Have you ever wondered why the world is largely monogamous today? This is an especially interesting question if you consider that historically monogamy did not always enjoy such superiority, not even in Western societies. What happened?
While at first it may sound strange, as with everything else incentives play a great role in who marries whom. Thus marriage has been extensively studied by economists as well as sociologists. In this article, I briefly present the three main theories on why humanity has made this transition from polygamy to monogamy. This will include answering: why was polygamy more common in the past, and what made it disappear.
The first theory concerns the quality-quantity tradeoff of children (see for instance Gould, Moav and Simhon, 2005). This is a relatively well-known theory, which has many implications (for instance for economic development), but it may have also played a role in forming marriages. The idea is that prior to the Industrial Revolution, people favored having many children (quantity) because they needed their workforce on the fields, and because mortality was high. During and after the Industrial Revolution, however, people gradually preferred having fewer children and would rather invest in their education (quality). This is because during these times, returns to education rose significantly, thus the value of education increased. The reason for not being able to raise many educated children was simply the fact that education was relatively expensive. Therefore, birth rates dropped. A decrease in the mortality rate (due to advances in medicine) probably reinforced this trend as well.
So how is this all connected to marriage? Just consider the fact that an important advantage of polygamy over monogamy is that having multiple wives can dramatically increase the number of offspring one has. Thus clearly, one can expect that in a society where the quantity of children is important, polygamy can be advantageous. But once quality becomes more important, monogamy prevails. This can simply be driven by the fact that people wanted fewer children. But it could also be influenced by the fact that men were seeking more educated and/or richer women because they were in a better position to raise a “high-quality” child. But these women may have been more reluctant than their uneducated and/or poor counterparts to be in a polygamous relationship.
The second theory is a little bit more technical, but also interesting (see Lagerlof, 2005). For simplicity, consider a society with rich and poor men, and women who are all identical. Suppose that wives have a “price”, that the husband needs to “pay” if he wants a woman to marry him. Now, this may not be a monetary price (although in traditional societies, grooms often paid money/gave gifts to the bride’s family), it can also be a representation of what the man can give the woman in a marriage (resources, etc.). One can reasonably assume that rich men had an easier time attracting wives because they could more easily pay that price.
Define the total benefit from marriage as how well both the husband and the wife feel in that marriage. Because of the larger abundance of resources, it is quite likely that the total benefit from marriage was higher in a rich man’s than in a poor man’s marriage. But the “price” of women was identical for all women (because women are identical). Now, define the total marriage surplus as the total benefit from marriage minus the price of the wife. Clearly, the total marriage surplus is then larger for rich men and their wives (because the benefit is larger) than for poor men and their wives. Suppose now that the rich are much richer than the poor (like in most of pre-Industrial Revolution history). Then the rich person’s surplus from getting a second a wife (while lower than his surplus from getting a first wife) can be still larger than the poor man’s surplus from getting a first wife.
Thus rich men will “pay” more for their second wives than poor men can “pay” for their first wives. The result: rich men will be polygamous, poor men will be monogamous or single. As history progressed, inequality between rich and poor has decreased substantially, perhaps moving up the poor man’s surplus from a first wife above the rich man’s surplus from a second wife. Therefore, women would rather marry a poor man in a monogamous relationship than a rich man in a polygamous relationship.
An interesting, sort of weird theory which may sound preposterous to some, but I believe in a historical context it makes some sense. For instance, consider a society say in the Early Middle Ages. Most people are poor, but there are some rich people. Women, whose main goal in life (whether voluntarily or involuntarily) is to bear children, will want to get married to accomplish their goal. Then they have a choice: either be the second wife of a rich man who can give them all the amenities they desire, or be the first wife of a poor man who may have a hard time providing for the family. On average, it is quite plausible that more women will choose the former than the latter.
Now as time passed, two things changed: (1) poor men became relatively richer, which enabled them to easily provide for the basic needs of their family; (2) women became more empowered whose primary goal in life need not be getting married and having children anymore. But without these two developments, polygamy seems like a reasonable choice especially for poor women.
A third theory is also concerned with the poor-rich divide (see Lagerlof, 2010). This theory is largely based on a paper by Acemoglu and Robinson (2000) in which the authors argue that Western societies extended voting rights/democracy, and reduced inequality to avoid a possible revolution by the underclasses. Again, consider our society with rich and poor men with large inequalities. As established above, this means that the rich can get two or more wives, while the poor can usually get at most one or none. With roughly even sex ratios, it’s then easy to see that this would imply lots of single poor men. Even married poor men would see most of their benefits from marriage disappear because of their bride’s price. This would make the poor quite unhappy, which could lead to a revolt. In this view, prohibiting polygamy is a strategic decision by the political elite to maintain order.
Perhaps the best example of this theory could be the Roman Catholic church. Although there is no conclusive evidence whether the church was thinking strategically, or religiously when they decided that polygamy is a no-go, one can reasonably think that a religion, which was about to become a mass religion upon which whole empires were built, made such a decision to prevent potential uprisings among the poor population. An alternative, but equally strategic, motive could have been that the rising Christian church wanted to distinguish itself from other existing religions, which at the time did not forbid polygamy.
To sum up, the three theories together imply that polygamy was crushed by increasing returns to education, decreasing inequality between rich and poor, and by strategic decisions of the ruling elite.