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In humans, males and females have a (theoretical - in practice there are more women) sex distribution of 50:50 in any given population (which is large enough to account for the law of large numbers) due to its biology.

  • Suppose, this ratio was 3:1 males:females, so that we now have 3 males for every 1 female.
  • Suppose this ratio is derived from our biology (so not a result of technology or societal custom) and appeared early on in our evolution.

How would human society have evolved as result of this?

Consider

  • Mating/relationships
  • Societal structure
  • Gender roles
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    $\begingroup$ 2 out of 3 males will kill themselves fighting for the 1 female, leaving us with a nicely balanced 1-1 ratio once again. ?I'm not sure if I'm being sarcastic with this comment? $\endgroup$ – Twelfth Apr 7 '15 at 0:39
  • $\begingroup$ @Twelfth While competition between males is a valid consideration I think you're oversimplifying things just a tad. $\endgroup$ – Dider Apr 7 '15 at 0:49
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    $\begingroup$ Heh, it was semi-sarcastic :) If you are talking ancient times though, the concept of raiding a village/town/city to kill the men and steal all of their woman becomes much more of a possibility. $\endgroup$ – Twelfth Apr 7 '15 at 0:51
  • $\begingroup$ Why not kill some male babies at birth to even it all up. $\endgroup$ – Robin Apr 7 '15 at 9:47
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    $\begingroup$ Wasn't this the situation of Australia a couple centuries ago? As former british prison colony it had unreasonably high male:female proportions. IIRC the effect was that women were very privileged and respected. $\endgroup$ – SF. Apr 7 '15 at 22:18
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I hate to say it, but if such a trait evolved it would quickly un-evolve.

Consider pregnancy. Each pregnancy is caused by exactly one man and exactly one woman. It is impossible to increase the absolute number of pregnancies caused by men without increasing the absolute number caused by women. This is fairly clear. In any society, the absolute number of pregnancies caused by men must be divided amongst the number of men in the society. Likewise women. If there are N total pregnancies, C women and 3*C men, it is clear that the expected pregnancies per woman is N/C while the expected pregnancies per man is N/3C. The total expected pregnancies resulting from having one child with this gender ratio is (N/C + 3 * N/3C) / 4. Which reduces to N/2C.

Now, consider a mutation that changes the ratio from 3:1 to 2:1. The total expected pregnancies resulting from the person bearing that mutation would be (N/C + 2 * N/3C) / 3. Which reduces to 5N/9c. The person bearing this mutation would expect to have about 11% more grandchildren than the person not bearing it. And their grandchildren will have 11% more grandchildren. And so on. This really isn't a trivial increase in fitness, and the mutations needed for it to occur are relatively simple. Going back to 1:1 requires an even simpler mutation (just break the old mutation that caused the 3:1 ratio) while providing even more fitness.

3:1 birth ratios just aren't stable.

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  • $\begingroup$ This is true only if number of pregnancies is the limiting factor for population growth. Male dominated sex ratios can be stable and beneficial, however, if child rearing is the limiting factor and is primarily done by males. $\endgroup$ – ckersch Apr 8 '15 at 14:20
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    $\begingroup$ No, this is true regardless of that. Evolution is profoundly selfish, it just doesn't care if it's making the whole society worse off by ever so slightly decreasing the number of caregivers, all it cares about is that the person bearing the mutation is 11% more effective than their neighbor who is not. $\endgroup$ – Saidoro Apr 8 '15 at 15:40
  • $\begingroup$ Evolution is selfish, which is why groups of males that aid one another in raising offspring tend to all be related. It's a phenomenon known as kin selection, in which an organism can further its genes by helping raise the offspring of its siblings. en.wikipedia.org/wiki/Kin_selection $\endgroup$ – ckersch Apr 8 '15 at 15:51
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    $\begingroup$ Kin selection isn't relevant here. Males who have the choice will invest about twice as many resources into their own children as those of their siblings because they have about twice as much of their own DNA as their siblings do. Further, there is no particular reason why they should invest more effort into the children of their female siblings than their male siblings. Children born to the 2:1ers will still have approximately as much child rearing being done as the children of the 3:1ers. Again, a mutation can be awful for society while also being highly successful. $\endgroup$ – Saidoro Apr 8 '15 at 17:33
  • $\begingroup$ The concept described here is called Fishers principle en.wikipedia.org/wiki/Fisher%27s_principle and it's has been proven to hold up against mutations that affect sex ratio. There is a way that a 3/1 sex ratio can happen though, if the sex ratio at reproductive age is significantly different, that is to say most of the males die before they reach adulthood. If only 1/9 of all males survive to adulthood then each male that reaches reproductive age has 3 times the mating successes to compensate for 1/3 surviving. Possibly due to males killing competition at young age? $\endgroup$ – dsollen Sep 29 '16 at 22:24
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According to this Wikipedia page, there is already an imbalance in sex ratio towards males (1.1:1.0). So I'm not sure the even distribution is practical towards sex ratio, at least in Homo sapien.

You want to know if this imbalance is taken to the extreme.

Mating/relationships

This can go one of two ways.

(A) If modern Homo sapien continues a focus on evolutionary advantage, males will be highly possessive of females. They will become hyper aggressive towards males who enter their territory, becoming more likely to attack if they feel their claim over a female is threatened or disputed. Females will largely become trophies, rarely leaving the male's side while in the public space. Males will certainly feel they deserve sexual intercourse with their female and the female has no right to deny him.

(B) If modern Homo sapien deviates from evolutionary necessity, females will be communal property. Males may not lay claim to a female for more than a night. Females will be expected to mate with as many different males as possible, to ensure diversity in the species. As a result of this artificial construct, females will control the relationship and males will take on an eye candy role. Males will have to prove they are better than other members of the species, much in the same way that animals display plumage or perform exotic dances to impress.

Social structure

If situation (A) manifests, society is going to be comprised of closed-door communities where males are dominant players. Females will be secluded from society, not permitted to wander on their own. Males won't even trust their female(s) to guards, since the guards are either male and have reason to break faith or they're female and are at the same risk as their charge. Males will flaunt their female(s) when they can, with more appealing individuals appearing with the male at every opportunity. Males may never leave home without their most appealing female under their arm. Males, of course, will be suspicious of each other, so there will be far fewer grand celebrations or gatherings at local bars. Sports will take on new connotations, perhaps where females are part of the reward. Females will certainly become a teaser for males to use to keep other males in their influence: "If you do this for me, I'll let you sleep with Amelia for a week."

If situation (B) arises, females will more often hold positions of power. Sex will be given out as special favors and rewards. Females will be free to choose who advances on their merits, rewarding loyalty and intelligence with less emphasis on who knows who. This kind of society will be far more stable, as males will try to keep females in a good mood to raise their own chance to have sexual intercourse and pass on their genes. There won't be much upheaval, as the minority will control the majority of power and influence. Males will still have the greater ability to flaunt their sexual endeavors. Females would be expected and almost required to have sex with as many males as they could, but males have limited opportunity. When a male manages to have sexual intercourse with a female, he can flaunt that reward/privilege.

Gender roles

In both cases I've outlined above, females are still the "eye candy," so I'll explore a few other areas of general society.

In society (A), males run the government exclusively. They're in charge of stores, organizations, and companies. There will be few females in any field that involves entering the public domain. They are unlikely to obtain a higher education prior to the development of the Internet. Their efforts will be dedicated to home-bound activities, such as knitting, painting, and animal husbandry.

In society (B), females will hold a majority of influence in government, even when they don't hold a majority of seats. Females will be given priority in higher education, possibly leading to higher numbers of females in the science, technology, engineering, and math (STEM) fields; these fields are currently dominated by males because males are statistically better at spacial and analytical skills.


As has been noted in the comments, this answer presents only the extremes of male and female behavior in this scenario. Examining the full spectrum of human sexuality and sexual behavior in this scenario, or any scenario, to be honest, is far too broad and deep a topic for an appropriate answer on this site. The two situations I present are intended to serve as a guide, not as an absolute answer.

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  • $\begingroup$ I like this but, just a consideration, in the second scenario could the female not take on a hyper-dictator type role. Demanding sex or subservience in return for bestowing sexual or emotional relationships? $\endgroup$ – Venture2099 Apr 7 '15 at 7:45
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    $\begingroup$ @Venture2099 Certainly a possibility, but unlikely. Females are rarely of the power-hungry, dictatorial-rule variety. Evolution has always had males compete for mates, so it's the male that becomes the aggressor. Females among Homo sapien tend towards strong emotional bonds and care for their fellows. This ratio difference will throw these to the extremes. There will probably be some society where there is a hyper-aggressive male dictator, regardless of the authority of females. $\endgroup$ – Frostfyre Apr 7 '15 at 12:03
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    $\begingroup$ That's a pretty black and white way to see genders. I don't see sex driving things THAT much because you can be 150% sure that males would also mate with males, regardless of how you feel about that, because -> History. So right off the bat, the ratio affects a lot more the procreation aspect of the issue than sexual desire. $\endgroup$ – Spacemonkey Apr 7 '15 at 17:40
  • $\begingroup$ @Spacemonkey Given that sexuality and sexual endeavors are an important part of human culture, there's an abundance of full-fledged books that can provide a wider view than the generic perspective I've presented here. The human sexuality page on Wikipedia has copious amount more information. $\endgroup$ – Frostfyre Apr 7 '15 at 17:50
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    $\begingroup$ This sort of sounds like chimps vs bonobos... Chimps being group A and bonobos being group B. $\endgroup$ – apaul Apr 7 '15 at 23:08
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Sex-selection abortions in India and China are producing this situation already. Results:

  • Increased crime and violence. (Family life civilizes men.)

  • Increased sex-slavery and kidnapping of girls to satisfy the men who can't get a wife.

  • More wars to capture women from other countries.

  • Women's freedom of movement is restricted to protect them from predatory men.

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    $\begingroup$ I'm sorry, I can't agree with this, it feels like it's applying a superficial view of gender roles/stereotypes of today only, and is not accurate to changes. First I have seen no proof that having a family has a significant impact on ones tendency to crime or violence. Second sex slavery wouldn't happen once women were so scarce because men would be competing over the females, not conspiring to share them, kidnappings could go up, but not by much, this is controlled by culture and generally those that rape in a culture are just as prone to do it if they have a wife as if they don't. $\endgroup$ – dsollen Sep 29 '16 at 23:03
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    $\begingroup$ Likewise wars to capture women wouldn't happen unless rape of women were the standard of an entire civilization, and that degree of rape doesn't happen in large organized social groups with strong rule of law (because a man doesn't want his daughter raped and so passes laws to protect her, and because rape-prone species won't evolve human intellect or societies!), and such countries usually aren't organized enough for large scale war. Last bullet point is semi-true, but not due to predatory men. Realistically men would evolve to fight other men for mates, not rape. $\endgroup$ – dsollen Sep 29 '16 at 23:10
  • $\begingroup$ See healthymarriageinfo.org/for-the-media/news/news-detail/… which comments on a study about how married people commit fewer crimes, which is an indication of becoming "civilized". $\endgroup$ – Paul Chernoch Oct 14 '16 at 18:13
  • $\begingroup$ @dsollen Boy, you're in for a surprise if you ever went to parts of India where the sex ratio is ~800:1000 $\endgroup$ – insanity Jun 15 '17 at 11:30
  • $\begingroup$ @PaulChernoch You're conflating correlation with causation. That could also be explained by less violent people being more inclined to get married or some third variable that predicts both nonviolence and marriage. $\endgroup$ – Beefster Dec 7 '18 at 23:30
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The first and foremost question to look at is why this is useful.

Luckily, as always, we can look to nature for examples of animals that behave like this in the wild. Interestingly, there are two major patterns that seem to come up in populations with male-skewed sex ratios: environments in which there are lots of resources and environments where it is very difficult to raise offspring.

Babies for everyone! Environments with lots of resources:

Our case study for this situation is the African jacana. Jacanas live in a resource rich environment in which the cost of producing eggs (for a female) is negligible. With plenty of food, females can produce lots of eggs quickly, but the limiting factor is parenting.

Jacanas, unlike many species which approach this by having a single male with a harem of females, improve their reproductive success by doing the opposite. A single female jacana will have a harem of many males. She will sequentially produce eggs with all of them and they will all care for their own chicks. Females spend their time (when not making eggs) competing for males, since the reproductive success of a female is tied to the number of males in her harem.

If humans did this, it would be likely that babies would be born sooner and smaller, with more development taking place outside the womb. Men would probably evolve the ability to lactate in order to feed their babies, and women would be bigger than men. Since humans are group dwelling animals, it's likely that they would live in groups of mostly unrelated men, with one or several related females leading the family group. Men would probably do most of the work involved with obtaining food, in a mirror image of lion prides. Females without harems would probably form into small bachelorette groups of related women, which would rove the countryside hunting for unclaimed men to raise their children.

Everyone cares for the same babies. Environments with few resources:

Interestingly, the other situation in which strongly skewed male sex ratios are seen is in environments in which raising offspring is difficult. Examples of animals that follow this strategy are wild canids and alpine marmots.

In rougher environments, having more helpers to care for the offspring of a single dominant alpha increases the chances that the offspring survive. Having these helpers leads to what is known as a cooperative breeding strategy. While not all cooperative breeders have more males than females, in those that do, the males in a group are almost always related. Subordinate brothers and sons of a dominant male help him raise children. Unlike in jacanas, the female helps raise the children as well.

If this were the case in humans, we would live gathered into bands of related men around a few (probably related) dominant females. Male children would tend to stick around the group unless there were too many males for the number of females in which case the entire group would likely split up and follow a few different women. Female children would be driven off by there mothers once they started to get uppity. These women would then go off on their own, try to find other groups, and then kill the women leading those groups to take their men for their own. Again, they'd probably be bigger than the men since they'd be under evolutionary pressure to fight off marauding women intent on stealing their spots.

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If that were the only change in the society, I imagine the status quo would be very violent.

Males of most mammal species including humans have evolved to be the larger and stronger gender on average. It's a basic property of the sex hormones; sexually mature men build muscle mass more easily due to testosterone, while estrogen favors storage of fat. This is because many mammalian species have developed a basic strategy of "females breed, males protect". Even where this isn't the case, an even more basic strategy of fighting other males for the right to mate with a female (survival of the fittest) is inherited from older vertebrate classes and from invertebrates like insects.

In a situation where there are 3 men for every woman, but the men retain the strength typical of the gender, this competition would boil over frequently, resulting, on average, in 2/3 of sexually-mature men dying childless, either being killed in a fight for a mate, or being driven off alive but with blue balls to die alone in the wilderness.

There is another, more altruistic alternative, however it's hard to see how it would develop in the ultimately competitive Earth evolutionary strategy. That is that an average of three men "team up" to protect and take turns reproducing with a woman. The advantage of the strategy is increased protection and provision for each child; with three men to hunt for every pregnant woman and child, that kid's got a pretty good chance. The disadvantage is game theory; in a primitive, survival-oriented society, being the first of the three men in the household to mate is still advantageous, because who knows if you'll still be alive tomorrow? That will still cause competition in the household, which ultimately makes this a rather fragile system of cooperation. We see this in a lot of polyamorous relationships that have developed in recent decades, especially involving multiple men; sooner or later, one man will get jealous of the relationship that the other man has with the woman, and break off on his own. The one-to-one system that humans nominally encourage allows for the same societal development and mutual protection, with less of the innate competition ("I protect your wife and kid, you protect mine, but we keep our hands off each other's woman otherwise").

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