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Monogamy Made Us Human
In theory and in practice, it’s what keeps a society stable.


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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.

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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).



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