In the course of history, approximately 85 percent of societies have practiced polygamy. Pushed by advocates of same-sex marriage and multiculturalism, some scholars, such as the signers of “Beyond Gay Marriage,” argue that it is irrational and bigoted for contemporary society to limit marriage to just two people. However, there is no bigotry in treating different things differently, and there are many important differences between polygamy and monogamy in practice as well as in principle.
There are three main forms of polygamous relationships: polygyny, polyandry, and polygynandry. In polygyny, by far the most common form of polygamy, one man may marry a number of wives. In polyandry, one wife has two or more husbands. This form of polygamy is extremely unusual, and often takes the form of two brothers marrying the same woman. In polygynandry, two or more wives marry to two or more husbands. Polygynandry is even more rare than polyandry, but will be similar in some respects to polygyny, insofar as a man has more than one wife. Since both polygynandry and polyandry are virtually non-existent, I’ll focus on the more common case of one man with multiple wives, and use the more common term polygamy to describe this arrangement.
Now let us turn to the practical considerations drawn from human experience. Recent empirical research suggests that, in virtually every respect, polygamy is socially detrimental—to society in general, to men, to women, and to children. These problems arise because of the nature of human reproduction.
In human reproduction, slightly more male than female babies are born (approximately 105 boys to 100 girls). As boys are more likely to die of natural causes as infants and from violence before they marry and reproduce, ceteris paribus, at any given marriageable age, there will be approximately 50% males and 50% females. Given roughly equal numbers of males and females as found in nature, polygamy and monogamy shape society in radically different ways. In a monogamous society, for each man there is a corresponding woman. William Tucker notes that this gives “every man [and every woman] a reasonable chance to mate.” By contrast, in a polygamous society, some men take multiple wives, but this leaves other men with greatly diminished prospects of marriage or an exclusion from mating altogether. The question under consideration, then, is what social effects does this arrangement bring?
In their 2012 article, “The Puzzle of Monogamous Marriage” appearing in Philosophical Transactions of the Royal Society, Joseph Henrich, Robert Boyd, and Peter J. Richerson used converging lines of evidence from the social sciences to compare polygamous and monogamous societies. They found that polygamous societies differ from monogamous societies in terms of violent crimes, female educational attainment, domestic violence, parental investment in children, and economic productivity.
A wealth of sociological information points to the fact that single men commit the vast majority of violent crimes. Women and married men seldom murder, rob, rape, and assault in comparison to single men. So, since there are many more single men in polygamous societies, polygamous societies have higher rates of violent crime. As Henrich and colleagues note:
Faced with high levels of intra-sexual competition and little chance of obtaining even one long-term mate, unmarried, low-status men will heavily discount the future and more readily engage in risky status-elevating and sex-seeking behaviors. This will result in higher rates of murder, theft, rape, social disruption, kidnapping (especially of females), sexual slavery and prostitution.
With little reason to invest in the established social order, single males are more likely to turn away from activities conducive to long-term productivity and turn toward the quick thrill, if not a violent overthrow of the established social order. These tendencies are detrimental to society as a whole, including to single men who are the most common victims of theft, violent assault, and murder.
In a polygamous society, the age of marriage will be lower for females than in a monogamous society. With a relative scarcity of possible mates of their own age, men seek wives among women of younger ages. Early marriage in turn leads to much higher rates of reproduction. Rather than delaying marriage and childbearing until their twenties or thirties, women marry and have children as teenagers. In modern social conditions, teen motherhood is detrimental for both these young women and their families. For a female teen, marriage to a much older man makes it unlikely that she will have an equal partnership with her husband and makes the completion of her education difficult, if not impossible. Indeed, marriage at a young age to a much older man is also linked to lethal domestic violence. In the words of one study:
The larger the age gap, the more likely it is that a husband will kill his wife, and vice‐versa (the young wife murders her husband). … This suggests that polygyny is relatively (potentially) much more dangerous than monogamous relations because age gaps of 16 years are not uncommon when accumulating young wives.
The difference in age exacerbates gender differences, and, for men, is more likely to give rise to jealous fears that their young wives will be unfaithful.
The phenomenon of "co-wives": (a misnomer since polygamy typically involves a hierarchy among the wives) also undermines the well-being of women. The senior wives worry that they will be replaced by younger wives, and the younger wives in turn worry about the power exerted in the home by senior wives. Research indicates that levels of domestic strife and violence are higher in polygamous homes than in monogamous homes as wives seek to preserve their place with their shared husband as well as struggle to secure resources for their own biological children. As Henrich and colleagues point out:
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, and no hint that women’s access to the means of production had any mitigating impact on conflict.
These conflicts lead polygamous family units, particularly those with three or more wives, to have in general higher rates of divorce than monogamous couples. In the supplementary materials to their article, Henrich, Boyd, and Richerson point out: “Systematic and controlled analyses from polygynous societies generally show higher divorce rates for polygynous vs. monogamous marriages in the same society. … Relative to monogamous families, polygynous families with more than two wives are five times more likely to divorce.”
As bad as polygamy is for women, it is perhaps even worse for the well-being of children. Because the polygamous wives tend to be younger and less well educated, their children suffer in not having more mature mothers, as would be more typical of their counterparts in a monogamous society. The children suffer also from having multiple stepmothers involved in ongoing struggles with each other. Half-siblings must compete for limited resources while having weaker genetic bonds to mitigate the conflict. While these extended-family relationships could in theory be a source of support, more often they endanger children. Henrich’s study explained:
Much empirical work in monogamous societies indicates that higher degrees of relatedness among household members are associated with lower rates of abuse, neglect and homicide. 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."
Polygamous families are also more likely than monogamous families to be in poverty, since typically only one breadwinner supports numerous children.
Polygamous societies also dilute the investment of fathers in their children in at least two ways. First, because marriage to other young women is still an option, a husband’s resources of time, attention, and money are diverted away from his own children and toward finding new mates. Secondly, in virtue of the greater number of children in the polygamous family, it becomes increasingly difficult to give each child sufficient time and attention. Indeed, some fathers of polygamous families have so many children that they do not even know each child’s name. This dilution of paternal investment is similar in effect to being raised by a single mother with all its attendant risk factors (especially for males) for drug abuse, trouble with the law, and dropping out of school.
A final harm brought on by polygamy is economic. Henrich’s study notes:
When males cannot invest in obtaining more wives (because of imposed monogamy) they invest and save in ways that generate both reduced population growth and more rapid economic expansion (increasing GDP per capita). Thus … the nearly threefold increase in GDP per capita between Comparable Monogamous Countries and Highly Polygynous Countries is partially caused by legally imposed monogamy.
Economic well-being contributes in turn to the stability of families which is a benefit to men, women, and children alike.
Finally, even aside from the sociological data, there is an inherent inequality in polygamous marriage. In monogamous marriage, spouses give themselves as spouses to each other unreservedly, unconditionally, and entirely. Now, giving oneself as a husband or wife to one’s spouse does not exclude giving of oneself in ways that are not distinctly marital to other people (such as playing tennis with a business partner, or going to the movies with a group of friends). Part of the marriage vow is the promise of sexual fidelity, the bodily manifestation of one’s commitment as spouse entirely to the spouse and to the spouse alone.
In a polygamous marriage, the man does not give himself qua husband entirely to his wife. A polygamous husband gives himself qua husband to however many wives he has. Wives, by contrast, are expected to reserve themselves in a sexual way for their husband alone. Moreover, wives face inequality among themselves as “senior wives” enjoy rank above “junior wives.” The polygamous relationship can never attain the mutual and complete self-donation of spouses in monogamous marriage because it is intrinsically impossible to reserve oneself in a sexual way entirely for one person and at the same time reserve oneself in a sexual way entirely for a different person (or persons). Marriage understood as a comprehensive union can exist only between two persons, and never more than two persons. Society, therefore, has good reason not simply to proscribe polygamy, but to endorse monogamy.
Christopher Kaczor is Professor of Philosophy at Loyola Marymount University and the author of The Ethics of Abortion: Women’s Rights, Human Life, and the Question of Justice (Routledge 2011).