Men who have lots of babies with lots of different women seem to be having a moment right now. Between Elon Musk’s outspoken pronatalism and Andrew Tate’s penchant for sexual aggression, it’s easy to see how somebody might think that we may be about to enter a new social phase: the death of monogamy. And indeed, it’s trivially easy to find discussions of emergent polygyny in the more manospheric corners of Reddit and in incel Discord servers, which describe “Chads” with numerous sexual partners as just one manifestation of this dynamic.

This all raises an interesting question for a demographer like me: is polygyny the future? Will we see, as we did during the expansion of steppe ancestries in the Bronze Age, a dramatic increase in the inequality of male reproductive success, with a few men having dozens of children by many women, while twenty, thirty, or perhaps even fifty percent of men have no descendants at all?

Humans have pretty much always been monogamous. We’ve been coupling up since before we were even humans. We know this from numerous different sources. Across mammalian species, the ratio of male-to-female body mass is a strong predictor of the intensity of male competition for mates, which in turn is a strong predictor of whether males tend to be monogamous or polygynous. Humans have male-to-female body mass ratios most like those of other monogamous primates. Likewise, animal species in which single alpha males maintain harems of females tend to have prominent built-in weaponry: horns, antlers, claws, extra bone plates on the skull, extremely sharp teeth. Humans have none of that. Instead of claws, we have fingernails that break easily, and hunter-gatherer communities rarely even show ethnographic evidence of boxing traditions: humans appear to be physically adapted not to fight to the death over mates. There is more such biophysical evidence: human testosterone levels, penis sizes, sex ratios at birth, and displays of aggression all support the idea that humans have been more-or-less monogamous for as far back in our genetic record as one can go, probably even before the advent of Homo sapiens.

Modern genetics can tell us approximately how many independent mitochondrial DNA and Y-chromosomal lines left descendants in the human genetic record at different time periods over the past 150,000 years. We can calculate the ratio of these, and thus know how many breeding men there were per breeding woman at any one time. For modern genetic databases, the ratio is around 2–3 mtDNA lines per Y-chromosomal line. For the period between 150,000 and 60,000 years ago, the ratio was also around 2–3 mtDNA lines per Y-chromosomal line. The ratio rose to about 3–4 around 60,000 years ago and remained there until about 20,000 years ago. Then it took off like a rocket. By about 4,000 BC, there might have been as many as 12–20 mtDNA lines per Y-chromosomal line. Since 4,000 BC, however, that number has dropped back to the levels we observe today.

So, what happened? During the Upper Palaeolithic period, there were rapid developments in a wide range of human technologies, including weapons. This period saw a massive improvement in the quality of stone blades, and the proliferation of projectile weapons like atlatls and bows. At the same time, a separate line of genetic evidence shows that the average age of fatherhood declined sharply. The revolution in weapons technology made roving males vastly more lethal and dangerous than they had been before. These men did what men with massive superiority in violent capabilities do: they killed other men and acquired harems. Where mobile human societies retained major advantages, polygyny lasted longer: even today, polygyny is most common in pastoralist and nomadic societies, and polygyny was most extreme in nomadic, pastoralist societies 6,000 years ago as well.

This period left a major genetic imprint on modern humans, of course. But the rapacious men of 6,000 years ago weren’t breeding with equally rapacious women: they bred with monogamous women, who perpetuated ancient genes for monogamy. Today, then, while you carry plenty of DNA from marauding Palaeolithic and Iron Age pastoralists who hunted for brides, more ancient ancestry predominates. You still share most of your genes with humans from 20,000 years ago, at the dawn of the explosion in polygyny.

Polygyny, then, became widespread during one episode of human history. In many areas, various more militarily capable groups almost completely eradicated prior populations, especially the males—but only almost completely: Neanderthals may be extinct, but almost all humans outside of Africa carry Neanderthal DNA. The ancestors of modern Asians may have wiped out the Denisovans, yet most Asians carry some Denisovan DNA. Population replacement is almost always accompanied by some degree of population mixing.

And, of course, polygyny didn’t stick. Today’s polygynous pastoral nomads have been relegated to a handful of pockets of steppe lands in central Asia, together with a broad swathe of the Sahel, where continuing low economic productivity and the moral imprimatur of Islam have given polygyny a second wind.

So, why did polygyny die out? Because it sucks. Watch a bull moose in rutting season and you’ll quickly realise that polygyny is a dead-end reproductive strategy. It is incredibly wasteful for a species to produce large numbers of males who will never reproduce.

While that kind of reproductive strategy can be stable under various conditions that are common among other animals, it makes no sense for a species like ours with a long period of immaturity during which parents have to make major investments of time and energy in their children in order to ensure they survive into adulthood.

Moreover, since human competition is almost always team-based rather than just head-to-head—teamwork is just as important in war and hunting as it is in domesticity and farming—human societies benefit from keeping as many team members around as possible. Very few other species fight as organised armies. Ants and bees do—and entire ant and bee colonies may have only a single queen who mates with tens of thousands of males. For species that operate cooperatively, polygyny is idiotic.

The upshot of this is that the most successful human societies are those that can produce and sustain lots of human muscle and many human brains. Monogamous societies are better at that because monogamous women have more babies and those babies are likelier to be healthy. We can see this when we compare population growth trends in ancient Greece and Rome with those of Persia and of their pastoralist barbarian neighbours: the Romans and Greeks were able to field armies and replace casualties at an extraordinary rate compared to the Persians, Celts, and Gauls. These societies all had various degrees of polygyny, especially for high-status men. The foes who gave Rome the most trouble, the Carthaginians, were apparently monogamists: Punic stelae containing family member lists never mention more than one wife, whereas, as Caesar notes in his Gallic Wars, Celtic chieftains often kept many wives. Likewise, the Greeks famously regarded Persian polygyny with disdain.

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Another classic case of the repeated failure of polygynous societies to persist can be seen in the Carpathian basin of Europe. This region was governed by the monogamous Romans as the province of Dacia for 170 years before being abandoned in the 270s. By the 370s, it had been conquered by the polygynous, nomadic, pastoralist Huns under Attila, though Hunnic dominance didn’t last. The Avars (polygynous, nomadic, pastoralist) came next, around 600 AD, though their khaganate collapsed around 800 AD under constant military pressure from the monogamous Franks to their west. The region would briefly be free of pastoralist predation until around 862, when the Magyars (Hungarians) arrived—they, too, were polygynous, nomadic, and pastoralist. Magyar rule lasted longer than that of the other tribes because they converted to Christianity and became monogamous. By the time the Ottomans—polygynous, and originally nomadic pastoralists—showed up on the borders of the Carpathian basin in the 1500s, their polygyny was seen as barbaric and foreign, even though three successive waves of polygynous invaders had successfully ruled the region for half a millennium.

We find a similar pattern whenever we have historic accounts of contestations between polygynous and monogamous societies: often, monogamous societies run roughshod over polygynous ones and convert them to monogamy. Even when polygynous societies conquer monogamists, they are as likely to be converted to monogamy as to propagate polygyny. Polygynous Muslims conquered much of Spain and the Balkans, yet today those regions are as monogamous as the rest of Europe. The victory of monogamy occurs most often through a mixture of greater internal cohesion and larger numbers. Because the social norm that each man can only have one wife removes a huge source of possible competition between men, monogamy boosts internal cohesion. It reduces aggression between men within a given society, enabling this aggression to be projected outwards towards external foes.

It may not be obvious why monogamy should be associated with more births per woman. And yet it is: around the world, monogamy is the most fecund marital arrangement. In the United States, devotedly monogamous Amish and Ultra-Orthodox Jews have, by far, the highest fertility rates. In Finland, Laestadian Lutherans retain stable fertility at around three or four children per woman, even as Finnish fertility in general is collapsing: the Laestadians are a devout, pietistic sect, but they are economically and socially integrated, and politically organised—and, most importantly, they are sexually conservative, requiring strict monogamy of their members. Even within more mainstream American society, the more determinedly monogamous evangelical Protestants have higher fertility rates than the nonreligious. Back when Europe had high fertility rates, fewer than 2 percent of all babies came from cases of extra-pair fertility.

On closer inspection, America’s two most prominent elite polygynists are no exceptions to the rule that polygyny does not much improve reproductive fitness. Elon Musk has thirteen children by four women. His brother, Kimbal Musk, has three children from three relationships. His sister, Tosca Musk, has two children. Thus, the Musk family has produced eighteen children from eight mothers, giving an average fertility rate of 2.25 children per woman, while the average completed fertility rate in the United States was around 2.1 children per woman in 2022. The Musk family fertility rate is therefore about average. Andrew Tate claims to have fathered ten to twelve children by an unspecified number of women—though none of his claims have been confirmed; his brother and sister each have two children. Very likely, the Tate family lineage could be averaging even less than two children per woman if Andrew Tate’s “ten or twelve” children were with more than three or four different women.

Thus, it turns out that many men who are highly polygynous have partners who have average or even below-average fertility rates. Polygynous men spread their own genes at the expense not only of other men’s genes, but also at the expense of the reproductive success of their mates.

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This is true globally. In Africa, where polygyny remains common in many countries, otherwise similar women in polygynous relationships have fewer children than their religiously and economically similar monogamous neighbours. The difference is not enormous, but it doesn’t have to be. Social norms around marriage and mating tend to be pretty sticky across time, and so if monogamous couples are even slightly more fertile across many generations this will lead to monogamists becoming more numerous than polygynists. In almost every country where polygyny existed forty years ago, it is less common today.

In the United States, data from the National Surveys of Family Growth inform us which men have had the most babies: men who have had one wife for a long time and aren’t sleeping around. Among men over age forty (an age chosen to capture completed fertility), fertility is highest for married men who have only ever had one sexual partner and declines as lifetime partner count rises. Likewise, among married men over forty, fertility was highest for those who reported having sex with exactly one women in the prior three months: men with no sexual partners obviously had lower fertility, but men with multiple sexual partners also had lower fertility. Serial monogamy also decreases fertility: the only men with above-replacement fertility are men whose average union has lasted longer than ten years, and lifetime accumulated fertility rates decline the shorter a man’s average unions are. Elite status doesn’t change this: monogamy is best for rich and poor men, highly educated and less highly educated men.

Thus, monogamy is the only serious path to healthy fertility rates. Polygyny reduces fertility.

One of the reasons society should favour the tried-and-true norm of monogamous marriage is that it is the most fecund arrangement, most likely to produce children in the kind and quantity we need. The future belongs to monogamists. Stable unions based on loving commitment produce more babies and happier and healthier babies and provide a strong basis for social cohesion. In evolutionary terms, monogamists are fit.