September 20, 2004
Humans Have Been Polygamous For Most Of History

More men than women have failed to reproduce in each generation.

Men and women differed in their participation in reproduction, the researchers report. More men than women get squeezed out of the mating game. As a result, twice as many women as men passed their genes to the next generation.

"It is a pattern that's built up over time. The norm through human evolution is for more women to have children than men," said Jason Wilder, a postdoctoral fellow in UA's Arizona Research Laboratories and lead author on the research articles. "There are men around who aren't able to have children, because they are being outcompeted by more successful males."

Co-author Michael Hammer, a research scientist in UA's Arizona Research Laboratories, said, "We may think of ourselves as a monogamous species, but we're coming from an evolutionary history that's probably slightly polygamous. If we're shifting toward monogamy, it's so recent it hasn't left an imprint on our genome."

Or the same reproductive behavior is continuing, but in a culturally accepted fashion, Wilder said. "The modern version that we generally don't find offensive is that men tend to remarry and have more children much more often than women do."


The team's research also overturns the long-accepted idea that, on average, women's genes traveled farther from their birthplace than did men's. That idea was based on a common marriage practice called patrilocality, wherein women tended to move from their natal village to their husbands' village.

If anything, men and their genes moved farther overall, the new research indicates.

To sort out how far men and women's genes traveled, the UA researchers used DNA from the Y chromosome, which is passed from father to son. Women's lineages were traced using mitochondrial DNA, which passes from mother to daughter.

The researchers report their findings in two related articles, one in the online early edition of the October issue of Nature Genetics and one in an upcoming edition of Molecular Biology and Evolution. The research was funded by the National Institutes of Health.

Scientists have puzzled over the fact that men's common ancestor, dubbed Y-chromosome Adam, seems to have lived around 100 thousand years ago, whereas women's common ancestor, known as mitochondrial Eve, lived almost 200 thousand years ago.

Worldwide, the DNA from the Y chromosome has much less genetic variability than does mitochondrial DNA.

"We wanted to know what shapes the patterns of Y-chromosome and mitochondrial DNA variation," said Wilder. "What can we learn about human behavior?"

To find out, Wilder, Hammer and Zahra Mobasher, a research specialist at UA's Arizona Research Laboratories, tested Y-chromosome DNA and mitochondrial DNA from three far-flung populations of humans: the Khoisan of southern Africa, Mongolian Khalks and highland Papua New Guineans. For each group, DNA from 24 or 25 people was tested.

Previously, researchers assumed equal numbers of men and women procreated. Based on that assumption, scientists explained the relative youth and low variability of the Y chromosome by suggesting that a beneficial mutation on the Y had swept through the whole world. However, the genetic patterns the UA researchers found contradicts those ideas.

If a beneficial mutation had swept through the males, men's common paternal ancestor would be the same age no matter where the UA researchers looked. Instead, the age of men's common ancestor differs between the southern African, Mongolian and Papua New Guinean populations studied. The finding tends to rule out some global beneficial mutation as the reason Y-chromosome DNA is less variable than mitochondrial DNA.

"Because we don't think the pattern we see was caused by an event that swept across the globe, we had to re-examine our assumptions about whether equal numbers of men and women are mating," Wilder said.

The team thinks the genetic patterns are all about sex.

Or lack thereof. Lots more men than women are childless, and it has ever been thus, the researchers say.

My guess is that the legalization and increasing use of divorce has increased the gap between what percentage of women and what percentage of men manage to reproduce in each generation. The most sucessful men have a legally sanctioned way to have a series of wives while the least successful men become street people. Nature is harsh. Why we refer to such an unforgiving and brutal reality as mother nature is beyond me. What is loving and kind about natural selection and evolution in action? Nothing that I can see.

My further guess is that cheap DNA sequencing will reduce the percentage of men who manage to reproduce as women use DNA sequencing results to choose sperm donors.

In the longer run people will genetically engineer their offspring. So they will voluntarily reduce the amount of their genetic sequences that they pass along to their offspring. Also, some will clone themselves as a way to increase the amount of their own DNA that they pass along.

Update: Greater female reproductive success is likely to accelerate for another reason. Selective abortion of female fetuses is creating a shortage of female births and adult women in China, in Taiwan, and in India. This trend will probably spread to additional countries as well. A shortage of females will very likely select for genes carried by males who become more successful. Genetic variations that select for intelligence, drive, and a certain degree of masculine aggressiveness will likely be selected for. Physical attractiveness will be selected for more strongly as well. Out-going personalities might also be getting selected for.

Share |      Randall Parker, 2004 September 20 02:29 AM  Trends, Human Evolution

Patri Friedman said at September 20, 2004 2:53 AM:

This is basic evolutionary biology. The fact that some males have extremely high reproductive success must mean that some males don't reproduce. Men are high variance, women are low variance. (And it all stems from their differing contributions to reproduction).

I can't wait to see the looks on the faces of people who believe in "equality of the sexes" if we ever meet sexually-reproducing aliens...because they are going to have gender differences which are very similar to ours.

eli said at September 20, 2004 9:27 AM:

The reason some males are "more successful" is pretty obvious: lots of males get killed in wars, compared to females.

Kurt said at September 20, 2004 9:42 AM:

So much for the social conservatives who say that monogamous marriage has been a "natural" institution for "thousands of years".

Invisible Scientist said at September 20, 2004 11:53 AM:

If my memory is correct, there are some articles that say that as much as
10 % of the population of the western industrialized countries, happens to be

Invisible Scientist said at September 20, 2004 12:45 PM:

But I must also add that technology is making the society even more
selective than before. The rate at which there is separation between
the poor and the rich is increasing with technology. And in the future the
pressures will increase dramatically, in such a way that people will become
obsolete very fast.

jgordon said at September 20, 2004 2:43 PM:

Marriage has likely never been a "natural" institution except perhaps in the sense of short term serial monogamy. In fact one of the purposes of marriage was probably to put a societal check on the tendency for a small percentage of highly successful males to monopolize all of the females. From a certain point of view, monogamous marriage is a kind of socialism that redistributes (by preventing polygamy) a scarce resource (namely females). I suspect this was originally implemented as a means to reduce friction and eventual warfare over reproductive access to females. As Randall has commented in his post increasing use of divorce seems to be eroding this particular use of marriage.

jgordon said at September 20, 2004 2:46 PM:

One more thought. Might it be possible that in the future some new social institution will arise to attempt to limit the increase that technology has on selectiveness?

Randall Parker said at September 20, 2004 3:10 PM:

What sort of institution could do that? I think it would take government power to accomplish such a goal and tremendous amounts of government power at that.

Invisible Scientist said at September 20, 2004 7:48 PM:

Someone says above that marriage as an institution, was for the purpose of reducing social
competition for females, to prevent warfare, and that the current rate of divorce is eroding this
use of marriage. But the current competitive situation might explain why the world is going into
war now. Maybe the current military tension between civilizations, is also connected with this theory.
And if this is true, the future looks very warlike.

jgordon said at September 21, 2004 11:19 AM:

You're probably right that it would require fairly intrusive government power to accomplish. However, I personally make no predictions as to what form the institution would be. I just offer that in an increasingly selective future, great masses of people, seeing themselves become increasingly disenfranchised from reproductive success, will form alliance and use the power of numbers to rectify the situation (Or at least attempt to rectify the situation.)

Invisible Scientist:
You may be right that the current tensions in the world are at least somewhat related to increased competition for reproductive access. I guess one would need data on the ratio of young males to young available females in the societies in question. I don't have any data to offer.
As an aside, I'm wary of trying to explain everything in terms of 'sex as destiny'; humans, being complex, do have other motivations. However, I have to admit that if you buy into evolutionary psychology (which I do), it can be tough to avoid 'sex as destiny.'

Patri Friedman said at September 21, 2004 6:32 PM:

Invisible - I think its 5% - 20%, depending on country. Also, there is far less military tension between countries now than at most times in history.

Eli - wars are a symptom, not a cause. They are one of many ways that men compete for status and resources.

jgordon - I encountered the theory about monogamy as socialism in _The Moral Animal_. The suggestion there was that it was not just about keeping men from fighting over women, but keeping there from being a large population of young, low-status, single men. The rate of violence and crime among that population is vastly higher than among married men.

Fˆz said at September 22, 2004 2:04 PM:

Handing control over procreation to one gender exclusively has not benefited the race.

I remark on this post here.

[[lots of males get killed in wars, compared to females.]] In the past that may have been true, and was part of a feedback pattern that reinforced a paternalist order that controlled access to sex. It's diminished now to the noise floor, as combatant casualties decline and non-combatant deaths escalate.

[[the current tensions in the world are at least somewhat related to increased competition for reproductive access.]] Without question, this is a motivator for Middle Eastern men to participate in terrorist acts that culminate in their deaths.

Luke Lea said at September 24, 2004 9:53 AM:

We need to make a distinction between marriage, which is a public institution, and sexual intercourse leading to pregnancy, which is a private act, and may or may not occur inside a marriage. Thus, to call the different effective population sizes of our male and female ancestors evidence for "polygamy" in the past is misleading; it is more likely evidence for widespread infidelity by women married to low status men during the fertile time of the month. Helen Fisher, I recall documents the greater waywardness of modern young married women during this part of their fertility cycle in the contemporary bar scene, and there is reason to suppose that something similar may be a hard-wired, evolutionary trait, at least in my opinion. It just makes too much sense. The progeny of those who follwed such a (probably unconcious) strategy would have more offspring than those who didn't. I would like to hear the contrary arguments.

Tj Green said at September 24, 2004 6:18 PM:

Sex has made us aggressive.This is what I have against the "Terminator" senario.Computers would evolve without sex,and therfore would not be aggressive.Our recent history has consisted of bands,or tribes,with psychopathic leaders.Once one tribe had developed a technical advantage,then they would attack the other tribe.It would have been the technical advance of farming that changed the rules of the game.Growing population,and changing the enviroment,created diseases like malaria.To combat this threat we see the prominance of sickle cell in Africa,and thalasaemia in Asia,and cystic fibrosis in Europe,to combat typhoid and cholera.Tay-sachs for tuberculosis. We all carry lethal genes,that are there for the survival of our species.The psychopaths were/are wrong,killing each other is suicidal.

Dave Gore said at October 30, 2005 8:10 AM:

The statement "twice as many women as men passed their genes to the next generation" cannot be true. The greatest percentage this would allow is 100% of the women and 50% of the men. Do you seriously propose that only 50% of men have passed on their genes to the next generation? A more plausible statement would be "twice as many men as women failed to pass on their genes to the next generation." The percentages passing on their genes then might be 87% for women and 74% for men, or 80% overall. This 80% overall figure was cited by Steve Olson in his article "The Royal We" in The Atlantic Monthly.

CASpears said at March 19, 2006 8:34 AM:

Some of this may be misleading. First off, men tend to die more and earlier than women naturally...for various reasons. Who knows maybe women are more important biologically. Men engage in more risky behaviors, such as speeding in cars, dangerous sports, etc. Men are more likely to die in war, and in the past this was common. Young men who had no yet reproduced are most likely to die, now that is at 18 or so, but in the past that would have been maybe at 13. It is true that in most societies that were semi-tropical to tropical men of means married multiple women, or had one formal wife, and mistresses or concubines. This was even common in the bible...which very few ministers seem to point out today. In fact I can't think of a passage in the bible where it says that a man can not marry more than one woman at one time. It only speaks about what the relationship should be like between a man and his wife, not how many wives he can have.
Marriage as we know it is an invention of European design...well in this countries, in Europe and other temperate climates, people could not produce the surplus wealth based on year round agriculture to maintain a household of multiple wives and children, so that did not before part of the culture in most areas of Europe. When they colonized most of the world, starting in the 15 century they took this ideal with them and said it was due to Christianity, but this is false. This was a pre-Christian economic convention. There were still Christians in Ethiopia and parts of the Middle East that had multiple wives during this time...being that most Christians lived in Europe, their standard became the dominant Christian behavior and they enforced this. Our society and the society of most countries now is based on this idea of a "nuclear" family...a concept which came from Western Europe. One now can not live in this type of society and afford multiple wives or households.
In most of the world the idea of marriage as one woman and one man was not dominant until maybe the 16 hundreds, even if then...maybe the 17 hundreds.
Due to divorce we might be seeing a reverse of this trend. Divorce, at least in this country being more socially acceptable, divorced men have a higher remarriage rate than divorced women (usually because if children are involved, they live with the woman, and are a burden, because a large percentage of men don't want to deal with another man's children, whereas a divorced man with children not living with him can more easily find a wife who can accept the children of the previous marriage coming over on the weekend, whatever...) so it seems that men are returning to the opportunity of having multiple wives and children by them, even if not at the same time...but this often causes economic problems for that man, because the society is still not set up to be like this.
As much as hate to use this an example, it is relative I think. African Americans due to the amount of men who are imprison, gay, die prematurely due to violence, etc are in a situation that is similar to people in ancient times. Quite simply, a shortage of men. This is well known among black women in this country. There is no shortage of men in America, but due to social barriers, most black women try to find a black men and do not date out of their ethnicity. The poor the black community and the poorer the black woman the more obvious this is, and this might be the reason that many lower class black women, who are often referred to as ghetto, appear to dress and act hypersexual. They know they are in great competition for available men who might want them with other women. Maybe this is also why these women are more likely to sleep with multiple men, hoping to get one or keep one, and also why they are more willing to “take another woman’s man.? They are faced with a biological imperative to reproduce and they have a cultural norm that comes from Europe that says they should be with one man and live with him in love forever and ever, but that is not the reality of their situation. Black men in these positions also realize they have an advantage and can “get away with whoring around? because they are a commodity that is rare. Whereas a typical white American man who is not endangered by any means and common, has to up his game to get the average white woman that he can be comfortable with because she has a wider choice.

Nancy Albert said at September 14, 2007 11:16 AM:

It is too bad no Muslims were genetically analyzed, because each generation loses 25% of male genetic material, given that half the men get two wives. Additionally, 30% of marriages are to first cousins so there is an alarming rate of genetic problems.

The need to dispose of extraneous young men in order to maintain a stable society drives warfare and explains the brutal history of Muslims and their present level of violence against each other. I believe the Torah established monogamy as the guiding principle of Judeo Christian culture, not latter-day European wealth.

robogirl said at April 10, 2008 7:09 PM:

Mitochondrial Eve is our most recent common ancester through the all female line, not our only common ancestor.Due to fact that mitochondrial DNA was originaly bacteria traceing the female line with mitochondrial DNA will only take us back so far. If Adam can be traced back 100 thousand years there had to be women, and I think it's safe to say Eve had a mother.

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