Whether you are in a monogamous marriage or a polygynous marriage makes a huge difference for a man, but not for a woman.

Throughout evolutionary history, humans were mildly polygynous. A species-typical degree of polygyny correlates with the extent of sexual dimorphism in size (the extent to which the male is larger than the female). The more sexually dimorphic the species, the more polygynous it is. This is either because males of polygynous species become larger in order to compete with other males and monopolize females, or because females of polygynous species become smaller in order to mature early and start .

I personally believe it’s the latter; I believe men and women could potentially be the same size except for the fact all human societies are invariably polygynous to various degrees. 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.

At any rate, what is indisputable is the positive correlation 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. On this scale, humans are mildly polygynous, not as polygynous as gorillas, 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%) practice polygyny, with only 16.14% practicing monogamy and .47% practicing . While present-day hunter-gatherer societies are not exactly the same as our ancestors in every detail, they are our best analogs available for us to study. The fact that polygyny is 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. Given a roughly 50-50 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 even be smaller. So the proportion of polygynous men in any society must always be lower than 50%. Most men in polygynous societies either have 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 they 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. As I discuss in earlier posts on the testicles and the penis, there is clear anatomical evidence on men’s body to suggest that women have always been mildly promiscuous.

Under polygyny, one man is married to several women, so a woman in a polygynous marriage still (legitimately) mates only with one man 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 while women have mated with only one man.

Sexual exclusivity prescribed under socially imposed monogamy today is therefore evolutionarily novel for men, but not for women. The Hypothesis 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 may not affect women’s likelihood of espousing the value of sexual exclusivity.

Consistent with this prediction of the Hypothesis, data from a large, representative American sample shows that more intelligent boys are more likely to grow up to value sexual exclusivity in early adulthood than less intelligent boys. In contrast, IQ does not affect girls’ value on sexual exclusivity in early adulthood. The effect of intelligence on the value of sexual exclusivity is more than four times as strong among men than among women. Among women, the association is not statistically significant.