Polygyny Justified

I can not help commenting on the polygyny issue being hotly debated recently. What the fuss it’s all about anyway? Islam allows a man to have 4 wives, period. The issue should be whether women have been empowered and protected by law to express their dissatisfaction with their husbands’ decision, by filing for divorce without losing their financial rights and the rights to foster the children. If you can’t keep your man from marrying other woman, let him pay dearly for his decision. That should make him think twice.

But again, I guess you’ve heard enough about that. From other directions, my friend the economist has briefly discussed the rationale behind polygyny. Now I’d like to discuss polygyny from evolutionary biologist position. That is, trying to explain whether there is any functional advantage of polygyny for our species’ continued existence. I based this largely from Jared Diamond’s work.

Jared Diamond, renown for his book “Guns, Germs & Steel”, wrote “The Third Chimpanzee” back in 1991. In this book he argues that since we are just another species of mammal (we share more than 98% of our genes with 2 species of chimpanzee—hence the title) many of our traits can be traced back from our animal heritage. One of these traits is human’s sexuality.

Evolution process has created a unique species, where as a result of language capability and higher intelligence human developed a dependency on tools for its survival. Thus rather than developed claws and fangs to catch its preys, human created tools and organized hunting, resulting in a more effective way to get protein source compared with other carnivorous species.

But it also has its trade-off. Higher intelligence is a result of an increased brain’s size, hence larger capacity of the container (cranium / head) is needed. But there is no way that female pelvis can accommodate a full-grown head during birth. The solution is by getting the infant out when size of the head can still be accommodated by the pelvis. As a consequence, human’s infant needs considerable time to reach maturity compared with other species.

In hunter-gatherer society rearing a child is an expensive activity which can only be done by a team of a father and a mother. The couple needs to stay together for a considerable period to do this. But before that, human father who will invest heavily on the upbringing of his offspring needs to have some confidence in his paternity. This is the rationale behind marriage institution, and it can be concluded that monogamous marriage is the norm for human species.

What about other alternatives? Promiscuousness would not work. In promiscuous environment there is no certainty on the fatherhood and no father would want to rear other’s child. So while promiscuousness maximized the probability of genes passed on, it is not efficient since the survival of a child single handedly raised by the mother can not be guaranteed. Polygyny maximized both the probability of genes passed on and survival rate of the offspring, but it’s very expensive for hunter-gather society. Until our ancestors invented agriculture polgygyny institution is largely unsustainable. Hence for a long period of our history monogamous marriage remains the most efficient way to ensure the continuity of our existence.

Then human re-invented polgyny institution and extra-marital sex. Why? Because sex is fun? Of course sex is always fun, otherwise no one would be interested to do so. But from the evolution theory point of view, the fun part is just an incentive / by-product while the main objective of copulation is to pass on your genes through reproduction. If now human only look for the fun part it’s because human social preference has evolved. Now the by-product has become more important than the objective.

Back to polygyny. Biologically a man potentially can sire far more offspring than a woman. The record lifetime number of offspring for a man is 888 from 500 wives, sired by Emperor Moulay Ismail of Morocco, while the corresponding record for a woman is only 69 (several triplets). In one polyandrous society, the Tre-ba of Tibet, women with 2 husbands average fewer children than women with one husband. In contrast in 19th century American Mormon society, men with one wife averaged only 7 children but men with two wives averaged 16 children, and those with three wives averaged 20. Polygynous Mormon men as a group averaged 2.4 wives and 15 children. Similarly, among the polygynous Temne people of Sierra Leone, a man’s average number of children increases from 1.7 to 7 as his number of wives increases from 1 to 5.

From that it is clear that when number of offspring is the objective, polygyny is superior to monogamous marriage. While the norm is monogamy given the limited resources, polygyny is preferred when things become more affordable. And as I mentioned before, after our ancestors invented agriculture and left the hunter-gatherer way of life, polygyny became more sustainable. At least for kings and nobles initially, but as the world GDP / capita exploded in the last 2000 years more commoners also practicing polygyny, albeit still in smaller percentage compared with monogamous marriage.

What about extra-marital sex? In short it’s cheating. Well, of course it’s cheating, but what I mean, for a man it’s the most economical way to pass on his gene and let other man unconsciously rearing his offspring. As what is the biological incentive for women to be involved in extra-marital sex remains unknown, since most of the time the seed from one man is sufficient for a woman to produce an offspring.

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As for myself, I’m indifferent towards polygyny, that is I don’t agree nor disagree with the practice. Ethically it is debatable, but what interesting here is to see the nature of polygyny from science point of view. About this issue Jarred Diamond succinctly says:

In short, we evolved, like other animals to win the reproduction game. That contest has a single aim, to leave as may descendants as possible. Much of the legacy of that game strategy is still with us. But we have also chosen to pursue ethical goals, which can conflict with the goals and methods of the sexual contest. Having that choice among goals represents one of our most radical departures from other animals.

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