Humans Are Naturally Polygynous, Not Monogamous - Why More Intelligent Men (but Not More Intelligent Women) Value Sexual Exclusivity

The Intelligence Paradox: Why the Intelligent Choice Isn't Always the Smart One - Satoshi Kanazawa 2012

Humans Are Naturally Polygynous, Not Monogamous
Why More Intelligent Men (but Not More Intelligent Women) Value Sexual Exclusivity

Because there is still a lot of confusion over terminology, I feel I must repeat what I said in my earlier book and first clarify different institutions of marriage.1 Monogamy is the marriage of one man to one woman, polygyny is the marriage of one man to more than one woman, and polyandry is the marriage of one woman to more than one man. Polygamy, even though it is often used in common discourse as a synonym for polygyny, refers to both polygyny and polyandry. It is therefore ambiguous as to what polygamy refers to, and the word should be avoided in any scientific discussion, unless it refers specifically to both polygyny and polyandry simultaneously. The reason people use “polygyny” and “polygamy” interchangeably is because, for reasons I explain in the earlier book,2 there are very few polyandrous societies in the world (polyandry contains the seed of its own extinction), and virtually all polygamous societies are indeed polygynous.

Throughout most of human evolutionary history, our ancestors were mildly polygynous, not monogamous. Now how do we know this? Marriage institutions, unlike skeletons, do not leave fossil records. So how do we know what kind of marriage institution (polygyny vs. monogamy) our ancestors practiced? From their skeletons; that's how we know.

How polygynous members of a species are in general correlates with the extent of sexual dimorphism in size (the average size difference between the male and the female). The more sexually dimorphic the species (where the males are bigger than the females), the more polygynous the species.3 This is either because males of polygynous species become larger in order to compete with other males and monopolize females,4 or because females of polygynous species become smaller in order to mature early and start mating.5 Sexual selection can also create sexual dimorphism in size, if women prefer taller men as mates and/or if men prefer shorter women as mates.

I personally believe that polygyny is key to sexual dimorphism in size among humans.6 I believe men and women could potentially be the same size in a perfectly monogamous society. But all human societies are invariably polygynous to various degrees.7 In fact, women's (but not men’s) average height in society is partly determined by its degree of polygyny. The more polygynous the society, the shorter women are on average, while men's average height is unaffected.8

At any rate, what is indisputable is the positive association between the degree of polygyny and the degree of sexual dimorphism in size, both across species and across human societies. Thus strictly monogamous gibbons are sexually monomorphic (males and females are about the same size), whereas highly polygynous gorillas are equally highly sexually dimorphic in size.

Southern elephant seals (Mirounga leonina) represent the extreme of this association. Male southern elephant seals on average maintain about 50 females in their harems. In other words, only 2% of male southern elephant seals get to reproduce each breeding season, and the other 98% end up being complete reproductive losers. Theirs is an extremely polygynous breeding system. As a result, male southern elephant seals are nearly eight times as large by weight as female elephant seals.9 In fact, many female southern elephant seals are often crushed to death under the weight of the male during copulation.

Fortunately, the average human male is only 17% heavier than the average human female.10 So, on this scale, humans are mildly polygynous, not as polygynous as gorillas (let alone southern elephant seals), but not strictly monogamous like gibbons either.

Consistent with this comparative evidence, a comprehensive survey of traditional societies shows that an overwhelming majority (83.39%) of them practice polygyny, with only 16.14% practicing monogamy and .47% practicing polyandry.11 The fact that polygyny is so widespread in such societies, combined with the comparative evidence discussed above, strongly suggest that our ancestors might have practiced polygyny throughout most of human evolutionary history.

Of course, polygynous marriage in any society is mathematically limited to a minority of men.12 Given a roughly 50—50 sex ratio, the highest proportion of men in polygynous marriage in any society is 50%. If half the men each take two wives, the other half must remain wifeless. If some men take more than two wives, more men must remain wifeless and the proportion of polygynous men will be even smaller. So the proportion of polygynous men in any society must always be lower than 50%. Most men in polygynous societies either have only one wife or no wife at all.

However, at least some men throughout evolutionary history were polygynous, and we are disproportionately descended from polygynous men with a large number of wives, because such men had more children than monogamous or wifeless men. Nor does the human evolutionary history of mild polygyny mean that women have always remained faithful to their legitimate husband. There is clear anatomical evidence in men's genitals to suggest that women have always been mildly promiscuous over human evolutionary history.13

As you can see in Figure 7.1, under polygyny, one man is married to several women, so a woman in a polygynous marriage still (legitimately) mates with only one man, just as a woman in a monogamous marriage does. So a woman in a polygynous marriage and a woman in a monogamous marriage are both (supposed to be) sexually exclusive to one man. In sharp contrast, a man in a polygynous marriage concurrently mates with several women, quite unlike a man in a monogamous marriage, who mates with only one woman. So throughout human evolutionary history, men have mated with several women concurrently while women have (legitimately) mated with only one man.

Figure 7.1 Marriage institutions: Monogamy vs. polygyny


Sexual exclusivity prescribed under socially imposed monogamy today is therefore evolutionarily novel for men, but not for women. The Intelligence Paradox would therefore predict that more intelligent men may value sexual exclusivity—having only one sexual partner in a committed relationship—more than less intelligent men, but intelligence may not affect women's likelihood of espousing value of sexual exclusivity.

Consistent with this prediction of the Intelligence Paradox, an analysis of the Add Health data shows that more intelligent boys are more likely to grow up to value sexual exclusivity in early adulthood than less intelligent men. Even after statistically controlling for the effects of such relevant factors and potential confounds as age, race, education, income, religion, and the number of times the respondent has been married, the more intelligent they are in junior high and high school, the more they value sexual exclusivity in a relationship seven years later. In contrast, net of the same control variables, childhood IQ does not affect women's value on sexual exclusivity in early adulthood (in their 20s). More intelligent girls do not grow up to value sexual exclusivity in a relationship more than less intelligent girls. The effect of childhood intelligence on the value of sexual exclusivity is more than four times as strong among men as among women. Among women, the association between childhood intelligence and adult value on sexual exclusivity is not statistically significant at all.