An article about monogamy by Dr. Kate Lister, who goes by the handle @WhoresofYore, was trending on Twitter on Friday. In the first paragraph, Lister asks, “Given the obvious interest in non-monogamy, you have to wonder, when did monogamy become the norm?” Unfortunately, she never really answers the question.
So I’ll try.
The answer is that monogamy became the norm for humans millions of years ago. About 10,000 years ago, it fell out of favor, and then just in the last few hundred years or so, monogamy came back. That’s a complicated answer, but we are a complicated species.
Dr. Lister is right that monogamy is rare in the animal kingdom and rarer still among mammals. Birds are the only class of animal where monogamy is common — 90% of bird species are monogamous — and even then birds cheat on each other far more often than scientists used to believe. It wasn’t until scientists could compare a subject’s DNA that they began to test whose hatchlings belonged to who. But once they did, they found out that the percentage of extra-paternal offspring in many bird species exceeded 20%. By contrast, in human societies that practice monogamous social norms, extra-paternal births vary between 11% and 2.5%.
So compared to other “monogamous” species, we humans are actually pretty good at it!
Turning to primates, monogamy is rare, but a pattern emerges when you look at how each of the primate families operates. Dr. Lister mentions that gibbons are monogamous, but they are actually far more “monogamish” than we are. They only pair off for a couple of years at a time, and even then, they usually cheat on their spouses.
That is not true of tamarins, who mate for life and are very loyal to their spouses. We can contrast the monogamous tamarins, and the monogamist gibbons, with the polyandrous chimpanzees, who have sex with everybody. Looking at the family structures of these species, we then see that the monogamous tamarin males provide just as much care to their offspring as tamarin females do, gibbon males help out a little but provide nowhere near as much care as female gibbons, and chimpanzee males don’t help at all.
The more monogamous a species is, the more likely it is that mom and dad work together to raise the young.
And it was the growing cooperation between our increasingly monogamous ancestors that sets us apart from the rest of the primates and gave us the calories we needed post-birth to grow bigger brains and conquer the world.
All this began to change about 10,000 years ago when those humans who began to adopt agriculture became more numerous and more powerful than their wandering relatives. This transition from mobile hunting and gathering to sedentary agriculture was the most violent in human history. The fossil record shows that, as a percentage of total population, more men died from war in these years than in any other time in human history, including World War I and World War II.
Those conquered but not killed in these battles usually became slaves, and those slaves were not equally distributed among the victors. Only the richest and most powerful families could afford to buy and keep slaves, so as the size and complexity of these societies grew from city-states into empires, a familiar pattern took shape. Anthropologist Dr. Laura Betzig explained that in any large ancient empire “powerful men mate with hundreds of women, pass their power on to a son by a legitimate wife, and take the lives of men who get in their way.”
And so polygamy became the dominant human mating pattern for thousands of years. This only began to change with the advent of Christianity — as monogamous as the Romans and Greeks were among citizens, sex with slaves was accepted and even encouraged. But it’s not as if Jesus delivered the Sermon on the Mount and then every European became monogamous the next day. It took time, centuries even, for the Church to impose or, really, reimpose, monogamy on the West, but it did eventually become the norm.
The rest of the world was still dominantly polygamous though. The Mongols, Aztecs, Mali, Vikings, and Babylonians were all highly polygamous societies. It wasn’t until just recently that a majority of humans lived in societies where polygamy wasn’t accepted. Japan outlawed polygamy in 1880, Russia in 1920, Thailand in 1935, China in 1950, India in 1955, and Nepal in 1963. And even then, a full 25% of humans still live in polygamous societies today.
The richest, most powerful, most technologically advanced societies are all monogamous. And that is because monogamy just produces a better environment than polygamy for human cooperation and development.
The big problem with polygamy is that it doesn’t take a lot of wealthy men collecting two or three wives before there are large percentages of men with no wife at all. Large numbers of unmarried men is bad for any society. Unmarried men are more likely than married men to gamble, use drugs, and commit rape, robbery, and murder.
Polygamous households are also far more chaotic than monogamous ones. Nations with strong monogamous norms have less domestic violence, less maternal mortality, and less sex trafficking than polygamous societies. Polygamous households have also been shown to have higher rates of abuse, neglect, and homicide of children than monogamous ones.
None of this means that monogamy is easy for any individual to practice. Like anything worth doing in life, monogamy takes work. You have to be patient and communicate and listen to your spouse — really listen, not just look at your phone while they talk. You have to be brave and share your innermost feelings and leave yourself vulnerable to being hurt. And yes, if you are lucky enough to be uncommonly wealthy or physically attractive, you may have to forgo the occasional extramarital sexual opportunity.
But if you find the right person, all that work is definitely worth it.
