Tuesday, 7 August 2007

Why did humans end up with a 50-50 sex ratio?

What an outstanding question!

Ok, first the boring part - how do humans end up with a 50-50 sex ratio?

Each of us has two sex chromosomes. A woman is XX, and a man is XY. An egg or sperm cell has half of our genes in it (so that when the two halves meet in fertilisation, a full set is created). Thus, an egg cell, made by the woman, must have one of her Xs in it. A sperm cell, made by a man, can have either his X or his Y chromosome. As it happens, one Y chromosome is enough to direct the body to make a male. The options, therefore, are (female contribution first, of course):

  • The one of the female's Xs + the male's X = female
  • The one of the female's Xs + the males' Y = male
  • The other of the female's Xs + the male's X = female
  • The other of the female's Xs + the male's Y = male

These are the only four options, so it is easy to see that you have equal odds of producing males or females.

But, the above answer misses the point slightly. Yes, the XX, XY situation guarantees a 50-50 split, but why did THAT system evolve? After all, it would theoretically be just as easy to evolve a different system that, say, made the split 72-28, for example. What is it that is so evolutionarily favourable about the 50-50 sex ratio?

The puzzle was solved by the British biologist R.A. Ficher, and I'll try to paraphrase it here. Imagine that there was a genetically inherited ratio of three times as many females as males (75-25). Fisher realised that this would be unsustainable. Why? Well, consider what would happen when they mated.

A female HAS to mate with a male in order to produce offspring. However, in the above example there are much more females than there are males. Thus, any particular male is likely (on average) to leave MORE descendants than a female is.

Imagine a hypothetical case of 100 birds where there are 75 matings (to make the maths easy). Therefore (since the ratio is 75-25), each female must have mated on average once. But, since there are only 25 males, and 75 matings, each male must have mated an average of... THREE TIMES.

Therefore, each individual member of the minority sex (males in this case) will tend to leave behind a disproportionate number of descendants (genes), compared to the majority sex.

So consider the 'options' to a parent. Clearly, if you want to spread your genes, it would be better to make male offspring, since you would leave an average of THREE TIMES more genes behind. And so it becomes evolutionarily favourable to produce male offspring. Since this is a successful strategy, it will tend to spread, meaning that more and more parents will produces males... which equalises the sex ratio!

The above is true no matter what the initial sex-ratio tendency. The only evolutionarily stable situation is 50-50, since any minority sex automatically becomes a more profitable way to propagate your genes. And this in turn favours a bias towards making more of the minority sex, which eventually equalises out the ratios.

Well, almost. This answers your question about humans, but for other animals that differ in their sex ratios, you need to use Fisher's broader idea that it's actually the INVESTMENT in males and females, rather than their actual numbers, that will tend to be 50-50. For example, if it is twice as 'expensive' to produce a male than a female, the stable sex ratio will be 2:1 in favour of the females. But this post is long enough already - if you want more detail, drop me a line.


  1. Alvin recently sent me the following email:

    Hi doc,

    fascinating website as ever!

    just one quick question about something not entirely clear in your answer: could you please explain why it is that males are more likely to produce male offspring, as implied by your answer? how would evolution help ensure that more males are born in this example?


  2. Hi Alvin

    Thanks for the kind words!

    In response to your question: I didn't mean to imply that males (as opposed to females) were more likely to produce males. Rather, what I meant to say was that, given the example's premises:

    1) males were the minority sex (by a 25 to 75 margin)
    2) making offspring requires both one male and one female

    then males would be do more mating on average than females. This would be true (as an average) no matter how many matings actually took place - since for each mating there would be a 1-in-25 chance for a particular male to be involved, and only a 1-in-75 chance for a particular female.

    With that in mind let's personify the 'choices' open to a parent (of either sex). If they were to produce a male, their odds of leaving behind their genes would be three times as great if they left behind a male. Put another way, they would leave an average of three times as many of their own genes if they gave birth to a male. This is because (as above) their male offspring would be more likely (by three times) to mate than a female offspring would.

    In this environment, it would pay an animal if, by chance mutation, it were able to produce males more often as females. Three times as many males as females would be ideal, but any mutation in this direction would be fine. Any such mutation would tend to be favoured by evolution, thus resulting in the species, over time, tending to have more males than females.

    But the story wouldn't end here - as the population became filled by more and more males, the advantage in producing males would be negated. No longer would a male child have significantly higher odds of mating than a female. Of course, if there were ever more males than females, then females would naturally be evolutionarily favoured, for the same reason as the males initially were.

    So the only stable strategy is a 50:50 sex ratio. Any other sex ratio is automatically pushed towards this 50:50 state - by evolution's automatic tendency (that we've discussed above) to favour the generation of the minority sex.

    Hope this makes more sense. Let me know if there are still problems, cos I'm not convinced I've explained it very well!