Oh I remember that. Men who spend long periods of time away from home father more girls or something? I’m not sure if it’s ever really been confirmed but it’s an interesting theory :) some mechanism to do with sperm turnover? So if it’s sitting around more then one type degrades faster? I can’t remember the details.

Couple of thoughts:

Mrs.TerryP' I don't know if your question got answered, but if you're still curious, here goes: say two boys are born, John and Peter. Let's say there is a gene that codes for better frontal lobe control and less impulsive behavior. Say Peter has the gene and John does not. Little Johnny grows up to be reckless and dies at age 17 before fathering children. Peter, more cautious, matures abx has five children. Effectively, ALL of Peter's genetic material gets passed on and NONE of Johnny's, thereby increasing the prevalence of one individual's DNA.

Also, there is no question that technology is starting to skew this typical population ratio, as evidenced by China and India, with fairly significant shifts in a blink of an eye. Social pressures are now adjusting and it will be interesting to see if an equilibrium gets reached again and how long that takes. (Could be rapid if China decided to start a war given an "expendable" population of young men!)

Talking natural sex selection is interesting. Let's not forget the role of the vagina and the woman's body! Female sperm are more robust and live longer: an acidic pH of the vagina (I.e. a good healthy vagina with lots of lactobacilli) can increase the chances of conceiving a girl. Also, timing of intercourse with regards to ovulation can affect

Oops! Timing of intercourse can affect sex if the baby as well. Having sex closer to ovulation theoretically gives male sperm a better shot.

I'm not sure on why we do the nearly 50:50 ratio, seems to be some good (non cultural) explanations.

I find it fascinating that we all have DOUBLE the amount of female ancestors than we do have males. So basically a LOT of males do not reproduce at all, and some males reproduce prolifically.