Monogamy is a mystery. A Martian zoologist, visiting Earth and noting our basic biology — not to mention our frequent philandering — would conclude that monogamy is not "natural" to the human species. So why is it so widely promulgated, especially in the modern Western world?
Of course, just because monogamy isn't natural doesn't mean it isn't possible or even desirable. Our species is often at its best when we do things that don't come easily, such as learning to play the violin or, perhaps, sticking with one mate till death do us part. But the question remains: Why does society urge upon us a lifestyle that goes against something so basic as our demonstrable interest in multiple sexual partners?
Although 100% monogamy is exceedingly rare among animal species, it is found in a few cases, my personal favorite being a parasitic worm known as Diplozoon paradoxum that inhabits freshwater fish. Males and females meet as adolescents, whereupon their bodies fuse together, and they remain sexually faithful even beyond death. Among the handful of mammals that are mostly monogamous, the "adaptive benefit" is two-parent child care, something that our own species finds especially important, given that human babies are profoundly helpless at birth and remain needy long after that.
But there is also an obvious evolutionary payoff for males (of pretty much any species, including Homo sapiens) to have multiple sexual partners. More matings mean more reproductive opportunities, more chances for genetic survival, even if baby-making isn't generally the avowed intent of sex these days. It should come as no surprise that early on, more than 80% of human societies were preferentially polygynous — one male mating with many females.
For females in general and human females in particular, the biological benefit of polyandry — one female, many males — is less clear, but this hasn't dampened many women's enthusiasm. Reproductively, it may represent a route to amassing more protection or resources, or it may be a mechanism for finding ever-fitter genes for ever-fitter offspring.
On balance, humans are somewhat polygynous and — paradoxically — somewhat polyandrous too. This does not mean that we are wildly promiscuous, but it indicates that as a species we show the characteristic evolutionary imprint of polygamy, not lifelong fidelity to one sexual partner.
Evolutionary biology can't exactly say why so many humans gave up polygamy and made the cultural move to monogamy. We just don't know, although there are theories and subtheories. One is a more nuanced version of the two-parent advantage: Monogamy may improve the survival rates of offspring because males will know which babies are theirs and will therefore be more likely to support them. There is much biological wisdom behind the saying "Mommy's babies, Daddy's maybes," and monogamy diminishes the maybe.
I would venture an additional impetus.
Imagine a polygynous society — the most common human polygamous arrangement — in which the average harem is made up of, say, 10 women. This means that for each male harem-keeper, there are nine unsuccessful, resentful bachelors liable to make trouble.
The simple reality is that polygyny is disadvantageous for humans because it pretty much ensures a complement of sexually and reproductively frustrated men. (As it turns out, it may also be bad for women in that it increases female vs. female competition for reproduction, but that's another story.)
This bad news about polygyny, by the way, runs counter to the lascivious notions of many males who regret they weren't alive in our distant harem-keeping days. Their imaginings are actually rather comical, analogous to someone remembering a past life as Emperor Napoleon when the overwhelming statistical likelihood is that, were past lives real, he would have been the poor boob who froze to death on the Russian steppes. Instead of satiated sultans, most men in polygynous cultures would have been miserable celibates.
In short, in addition to its benefits when it comes to child-rearing, monogamy is a great democratizing institution. Compared to polygyny, it enables many more men to have a wife and a chance at a family. So we can hypothesize that Western society adopted monogamy as a trade-off in which powerful men essentially agreed to forego the obvious advantages of the seraglio in return for wider social peace and harmony.
I think monogamy is part of our egalitarian ethos; indeed, it may have set the stage for social equality. Consider the contribution that socio-sexual frustration has apparently made to unrest in the Arab world, where limited economic prospects contribute to limited mating opportunities.
Similarly, look at the situation of modern China, where an excess of men seems poised to generate unpredictable disorder.
Within the United States, Bill Gates could surely afford a few hundred wives, and the anthropological record is clear that in many societies, for a lot of human history, men with his wealth literally did just that. Of course, Gates just doesn't seem the type, but that is probably testimony to our cultural norms rather than how biology whispers within us.
At the same time, monogamy doesn't guarantee democratic or egalitarian outcomes, any more than a public promise of sexual exclusivity on one's wedding day guarantees it in someone's private life.
And just as biology pushes men in particular to depart from monogamy, it seems plausible that strong socio-evolutionary factors push back. One is dual parenting. And the second may well be the degree to which a one-man, one-woman system gives those of us with fewer assets than a billionaire the opportunity to partake of our own personal harem, even if it's a harem of one.
David P. Barash is an evolutionary biologist and professor of psychology at the University of Washington; his most recent book is "Out of Eden: The Surprising Consequences of Polygamy."