Monogamy

• 1. Introduction

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  Monogamy, at its simplest, describes a relationship between two individuals who form an exclusive intimate partnership. Yet the word conceals a tangle of meanings so deep that biologists, anthropologists, and social scientists rarely agree on what it actually refers to. A biologist studying birds means something different by monogamy than a cultural anthropologist studying marriage customs in rural Ethiopia, and both mean something different from a lawyer drafting a marriage contract in Tokyo. The Greek roots are straightforward: monos, "one", and gamos, "marriage". But what follows from those roots is anything but simple. Is monogamy a matter of who you live with, who you sleep with, or who fathered your children? The answer, as it turns out, depends enormously on who you ask and when in history you are asking. What drove the shift toward monogamy in human prehistory? How do prairie voles and fairywrens fit into the story? And what did Augustus Caesar, Friedrich Engels, and the founders of communist China all make of the institution? Those are the questions this documentary will answer.

• 2. Four Meanings, One Word

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  Biologists, biological anthropologists, and behavioral ecologists tend to use monogamy in the sense of sexual, if not genetic, exclusivity. When cultural or social anthropologists use the same term, they generally mean social or marital monogamy. The gap between those two usages has caused considerable confusion in research. Genetic monogamy is the most demanding definition: it requires DNA evidence confirming that a pair reproduce exclusively with each other. Sexual monogamy simply means two partners having no outside sex partners. Social monogamy describes two individuals sharing a home, a sexual relationship, and basic resources such as shelter, food, and parenting responsibilities. Marital monogamy applies within the institution of marriage itself, and can be further split into classical monogamy, which the source defines as "a single relationship between people who marry as virgins, remain sexually exclusive their entire lives, and become celibate upon the death of the partner", and serial monogamy, meaning marriage to only one other person at a time. The 2012 work that redefined practices as either formal or informal polyandry illustrates how much these definitions matter in practice. Researchers examining 53 communities studied between 1912 and 2010 found that polyandry was far more common worldwide than previously believed, once a broader definition was applied. The choice of definition, in other words, does not merely affect how scientists categorise a practice. It can overturn the received picture of how common that practice actually is.

• 3. By the Numbers

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  George P. Murdock's Ethnographic Atlas, which recorded 1,231 societies from around the world, found that only 186 were monogamous; 453 had occasional polygyny; 588 had more frequent polygyny; and just 4 had polyandry. At first glance, monogamy looks like a minority practice. But Murdock's data did not account for the relative population of each society, and the actual practice of polygamy in a tolerant society may be quite low, with the majority of aspirant polygamists living in de facto monogamous marriages. The Standard Cross-Cultural Sample, which describes extramarital sex in over 50 pre-industrial cultures, offers a further layer of detail. Among men, extramarital sex was described as "universal" in 6 cultures, "moderate" in 29, "occasional" in 6, and "uncommon" in 10. Among women the proportions shifted: "universal" in 6 cultures, "moderate" in 23, "occasional" in 9, and "uncommon" in 15. Research by Colleen Hoffon of 566 homosexual male couples from the San Francisco Bay Area in 2010 found that 45% had monogamous relationships. The Human Rights Campaign, drawing on a Rockway Institute report, noted that over 80% of the homosexuals surveyed expected to be in a monogamous relationship after age 30. Genetic monogamy tells yet another story. Simmons, Firman, Rhodes, and Peters reviewed 11 published studies across locations in the United States, France, Switzerland, the United Kingdom, Mexico, and among the native Yanomami Indians of the Amazon forest. Rates of extrapair paternity ranged from 0.03% to 11.8%, with a median of 1.8%. A separate review of 17 studies by Bellis, Hughes, Hughes, and Ashton found a median of 3.7% extrapair paternity. Taken together, these reviews suggest a range of 96% to 98% genetic monogamy. Sociologist Michael Gilding traced media claims of rates as high as 30% back to an informal remark at a 1972 conference.

• 4. The Biological Case

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  In any species, three main aspects combine to promote a monogamous mating system: paternal care, resource access, and mate choice. In humans, the main theoretical drivers are paternal care and extreme ecological stresses. Paternal care matters especially in humans because of the extra nutritional demands of larger brains and a lengthier developmental period. The evidence, however, is more complicated than a simple cause-and-effect. Monogamy is correlated with paternal care, as shown by Marlowe, but not caused by it, because humans can reduce the need for bi-parental care through the help of siblings and other family members. Human intelligence and material culture also allows for better adaptation to rough ecological conditions, weakening the link between harsh climates and monogamous marriage. Paleoanthropological and genetic studies offer two conflicting timelines for when monogamy evolved. Fossil evidence examined by Reno et al. suggested that the sexual dimorphism of Australopithecus afarensis, a human ancestor from approximately 3.9 to 3.0 million years ago, fell within the modern human range, hinting that reduced male-male competition may have appeared very early. However, Gordon, Green, and Richmond argued that A. afarensis was more sexually dimorphic than modern humans and chimpanzees, with levels closer to orangutans and gorillas. Homo habilis, living approximately 2.3 million years ago, was the most sexually dimorphic early hominid. Plavcan and van Schaik concluded that the overall picture of sexual dimorphism in australopithecines does not clearly indicate any specific mating system. Genetic studies, by contrast, suggest that monogamy increased much more recently, within the last 10,000 to 20,000 years. The San people of Southern Africa, currently the oldest ethnic group on the continent where Homo sapiens emerged, practice serial monogamy as their norm, with a hunter able to take a second wife only if he can obtain enough food to support her.

• 5. Agriculture, Property, and Power

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  Anthropologist Jack Goody's comparative study using the Ethnographic Atlas demonstrated that monogamy is part of a cultural complex running from Japan to Ireland, linked statistically to intensive plough agriculture and to systems in which property can be inherited by children of both sexes. Drawing on Ester Boserup's work, Goody noted that in plough agriculture, farming is largely men's work and is associated with private property; marriage tends toward monogamy to keep property within the nuclear family. A molecular genetic study of global human genetic diversity found that sexual polygyny was typical until the shift to sedentary farming communities approximately 10,000 to 5,000 years ago in Europe and Asia, and more recently in Africa and the Americas. The genetic record from that transition period reveals a striking anomaly: at around 8,000 years ago, the average man with modern descendants appears to have had children with approximately 17 women. Researchers speculate that this reflects a brief period of extreme power and resource hoarding during the early sedentary agricultural phase, followed by a rapid correction. The movement back toward around 4.5 women per man, accompanied by evidence for growing monogamy as agriculture progressed, remains difficult to explain. Some researchers point to catastrophic male mortality; others suggest that warfare among patrilineal clan-based societies eradicated entire male bloodlines, while women were absorbed into victorious groups. Laura Betzig argued that in six large, highly stratified early states, including Mesopotamia, Egypt, Aztec Mexico, Inca Peru, India, and China, commoners were generally monogamous while elites practiced de facto polygyny. Betzig also argued that Augustus Caesar encouraged marriage and reproduction to force the aristocracy to divide their wealth, while the aristocrats responded by limiting their legitimate children to protect their legacies, all while maintaining many extra-pair copulations.

• 6. Law, Religion, and Politics

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  Ancient Egypt shows one of the earliest documented cases of monogamy as a social norm. A Nineteenth Dynasty official recorded, as proof of his devotion, that he had remained married to his wife since their youth even after he had become very successful. Egyptian women had the legal right to request a divorce if their husband took a second wife. Tomb inscriptions typically identify wives with the phrase "His wife X, his beloved", and wisdom texts such as the Instruction of Ptahhotep, the Instruction of Any, and the Instruction of Ankhsheshonq all counsel fidelity, with the last explicitly stating that it is wrong to abandon a wife because she cannot become pregnant. In ancient Israel, the Dead Sea Sect's Damascus Document, one of the Dead Sea Scrolls, interpreted the Pentateuch as prohibiting polygamy. A synod convened by Gershom ben Judah around 1000 CE banned polygamy among Ashkenazi and Sephardic Jews. In the contemporary era, Japan banned polygyny in 1880, China in 1953, India in 1955, and Nepal in 1963. The United Nations General Assembly adopted the Convention on the Elimination of All Forms of Discrimination Against Women in 1979, with Article 16 requiring nations to give women and men equal rights in marriage. In China, the 1950 Marriage Law outlawed arranged marriages and established free choice of partners, monogamy, and equal rights for both sexes as the legal framework, with the government running an extensive campaign of marriage-law education through the Communist Party, women's federations, trade unions, the armed forces, and schools. Friedrich Engels, in The Origin of the Family, Private Property and the State in 1884, had argued that compulsory monogamy could only increase prostitution and general immorality, describing the monogamous family as distinguished from the pairing family by "the far greater durability of wedlock, which can no longer be dissolved at the pleasure of either party". The communist revolutionaries in China ultimately rejected that view in favour of monogamy as a guarantee of equal rights.

• 7. Animals and the Lab

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  Over 90% of avian species are socially monogamous, compared to only 3% of mammalian species and up to 15% of primate species. Social monogamy has also been observed in reptiles, fish, and insects, and even in a parasitic worm, Schistosoma mansoni, which in its female-male pairings in the human body is monogamous. Patricia Adair Gowaty estimated that out of 180 different species of socially monogamous songbirds, only 10% are sexually monogamous. Among fairywrens of the species Malurus splendens and Malurus cyaneus, more than 65% of chicks are fathered by males outside the supposed breeding pair. Popular science author Matt Ridley, in The Red Queen: Sex and the Evolution of Human Nature, described the human mating system as "monogamy plagued by adultery". The prairie vole stands out as the most studied mammalian example of social monogamy. Male prairie voles with the longest strings of a particular microsatellite DNA spend more time with their mates and pups than males with shorter strings. Physiologically, pair-bonding behavior has been linked to levels of vasopressin, dopamine, and oxytocin in the brain. In experiments, administering oxytocin and vasopressin directly promoted affiliative behavior in prairie voles but not in the similar, non-monogamous montane vole. The key difference is the location, density, and distribution of receptors for these substances: only in prairie voles are oxytocin and vasopressin receptors located along the mesolimbic dopamine reward pathway, which appears to condition the animal to its mate's scent while consolidating the social memory of mating. A separate experiment introduced a single gene into the brains of adult male meadow voles by virus, shifting their behaviour to resemble that of the typically monogamous prairie vole, though other scientists have disputed the gene's relationship to monogamy and questioned whether the human version plays an analogous role.