Imagine you took all the men in a particular society and lined them up according to their level of income. On one side of the line you would have the extremely wealthy, the Bill Gateses of the group, and on the other side you would have the extremely poor—perhaps those who live in cardboard boxes. Now imagine that you line up all the women as well with the intention of matching the men with the women in matrimony. The men make an offer of marriage and the women choose to refuse or accept those offers. Now this is an economic story, so what the women really care about is the standard of living she is going to have if she accepts the offer.
The women who are lucky enough to be matched up on the rich end of the line are going to be pretty pleased with the way things worked out, and, since all that matters is income, will accept their offers. The women at the bottom end, though, have a tough decision to make. If they accept the offer, they are going to be married to a man who can provide them no standard of living. If they reject the offer and the society is monogamous, then they will be forced to live with their parents—or some economic model equivalent. If they reject the offer and the society is not strictly monogamous, though, then maybe they have another choice; they could become the second, or third, or fourth wife of the Bill Gates type. Maybe some women would prefer to live with their parents, or marry the really poor guy, but many wouldn’t mind if there were a couple of extra wives around if it meant being married to the guy with all the resources.
This is a long way of saying that when it comes to marriage, most economists would expect that in societies with high income inequality, polygyny (one man with multiple wives) should exist. The reality, though, is that prosperous nations often have high income inequality but rarely do they have polygyny. So here we have a mystery, it’s the mystery of monogamy (which I once joked was the “Myth of Monogamy” which is a different, slightly embarrassing, story).
Now you might be thinking that in wealthy nations women don’t need a man to support them, that they can educate themselves, earn their own income, buy their own property and, as a result, are free to marry the guy in the cardboard box if they like to without really suffering. All true. So from a modern perspective this story probably doesn’t make as much sense. Our marriage institutions have been historically determined though, and monogamy was established long before women could go school, earn income, or own property. Nowhere was there more inequality than in England before the Industrial Revolution, but that country has a long history of legally imposed monogamy.
So where there is an economic mystery there are economists who hope to explain it. There are several potential explanations, which I will perhaps return to another time, but there is simple one that I like in particular which argues that if rich men take all the wives for themselves the poor men don’t like it very much.* In fact it would probably make them very angry. If you are the ruler of a country with high levels of inequality, and you worry that these very angry men will storm your palace and take your power, then you pass laws making sure that this doesn’t happen; you are willing to forgo having the extra wives for peace in your kingdom.
That is the story for now, but as an aside there is yet another choice for the women who do not want to accept the offers of marriage from the poor guys, live with their parents or be in a household with multiple wives... they could choose to be a prostitute. It is not a coincidence that prostitution rates are higher in countries with polygynous marriage institutions. The excess of men without wives in those countries increases the demand for prostitution services and, importantly here, it drives up the price that a woman is paid to be a prostitute because she has an alternative choice, she could marry a wealthy man, albeit as a second or third wife. The two factors make prostitution more profitable, and more prevalent, in polygynous societies.
* Lagerlöf, Nils Peter (2010). “Pacifying Monogamy: The Mystery Revisited.” Journal of Economic Growth vol(15).