In the 19th century, Mormon polygamy shocked the American public, and Utah was only granted statehood on the condition that it prohibit it forever. Today, 80 percent of Americans still think it’s wrong according to a 2020 Gallup poll, but that number is trending downwards. In Canada, a 2018 Ipsos poll found that 36 percent of respondents approved of decriminalizing polygamy. Polyamory and polygamy are both gaining mainstream media respect, and are often treated as equivalent, but they aren’t. Legalization of polygamy has long been a cause aided by reality TV and TV dramas. A blog post at the Daily Kos in 2013:

Rarely does a reality TV show open your eyes to your own prejudices. Sister Wives is that rare exception. Kody Brown and his four wives are fundamental[ist] Mormons living a polygamous lifestyle. Over the past year I’ve watched the Browns marry wife #4, raise their kids, deal with police investigations, and move to Las Vegas. They’ve really changed how I thought about polygamy.

Court cases, including one brought by Kody Brown and his wives, have sought legalization. In 2020, Utah passed a law that decriminalized polygamy, but few voices have been heard wondering if Utah is meeting its obligation to “prohibit” it.

Legalization gets support not only from polygamous religious sects, but also from some scholars, lawyers, philosophers, and cultural critics representing a wide span of political, moral, and religious attitudes. Pop science lends credence to these views by describing monogamy as a bad fit for biological human nature and a potent source of personal misery. Supporters point out that men can have scores of babies by scores of women and walk away, while women can have only a small number of babies—as many with one man as with 20—and must feed and care for them.

Despite the keen interest of the commentariat, public debates about polygamy tend to be disturbingly fact-free, and driven by terminological and other confusions. “Polygamy” refers to any system of plural marriage, polyandrous or polygynous, but polyandry is so rare that for practical purposes polygamy is polygyny. For zoologists, “polygyny” means only that males of a given species mate with more than one female. But when we discuss norms, laws, and social practices, we’re talking about social polygyny. Social polygyny is a set of norms that permit men to be married to many wives at the same time, but restrict women to one husband at a time, as in fundamentalist Mormon practice.

Zoologically speaking, a species would be polygynous even if males stuck exclusively to one mate, and re-mated only upon the mate’s death. But socially speaking, this would amount to a strict form of serial monogamy, and it would have vastly different social effects from polygyny. Polygamy and polyamory get lumped together in argument, but polygamy is a form of marriage and polyamory is primarily a sexual ethic. Some polyamorists now call for legalizing polyamorous marriage. But polyamorous marriage, too, presents a different set of social and legal questions from polygyny—unless, of course, the form of polyamorous marriage sought was, or inevitably would become, a de facto form of polygyny.

In their monograph The Myth of Monogamy: Fidelity and Infidelity in Animals and People, David P. Barash and Judith Eve Lipton argue that polygyny is practiced in 85 percent of human societies, and that it is therefore the biologically “natural” way that human beings mate. Monogamy, on the other hand, is miserable and hard and has high rates of infidelity and divorce. But the most common type of human mating relationship is serial monogamy, and the way most human beings have ever lived can’t really be unnatural. In any case, what is or is not natural does not help us decide how we ought to live.

Others claim that Christianity’s historical role in rooting out polygamy indicates that monogamy is a remnant of oppressive, outdated Christian sexual morality, like Christian doctrine on divorce and homosexuality. But church opposition to polygamy wasn’t based on sexual morality. Nor would the church’s approval necessarily show that monogamy is a bad idea.

The total US polygynous population is estimated to be as high as 150,000, but hard data is in short supply. The majority are probably fundamentalist Mormons, with 20,000 to 60,000 members. But Muslim polygynists are reckoned at anywhere from 20,000 to 100,000 or more. The two largest of the many Mormon fundamentalist sects are the Fundamentalist Church of Jesus Christ of Latter-Day Saints (FLDS) and the less extreme Apostolic United Brethren (AUB), to which Kody Brown belongs. Serious abuses by the Texas FLDS group headed by Warren Jeffs led to arrests and convictions that made the news 14 years ago. Mormon fundamentalist enclaves are found in Canada, Mexico, Utah, Arizona, Texas, South Dakota, North Dakota, Minnesota, Colorado, Wyoming, Nevada, and Missouri.

Arguments offered in support of polygyny

Most of us believe that people should be free to live as they please, so long as they’re not hurting anyone else. This conviction is used to support arguments in favor of polygamy like these:

• Polygyny saves many women from being mateless and childless.

• Many would prefer to be the plural wife of a fine man rather than the single mate of an ordinary or inferior one.

• Many would welcome co-wives to share childcare and household chores and to offer supportive affection and companionship.

• Polygyny creates a larger pool of available men from which monogamous women may choose, enabling them to get better mates.

• Polygyny would reduce divorce, marital infidelity, and the number of fatherless children. A polygynous man seeking sexual variety needn’t divorce one wife to add a new one and needn’t seek extramarital affairs which might produce children he abandons.

• The men left mateless will be those least desirable as mates and fathers—poorer, weaker, badly behaved, and less educated. They won’t be too numerous because polygyny rates are generally low.

• If polygyny causes harms, mitigation of those harms, not prohibition, should be the response.

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Overall, defenses of polygamy tend to be heavy on doctrinal and constitutional argument, and light on—or dismissive of—facts. However, these arguments must be considered alongside an immense body of empirical evidence. Samples of that evidence can be found here, here, and here, but I will summarize the salient conclusions about polygyny’s effects.

Effects on men

• Polygyny, even at low-sounding percentage rates, creates a shortage of unmarried women and an excess of unmarried men. In a simple society of 100 people, suppose only three out of 50 men (or six percent) are polygynous. Suppose two of them have three wives each and the third, a very rich man, has 10. That means those three men marry 32 percent of the women, leaving just 34 for the remaining 47 men. Over a quarter of the men will have no wives. Even these modest levels of polygyny are sometimes exceeded in real life. In Burkina Faso and Nigeria, 36 percent and 28 percent of households, respectively, are polygynous; in Togo 54 percent of married women are in polygynous marriages. Some United States polygynous communities have had very high rates.

• Polygyny creates intense male competition for mates. In polygynous societies, all married men are still in the marriage market because they can always marry more wives. In a monogamous society, once a man marries, he exits the competition.

• In polygynous societies, competition among men for mates motivates male control over their sisters, daughters, and wives. Sisters and daughters can be traded for a new wife or for money, goods, labor, favors, and political or military support from other men. To consolidate these deals, they need the power to force the girls and women to marry. Polygynous husbands may seek control over their wives so as to compel their labor, and also because their wives’ sexual opportunities with them are limited compared to those of monogamous women’s. Young wives, meanwhile, may prefer young men to old husbands.

• In societies where women and children are important as laborers for their husbands and fathers—as they often are in polygynous relationships in the United States—competition for wives increases, creating a larger number of unmarried males.

• Unmarried men in both polygynous and monogamous societies commit far more crimes than married men. Having a wife reduces the chance that a man will commit a crime by half. (Unmarried cohabitation doesn’t reduce crime.)

• A study in Malawi found that polygynous husbands had more extramarital affairs than monogamous husbands.

Effects on women

• Women in polygynous societies have low status and less freedom than women in monogamous societies.

• Arguments that many women favor polygyny and should be free to choose it assume that in polygynous societies women would be free to choose their husbands. They also assume that in polygynous societies women who “chose” monogamy could prevent a husband from marrying again. They assume that women’s far stronger rights, status, and freedoms in monogamous societies and traditions don’t depend upon monogamy.

• Women in polygynous societies have worse health, shorter lives, lower education, and a higher suicide rate than women in monogamy.

• Co-wives must compete with each other for love, sex, and money, and for education, inheritances, and paternal care and attention for their children. One study reports, “Co-wife conflict is ubiquitous in polygynous households. From anthropology, a review of ethnographic data from 69 non-sororal polygynous societies from around the globe reveals no case where co-wife relations could be described as harmonious...” A clear majority of monogamous households (though certainly not all) are happy or satisfied.

• Polygynous wives suffer more violence and abuse. The age gap between polygynous husbands and their younger wives often increases marital conflict.

• The shortage of women in polygyny forces men to seek younger and younger wives. Women’s average age at marriage is 10 years younger than in monogamy, they are much younger when they first give birth, and they have a wider marital age gap.

• Early marriage and child-bearing contribute heavily to poorer mental and physical health for women—more depression, nervous breakdowns, and higher mortality and shorter lives than monogamous women.

Effects on children

• Children in polygynous relationships receive much less paternal attention and care. Polygyny reduces a man’s investment of time and resources in each of his more numerous offspring. One man with several wives and a lot of children can’t give any of them the time and attention a monogamous man can give each member of his family. (So far, Kody Brown seems to have 18 children. FLDS recommends that each man takes at least three wives; its leaders sometimes have staggering numbers—for example, Canadian FLDS leader Winston Blackmore has 150 children by 27 wives.)

• Excess boys have been abandoned or forced out of some fundamentalist Mormon communities.

• Children in polygynous families suffer more abuse, neglect, and violence than those in monogamous families, and have more physical, emotional, and behavioral problems.

• Infant mortality is much higher in polygynous than in comparable monogamous societies. Within polygynous societies, children in monogamous families also have lower mortality rates than those in polygynous families. In 19th century American polygyny, the children of poor men’s monogamous marriages had significantly better survival rates than the children of rich polygynists.

• A study reports: “In Africa, diverse studies show that, relative to children from monogamous households, children from polygynous household[s] risk diminished nutritional status, poorer health outcomes, and diminished mortality. … [B]oth infant and child mortality in [highly polygynous countries] are roughly twice that of [comparable monogamous countries].”

• Conflict in polygamous households endangers children by creating higher levels of violence and constant stress. Often it will be a co-wife and not the father who abuses or murders a child. A good deal of evidence confirms that, on average, children are in far more danger in families comprised of many unrelated adult-child “dyads” (two-person pairs) and less-related child-child dyads: “Living in the same household with genetically unrelated adults is the single biggest risk factor for abuse, neglect and homicide of children. Stepmothers are 2.4 times more likely to kill their stepchildren than birth mothers, and children living with an unrelated parent are between 15 and 77 times more likely to die ‘accidentally.’” In a monogamous household, there is only one unrelated dyad, the husband and wife. In a household with three wives, each of whom has three children, there are 51 less-related and unrelated dyads.

Social and national effects

• On every key measure of social welfare, polygynous societies fare worse than monogamous ones. They have more crime, poverty, social disorder and disruption, and substance abuse, and less education, worse health, higher mortality, and less freedom.

• The effects of polygyny arise wherever it is practiced, regardless of the race, culture, or religion of its people. Polygyny in North America does not have different effects from polygyny elsewhere.

• In polygynous societies, unmarried, low-status men with little to lose may gang up and engage in risky, status-elevating, sex-seeking, and illegal behavior—rowdiness, substance abuse, theft, assault, murder, rape, kidnapping (especially of females), sexual slavery, and prostitution. Some may become “marauders who go on raids to steal wealth and wives, while raping and pillaging.” Some observers think that polygyny explains the rise of Boko Haram and its kidnappings of young Nigerian girls.

• Polygynous communities increase in population faster than monogamous ones. When polygynists congregate in one locale, they may (and in the US, have) negatively affect their neighbors’ economies, politics, culture, schools, welfare systems, policing, and government services, whether by voting power or otherwise. Patriarchal men may control how their wives and children vote, or seek wives among local girls and women, contributing to the spread of polygyny.

• Polygynous societies have fewer freedoms and less effective, less stable, and less democratic governments than monogamous ones.

• Divorce is “as prevalent” in polygynous marriages as in monogamous ones.

• The richest, most advanced societies in the world are all monogamous. Monogamous societies are richer even when compared with polygynous countries at similar levels of development. Why? It’s hypothesized that, with its better-raised children and its equal mating opportunities, monogamy reduces, and polygyny increases, crime, social disorder, political upheavals, civil war, and cross-border wars. And by increasing women’s welfare, monogamy increases their productivity, which in turn boosts the welfare and future productivity of their children—adding to the society’s competitive power. Freed from fighting, warfare, and the drain on time and resources spent acquiring and managing several wives and a huge family of children, monogamous men can engage in commercial and other pursuits that enrich their society and give it an edge in inter-group economic competition.

The legal debate

Courts, like some scholars and culture critics, may ignore these realities. A 2011 legal case in British Columbia—which has a sizeable FLDS community with strong ties to the FLDS in the United States—and a 2013 case in the federal District Court in Utah provide a convenient, if dismaying, illustration of the differences between fact-based and fact-free approaches.