Negative externalities in marriage markets and polygamy

Democracy in America says that it’s time to think about legalizing polygamous marriages:


I’m an economist (Well, a grad student. But still.) I don’t claim to know the state’s rationale for anything, let alone marriage policy. My take is that policy tends to emanate from what the median voter wants, rather than from any kind of cost-benefit analysis. But if the question is about the costs and benefits of gay marriage and polygamy, then indeed that calculus is quite different: polygamous marriage differs sharply from homosexual marriage in that causes a damaging imbalance in marriage markets.

To fix concepts, let’s be clear that when people say “polygamy” they mean “polygyny”, or the pairing of one husband with multiple wives. Polygyny is far more common, both across human history and today. It is sustainable only in fast-growing populations, for reasons that are obvious if you think about them for a second: the sex ratio among US adults is 1.00, meaning there is almost exactly one man for every woman. That means everyone can get married if we all happen to be straight and monogamous (and have sufficiently malleable standards for mates). Suppose the population were a fixed size, and the number of men exceeded the number of women by just one lonely guy. He’d be lonely indeed: the marriage market would not clear – you can only marry one person – and he’d be desperate. He would bid the effective price of a male partner down, way down, in order to get married at any cost. This would leave women with all the bargaining power, and still leave one man unmarried.*

If the population is growing, and men marry down in age, then polygyny can work out just fine. There’s always a new crop of young women to marry and everyone can find a spouse, even if some men take four wives. But population growth is very low in most of the developed world, which probably has much to do with its low rates of polygyny. In the modern US, any appreciable number of polygynous marriages would leave huge numbers of men out in the cold.

This would have negative effects on men in marriages as well, as those excluded from marriage fight for partners: “Don’t want to marry me? What if I raise the kids and also hold down three jobs?” Sometimes these desperate attempts actually create polyandry: one of my favorite courses in college was taught by a professor who research showed that polygyny in pre-modern China led to multiple-husband marriages. The fact that fundamentalist Mormons “discard their surplus boys” is not some random horrible thing they do, unrelated to their marriage practices. It is an essential component of polygynous marital patterns. The men in control of fundamentalist LDS communities do it in order to keep the marriage market favorable for themselves.

This is fundamentally different from gay marriage. Homosexuals and bisexuals are a small share of the US population, and homosexual identity is roughly equally common among men and women. That means that legalizing gay marriage, inasmuch as it encourages people to leave the heterosexual marriage market**, will not lead to imbalance. Legalizing polygamous marriages will.

The state has a compelling policy interest in discouraging plural marriages, inasmuch as such marriages are overwhelmingly polygynous and not polyandrous. A classical liberal would argue that you have the right to do whatever you like as long as you do not hurt others – this is why economists think that markets should operate free of government interference, most of the time. But sometimes what you do in a market harms others. If you build a bar next to my house, the noise affects me: it imposes a negative externality. In principle, even if all the abuses of women and children that so often accompany American polygamy were erased, the very institution of polygamy damages society as a whole. By leaving the remainder of the marriage market imbalanced, it harms not just the men left out of marriages but every male.

*This is not hypothetical – it’s a real issue, that actually happens. Kerwin Charles documented it in some black communities in the US, where high rates of incarceration among men have left women at a severe disadvantage. As any aspiring medical resident can tell you, matching markets are brutal. Any imbalance in the two sides makes life very unpleasant.
**It also seems unlikely that homosexuals would marry the opposite sex in the absence of a ban on their marrying, whereas people forbidden to marry multiple people will likely still marry one.

2 thoughts on “Negative externalities in marriage markets and polygamy”


    You’re mostly right. If plural marriage comes to the U.S. (or any European country), it will likely allow any combination of partners, including same-sex marriages of n>2. However, the net effect will still be that legalizing plural marriage will result in more MFF(f…) marriages than MMF marriages, even if all existing partners in a marriage are required to give consent to the addition. (Neither legalizing polyandry nor requiring consent is a common feature in existing polygamous societies.)

    My observations among the Bay Area’s “polyamorous” community show that MMF actually seems to be more stable than MFF, but that’s drawing from a population of fairly socially liberal and sexually experimental people. And actual three-person committed relationships living in one household are fairly rare even in that community. So while what I see around me would probably balance out if plural marriage were legal, I doubt that would be the end result overall.

    Also worth looking at: not all gay people are exclusively homosexual. How many bisexual men and women would plural marriage bring back into the heterosexual marriage market? Would those numbers balance, or at all offset the woman shortage that polygyny would create?

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