A reader has raised a couple of questions. I’ll begin with the first one:
Let's suppose people from other parts of the world move to a tropical environment. Should we expect a shift toward polygynous habits within a couple of millennia under the influence of tropical agriculture?
People would shift toward more polygyny, but they would do so by pushing their envelopes of behavioral plasticity. A couple of millennia doesn’t seem to be enough time to shift the genetic goalposts.
Some 9,000 years ago, Amerindians began to develop agriculture in the tropical New World. These tropical agriculturalists, like the Yanomamo of Amazonia, show the predicted changes in behavior: higher female self-reliance in food production, lower paternal investment, and higher polygyny rates. Between 10% and 20% of all Yanomamo males are polygynous at any one time (Hames, 1995). Nonetheless, this is still lower than the 20-50% we see in sub-Saharan Africa.
It seems, then, that natural selection hasn’t had enough time to create a strong predisposition to polygyny in tropical Amerindians, at least not as strong as what we see in sub-Saharan Africa. In addition, their ancestors may have started off with a very low predisposition to polygyny, having entered the Americas by way of an Arctic environment that selected for high paternal investment. Even in the tropics, Amerindians still have an Arctic-adapted anatomy and this evolutionary conservatism may extend to behavior (Holliday, 1997, pp. 425-426).
In polygynous societies, women choose men. In societies with an unbalanced sex ratio (females>males wise) men choose women. How would you describe the actual state of our society according on this scale?
If we look at reproductive age brackets (20 – 40 years), there are more single men than single women. This is a relatively recent phenomenon and seems to date from the late 1980s. The 1992 U.S. census found the following ratios of single men to single women per age group:
Under 25 – 111
25 to 29 years – 128
30 to 34 years – 121
35 – 39 years – 109
40 – 44 years – 83
45 – 64 years – 64
65 years and over – 30
The imbalance is even greater among 20 to 40 year-olds if we look only at childless singles. It is greater still among white Americans (the black American population has a surplus of single women because of high mortality and incarceration rates among young black men).
Not long ago, the reverse was true. There were more single women than single men in all age groups. If a woman was still unmarried at 25, people considered her doomed to spinsterhood.
What happened? First, the death rate has fallen dramatically among young men. No major wars have occurred since 1945. Highway and workplace accidents have steadily declined. Alcohol is no longer the grim reaper that it once was. Today, so few young men are dying that we now have a serious shortage of organs for transplantation.
Second, divorce laws have liberalized throughout the Western world. It’s now much easier for a man to divorce his aging wife and marry a younger one. And this is the pattern we generally see with second marriages.
Third, it is economically easier, and socially more acceptable, for a woman to have children out of wedlock. Many of these single mothers then drop out of the marriage market, partly because they find it difficult to date men and be a parent at the same time. As well, men are often reluctant to care for children who are not their own, especially if (as is often the case) the woman is not interested in having any more.
In short, marriage-minded men are now in a seller’s market and not a buyer’s market. This is a big change, and one that has aroused surprisingly little comment from social scientists. Yet I see the evidence all around me—the thirty-something man who has a good job, no bad habits, and still no woman in his life. How come? Usually, he’ll duck the question and put on a brave face. Or he’ll blame himself: his low-paying job, his lack of personality, or his looks.
Yet these reasons were not operational when my parents were young—less than a half-century ago. If a man wasn’t married then, it was because he didn’t want to be. Or because he was a ‘bum’ or a ‘psycho’. How times have changed …
Hames, R. (1995). Yanomamö, Varying Adaptations of Foraging Horticulturalists, Originally prepared for Just in Time Anthropology series, Prentice Hall and Simon & Schuster, supplemental readings for Ember and Ember's Anthropology, 8th edition.
Holliday, T.W. (1997). Body proportions in Late Pleistocene Europe and modern human origins. Journal of Human Evolution, 32, 423-447.