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Females biologically do invest more in each child than males. That is, they have to carry the offspring inside their body, eat for two, and all that chore. Exactly for that reason, a female is under selective pressure to be polygamous.

The right word is actually polyandrous, and i’ll get to that. But let me say that, yes, i know most everyone thinks it is the other way around.

Also, this is a reaction to “The Red Queen: sex and the evolution of human nature“, a book by Matt Ridley.

If you are not familiar with evolution of sex debate, it’s something like this. Despite the vast majority of multicellular organisms reproducing through sex, it is difficult to understand how natural selection could lead to it. This seems to be a hot — and undecided — debate in biology right now. There are many possible explanations, none of them easily refutable.

The problem basically is that sex is expensive: you need two organisms to produce offspring, instead of one. Also, your genes shouldn’t like to have half of them left behind instead of passed into the next generation. This is like each son you have is just a half-son.

As the genes on the son are a complex mix of his parents’, it seems reasonable that sex is worthwhile because it generates genetic variance. Otherwise, it would be easy to simply carry two sets of chromosomes ourselves and mix them without the fuss of match-making. Or be self-inseminating hermaphrodites. Something like that.

When a woman bears a child, the genetic variance added to the next generation is 1/2 genome. Half an individual’s amount of genes added to the mix. A second child from the same father will add only around 1/4: some of the father’s genes the first born already had. With three kids, it’s 1/8 genome, and so on.

Obviously a perfect set of chromosomes (son) can die for sheer dumb lack of luck. So it is advantageous to produce lots of child, even if their genetic load is the same. But this is no different from asexual reproduction: sheer force in numbers, and if you’re going that road it is more sensible to pass along all your genes into each suckling, not only a half.

From a genetic point of view, if we are not getting extra variance it is better to go asex. If you will have 10 sons with the same genetic richness, it is better to have 20 asex sons that are technically copies of yourself.

The female will spend the whole gestation period every time she is impregnated, and if she finds a new genetic source (available mate) during gestation she is unable to breed. So, the more she has to invest into each child, the more she should play for variance. Ergo, promiscuity.

Having polygamist sex with every male she finds will only be slightly more difficult (due to matchmaking cost) but will result in a progeny with much more varied genes with more chance to succeed. (There actually seems to be evidence that she will have more progeny with more partners…)

The other argument in favour of female monogamy is that she will try to get the better genes she can to procreate. This implies genetic variance is less important than the surplus advantage the best male available has over the second best. It is not the value of the chosen mate per se, but his excess to all the others. And this difference needs to exceed the penalty of sex: half the genome not going to the son. Very unlikely.

On the other hand, investing very little leaves the male depending on he female. If he walks out on her: what does she care? It is not like other males she’ll encounter will be pregnant and unable to give her that bit drop of semen. In many species the female actually can make children without the male.

Say we were going to buy together a television that costs $100. I am going to pay $1 and you $99. Would you be worried of me giving up? Would i worry that you do?

So the male has all the incentive to try to keep the woman on to the deal. And the direct way of doing that is simply keep asking: being around as much as he can, mating as often as she desires. And he can also try to cheat, for example with sperm competition or finding ways to monopolize the females.

That last strategy leads easily to monogamy, in my opinion, but let’s see what might tempt the male in the direction of promiscuity.

It is assumed the male can have more than one kid at the same time, with two females pregnant from his own sperm. He can have a whole harem. This is referred to as an “genetic lottery” where having an disproportional number of progeny is the unbelievable prize. But there is one problem with that: fertilization rate.

In those species where the female does indeed “invest” more in childmaking there are also complications of the mating process that make fertilization fallible. Only some of the times a couple have intercourse they will produce a litter. Ridley provides the following stats: Gorillas mate 10 times for each baby, Chimpanzees 500 to 1000 and Bonobos around 3000.

If we model intercourse as a binomial distribution, which is like assuming each mating is like throwing a dice, what do we get? Say two Gorillas, each mating 50 times in their lives, the first with only one female, the other with 50 different ones. The first is reasonably sure of having 5 children. The second has 0.5% chances of having 0 kids, 3% chance of one, 11% chance of two, and so on. His chances of having 10 kids are around 0.04%. In fact, the expected result is 5 sons.

This model is very naive, but it should show that promiscuity is not guaranteed to be better for the male. The “fuck as often you can with as much females as you can” strategy might have the same expected outcome that “find a good female and fuck her as much as you can”. In fact, it might be the same outcome with a bigger risk.

The assumption is that, after the female becomes pregnant, the male can go away and find another partner. But this could go either way. He might find another female willing to have sex, or he might not. Assuming the population to be roughly divided between males and females, there is a fat chance that many of the fertile women will be already relating to other males.

Again, to be able to say that the polygamist strategy is necessarily better we need to consider other complications in the species mating system as a whole, and maybe even all the other aspects of their behaviour. In some cases it might pay to be promiscuous, in others not. In any case, it seems unlikely that any strategy is the nature of maleness itself.

(There are many species that practice harem building, certainly. There are also those that make lifelong couples. In any case, i have a particular — and indemonstrable — hunch that even when males have more than one mate they try to have an stable and lasting — why not say it? — and “meaningful” relationship with each, instead of being exactly promiscuous.)

Despite all that, the idea that male = polygamist and female = monogamist is held as obvious. It is taken for granted. It is so easily accepted. In fact, it is even sometimes called a “Principle”! The Bateman Principle.

This principle is accepted not because it is watertight or for being based on good evidence. It is just accepted because it confirms our social biases. This women want love and fidelity story is just a convenient lie. Yet another fairy tale. But a deeply entrenched one.

For example, consider that in common usage monogamy is taken for antonym of promiscuity. Polygamy and monogamy refer only to the male having one or more mates, and the words for females are practically ignored: polyandry and monoandry have 1000 times less hits on Google than their counterparts. It is like only the male has the option of infidelity.

I think this shows that instead of talking about monogamy/polygamy as whole mating systems, we are only considering those things from our biased and ethnocentric point of view.

And this becomes alarming when we try to force biological data to confirm our prejudices!

It is hard to swallow Matt Ridley pretending to be judgement-free while saying that women are “naturally” interested in “older, rich, powerful men”. Or that it is “natural” that men strive for status and women strive for being mothers.

I am certainly not opposed to make inferences from biology into ethics and our lives, in general. But you can’t make those inferences blindly. You can’t be stupid, to make it simpler.

And laying rules about the natural preferences for each gender is stupid.

As a side note: maybe the best question is not [why sex?] but instead [why NOT sex (as horizontal-gene-swapping)?]. In other words, what is the advantage for a given DNA-based being to monopolize it’s own genetic repertoire?


  1. If I recall, the size of the testes correlate with the promiscuity of the females in a given species. Again, if I recall, our species is about half-way between chimps and bonobo — which seems to indicate human females were fairly promiscuous during much of our previous evolution.

  2. I’ll check the exact proportion latter, but i am thinking ours is a little smaller than chimps’. Which says very promiscuous, alarmingly so really but not the champ.

    But obviously it points at female promiscuity. There are lots of data in this direction. There are a fair amount on the other, also, to be fair. But Matt Ridley definitely ignores all that goes against it.

    He evens talks a lot about sperm competition FAILING to mention the obvious significance it can have for his whole female=monogamous theory.

    I mean, come on, the book has a chapter called “monogamy and the nature of women” and another by “polygamy and the nature of men”. It just makes me nuts!!!

    And it is not even him, only, this point of view is fairly well accepted in general. Arg!

  3. Why I was not following your blog already, I’ll never know.

    Incidentally, there’s a book out ther right now, “Sex at Dawn”, that looks exactly into how gender roles evolded to what we have today, and makin a case against current monogamy-based relationships.


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