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What factors could skew the sex ratio of an intelligent species, especially a humanoid/human sub-race? For example, say one race has markedly more females born than males, and another race has the opposite, many more males than females. The race is unimportant, whether elves and dwarves, nymphs and goblins, respectively, etc.

What would lead to such scenario when births of one sex are so much more common than births of the other, assuming the parents aren't deliberately altering the ratio somehow?

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    $\begingroup$ Intelligence does not matter. What matters is the sex determination mechanism. For example, in many reptiles sex is determined by the temperature at which the eggs develop; there is a species of turtles, the green sea turtle (Chelonia midas), which uses this mechanism for sex determination, and, as a consequence of climate warming, is being confronted with an extremely skewed sex ratio. $\endgroup$ – AlexP Jan 20 '18 at 15:21
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    $\begingroup$ And, please, you don't want "gender ratio" (which is meaningless), you want "sex ratio". "Gender" is a social attribute; it controls whether a person is to be addressed as Madame or Monsieur, whether they wear skirts or trousers, whether they keep their hair long or short. The biological attribute is "sex". $\endgroup$ – AlexP Jan 20 '18 at 15:24
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    $\begingroup$ Are you asking for a biological reason or a historical reason? In other words, is this a species that naturally has an imbalance or one that has become imbalanced? $\endgroup$ – SRM Jan 20 '18 at 15:26
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    $\begingroup$ @AlexP : that is a modern ideological signaling. Many people still use gender to refer to male or female biology, and sex as the act itself. Just because you along with some people switched over to a recent fad doesn't mean you have to push it down the throat of others. $\endgroup$ – vsz Apr 8 at 7:47
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    $\begingroup$ @Demigan: My original comment (one year ago) was simply a notice that the standard phrase is "sex ratio" and not "gender ratio" as it was originally in the question. The point was that all relevant literature, including for example Wikipedia, calls it "sex ratio", and searching for "gender ratio" would be less useful. No further implications were intended; and the comment was indeed accepted and the question edited. I couldn't care less about what the English word for "sex" is; it's not my mother tongue or everyday language, and I have no affective relation to it. $\endgroup$ – AlexP Apr 8 at 11:42

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Disease linked to sex chromosome.

Here is how to do achieve this using the chromosomal system humans use. Humans determine sex with chromosomes: males are XY and females XX. There are diseases transmitted on the X chromosomes which mostly affect boys. An example is chronic granulomatous disease.
x linked disease http://disorders.eyes.arizona.edu/disorders/nystagmus-1-congenital-x-linked

If a boy is born with the mutation on his single X chromosome, his immune system will be crippled. A girl, however, has another X which can rescue her immune system with the normal gene. This girl will, however, be a carrier - any boy she conceives will have a 50% chance of getting the X with the mutation. A girl born with 2 X chromosomes both with the mutation will be affected but this implies that her father lived to reproductive age despite having the disease.

If you make this a disease that is lethal in utero, the 50% of males conceived who get a bad X will miscarry. So assuming that the ratio is 50/50 normally, this would produce a ratio of 75 females to 25 males; half the males die.


The converse would be a disease on the X chromosome that involved dose - for example, a mutant gene that made some product that was detrimental in high doses. To make it work the mutant gene product could increase reproductive fitness in moderate doses - the heterozygote state. For example, a gene on the X which led to greater fierceness or wakefulness or strength. Male goblins (XY) who have a mutant X would dominate the smaller goblins, and sire more of the children. The mutant X would spread through the population. Mothers with 1 mutant X would likewise be robust and get more than their share of resources. But if the mother had one mutant X and one normal, and conceived a baby goblin with a mutant X father, any double mutant X female conceived would have too much, and die in utero. The same scenario as the male one above 75% males and 25% females because double mutant X females die.

Sickle cell disease works this way (although is not transmitted on the X chromosome).

sickle cell genetics diagram A heterozygote individual with one mutant sickle cell gene is resistant to malaria and has a reproductive advantage. But 2 sickle cell genes (the homozygote) historically led to death in childhood. The fitness advantage of the heterozygote state outweighed the fitness disadvantage when 25% of offspring from 2 her parents die young - so the gene for sickle cell spread through the population.

I am thinking now about the Empress goblin, who is a double mutant x and so much stronger/fiercer than a single mutant x male, and considerably more than a normal goblin. Since before her birth she has been prevented from dying by constant medication that reduces the effect of the double dose X.

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    $\begingroup$ Case 1 will not work. You cannot have all women to be carriers of this disease: Even if every female in a starting population has it only some daughters will be carriers. And depending on your reproduction patterns the number of carriers may start dropping over time. For example, women from families known for inability to produce many male heirs will be less desirable brides. Eventually, the sex ratios return to 50/50. $\endgroup$ – Olga Jan 21 '18 at 18:40
  • $\begingroup$ Case 2 looks plausible but most likely result in three distinct genders associated with biological sex and chromosomes: Male (Xmutant Y), female (Xmutant X), and 'neutral' (XY). $\endgroup$ – Olga Jan 21 '18 at 18:51
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    $\begingroup$ One real-life example of a skewed sex ratio produced by such a genetic mechanism is the wood lemming (Myopus schisticolor), which has a sex ratio of about 3:1. They have a mutant X* chromosome that both overrides the effect of the Y chromosome (causing X*Y lemmings to be physically female) and has meiotic drive in such X*Y females (causing them to only produce X* ova). So the population consists of XY males and XX, X*X and X*Y females, the latter giving birth only to (X*X or X*Y) daughters. $\endgroup$ – Ilmari Karonen Apr 9 at 20:03
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It might seem like chromosomal sex determination would result in a 50/50 sex ratio (at least, at conception--sex-linked differences in survivability may skew that ratio even before birth, as outlined in Will's answer).

Nevertheless, the human primary sex ratio (at conception) is just ever so slightly tilted towards females... and then switches by birth, to an excess of approximately 7 males for every 100 hundred females. Since males post-birth have higher mortality rates and lower life expectancies than females, it has long been thought that this difference is an evolutionary response to ensure that the sex ratio around reproductive maturity is close to 50/50. But whatever the reason, it's clear that just having chromosomal sex determination in humans isn't good enough, by itself, to ensure a 50/50 split. Yes, it helps to start there (where other determination systems may not), but there are clearly other mechanisms at play, and there's no reason those mechanisms couldn't force any other arbitrary ratio, if there's a good reason for it.

So, you don't need to worry about the underlying fundamental biological sex determination mechanism, or chromosomal structure, or any of that. What you do need to worry about is

a) Factors that would provide an evolutionary pressure towards skewed ratios, by whatever biological mechanism;

b) Over what sub-population you are measuring the ratio. Do you want skewed ratios over the entire population, regardless of age? Do you want consistently skewed ratios across all age groups? Do you want skewed ratios for some specific age groups? Each of these options may be selected for entirely different reasons!

Since the question mentions more males being born vs. females, or vice-versa, I'll presume you don't actually care that much about total population figures, or ratios at other specific ages. In that case, the obvious pressures would be extreme mortality for one gender over the other; if, for example, men die at a much higher rate that women, for innate (e.g., genetic health) or social (e.g., war) reasons, but the species is generally (serially) monogamous, you'd expect to see a lot more boys born. But if, say, women have extremely high rates of mortality in childbirth, you'd expect the opposite--most babies would need to be girls, to make up for the ones who die early.

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  • $\begingroup$ Human sex ratio at birth favors boys over girls weakly, about 1.06 to 1. Then various factors come into play and for people over 65 years of age the sex ratio strongly favors women, about 0.7 men for 1 woman in western countries. $\endgroup$ – AlexP Jan 21 '18 at 1:49
  • $\begingroup$ Eh, 1.07 vs. 1.06... close enough. The point is still valid. Sex ratios at birth are not fixed by the nature of chromosomal sex determination. $\endgroup$ – Logan R. Kearsley Jan 21 '18 at 2:28
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    $\begingroup$ The comment was in support of the answer. The point is that sex ratio at birth and sex ratio later in life are not necessarily the same. $\endgroup$ – AlexP Jan 21 '18 at 3:27
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China is slanted heavily male because of the One-Child Policy that was in effect for so long. Russia is heavily female because of the shortened lifespan of men, generally attributed to alcoholism. There are so many varieties of ways to create gender-imbalance, it is hard to recommend one without more limits on your problem. For example, the China imbalance affects breeding-age population; the Russia imbalance does not.

What you need is some cultural activity that is gender-specific and life-threatening. Something like toxic makeup/tattoo work, or perilous hunting trips, or a fatal rite-of-passage, to list a few examples. Try to identify why the sexes in your society are divided and then identify a dangerous activity that sits on only one side of that divide.

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Have chromosome triplets. Where one of three chromosomes being an X chromosome is enough to make a newborn a genetic female but to be a genetic male it needs two of three chromosomes to be Y chromosomes.

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    $\begingroup$ Triploid is tough but tetraploid can work well and can be adjusted for any desired gender ratio. $\endgroup$ – arp Jun 6 '18 at 19:36
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These are great answers, but I was getting a different take on the question. Biology has lots of ways to generate such differences. I am going to look at factors that would put pressures on an entire species to skew without eliminating individuals.

  • reproductive advantage- the rate limiting step in population growth is how many females you have. I've bred mice, and you need one selected male and dozens of females. I imagine androgynous elves where reproductive rates might be slow and you need to maximize the number of females to repopulate. You could have all of a species born female, and only the rulers transition to male. In certain social fish, if the dominant male dies the alpha female becomes male an takes on the role.
  • environmental crisis-locusts undergo a radical transformation to environmental deprivation. Envision an orc-like race where bad conditions cause females to give birth to only males, or even hyper-males (think ogre-orcs). The large number of males lead to invasions of nearby territories and the extermination of the local males in favor of the invaders. The stressed orc genes spread & survive. In humans, there are sociology studies suggesting increased percentages of males in a population leads to increased likelihood of wars (this is admittedly actively debated). And there are genetic studies showing human migrations into populated areas where a preexisting population's genes are incorporated into the subsequent population, but the Y chromosomes disappear. Kill all the males and there's no competition.
  • Obligate parthenogenesis - There are species of reptile where at some point all the males died and only females are left.Evolutionarily there are long-term consequences, but short-term this maximizes the ability to exploit favorable environments by maximizing reproduction. You could require mating with related species to 'catalyze' the process if desired. There could be some kind of genetic transfer (imagine a CRISPR-like arrangement where the child gets a sample of paternal DNA and acquires the same hair/eye color)
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https://bigislandbees.com/blogs/bee-blog/14137353-bee-hive-hierarchy-and-activities

Look at bees dude, they have way more females than males. I would assume it has to how specialized the drones are meant for breeding while the females don't actually have to bear any children except for the queen.

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  • $\begingroup$ The details of bee reproduction are actually a lot more interesting than this answer or the article it links to do it justice. But the gist of it is that unfertilized bee eggs become drones while fertilized bee eggs become workers or queens. Also, drones only have half as much DNA as workers/queens. Calling one male and the other female might in fact be misleading, because there are no such things as X and Y chromosomes in bees. Genetically speaking, bee reproduction works completely different than vertebrate reproduction. $\endgroup$ – Philipp Apr 8 at 13:22
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The setting's gods deliberately created them that way.

A simple answer is that when the gods of this world were creating it, they decided to create a particular race (or multiple races) with an imbalanced gender ratio.

Why they might choose to do so is an open question - maybe they've got an ideological reason to do so like a combative god wanting an all-male race of goblins to continually fight other races to abduct their women for breeding. Maybe they're an ascended mortal from another world and want to give the men of this new world each the harem of women they would've wanted as a mortal man. Maybe they just think it'd be a neat social experiment. Who is going to argue with them over it, aside from perhaps other gods?

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  • $\begingroup$ The race can be created this way, but unless there's some pressure that forces them to stay that way, they won't, so this solution isn't appropriate for a species that's been around for many generations. Even if humans sprang onto the planet with a major sex imbalance, it would equalize over time due to the fact that biological and evolutionary pressures are pretty similar between the sexes. $\endgroup$ – Nuclear Wang Apr 8 at 19:17
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Sequential Hermaphroditism is a trait that occurs in many fish, gastropods or plants, by which their sex changes at a certain age, size, or social rank.

Protandry means all offspring are born male, but some later become female. For example, a group of clownfish has one female, at the top of the social ladder. When this female dies, the dominant male then changes sex to become female.

Protogyny (which is the more common form) is where all offspring are born female, but some later become male. A California sheephead will change from female to male once it reaches a certain size.

So, all offspring of your species are born one gender. At some point in their life, they may reach a trigger condition that causes them to swap to the other gender. The skew then comes from what that trigger is - there are far more adults over 20 than people under 20 in the world. There are far more people at the bottom of the social pyramid than at the top.

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The process is already there, you just need the right evolutionary pressures

A lot of fancy answers about chromosomes etc overlook the fundamental fact that a species can make or reject sperm in ratios of whatever is needed. An interesting thing about humans for example is that we use hormones to naturally increases/decrease the male to female ratios of births to maintain about a 50:50 split even following things like wars where a lot of men die which should result in severe gender inequalities.

A 50:50 split is an important ratio for a species that evolved to take advantage of monogamous relationships because it maximized your beeded potential; however, this is not the only survival strategy that works. A species that has evolves to have harems of females coupled with a low birth rate per female may naturally produce less males. Another case where you have less males is when a "queen" can super breed. There are genetic advantages to the X chromosome due to extra redundancies that make a mostly female species advantageous when breeding is done only by a select few members of society (like ants or bees). In contrast many species (particularly mammals) tend to have secondary characteristics associated with the Y chromosome which can lead to a physically stronger mostly male population which could also in theory be advantageous when you have a breeding queen scenario.

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All you need is a Biological Mechanism to make gender transmissible and an Evolutionary Pressure to cause the skew OR just an Environmental Factor that impacts gender

Human males already have an impact on the sex ratio of their children. Some men carry genes that predispose them to daughters or sons - https://www.sciencedaily.com/releases/2008/12/081211121835.htm. (some men have more X sperm than Y sperm, others more Y than X and some equal amounts). All you need is a reason for one genetic variant to be more advantageous than the other to end up with a large sex ratio skew. Say almost all the men are killed in continual wars, then "Bob" who managed to survive and has a predisoposition to having sons will have loads of grandchildren because all his sons will father many kids because there are so many women available, while "Tim" who survived the war has a predisposition to having daughters none of whom can find husbands so he has no grandchildren to pass his "daughter" genes to. You eventually get a population that skews heavily to producing Y chromosomes, and therefor baby boys. There are many other reasons why sex ratios are likely to skew at birth - though in most species they settle around 50% because anytime there are only a handful of sex "A" they are guaranteed to find mates while sex "B" is not, so sex "A" passes on their genes at a higher frequency including the genes that made them sex "A" - this happens until the ratios even out.

You could also have the sex ratios dependent on the environment (for example a lot of reptile species have sex dependent on temperature) maybe there were certain temperatures that the species evolved with (that resulted in 50/50 sex ratios) but now the temperature has shifted so that there are way more males than females. There is already concern that some species may start to struggle due to climate change.

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A straightforward mechanism for hominids is that multiple births are much more common for one sex than the other. For instance, if males were (almost) always singles but females were (almost) always twins or triplets, or vice versa, a significant imbalance is inevitable. We know there's a genetic component to multiples, so all you need is a mutation that makes it somehow sex-linked. Or maybe your species originally always had multiples (like armadillos) and the mutation was having singles for one sex.

If a society wants more of a given sex, then those who have multiples of that sex (and not of the other) would be prized mates and their mutation would rapidly spread. It just happens in your story that one race prizes male children and the other prizes female children, so you can easily evolve opposite results.

This could relate to other differences in their societies. For instance, long-lived elves might prefer fewer females to limit population growth, whereas short-lived orcs might prefer more females so they can continually replace warriors lost in battle. Or maybe the sex imbalance is what caused elves to be peaceful and orcs to be warlike. Cause and effect can be tricky.

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What factors could skew the sex ratio of an intelligent species (...)

The species as a whole is intelligent, but one of the genders is not. This happens in the real world, for example, with humans.

As anedoctal evidence, the ratio of men to women in the cast of Jackass tends to infinity:zero.

That is a poor sample group, though, so I did some further research. Statistics show that for every man that wins a Darwin Award, approximately 0.127 women are similarly awarded. And this is why there are more women than men alive at any moment - males are more prone to engage in idiotic behaviour, which causes them to die stupidly more often.

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  • $\begingroup$ Your Darwin Award statistic is off by a factor of 100. That link analyzes 282 male DA winners and 36 female DA winners, for a ratio of less than 10:1, not 1000:1. They also for some reason omit cases that involved both a man and a woman, which would put the ratio closer to 5:1. And let's not forget that males' genetic material is a lot more dangly and vulnerable than females', so equally stupid behavior between the sexes would likely still result in an imbalance in DA winners. $\endgroup$ – Nuclear Wang Apr 8 at 19:27
  • $\begingroup$ @NuclearWang ok, I will edit my answer. Thanks :) $\endgroup$ – Renan Apr 8 at 19:48

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