Stephen Macedo is Laurance S. Rockefeller Professor of Politics and the University Center for Human Values at Princeton University and the author of, Just Married: Same-Sex Couples, Monogamy, and the Future of Marriage (Princeton University Press, 2015)

MANY progressives are signing on to a central plank in Chief Justice John Roberts’s dissent in the recent same-sex marriage case. As W.W. put it in a recent post here, now that same-sex marriage has “blasted open” the “libertarian door,” liberals need “more rationally compelling arguments” to shut polygamists out. Indeed, the author asserts that accounts of polygamy’s social harms, including mine, have “the same speculative character as some conservative arguments against legal gay marriage.” And if polygamy in practice has always been patriarchal, the same could have been said of monogamous marriage until 40 (or 50?) years ago. If one can be reformed why not the other?

Others charge that mere “political pragmatism” led many same-sex marriage proponents to distance themselves from polygamy. In so doing, says Fredrik DeBoer, these erstwhile liberals ignore the principle that “consent is the measure of all things in sexual and romantic practice.” We will now reap the “long term cost” of the “incredibly flimsy” arguments against polygamy advanced on behalf of liberal monogamy, including same-sex marriage.

Yet W.W., Mr DeBoer, and other libertarians seem unaware of the extensive evidence of polygamy’s social harms, as well as the principled reasons for distinguishing plural from same-sex marriage. They are also too quick to conflate social tolerance, partial recognition and equal recognition of plural marriages. Finally, consent is not the only basis for drawing legal distinctions when it comes to “all things” marital. Let’s take these up briefly in turn.

If same-sex marriage is legal, why not polygamy? I explore the evidence of polygamy’s harms in a recently published book, drawing on many sources that include the well-informed 100,000-word opinion penned by Chief Justice Robert Bauman of the Supreme Court of British Columbia in its landmark case on polygamy. Assisted by eminent scholars and extensive first-hand testimony, Justice Bauman assembled “the most comprehensive judicial record on the subject ever produced.” That record demonstrates “a very strong basis for a reasoned apprehension of harm…inherent in the practice of polygamy”, harm “to women, to children, to society and to the institution of monogamous marriage.” The vast majority (85%) of the polygamous societies studied by anthropologists reveal the arrangement to be a marker of privilege and high status. In that form—as polygyny, one husband with multiple wives—plural marriage still prevails across much of the world. Polyandry, or a marriage involving multiple husbands, is rare and fragile and seems to exist mainly under highly unfavourable circumstances (as when brothers marry the same woman in order to keep intact a farm that can only support one family). Polygyny increases competition among men and the pool of unmarried males, which contributes to greater violence, risk taking, and substance abuse in society (if the top 10% of males take an average of three wives, 20% of men will be unable to marry). Complex plural families—composed of multiple wives and children related as half-siblings—are prone to jealousy and conflict. Even accounts sympathetic to polygamy identify jealousy as a big problem, and diminished genetic relatedness is a risk for sexual abuse. Polygamous families are characterised by much higher levels of violence and stress in the home, as well as worse health outcomes for women and children. Monogamy, in contrast, gives better-off men an incentive to invest their surplus resources in their children rather than in acquiring additional wives. Indeed, studies of Mormon polygamy in the 19th century found that the children of poorer men tended to enjoy better health and longevity because their fathers couldn’t afford to have multiple wives. Moreover, the “cruel arithmetic” of polygyny requires that many younger and lower-class males, potential competitors for younger wives, will be expelled from polygamous communities by older, higher-status males.

Because monogamy increases the surplus resources available to be invested in children and reduces conflict and competition, it enables greater social progress. The origins of monogamy are unknown, but it became the rule in ancient Greece and Rome and spread through Rome’s influence. The transition to institutionalised monogamy thus appears to have contributed to more egalitarian social relations, greater social progress and a fairer distribution of the opportunity to marry and create a family.

In addition to the large body of empirical evidence—the surface of which I have barely scratched here—there are principled reasons that distinguish polygamy and same-sex marriage.

First, common sense and evidence suggest that plural marriage is “an advantageous strategy available to those with the inclination and resources to pursue it,” in the words of Justice Bauman. Whereas most people experience sexual orientation as a given, any of us might form the preference for bringing a third person into our sexual relationships (and men seem most prone to this). In addition, equality in marriage requires reciprocity, as two people come to depend on one another and must negotiate ways of conducting wide segments of their lives together. Bringing in a third person—making the dyad a triad—compounds the complexity of all these negotiations and is a recipe for conflict.

A third consideration in favour of one spouse per person is distributive fairness. Most men and women strive mightily to find a suitable marriage partner. We should design our social and political institutions, here as elsewhere, to secure the conditions within which this opportunity is available to all on fair terms. Monogamy serves this important public purpose, while polygamy is associated in practice with status and gender hierarchies.

Finally, it was long argued that same-sex marriage would degrade marriage for all but no one ever explained how. Plural marriage, in contrast, changes the character of marriage deeply in a way that degrades its core meaning. Millions have entered into marital commitments with the assurance that marriage is monogamous: the law of marriage confirms it. Monogamous marital commitment therefore provides a basis for the mutual trust that allows two people to build a life in common. The option of legally recognised polygamy changes the public meaning of marriage for all by furnishing options for married couples that many—I’d wager most—would find unwelcome. Plural civil marriage would thus undermine assurances central to marriage as we know it, assurances that the vast majority find deeply valuable.

21st-century mating: Luv u l8r The libertarians are unlikely to be moved by any of this because, as W.W. says, liberal monogamists cannot show that the “patriarchal, inegalitarian aspects of historical plural marriage” are “incapable of being fixed”. True: you cannot prove a negative. But why is it not equally likely that a significant minority of well-off men could find plural marriage an exciting option (Newt Gingrich’s second wife has reported that he proposed an open marriage before divorcing her). Polygamy is already making a comeback in China in spite of the fact that it remains against the law, and in other places legal restrictions are being relaxed. W.W. and others observe that polygamous marriage will be deeply unattractive to all but a small minority of women. If polygamy remains a fringe phenomenon, as it is now in America, then there is nothing to worry about. This is one reason why we can safely tolerate polygamy, but I do not think we should go further and extend equal recognition. The law of monogamy reflects and reinforces sensible judgments about the patterns of life that are generally happy and successful. The real question is: why would anyone argue for the equal legal recognition of plural marriage given that it has existed for centuries but never taken the hoped-for egalitarian social form? Mr DeBoer asserts that, “Polyamory”, the name given to egalitarian adult romantic networks, “is a fact. People are living in group relationships today.” Yet he cites no studies of the prevalence of this arrangement—or its effects—because there are none. The evidence of polyamory is entirely a matter of anecdote, speculation and free-love advocacy. In no Western society is there any broad social movement for polygamous or “polyamorous rights” equivalent to the decades-long struggle for the rights of gay, lesbian and transgendered citizens. Responsible liberal societies ought to reform complex social institutions such as marriage on the basis of reliable evidence and widely available social knowledge. Let me be clear: I am not arguing that polygamy is “inherently immoral”, as some have asserted in the past with respect to both plural and same-sex relations. I also allow that in particular instances polygamous families can be successful and healthy. But the general human experience with polygamy–past and present–suggests to me that there are weighty and legitimate grounds for concern here that allow us to distinguish plural and same-sex marriage.

So I agree with the libertarians that consensual plural cohabitation should not be prosecuted. Tolerance has been the de facto policy across America for decades, though we may need clarification on that score. A clear policy of toleration would be sufficient to allow respectable polygamists now living in the shadows to emerge. No doubt we will continue to learn about this “experiment in living”. But equal legal recognition of polygamous marriage requires positive evidence that this is a general social form that serves the interests of adults, children and society generally in a way that is consistent with our basic constitutional commitments to equal liberty and fair opportunity. Whatever the libertarians might say, the public morality of a liberal society not only allows but requires reasonable efforts to preserve important social institutions that are conducive to equal freedom and fair opportunity for all.

The libertarians are right about one thing: no one knows how these matters will look 100 years from now. But why should we expect future generations to look favourably on radical social experiments conducted in the absence of supportive evidence? And why would we credit their judgment if they did? History’s arc, as best we can now tell, bends away from the plural marriage and toward monogamy, gender equality, and same-sex marriage.