The Economic Causes of Monogamy

The occasional 7-dwarf orgy notwithstanding (and you cannot convince me it never happened--I just know there was a night with a full moon and an opportunistic bottle of peach schnapps...), when most Western fairy tales end with "and they all lived happily ever after", they mean a prince and a princess. The ideal of one man and one woman united in marital bliss is so pervasive in the developed world that sometimes it takes an egghead (or a pervert) to question why.

That is exactly what three researchers (so eggheads it is) at Hebrew University have done. In a paper in this month's AER, Eric Gould, Omer Moav and Avi Simhon undertake to address the mystery of why the developed world is so uniformly monogamous, when the developing world (and much of human history) is polygynous.

The biological literature gave us the "threshold model" of polygyny--no doubt you're familiar with some or other description of it. It basically says that, since males (but not females) can increase their reproductive success by acquiring more mates, and since males have different resource endowments but all females are pretty much the same (a uterus on legs, essentially), it makes sense in the human context for wealthy men to procure multiple wives. He who has more to spend can buy access to more women (and hence produce more offspring), and those women are in turn better off sharing his wealth than partaking individually of a poor man's poverty.

Throughout much of human history, this has been the case. George Murdock recorded in his ethnographic atlas that 850 of the 1,170 societies he provided data on practiced polygyny. It is therefore something of a puzzle why modern industrialized societies are so monogamous. The observed correlation between development and monogamy has intrigued researchers across the social sciences. Many explanations have been floated, among them a decline in male inequality, a more balanced sex ratio as fewer men die in hard living, and increasing equality between the sexes. According to the authors, none of these seems sufficient. Laura Betzig is quoted in the paper:

That leaves me with my favorite question. When, and why did polygyny and despotism end, and monogamy and democracy begin? Some people have said the Roman Empire was monogamous. This evidence is not persuasive. Others have said monogamy began in the Middle Ages under the Catholic Church. But political, economic, and even reproductive inequality seem to have characterized medieval Europe too. It seems to me that one event changed all that: the switch to an industrial economy in Europe in the past few centuries.

Gould et al. seize this idea of an industrial explanation for the rise of monogamy in Western countries. They propose a model in which male inequality does indeed result in polygyny--as long as the inequality is in non-embodied wealth (land, for example). If nonlabor income comprises the majority of male earnings, then the important thing for him is to have many children (and to get them from many wives). If, on the other hand, labor income (and hence human capital, or being highly skilled) is important, AND (and this is key) if women are not identical in their ability to produce highly skilled children, then the equilibrium moves closer to monogamy as rising prices of high quality wives force men to choose quality over quantity. The key finding is therefore that "male inequality generates polygyny, but female inequality reduces it". What is more, the source of male inequality matters a great deal in determining the equilibrium level of polygyny.

That may have been an overly-hurried summary, so let me explain just the basics of their model. It's a pretty neat way of illustrating their hypothesis. In an economy there are two types of men: "skilled" and "unskilled". These men earn income from two sources: nonlabor (i.e. bequests from their fathers) and labor (which depends on their skill level). The women are also either skilled or unskilled, and they enter into marriage contracts with men (not necessarily monogamous) that provide them with a certain level of consumption and some offspring. A woman is able to have only two children (a boy, who will inherit, and a girl), and she wants to maximize the amount of human and physical capital left to the boy (for simplicity girls do not have any wealth).

Human capital is bequeathed to male children in the form of investment in their skill level. If parents decide to invest in their son, they pay a cost (of education, training, etc.), and he becomes "skilled". Crucially, the skill level of the parents determines how high this cost of investment is, and this is where the quality of a woman matters. The authors make a key assumption that skilled women are more efficient than unskilled women at producing skilled children. A skilled man and a skilled woman are together very efficient in producing skilled sons, and hence they pay a low cost. If only one parent is skilled (father or mother), the cost is higher. If both parents are unskilled, it is presumed impossible that they could ever produce a skilled child.

A man must choose (without exceeding his total income), consumption levels for himself and his wife/wives, the human capital level of each child (which requires paying the investment cost), and a bequest of physical capital to each boy, while making sure that his prospective wife/wives will accept the marriage contract (which could be different for skilled and unskilled women).

The authors choose two conditions for equilibrium, namely 1) that men maximize their utility subject to their constraints and 2) that the marriage market clears--all women marry.

The initial results should not be surprising--parents invest in educating their children if and only if the return exceeds the cost (so couples where both are skilled educate more often than couples where only one is skilled), and rich (skilled) men have at least as many wives as poor (unskilled) men.

To see how the source of male inequality affects the level of polygyny, the authors allow the return to skill (call it h) to vary. This is meant to capture what the authors envision as one of the most important differences between developed and developing countries: in developed countries human capital is paramount in determining a man's wealth (think of your average lawyer, i-banker, computer programmer), whereas in developing countries, the return on being high-skilled is much lower (farming, cattle-herding etc.). The results are as follows:

1) If the return to skill (h) is low, no one invests in skilled children, and rich (skilled) men have more wives than poor men.

When the value of human capital is sufficiently low, skilled men are not interested in producing quality children even with high-quality women, who can produce high-quality children at the lowest cost. Therefore, women in the marriage market are valued only for the quantity of children they can produce, which is assumed to be identical.

2) If h is within some intermediate range, then couples where both are skilled invest, couples where only one is skilled do not invest, and polygyny declines with h.

Skilled and unskilled women differ in the type of children they raise when h lies within this region, so the value of skilled women in the marriage market is not identical to unskilled women. Thus, skilled women are valued for the quality and not just the quantity of women they produce.

Figure 1A shows that some skilled men marry a certain number (>=1) of skilled women and have skilled children, while other skilled men marry a greater number of unskilled women and raise unskilled children. The reason the latter group members marry a larger number of women is because they are being compensated for the lower-quality children with higher quantity. This result is enabled by the lower cost of unskilled women in equilibrium--the "full price" (the consumption transfer plus bequest level and human capital investment) is lower for unskilled women because of the lower human capital of their children.

The key result (as you can see in the figure above) is that the composition of inequality is an important determinant of the rate of polygyny. As h increases (as male inequality is determined more and more by human capital rather than by physical capital, or in development terms, as a country industrializes), the average number of wives per skilled man decreases. It becomes more important for skilled men to have skilled wives to complement them in the production of skilled children, and as skilled women become more valuable, it becomes harder to afford many of them.

Another result (which any female grad student could tell you), is that assortative matching becomes more common in this intermediate range, as skilled men prefer to have children with skilled women (and vice versa).

This paper essentially takes the Becker and Lewis idea of fertility's being determined by trade-off between quality and quantity of children, and extends it to the choice of the children's mother. In short, if you're already choosing to have fewer, high-quality children, it's probably cheaper to have them with fewer, high-quality wives.

Now, the authors need to impose another restriction on the parameters to get absolute monogamy (one man and one woman only), but I'm not personally bothered all that much about getting to that result. I think it would be foolish to claim that western society is completely monogamous--we've got Francois Mitterand for concurrent polygyny, Donald Trump for sequential polygyny, and Elizabeth Taylor Hilton Wilding Todd Fisher Burton Burton Warner Fortensky for proving that girls can play that game too.

Snow White (provided she had a Master's degree) never had it so good.

It's also extremely important not to forget the role of cultural norms and values in determining how people feel about love and marriage. For my money, I'm less eager to dismiss the role of Christianity and its ideal of marital monogamy in setting the values of the Western world. But what this paper does is at least give us something to think about (however theoretical) in explaining the regularity of (relative) monogamy in the West. A number of international organizations, including the UN and the AU, have been promoting monogamy as a way to equalize the rights of husbands and wives. What this paper suggests is that exogenous enforcement of monogamy probably won't work (and it doesn't in much of West Africa, for example, where the illegality of polygyny is pretty much ignored). What is needed, as is the case with so many social ills, is economic development.