[The following is a guest post by Lauren K. Hall, Assistant Professor of Political Science at Rochester Institute of Technology.]

Christopher Freiman thinks it might be time to legalize polygamy now that same-sex marriage is a legally protected right nation-wide. I’m not so sure.

Freiman’s main contention is that “all of the standard objections to polygamous marriage can apply to heterosexual monogamous marriage as well.” This is true in a sense, but only if your objections are so broad as to be almost meaningless.

In reality, heterosexual or homosexual marriage is facultatively different from polygamy. The most obvious difference is that heterosexual marriage (to focus on his example) is a form of monogamy. Apart from the practical concerns of having to reformat the entire tax code, benefits structure, and the problems plural divorce and child custody would create for family courts, there are other more foundational reasons to be suspicious of extending marriage rights to polygamous families.

Polygamy is just different from monogamy. (When I use the term “polygamy” I really mean polygyny, or a man having multiple wives, since polyandry has been rare in the human experience.) So what does men having multiple wives do?   A few important things. The most dangerous is that it skews sex-ratios of available mates. Polygamy creates a dearth of fertile women and a surplus of unmarried men, since powerful men pull fertile women out of the marriage pool. The sex-ratio problem is particularly problematic in insular or closed societies where a dearth of women creates downward pressure on the age of marriageable women, encouraging child-marriage. The surplus of unmarried men is dangerous since such men must resort to ever more competitive and aggressive behaviors to secure mates, including in some cases, rape and abduction. Additionally, polygamy fractures paternal investment in offspring and leads to increased investment in sons at the expense of daughters.

Because marriageable females become scarce, polygamy concentrates reproductive power in the hands of powerful or wealthy men who can afford to buy or coerce women into relationships and control unmarried males. The scarcity of women may also lead to an increase in sex-trafficking as women are brought in from other communities to meet demand. The economic language is intentional here, since polygamy tends to support the treatment of women as commodities, to be bought, sold, and controlled by male relatives.

The reality is that polygamy, unlike monogamy of any stripe, is associated with externalities that are difficult for liberal societies to cope with. While it is difficult to untangle the various causal mechanisms at play, polygamy is associated with everything from increased maternal and infant mortality to increased incidences of child-marriage to decreased political power and educational levels for women. These patterns are seen not only in polygamous societies in developing countries, but also in polygamous communities in the U.S. and, interestingly, in polygamous primate species. Across the board, polygamy is associated with lowered health, education, and status outcomes for women. Monogamy, conversely, has no consistently negative social externalities of this kind.

There are, of course, alternatives to outright bans on polygamous unions. Availability of education for women and the existence of a strong middle class tend to be negatively correlated with polygamy. Unfortunately for the libertarian argument, however, the research on polygamy demonstrates that, while education of women and liberal attitudes probably help prevent the spread of polygamy, the most effective tool is banning it, either through the passive failure to recognize such marriages or through the more active pursuit of polygamous families in criminal trials. I don’t condone the latter, partly because it seems unnecessarily punitive and may drive such behavior underground, exacerbating abuses. But just because the active governmental role has issues does not mean the passive refusal to recognize such marriages is similarly problematic. In fact, it seems bizarre from a libertarian perspective to make the argument that once we allow some forms of government sanctioning of marriage that we must allow government to sanction all marriages, however deleterious their effects.

There’s another discussion to be had about serial monogamy and the effects of de facto polygamy on children, but leaving that aside, at the very least we should resist the idea that every family form is just as good as any other. We should also resist the contention that equal rights claims demand that we recognize any and all marriage forms just because. Monogamy is one of the bourgeois virtues that make liberal societies possible and because liberal societies require certain types of people, there are some decisions that individuals make that we do not have to either recognize or condone. If we care about voluntary cooperation and protection of individual rights, family forms that destroy both of those goods should be scrutinized and judged. Libertarianism does not entail relativism.