Genes, Culture, and Monogamy

Of all the human societies we know about, past and present, about 80% allow successful men to have more than one wife. In some of these, polygyny is common, while in others it is limited to kings or chiefs. Polygyny would seem to make sense in evolutionary terms, because the most successful men would have more offspring, and failures would be excluded from the gene pool.

However, in the modern period the world has been dominated by societies that enforce monogamy. In Europe, North America, and Japan, having more than one wife is a crime, and though we may not take this very seriously now, not so long ago we hanged men for bigamy.

Via Gene Expression, a new paper by Joseph Henrich et al. asks why. They have this to say:

In suppressing intrasexual competition and reducing the size of the pool of unmarried men, normative monogamy reduces crime rates, including rape, murder, assault, robbery and fraud, as well as decreasing personal abuses. By assuaging the competition for younger brides, normative monogamy decreases (i) the spousal age gap, (ii) fertility, and (iii) gender inequality. By shifting male efforts from seeking wives to paternal investment, normative monogamy increases savings, child investment and economic productivity. By increasing the relatedness within households, normative monogamy reduces intra-household conflict, leading to lower rates of child neglect, abuse, accidental death and homicide.

In other words, men who aren't fighting each other over wives do a lot of other good stuff for society.

I would also emphasize the increase in male social solidarity that results from every man having a woman. Modern polygynists are notoriously suspicious of other men, and have been known to kill them for just talking to one of their wives. (Or kill the wife.) Monogamous societies seem much less consumed with jealousy, especially when husbands and wives are of roughly equal age.

Binding men into monogamous marriages seems to go some distance toward taming their worst instincts; married men commit many fewer assaults and murders than single men, and they are more likely to have full-time jobs and contribute toward their children's upbringing. (Even children they had with other wives.)

So the model developed in both Europe and East Asia of monogamous marriage between people of nearly equal ages seems to work very well for modern societies. It remains an open question whether any society without such a system can achieve a modern level of economic and political development.

And that far more than compensates for any loss of individual evolutionary fitness. Your loosely organized tribesmen can be as tall, strong, and charismatic as you want, but they will still be slaughtered by the soldiers of a well-organized, bureaucratic state. People who can work together well will always beat people who can't.