When people imagine getting married, they often think about an everlasting bond with one person. Monogamy is so deeply embedded in our cultural perceptions of marriage that its essentialness to the institution is hardly questioned. Yet over the course of human history, polygamy has actually prevailed 85% of the time.

So why has monogamy spread so widely in the modern era?

Monogamous marriage is perplexing on various fronts. Historically speaking, it's rare. The anthropological record reveals that a mere 15% of societies have ever subscribed to the practice. While pair bonding between men and women stretches back to our ancestral past, monogamy is only a few centuries old—Nepal, for example, only outlawed polygamy in 1963. Moreover, economic disparity tends to be compatible with polygamous systems, as taking multiple wives has always been linked to wealth, power, and nobility. Yet the aristocrats who would have benefited most from polygamy didn't suppress monogamy. Even today, when humankind has more extreme wealth inequality than it has ever known, monogamy remains the norm. This is to say nothing of the evolutionary disharmony between the mating goals of males and females. Women, with their greater parental investment, tend to seek out men who demonstrate active interest in providing for and raising children; men often desire sexual variety and short-term unions.

In light of these factors, how did monogamy become culturally dominant?

A recent study led by Joseph Henrich of the University of British Columbia has found that monogamous unions have unexpected and far-reaching benefits. Until recently, polygamy enjoyed a flourishing history. In egalitarian contexts, this system poses negligible conflicts as “high-status” men may get an extra wife. On occasion, they may get three or four. But following the advent of agriculture, societies grew larger and more complex. They also grew socially unequal. The conditions were ripe for high-ranking men to take numerous wives, and polygamy levels spiked. At its apex, rulers of early empires had massive harems. And herein lies the problem.

When there's a shortage of available women, the competition among “low-status” men for mates becomes fierce, and they will resort to brutal methods to attain one. Henrich and colleagues report that in polygamous societies, rates of crime swell. However, when women are evenly distributed, men can turn their attention to more constructive activities like contributing to their families and communities. Evidently, this is why monogamy has succeeded: it's good for society. Put more technically, monogamous marriage is a product of group selection.

Group selection can emerge from the competition between populations, such as communities, states and nations. The theory goes that selective forces will favor the traits, social strategies and cultural norms that promote a population's success. The result is adaptations that benefit the group — even if they're at the expense of the individual. For complex societies, competitive success relies on factors like economic productivity, labor, quality parenting, and low crime levels. Polygamy, it turns out, undermines these group advantages. Yet monogamous marriage reinforces these competitive edges, which may have been instrumental in its dissemination across Europe and then the globe.

The authors present five reasons why monogamous marriage works so well on the group scale. The first three grow out of a single condition: diffusing the intensity of the competition between low-ranking men for wives. When there are enough women to go around, it seems society at large wins.

1. It reduces crime. With married men continuing to snatch up unmarried women, the number of unattached males inflates and competition between them escalates. With meager hopes of attaining even one long-term mate, low-ranking men divest in their futures. Subsequently, they throw caution to the wind, partaking in status- and sex-seeking behaviors. Studies show that unmarried men band together in groups and commit more risky and serious crimes than married men. An excess of single men increases all sorts of societal ills, including murder, theft, rape, social disruption, kidnapping of females, sexual slavery and prostitution. As a by-product, and for good measure, these men have a higher likelihood of abusing substances. By contrast, monogamy deflates the number of single men, thus lowering rates of crime, social disruption and possible substance abuse.

2. It is good for girls and women. Under polygamous conditions, the age of first marriage for females plunges and the spousal age gap widens. The thin supply of unmarried women and the vehement competition for them drives men of all ages to seek out young women and girls. The competition also presses men to leverage whatever means they have to attain a wife, including bartering and bargaining with the fathers and brothers of available women. These young women are then forced into oppressive homes, as older husbands guard them from other men (to insure paternity) and rule the household with an iron fist. Heightened competition also impels men to control their female relatives, as the demand for wives surges. Collectively, these dynamics breed societal problems, including the suppression of women's rights, gender inequality and domestic violence.

Conversely, monogamous unions relieve the pressure to push younger brides into the marriage pool, which in turn narrows the spousal age gap. It also ameliorates male strivings to control and guard women as well as gender inequality. Interestingly, the researchers note, universal monogamy may have also spurred the ascent of democratic institutions and ideas about equality between men and women.

3. It is good for fathers and children. If men can't effectively compete and obtain a wife, then they can't mate and raise children. Polygamy also dilutes the investment a father makes into any one of his multitude of children by multiple wives, since it shifts his attention away from raising children to obtaining more women. As such, children receive precious little from polygamous fathers.

On the flip side, monogamy creates the conditions for low-ranking men to marry as well as to save and invest for the future. The labor and talents of these otherwise risk-taking criminals and possible substance abusers are instead redirected into future-oriented investments in their families. In other words, these men focus on being husbands and fathers.

4. It is good for the economy. Research using decision models to investigate how polygamous vs. monogamous systems impact economic productivity come down in favor of monogamy. More specifically, if monogamy were to be enforced in polygamous societies, the analysis forecasts that saving rates rise, bride prices vanish and GDP per capita climbs. What's driving all of these positive outcomes? When men are diverted from obtaining additional wives or selling daughters, they have fewer children, invest in the workforce, and both save and consume more. Again, monogamy is advantageous for society as a whole.

5. It promotes more peaceful households. The authors maintain that polygamous homes are thick with conflict because co-wives are typically not related to one another. Research shows that unrelatedness is a risk factor for greater levels of abuse, violence and murder. One reason for these heightened threats is the competition that polygamy engenders between co-wives. For example, resources may not be evenly apportioned among the children, causing clashes between wives as they try to secure supplies for their own brood.

By contrast, monogamous marriage eliminates the issue of unrelatedness in the household. With the competition between co-wives removed, it decreases levels of abuse, disproportionate allocation of resources, and violence between adults and towards children. Studies indicate that genetic relatedness offers some safeguard against such conflicts in the household, since kin are less likely to be aggressive towards each other in these ways. A related home, apparently, is a safer one.

The investigators show that forsaking all others has benefits that reach well beyond personal happiness and has propelled human societies to thrive and prosper. Since monogamy has advanced nations and heterosexuals so well, how much more successful would societies be if monogamous marriage for homosexuals was also a cultural norm? Perhaps if “making it legal” was a given, gay people could channel their energies in similarly productive ways as straight individuals have, such as focusing on their families and communities. It might even discourage hate crimes and encourage acceptance. That would be good for everyone. It is something to think about.

Connect with Dr. Mehta at drvinitamehta.com and on twitter and Pinterest! You can find Dr. Mehta's other Psychology Today posts here.

Vinita Mehta, Ph.D. is a licensed Clinical Psychologist in Washington, DC, and an expert on relationships, managing anxiety and stress, and building health and resilience. Dr. Mehta provides speaking engagements for your organization and psychotherapy for adults.  She has successfully worked with individuals struggling with depression, anxiety, and life transitions, with a growing specialization in recovery from trauma and abuse. Dr. Mehta is also the author of the forthcoming book Paleo Love: How Our Stone Age Bodies Complicate Modern Relationships.

Journal Reference

Henrich, J., Boyd, R., & Richerson, P. (2012). The puzzle of monogamous marriage Philosophical Transactions of the Royal Society B: Biological Sciences, 367 (1589), 657-669 DOI: 10.1098/rstb.2011.0290

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