I'm often asked whether people are "naturally" monogamous, and similarly, whether monogamy itself is obsolete. The truth is that people aren't naturally monogamous and, indeed, rather than being obsolete, monogamy is relatively recent, and that's much of the problem! What's obsolete is polygamy, which was—and to some extent, still is—the "natural" default system for human beings. Monogamy definitely isn't natural, as attested by much of our biology, including the presence of sexual dimorphism (men being typically larger than women, with substantially more muscle mass) in our species.

In addition, men are significantly more inclined to violence than women, and boys become sexually and socially mature later than girls. All these traits are characteristic of animal species in which one male attempts to mate with multiple females; they are strongly consistent with male-male competition for such opportunities. Beyond this, the anthropology of human polygyny is quite clear, given that before the homogenization of so many traditional societies caused by Western colonialism, roughly 83% of indigenous human groups were preferentially harem-forming. For their part, women are typically less overt in their preference for multiple sexual partners, but they, too, find monogamy more restraining than Victorian stereotypes would suggest. Add this up, and there is no question that a visiting Martian observer would conclude that Homo sapiens is biologically polygamous.

Paradoxically, harem formation was especially hurtful to men, more than to women, since in any harem-forming species with an equal sex ratio, only a minority of men are mated, leaving most excluded and sexually frustrated, in addition to being subject to comparatively high levels of competition, sometimes violent and even lethal. On the other hand, it is clear that women in harem societies are generally poorly treated as well; although they are likely to end up mated to high status men, their actual reproductive success is often reduced by competition among co-wives, not to mention that they are often physically and emotionally abused and restricted by their own romantic and mating choices.

No one knows precisely how and why monogamy became culturally institutionalized, largely in the Western world, from which it has spread. One possibility is that monogamy represented a social exchange whereby powerful polygynists essentially agreed to forego their exclusive access to women, and to share marriage opportunities with otherwise excluded bachelors, in return for a degree of social harmony and relative peace. (From elephant seals to Homo sapiens, bachelor males are socially disruptive.) It is unclear to what extent women were influential in this "agreement," since it can at least be argued that they would be better off as the fourth or even twentieth wife of a very wealthy man than as the only wife of a pauper. In any event, rather than being an old and outmoded institution, it is a biological newcomer for the human species. And herein lies much of the stress associated with monogamy, which is definitely under siege, though not from radical feminism, some sort of home-wrecking homosexual agenda, or rampant, irresponsible pleasure-seeking, but from our biology itself.

Both genders are inclined to seek multiple sexual partners. For men, the underlying evolutionary calculus is obvious: more matings with different women can result in enhanced evolutionary fitness via a larger number of offspring. For women, the predisposing factors are more complex and nuanced, including possibly obtaining better genes for their children, improved access to material resources, prospecting for possible social advancement, and so forth. But for both men and women, the costs of non-monogamy can also be severe, including abandonment of one's children by the cuckolded spouse, as well as social ostracism and even violence. Indeed, for all the liability and strain involved in suppressing our biological inclinations for non-monogamy, the reality is that for human beings, monogamy offers distinct advantages as well. Notable among these is biparental care. In any species experiencing internal fertilization, males, but not females, are stuck with a profound biologically mandated uncertainty: mommy's babies, daddy's maybes. As a result, it is very rare for any species to engage in biparental care unless the males are guaranteed confidence of their genetic relatedness to the offspring, a confidence that monogamy alone can provide. And because human children need so much parental assistance, protection and investment, we, perhaps more than any other animals, are especially benefitted by monogamy (which, incidentally, needn't necessarily involve individuals of two different sexes).

It is important to emphasize that even though monogamy definitely isn't natural to human beings, and therefore isn't easy, it is nonetheless possible, as well as offering some important benefits. Many of humanity's most notable accomplishments—learning to play the violin, speak multiple languages, perform delicate surgery—are equally "unnatural", but are assuredly good. They also, like successful monogamy, require hard work. It is easy to do "what comes naturally"; animals do it all the time! Perhaps what makes human beings special is our ability to do things that are "unnatural," whether those things are obsolete or—like monogamy—are socially imposed and thus new to our evolutionary experience.