A Mormon polygamist family in 1888 (Wikipedia)

David P. Barash is an evolutionary biologist and professor of psychology at the University of Washington; his most recent book is Out of Eden: surprising consequences of polygamy (2016, Oxford University Press). A version of this post recently appeared in Psychology Today.

I’ve written – both in recent posts and my latest book – that people are polygynous (naturally harem-keeping: one man, many women). Paradoxically, we’re also polyandrous (one woman, many men), and I’ll write about that next. But for now, what’s the evidence for polygyny?

#1) In all polygynous species, males are physically larger than females. Basically, this is because polygyny produces a situation in which males compete with each other for access to females and in the biological arena such competition typically occurs via direct confrontations, in which the larger and stronger nearly always wins. If the species is strictly polygynous – that is, if polygyny is the only mating system (such as in elk, or gorillas) – then a small number of males get to father many offspring whereas most males are unmated reproductive failures. The greater the “degree of polygyny” (essentially, the larger the average harem size), the more bachelors.

The evidence is even stronger when we consider that there is a direct correlation between the degree of polygyny – average harem size – and the degree of “sexual dimorphism,” the extent to which males and females are different. This difference is most frequent in size, but also reflected in degree of ornamentation (combs, wattles, bright feathers, fancy antlers, etc.).  Gorillas, for example, are quite polygynous and males are much bigger than females; gibbons are nearly monogamous, and males and females are almost exactly the same size. And  human beings? Men are about 1.2 times larger than women, a difference that is even greater if we look at muscle mass, especially in arms and upper legs.

The disparity between the patterning of male and female reproductive success within polygynous species turns out to be very important.  Another, more technical way of saying this is that under polygyny, the “variance” in male reproductive success is high, whereas the variance in female reproductive success is low. Consider elk, for example: natural selection favors bulls who are physically imposing and therefore successful in bull-bull confrontations, because they are the ones whose genes are projected into the future, whereas there is no comparably unbalanced payoff for cows. For bulls, the situation is close to winner-take-all (all the females available in any given harem, along with all the reproductive success). For cows, everyone is, to some degree, a winner – although no one wins big time. One interesting result, incidentally, is that since they are largely freed from the tribulations of same-sex competition, females often get a curious benefit: they are more likely than males to be at the ecological optimum when it comes to body size. Males, on the other hand, since they are constrained by the rigors of sexual competition, are more likely to be too big for their own good.

Whenever species shows a consistent pattern of males larger and stronger than females, it’s a good bet that polygyny is involved. Greater male size isn’t by itself proof of polygyny, but it points in that direction. Note, as well, that in this and other cases, we are dealing with a statistical generalization, which is not invalidated by the fact that some men are indeed smaller than some women. The fact remains that by and large, men are larger and physically stronger than women. Not coincidentally, by the way, women are “stronger” in that they live longer, something probably due in large part to the rigors of male-male competition … itself due to polygyny.

#2) In all polygynous species, males aren’t just larger than females, they are more prone to aggression and violence, especially directed toward other males. In many cases, males are also outfitted with intimidating anatomy that contributes to their potential success: horns, antlers, large canines, sharp claws, etc. But once again, these accouterments only make sense insofar as their possessors are inclined to employ them.

It wouldn’t do for a bull (elk, seal, or baboon) no matter how large and imposing, to refrain from using his bulk when it comes to competing with other bulls. There is little evolutionary payoff to being a Ferdinand among bulls. No matter how imposing, he could refrain from the competitive fray, save himself the time and energy his colleagues expend in threatening, challenging and – if need be - fighting each other, not to mention the risk of being injured or killed in the process. Ferdinand would doubtless live a longer life, and probably a more pleasant one. But when he dies, his genes would die with him. Publish or perish.

Accordingly, just as polygyny generates sexual dimorphism in physical size, it works similarly with regard to behavior, and for the same basic reason. As with the male-female difference in physical size, male-female differences in violent behavior vary with the degree of polygyny. As expected, the male-female difference in aggressiveness and violence among highly polygynous species is very great. And moderately polygynous species? Here the difference is, not surprisingly, moderate.

Among human beings, men – starting as boys – are more aggressive and violence-prone than are women. I have found that the male-female difference in perpetrators of violent crime is about 10 to 1, consistent across every state in the US, and true of every country for which such data are available. Moreover, this difference is greater yet when proceeding from crimes that are less violent to more violent: the male-female difference in petty crime, for example, is very slight, greater when it comes to robbery, and greater yet with regard to assault and most dramatic in homicides. This is true even when the actual crime rates differ dramatically across different countries. Thus, the homicide rate in Iceland is about one percent that in Honduras, but the male:female ratio of those committing homicide is essentially unchanged. Overall cultural differences between Iceland and Honduras are very great, which doubtless explains the overall difference in homicide rates. However, male-female differences remain proportionately unchanged, just as male-female differences in human biology don’t vary between Iceland and Honduras, or indeed, any place people are found.

#3) In all polygynous species, females become sexually and socially mature at a younger age than do males. This phenomenon is superficially counter-intuitive, since when it comes to reproducing, females by definition are the ones who bear the greater physiological and anatomical burden: Eggs are much larger than sperm; among mammals, females, not males, are the ones who must build a placenta and then nourish their offspring from their own blood supply. Females, not males, undergo not only the demands of pregnancy and birth, they also provide all the calories available to their infants via nursing (and, little known to most people, lactation actually makes even greater energy demands than does gestation).

Based on these considerations, we would expect that if anything females would delay their sexual maturation until they are proportionally larger and stronger than males, since when it comes to producing children, the biologically mandated demands upon males are comparatively trivial: just a squirt of semen. But in fact not only are females typically smaller than males as we have seen, but they become sexually mature earlier rather than later because of yet another consequence of the power of polygyny. Male-male competition (mandated, as we have seen, for the harem-keeping sex), makes it highly disadvantageous for males to enter the competitive fray when they are too young, too small, and too inexperienced. A male who seeks to reproduce prematurely would literally be beaten up by his older, larger and more savvy competitors, whereas early-breeding females – who don’t have to deal with the same kind of socio-sexual competition – don’t suffer a comparable penalty.

And so, among polygynous species, females become sexually and socially mature earlier than do males. The technical term is “sexual bimaturism,” and anyone who has ever seen 8th, 9th or 10 graders, or simply been an adolescent will immediately recognize the phenomenon, whereby girls aged 12 to 16 are not only likely to be (temporarily) taller than their male classmates, but considerably more mature, socially as well as sexually. 

Once again, and as expected, the degree of sexual bimaturism among animals varies directly and consistently with their degree of polygyny. Sexual maturation occurs at roughly the same age for males and females in primate species that are monogamous: e.g., Latin American marmosets and owl monkeys.  Among polygynous species – e.g., rhesus macaques, squirrel monkeys and, indeed, nearly all primates (human as well as nonhuman) – males mature more slowly and thus, they reach social and sexual maturity later than do females, when they are considerably older and larger than “their” females, and also, not coincidentally, considerably older and larger than the other, less successful males. The sexual bimaturism so familiar to observers of Western teenagers (and to those teenagers themselves!) is a cross-cultural universal. And finally,

#4) We have the simple historical record, confirmed by anthropology. A noted cross-cultural survey of 849 societies found that prior to Western imperialism and colonial control over much of the world - which included the imposition of historically recent Judeo-Christian marital rules - 708 (83%) of indigenous human societies were preferentially polygynous. Among these, roughly one-half were usually polygynous and one-half, occasionally so. Of the remainder, 137 (16%) were officially monogamous  and fewer than 1% polyandrous.*

So, our biological polygyny can be considered to be essentially proved (Popperian would say, it has withstood every available attempt to disprove it). But as I’ll describe in my next post, women aren’t nearly as sexually passive as all this polygyny business might suggest.

*As it happens, these proportions are almost identical to that found among primates, too:monogamy occurs in roughly 15% of nonhuman primate species, and various degrees of polygyny found in nearly 85% with polyandry virtually unknown.