Among mammals a larger proportion of females than males reproduce, the extent of the imbalance signalled by gender differences in size. Elephant seal males are three times as massive as females, while gibbons are characterised by physical equality. The former play winner-take-all, amassing huge harems. Exclusive possession requires violence to enforce, a reason for the shorter life-expectancy of elephant seal males. In contrast the gibbon is a monogamist, entering into a cooperative pair bond to defend shared territory and raise offspring.

Evolution's logic by which the future belongs to the fecund is operative in both cases, but there's more than one way to skin the cat. Obviously the size difference in our own species is modest, so some anthropologists may emphasise pair bonds while others argue for a more fluid serial monogamy, but in both cases the presumed evolutionary norm is not extreme polygamy.

Despite these biological truisms, cultural anthropologists know that most societies not only accept polygamy, but idealise it, while evolutionary geneticists report super-male lineages such as that of Genghis Khan which are incredibly fertile. No one suggests that the conqueror was super-human in size, rather, he illustrates how societies can be converted into a winner-takes-all game. It is not true that Genghis Khan said the best thing in life was "To crush your enemies, see them driven before you, and to hear the lamentation of their women." But there is a reason why many find this myth plausible. Civilisation has borne witness to the rise of radical inequality (pdf), cultures where the accumulation of wealth and women are the pinnacles of achievement. Abraham, Jacob and Solomon were the fathers of nations by their many wives as well as slave-owning autocrats. Hunter-gatherers are no angels, but the structural constraints of their economic system renders it impossible for an ambitious male to control all of a band's wealth and support dozens of wives.

A custom's ubiquity does not speak to its virtue. Because of the adoption of Greco-Roman monogamous norms by western Christianity, Europeans are among the cultures which have rejected polygamy. During periods of great inequality of power, even self-styled autocrats such as Henry VIII took only one wife at a time. It was this European society, where elite males were peculiarly constrained in their marital excesses, which eventually led the economic revolution which has so reduced inequality in income over the past two centuries.

Even the gilded-age plutocrat never considered openly collecting women as a sign of power and privilege, so naturally the working man had a reasonable expectation of finding a mate.

Since 1970 the gains in income have gone mostly to the elites, as they did before 1800. A positive vision of a society characterised by a modicum of social and economic equality has given way to the liberal individualist ethos, where personal choice is supreme. Perhaps western societies will revert to the "normal" human type, and accept the inevitability of both radical inequality of marriage and income, becoming Saudis in cloudy climes.