A Mormon family, circa 1888. (Photo: Wikimedia Commons)

How hard-wired are human beings for polygamy? Is it our nature to love and commit to one person, or to doubt, to flirt, and, perhaps, to stray? David P. Barash's new book, Out of Eden, answers this question with a paradoxical "Yes."

Surveying the animal world, Barash, an evolutionary biologist and professor of psychology at the University of Washington, pulls together decades of research to show how some aspects of humanity we take for granted—the difference in size between the sexes (sexual dimorphism), girls' tendency to enter adolescence sooner (sexual bimaturism), and mens' propensity for violence—correlate with a natural but moderate tendency toward polygyny, in which males compete to amass harems of females. He contrasts highly polygynous species like elephant seals, in which a male may keep up to 40 females, with somewhat less polygamous human beings and mostly monogamous owl monkeys. And he points out that polyandry, in which a female mates with multiple males, isn't as rare as we thought. Females are just much more subtle about it.

Barash also makes a strong case for the biological, cultural, and personal benefits of informed monogamy. Being attracted to someone other than your mate doesn't mean you've settled on the wrong person; it's just a bug in your system.

One of the insights in your book is that polygamy and monogamy exist on a continuum. You quote E. O. Wilson as saying humans are moderately polygamous. How do you measure the extent of polygamy within a species?

There are a number of ways, actually. The most meaningful one would be when you literally look at the number of mates that are simultaneously maintained by a member of one sex. In the case of polygyny, which is male harem formation, you simply look at the number of females in an average harem.

You can also look it anatomically. If you look at those species that are highly polygynous, in which many females are mated to just one male, we find that they're actually highly sexually dimorphic, and there's a really close one-to-one correspondence between the degree of polygyny in terms of number of mates and the degree of sexual dimorphism.

In other words, the greater the size difference between the male and the female—the bigger the male is than the female, generally speaking, within a species—the greater the degree of polygyny?

Exactly. And the reason for this is really fairly important. It has to do with the degree of competition [within] that sex that has a higher variance in reproductive success, where you have a small number of individuals who are very successful, and relatively large numbers who are unsuccessful. The greater the degree of that disparity within a given sex, the greater the degree of competition, then, to be the successful ones. It's not always true but for the great majority of species, simply being bigger and stronger means you're more likely to be successful, and that, in turn, translates into a greater degree of sexual dimorphism. It's not just anatomical; it's behavioral as well.

I wanted to ask you about that behavioral aspect. What do homicide rates across the world tell us about our human propensity toward polygamy?

What they tell us is an unfortunate story, in that males around the globe, as a worldwide cross-cultural universal, are far and away the most violent sex. We see a dramatic disparity, with males being particularly violent toward other males, and, again, that's entirely consistent with our harem-keeping heritage.

Why is that?

It's a matter of the individuals of the harem-keeping sex competing among each other for access to those that are going to be kept.

Many of my social science colleagues will maintain this is purely a culturally mediated difference. And I have no doubt that it is culturally mediated. But the fact that culture is involved hardly explains the phenomenon, [and] certainly doesn't explain the cross-cultural universality. Why should culture always be involved in the same direction?

What's the relative size difference between men and women?

Males are approximately 1.2 times larger and heavier than women. That difference is greater yet, though, if you look at muscle mass. That might give some indication of harem size in the past.

Out of Eden: The Surprising Consequences of Polygamy. (Photo: Oxford University Press)

It's not just a matter of sexual dimorphism. Another really useful [indicator] would be what we call sexual bimaturism. Another one of these things that humans take for granted—kind of like the overwhelming propensity of men to [to commit acts of] violence relative to women—is the fact that girls become sexually and socially mature earlier than do boys. It's dramatic in middle school.

Males have a far greater behavioral burden when it comes to acquiring a mate. The greater the degree of male-male competition, the greater the payoff to waiting—or, another way of saying it, the greater the disadvantage of attempting to enter the social fray when you're too young, too small, possibly too inexperienced as well. A male elephant seal who attempts to compete with other elephant seals when he is still relatively young is going to get beaten up, maybe even killed.

So, in polygynous species, the males are bigger and more aggressive and violent because biology is essentially rewarding them. They win the evolutionary prize by getting to father lots of offspring with many women. But in other ways, you point out that, depending on the species, these overly large males pay for their bulk.

On balance, they're better off in the sense of [being] more fit evolutionarily if they're able to hold their own in competition. But if you just look at the ecological energetics, they're often too big for their own good. Too much of their anatomy is devoted to muscle, for instance, [and] not enough to their internal organs—including, quite possibly, brain power. I have to say, as I pursued this further, I am not only depressed at the degree of violence that men are predisposed to by virtue of their biology, I'm also increasingly inclined to think that women really are superior in all sorts of ways.

What is the benefit for a woman to behave polygamously?

Up until fairly recently, biologists like myself knew that males of almost any species are likely to look for multiple partners, but it was believed that females did not. And it's really only with the advent of DNA technology that we're able to look at the offspring of a given female and find that, my gosh, social monogamy and sexual monogamy are simply not the same thing. As though women didn't already know that.

There's no one-size-fits-all explanation for that. One of them is that the male in question may make inadequate sperm, may make insufficient quantities of sperm or maybe the sperm themselves are insufficiently viable, and so essentially females are taking out an insurance policy. Another one that seems at least as viable is that in having multiple mates, a female is obtaining extra resources.

Then there's also obtaining better genes, in the sense of mating with a male who may be more attractive sexually, as a result of which the male offspring produced may be themselves more attractive sexually. That's referred to among biologists as the "sexy son" hypothesis.

You write that though the research is not conclusive, there's some evidence our evolutionary ancestors were more polygamous than we are. Do you have any sense why humans would have evolved toward being somewhat less polygamous?

The biological evolution almost certainly has to do with the value of bi-parental care and the fact that our offspring are so extraordinarily helpless at birth that it's a hard slog to do single parenting. It's a lot easier if you have two committed parents.

The other part of your question would be historically and culturally, during more recent times, and the reasons for that are really unclear. Why is it that the Western religious ethic so strongly emphasizes monogamy? One hypothesis that I find really intriguing is [to] look at harem formation. When I give lectures about this, I can almost see the men in the audience licking their chops and saying, "Gee I wish I lived in those days." The implication being they would have a harem of beautiful women. The reality is they probably wouldn't.

Monogamy is a good deal for women, but even more so, it's a good deal for men—and that's somewhat counterintuitive—because monogamy is a sexually democratizing principle. In the absence of monogamy, the likelihood is that most men are reproductively excluded. And so one possibility is that the historical-cultural tradition favoring monogamy was in part a deal that powerful men made, whereby they said, "OK, I will forego my overt opportunity to be a big-time harem master in return for a degree of social peace."

In the process of writing this book, you were surprised at the extent to which you found yourself increasingly critical of the harm polygamy does in our world. Do you mean the social and economic inequity that a lot of young men experience, or the tendency for young women and girls to be taken as wives when men do take up harems? Or something else entirely?

If you look at the anthropology of it, there's quite a lot of convincing evidence that, in cases of human polygyny, the women in question don't do as well as if they were monogamously married. The payoff of having a committed husband and father of their offspring more than makes up for the presumed benefits of being with a wealthier guy. Then, if you add to that the social cost of often being forced to marry someone you don't choose, and often being under-age as well, not to mention the increased aggressiveness and disruption among the unmated males—I just think there's a whole lot to be said for monogamy.

You liken it to learning how to play the violin: not necessarily intuitive, and difficult, but nevertheless very worthwhile.

I think one can make a strong case that human beings are at their best when they strive to transcend some of their biological limitations.

There are those who say, "We're supposed to be monogamous—God made us that way, and if I find myself looking at multiple potential sexual partners, that must mean that I'm a horrible sinner, or a bad person, or I'm not cut out for monogamy." Well, no one's cut out for monogamy. That doesn't mean we shouldn't aim for it.

This interview has been edited for length and clarity.