Monogamy Made Us Human

by William Tucker
In theory and in practice, it’s what keeps a society stable.

Since antiquity, monogamy has been the general rule of Western civilizations. Yet people have always known that other mating systems are possible. The Greek gods practiced a very loose monogamy that bordered on marital chaos. Many of the early Hebrew patriarchs took multiple wives. Although monogamy was established in the legal codes of Greece and Rome and reinforced by the Catholic Church in the Middle Ages, it was well known that other cultures — mostly Islam in the Middle East — did not acknowledge it.

This became uncomfortably clear as European explorers pushed out among the tribes of Africa, the South Seas, and the American Plains, revealing that the practice of polygamy was almost universal outside the Christian West. Might polygamy’s roots be found in distant prehistory and perhaps even be part of our evolutionary history?

The first attempt to explain the origins of the human family in evolutionary terms came in 1861, two years after the publication of Darwin’s Origin of Species. Johann Bachofen, a Swiss law professor, published Das Mutterrech (“The Mother-Right”), “an investigation of the religious and juridical character of matriarchy in the ancient world.” Bachofen began with the simple observation that remains the most powerful argument of those, including many of today’s feminists, who see family as based in single motherhood. While maternity is always known, paternity is always a bit of a mystery. In a primitive world, the link between sex and paternity would have been obscure, and males would have had more difficulty laying claim to their offspring. The two-parent family was formed, said Bachofen, only when women became weary of rearing children alone and persuaded men to settle down and help. He buttressed his case for this early matriarchy with evidence from mythology and legend.

These speculations were expanded in the next decade by Lewis Henry Morgan, an upstate New York attorney and amateur anthropologist. Among Morgan’s clients were several Iroquois tribes. He became fascinated with their system of matrilineal tribal claims, each of which took the name of an animal. In 1871, in his book Ancient Society, Morgan postulated that these clans were actually the vestiges of “group marriage.” Noting that the Iroquois still practiced polygamy, particularly sororate marriage, where a man marries a group of sisters, Morgan conjectured that in ancient times whole families of brothers had married whole families of sisters, forming the totem clans.

Projecting this logic all the way back to the beginning of human history, Morgan asserted that, without knowledge of paternity or the restraints of later civilization, marriage would have been unknown. He described an indiscriminate form of mating that he called “the primal horde,” a dynamic that Jane Goodall in the 1980s would observe among chimpanzees, who practice sexual communism: Every male gets to mate with every female.

Morgan’s theory probably would not be remembered today if it had not caught the attention of the son of a prosperous British factory owner named Friedrich Engels. In 1884, Engels published, as an addendum to Marx’s Das Kapital, his book The Origin of the Family, Private Property, and the State in which he brought back the claims of primitive matriarchy, with a vengeance. He argued that the primal horde had actually been a lost paradise in which men and women lived in a utopia of sexual abandon before the chains of patriarchy were forged.

“The overthrow of the mother-right was the world-historical defeat of the female sex,” wrote Engels (emphasis in the original). Once paternity was recognized, “the man took command in the home also; the woman was degraded and reduced to servitude; she became the slave of his lust and a mere instrument for the production of children.” Once women became “property,” material goods were claimed as property too, and all the evils of capitalism quickly followed. The downfall of the primal horde, said Engels, was the anthropological equivalent of the expulsion from the Garden of Eden. It was for this reason that early Communism was often coupled with the idea of “free love.”

Then a few stray facts interrupted this speculation. It had always been assumed that the tribal societies discovered in tropical Africa and the forests of Indonesia and North and South America, all practicing what is called “hoe agriculture,” represented the earlier human societies. As explorers pushed farther into the forgotten corners of the world, however, they discovered a few remaining tribes that were still practicing hunting and gathering — the Bushmen of the Kalahari Desert, the Pygmies of Central Africa, the Aborigines of Australia. This led to an astonishing revelation: All turned out to be monogamous.

Apparently, it was the invention of agriculture and the accumulation of property and permanent wealth that had caused primitive agriculturalists to take up polygamy, as wealthier men began to acquire more women and male members could now be excluded from the tribe without great consequence. And so by the late 1920s a potential blueprint for the evolution of the family was coming into view: Monogamy was the original form of human bonding while polygamy was a later development. By the 1930s, European anthropologists such as A. R. Radcliffe-Brown were arguing that polygamy was in fact a backsliding, arriving only after the communal norms of hunter-gatherer societies had broken down.

What advantages does monogamy have over other forms of mating to begin with? Perhaps the key lies in another aspect of chimp communism that Goodall discovered after years of observation. Although at first it seemed insignificant, she gradually recognized its import, calling it “the forerunner of human marriage.” It was “the consort relationship.”

Goodall found that, once all the obligations for mating with every male had been met, females — and particularly high-ranking females — liked to sneak off into the jungle with a favorite male. The couple would stay away two or three days, even as long as a week, building a nest in the trees, grooming each other, exchanging signs of affection, and copulating frequently. As further research began to reveal secrets of chimp biology, it was discovered that females actually delay ovulation through the period in which they are mating at random so that they are more likely to conceive with their consorts. In this way, they preserve the aboriginal right of the female of every species, which is to mate with the male of her choice. The public show of uninhibited sexual activity is actually a ruse to deceive the lower-status males. Biologically, chimp females are designed to conceive with a preferred mate. Genetic tests have later shown that half the offspring in a troop are sired in these consort relationships.

So why do female chimps go through the exhausting ritual of mating with every male member of the troop if they really want to be mated with a single, dominant male? It is not a pleasant ritual for them. At the end of it they are often left battered and bleeding. Goodall even noted that young females are often reluctant to go out among the males during their first estrus and have to be urged on by the older females. Because they can generally be fertilized by only one male, females are much more selective about mating, while males — who have ample sperm and can mate with numerous females — are much more eager and indiscriminate. Why do chimp females act so differently?

This was the question that as a young graduate student Sarah Blaffer Hrdy began asking in the 1970s. Hrdy became intrigued by the phenomenon of infanticide among langurs, a species of colobus monkeys in the forests of Southeast Asia. She perceived a pattern now recognized as common to all polygamous species and tied directly to selfish-gene theory and the instinctive understanding males have about fostering their own offspring.

When an alpha male takes over a polygynous harem, no matter how small, he probably has only two or three years before he is displaced by another male. But in species such as the colobus monkey, an infant may nurse with the mother for four years, during which time her hormonal balance will prevent her from becoming fertile again. If the alpha is to take advantage of his dominance, he has only one choice. He must kill the offspring of the previous alpha and put the females to work producing his own.

Hrdy soon realized that while such infanticide was common among other primates, female chimps had evolved a strategy to prevent this. They confuse paternity. By carefully mating with every male member of the group, females give each one reason to think that he might be the father. This accommodation allows them to live in relative peace in the midst of a larger group of males once the infant is born. The females are protecting against infanticide. All of this of course demolishes the theory, going back to Bachofen and Engels, that males are unaware of their paternity. They do not have to understand the nature of intercourse in any theoretical sense. It is all bred into their nature. Male “mate guarding,” as it is known, is a universal behavior. The whole polymorphous polygamous mating system, then, is a set of rules to keep lower-status males loyal to the troop.

Let us conduct a thought experiment. What would happen today if a chimp troop moved out on the savanna and tried to survive in a treeless environment with predators everywhere? Would chimp sexual behavior change?

The first thing to note is that such a group would have to cling much more tightly together. Second, primal-horde mating would become extremely awkward and disruptive, if not impossible. In the relative sanctuary of the tropic forest, when a chimp female goes into estrus, all other activity stops and the males may spend close to a week following her around. On the savanna, however, there would be no such luxury. A chimp troop that spends whole days obsessed with a female in heat would have trouble finding food and leave itself extremely vulnerable to predators.

Third and most important, the consort relationship would no longer be possible. There would be no way to sneak off into the forest with a favored partner. No less than the primal horde, a male and female that left the troop for two or three days on an amorous “safari” would leave themselves highly exposed to predators.

The loss of the consort relationship, the source of half of chimp pregnancies, would be severely disruptive, particularly to high-status males. If they were forced to go back to standing in line with the rest of the troop, their mating success would be severely circumscribed. Given this situation, then, what two members of the troop, male and female, would have the most to gain from defying the social order and forming an exclusive pair bond?

Suppose a lower-status male tried to monopolize a high-status female. He wouldn’t have much luck. The other males would gang up on him and the female would resist as well, since she wants to mate with a more dominant male. So that wouldn’t work. What if a lower-status male tried to monopolize a lower-status female at his own level? His chances might be better. The other males might not object to the loss of a lower-status female. But the female would object because she would not want to be excluded from mating with higher-status males.

There remains then one other possibility. What if the dominant male and the dominant female decided to pair off, making the consort relationship public, so to speak, and defying the mores of the troop? The alpha male now has 100 percent assurance that he will be siring offspring with the dominant female. This is a significant improvement over the crapshoot where he must compete with all the other males in the promiscuous free-for-all. Granted, he might also want to mate with other females as well — but here we are encountering a story that recurs throughout human history. For the time being, he is improving his mating possibilities by monopolizing the most desirable female.

Meanwhile, for the alpha female there is also a vast improvement. She now has the assurance that the alpha male will be siring her offspring. She no longer has to undergo the ordeal of mating with every available male for more than a week. But one problem remains: What about infanticide? After she gives birth she might still encounter a sub-dominant male who knows he is not the father and wants to make her available for his offspring. Living in a large, tightly engaged group, this now becomes a perpetual problem — unless the alpha male stays with her. For the alpha couple, then, pairing off improves mating success for both of them — but only if he remains to guard his offspring after it is born. Since he knows for certain that it is his offspring, however, he will be willing to guard them. And so a permanent, monogamous relationship is born.

But what about the rest of the troop? What happens to them? Well, once the alpha couple have paired off, the beta couple now find themselves in the same position. They have the same advantages in forming a pair bond. Moreover, they have the example of the alpha couple to justify them. After that the gamma couple have the same advantages, and so on down the line — much the way it happens in high school. The important thing is that the solidarity of the troop is maintained. It is now possible for the males to get along with each other with only a minimum of sexual rivalry — unlike polygamy, where males are constantly competing for control of numerous females and the lowest-status males must be excluded.

For a group trying to live in close proximity, the example of the alpha couple becomes crucial. If the “king and queen” can be satisfied with each other, then everyone else can be satisfied as well. But if the alpha male collects a “harem,” then other males can have the same aspiration, and the free-for-all of unlimited sexual competition returns. This is another story that has recurred throughout human history.

Altogether, this is what in game theory is known as Nash Equilibrium, after the contribution of the great mathematician John Nash, the subject of the book and film A Beautiful Mind. Nash’s thesis, still the mainstay of all game theory, says that a system can achieve equilibrium when each player has achieved the best outcome he or she can under the existing rules. For a large heterosexual group with the same number of males and females, monogamy satisfies Nash Equilibrium. Each player has optimized his or her outcome under the rules of the existing system. The only way to improve an individual outcome is to break the rules. That causes other kinds of disruption and works to the disadvantage of the entire group, and so other members have incentive to constantly enforce the rules.

This is why human societies everywhere and throughout all time have enforced some kind of rules on marriage and have frowned on extramarital affairs. The stability of the group is at stake. If people start flaunting the rules of marriage, then the equilibrium is upset as growing numbers of males and females are left without mates. These individuals become disruptive, and the cohesion of the entire society is threatened. Monogamy does not maximize the interests of every participant. What it does is optimize everyone’s individual outcome in a way that maintains the integrity of the entire society, whose credo is “a girl for every boy, a boy for every girl.”

— William Tucker is a journalist and the author of several books, including Excluded Americans. This article is adapted from his most recent book, Marriage and Civilization: How Monogamy Made Us Human (Regnery).