David P. Barash at the History News Network:
For now, I want to focus on why monogamy has become so popular, at least in the modern Western world, and at least in theory, if not always in practice. Although monogamy is exceedingly rare in the animal world, it is found in a few cases, and nearly always, the payoff seems to be associated with the adaptive benefit of biparental childcare, something that Homo sapiens finds especially beneficial, given that we are unusual in that our offspring are profoundly helpless at birth, remaining needy for an extraordinarily long time. Nor is it absolutely necessary that the cooperating adults be man and woman; we know from abundant sociological data that two women or two men can do an excellent job, and that when it comes to child rearing, two – of any sex – are better than one. But we also know that prior to the cultural homogenization that followed European colonialism, more than 83% of human societies were preferentially polygamous, and that polygamy was also prominent in the ancient Near East from which that presumed Western move to monogamy originated.
So my question for now is: why did such a large segment of human society switch from polygamy to monogamy? And my first answer is: at present, we don’t know. My second answer is a guess, which goes as follows. (I propose it simply as a hypothesis, in the hope that readers will not only find it interesting but also useful in generating informed discussion and, if possible, meaningful research.)
Imagine a polygynous society with an average harem size of, say, ten. This means that for every male harem-keeper, there are nine unsuccessful, sexually and reproductively frustrated, resentful bachelors. The simply reality is that polygyny is disadvantageous not only for women – for complex reasons – but even more so for men, since with a 50/50 sex ratio, there are unmated men in proportion as polygyny obtains. This, btw, runs counter to the lascivious imaginings of many men, who, when I describe the evidence for primitive human polygyny, often express regret that they weren’t alive in those days, imagining that they would be a happy harem-holder.