"And now, polygamy," sighs Charles Krauthammer, in a recent Washington Post column. It's true. As if they didn't already have enough on their minds, Americans are going to have to debate polygamy. And not a moment too soon.

For generations, taboo kept polygamy out of sight and out of mind in America. But the taboo is crumbling. An HBO television series called "Big Love," which benignly portrays a one-husband, three-wife family in Utah, set off the latest round of polygamy talk. Even so, a federal lawsuit (now on appeal), the American Civil Liberties Union's stand for polygamy rights, and the rising voices of pro-polygamy groups such as TruthBearer.org (an evangelical Christian group) and Principle Voices (which Newsweek describes as "a Utah-based group run by wives from polygamous marriages") were already making the subject hard to duck.

So far, libertarians and lifestyle liberals approach polygamy as an individual-choice issue, while cultural conservatives use it as a bloody shirt to wave in the gay-marriage debate. The broad public opposes polygamy but is unsure why. What hardly anyone is doing is thinking about polygamy as social policy.

If the coming debate changes that, it will have done everyone a favor. For reasons that have everything to do with its own social dynamics and nothing to do with gay marriage, polygamy is a profoundly hazardous policy.

To understand why, begin with two crucial words. The first is "marriage." Group love (sometimes called polyamory) is already legal, and some people freely practice it. Polygamy asserts not a right to love several others but a right to marry them all. Because a marriage license is a state grant, polygamy is a matter of public policy, not just of personal preference.

The second crucial word is "polygyny." Unlike gay marriage, polygamy has been a common form of marriage since at least biblical times, and probably long before. In his 1994 book The Moral Animal: The New Science of Evolutionary Psychology, Robert Wright notes that a "huge majority" of the human societies for which anthropologists have data have been polygamous. Virtually all of those have been polygynous: that is, one husband, multiple wives. Polyandry (one wife, many husbands) is vanishingly rare. The real-world practice of polygamy seems to flow from men's desire to marry all the women they can have children with.

Moreover, in America today the main constituents for polygamous marriage are Mormons* and, as Newsweek reports, "a growing number of evangelical Christian and Muslim polygamists." These religious groups practice polygyny, not polyandry. Thus, in light of current American politics as well as copious anthropological experience, any responsible planner must assume that if polygamy were legalized, polygynous marriages would outnumber polyandrous ones — probably vastly.

Here is something else to consider: As far as I've been able to determine, no polygamous society has ever been a true liberal democracy, in anything like the modern sense. As societies move away from hierarchy and toward equal opportunity, they leave polygamy behind. They monogamize as they modernize. That may be a coincidence, but it seems more likely to be a logical outgrowth of the arithmetic of polygamy.

Other things being equal (and, to a good first approximation, they are), when one man marries two women, some other man marries no woman. When one man marries three women, two other men don't marry. When one man marries four women, three other men don't marry. Monogamy gives everyone a shot at marriage. Polygyny, by contrast, is a zero-sum game that skews the marriage market so that some men marry at the expense of others.

For the individuals affected, losing the opportunity to marry is a grave, even devastating, deprivation. (Just ask a gay American.) But the effects are still worse at the social level. Sexual imbalance in the marriage market has no good social consequences and many grim ones.

Two political scientists, Valerie M. Hudson and Andrea M. den Boer, ponder those consequences in their 2004 book Bare Branches: Security Implications of Asia's Surplus Male Population. Summarizing their findings in a Washington Post article, they write: "Scarcity of women leads to a situation in which men with advantages — money, skills, education — will marry, but men without such advantages — poor, unskilled, illiterate — will not. A permanent subclass of bare branches [unmarriageable men] from the lowest socioeconomic classes is created. In China and India, for example, by the year 2020 bare branches will make up 12 to 15 percent of the young adult male population."

The problem in China and India is sex-selective abortion (and sometimes infanticide), not polygamy; where the marriage market is concerned, however, the two are functional equivalents. In their book, Hudson and den Boer note that "bare branches are more likely than other males to turn to vice and violence." To get ahead, they "may turn to appropriation of resources, using force if necessary." Such men are ripe for recruitment by gangs, and in groups they "exhibit even more exaggerated risky and violent behavior." The result is "a significant increase in societal, and possibly intersocietal, violence."

Crime rates, according to the authors, tend to be higher in polygynous societies. Worse, "high-sex-ratio societies are governable only by authoritarian regimes capable of suppressing violence at home and exporting it abroad through colonization or war." In medieval Portugal, "the regime would send bare branches on foreign adventures of conquest and colonization." (An equivalent today may be jihad.) In 19th-century China, where as many as 25 percent of men were unable to marry, "these young men became natural recruits for bandit gangs and local militia," which nearly toppled the government. In what is now Taiwan, unattached males fomented regular revolts and became "entrepreneurs of violence."

Hudson and den Boer suggest that societies become inherently unstable when sex ratios reach something like 120 males to 100 females: in other words, when one-sixth of men are surplus goods on the marriage market. The United States as a whole would reach that ratio if, for example, 5 percent of men took two wives, 3 percent took three wives, and 2 percent took four wives — numbers that are quite imaginable, if polygamy were legal for a while. In particular communities — inner cities, for example — polygamy could take a toll much more quickly. Even a handful of "Solomons" (high-status men taking multiple wives) could create brigades of new recruits for street gangs and drug lords, the last thing those communities need.

Such problems are not merely theoretical. In northern Arizona, a polygamous Mormon sect has managed its surplus males by dumping them on the street —

literally. The sect, reports The Arizona Republic, "has orphaned more than 400 teenagers … in order to leave young women for marriage to the older men." The paper goes on to say that the boys "are dropped off in neighboring towns, facing hunger, homelessness, and homesickness, and most cripplingly, a belief in a future of suffering and darkness."

True, in modern America some polygynous marriages would probably be offset by group marriages or chain marriages involving multiple husbands, but there is no way to know how large such an offset might be. And remember: Every unbalanced polygynous marriage, other things being equal, leaves some man bereft of the opportunity to marry, which is no small cost to that man.

The social dynamics of zero-sum marriage are ugly. In a polygamous world, boys could no longer grow up taking marriage for granted. Many would instead see marriage as a trophy in a sometimes brutal competition for wives. Losers would understandably burn with resentment, and most young men, even those who eventually won, would fear losing. Although much has been said about polygamy's inegalitarian implications for women who share a husband, the greater victims of inequality would be men who never become husbands.

By this point it should be obvious that polygamy is, structurally and socially, the opposite of same-sex marriage, not its equivalent. Same-sex marriage stabilizes individuals, couples, communities, and society by extending marriage to many who now lack it. Polygamy destabilizes individuals, couples, communities, and society by withdrawing marriage from many who now have it.

As the public focuses on a subject it has not confronted for generations, the hazards of polygamy are likely to sink in. In time, debating polygamy will remind us why our ancestors were right to abolish it. The question is whether the debate will reach its stride soon enough to prevent polygamy

from winning a lazy acquiescence that it in no way deserves.

*Author's note: My wording left some readers under the impression that the modern Mormon church may endorse or practice polygamy. It does not. I should have made clearer that I was referring to certain people who claim to be Mormons, not to the church or mainstream practice.

© Copyright 2006 National Journal

Jonathan Rauch is a senior writer and columnist for National Journal and a frequent contributor to Reason. The article was originally published by National Journal.