The evolution of marriage over history

Marriage has changed a lot over the history of humankind. The earliest societies tended to be polygamous, then there was a switch to monogamy. We did not stop there, however. Currently, most societies practice serial monogamy. That is, monogamy with divorce and remarriage.

How can these transitions between marriage regimes be explained? Why did humanity start out as polygamous, and what prompted us to switch to monogamy, and then later on to serial monogamy? Answering such questions elegantly requires a unified theory of marriage, which is what this post is about.

Such a theory of marriage was developed by de la Croix and Mariani (2014) (more up-to-date version) in an amazing paper published in the Review of Economic Studies.

In an earlier post I talked about various theories as to why monogamy is more widespread today than polygamy. Indeed all those theories imply that sometime over the course of history most human societies switched to monogamy. But there is no single theoretical framework that encompasses that switch and the distinction between monogamy and serial monogamy at the same time. This is where de la Croix and Mariani’s paper comes in.

Let us start with a little history. It is largely accepted that most ancient societies were polygamous. This does not necessarily mean a widely practiced polygamy, but more like a tolerated one. In fact, it was only the rich who could generally (afford to) be polygamous. And more precisely, polygamy per se  (which can involve mutliple wives and/or husbands) was not the most widespread institution, instead it was polygyny (one male having multiple wives), which is a subset of polygamy.

After some time, however, strict monogamy was introduced and polygyny banned. This largely coincides with the spread of Christianity, at least in Europe. Finally, serial monogamy appeared with the rise of divorce laws. This is obviously a more modern phenomenon.

Below is a great figure from the paper (click to enlarge), which summarizes the history of marriage institutions. The left column shows important dates regarding the transition from polygamy (P) to monogamy (M). The horizontal line is when the authors say the switch – roughly speaking – happened. The right column contains similar information for the switch from monogamy to serial monogamy (S).

History of marriage institutions, EuropeNow let’s discuss the authors’ theory. The idea is as follows. Society comprises of four groups: rich females, rich males, poor females, poor males. Each of these groups have a preference ordering of the three possible marriage regimes: polygamy (P), monogamy (M) and serial monogamy (S). Their preference ordering may depend on the state of the economy (such as the fraction of people who are rich, etc.). In other words, the preferences of a group may change over time as the economy changes.

These group-level preferences are then aggregated into a society-level preference, as if people were voting in an election on which regime to implement. Of course, this is not to be interpreted as a literal vote. Clearly, nobody but rich males had actual political power for most of human history. But still, preferences of other groups were taken into account (albeit possibly with a lower weight) in order to prevent dissatisfaction with the ruling elites for one.

First, we’ll look at the preferences by group. Throughout this discussion, please refer to the figure below. To interpret it, note that the horizontal axis (mu) refers to the fraction of males that are rich; the vertical axis (phi) to the fraction of females that are rich. We’re only concerned with the part below the 45-degree line, because it is assumed throughout the analysis that the fraction of rich males is larger than the fraction of rich females (mu > phi). This also means that we restrict our attention to polygyny (one husband, multiple wives).

Preferences over marriage, by groupRich females, as can be seen in the leftmost panel of the figure above, always prefer serial monogamy. This result requires that the probability of a marriage going bad be high enough. Furthermore, their least preferred alternative (weakly so) is polygamy. This is because in the model, it is assumed that polygamy is always polygyny. And that the wives are so to speak somewhat jealous of each other, which has a negative effect on their utility. Rich females are indifferent between monogamy and polygamy when the fraction of rich males is high enough though, as can be seen above. But at this point, polygamy will never be practiced (as it isn’t even the preferred alternative of rich males).

Rich males always prefer polygamy to monogamy. This is because the former institution allows them to take advantage of their wealth on the marriage market more. Serial monogamy, however, is preferred to polygamy if the fraction of males that are rich is high. This is because, in this case, there are just simply too many rich males, so polygamy carries with it a higher risk of staying single. Serial monogamy can in this case be a better alternative, as it allows one to remarry if a marriage turns bad.

Poor females prefer polygamy to monogamy as long as there aren’t too many rich males. In this case, poor females clearly have a higher chance of marrying a rich man with polygamy. And they prefer being the second wife of a rich man to being the first of a poor. As the number of rich men increases, poor females prefer monogamy. The reason is that now they will have a decent chance of being the only wife of a rich man. But given that the number of rich men is not too high yet, in case they were to divorce, they would have too low of a chance to remarry rich as well. So they don’t like serial monogamy yet. They only prefer serial monogamy if the number of rich men increases even further.

Finally, poor males always prefer monogamy. They don’t like polygamy, because – for poor males – it carries with it a risk of being single. This is because rich males will marry polygamously first, and thus with equal sex ratios some poor men will bound to remain single. As for their second preferred alternative, this group prefers polygamy to serial monogamy only if the number of rich men is low enough so that the chance of staying single with polygamy is low.

Now let us look at how these group preferences are aggregated to create what the authors call a political equilibrium. Assume that there is majority voting, but that different groups can have different voting weights.

If we keep the fraction of rich males/females exogenous we can already see that the model can reproduce what we saw in history. With an initially low number of rich males, polygamy will be the prevailing institution. As the number of rich males increases, monogamy will prevail because of the preferences of poor females. Finally, as the number of rich people (both males and females) grows even further, serial monogamy becomes the constitution.

The authors go further though, and introduce some dynamics into the model. Suppose the share of rich men and women evolves endogenously (i.e. within the framework of the model). The probability of a child becoming rich depends on parental resources and wealth.

In particular, children of polygamous households have a lower chance of being rich (as consistent with empirical evidence). Furthermore, divorce also has a cost which drains the family’s resources and thus reduces the probability of becoming rich.

The dynamics of this model are summarized in the figure below. The triangle is colored according to the prevailing aggregated (society-level) preferences: light gray is the region of polygamy, gray is monogamy and dark gray is serial monogamy.

Evolution of marriage institutions over time

In the left panel, the arrows indicate where society is moving from a given point on the graph. We can see that in polygamy (light gray), the number of rich males is increasing (we’re moving right). This is because rich males have relatively more children (they’re polygamous after all) than poor males. Then as the number of rich males is increasing, we’re bound to enter the region of monogamy. In that case, both the number of rich males and females is increasing. This is because monogamy increases female social mobility because as opposed to polygamy, resources don’t have to be divided amongst so many children. So daughters have a higher chance of becoming rich now.

Since both males and females are getting richer, we’re going northeast on the graph. Then we hit the region of serial monogamy. This region slows social mobility down (because divorce costs drain family resources). And thus we converge to a steady state.

A simulated version of these dynamics can be seen in the right panel of the above figure. We start off from the southwest corner of the plot, and as the black dots show the dynamics take us through monogamy country all the way to our destination in the serial monogamy steady state.

There are various other things considered in the paper (which is well worth a read). For instance, the authors look at progressive enfranchisement: what if poor males and females are initially powerless, and then they gradually gain political power. They also look at what happens if they endogenize the transfer of resources within the household in a Beckerian way. In general, the dynamics presented above are relatively robust. They can arise in a variety of situations.

Thus this paper tells us that polygamy could have initially been the prevailing institution because there wasn’t much opposition to it from poor males and females when the share of rich males was low. Then as there were more and more rich males (for instance with the Urban Revolution in Western Europe), certain groups started preferring monogamy (e.g. poor females) and some institutions (e.g. Christianity) catered to these preferences. Even later (around the late 19th century), with even more rich males and females, several groups started preferring serial monogamy, bringing about a transition to that regime.

One potential concern that is not really addressed in the paper is whether this theory is universal enough to apply outside of Europe and the Western world in general. The evolution of marriage institutions presented above and explained by the paper may be very Europe-specific. Unfortunately, I don’t have the necessary historical knowledge to tell if this is the case. But if other regions showed similar patterns in marriage, that would be very encouraging for this theory. Even if not, it is still a great framework that explains at the very least the development of marriage institutions in the Western world.

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