Imagine a species almost identical to humans, with one exception. All sperm contained both X and Y chromosome, and every child has the potential to develop as either sex. However, the mother has some form of control to select the sex of the child during early development, say through controlling rather or not estrogen was introduced in the womb during early development to trigger female development.

This species evolved this way from the start, they are not using technology to control this change. As such their culture and psychology would be evolved around the presumption that sex choice was possible.

I'm wondering how the species would evolve, and in particular what it would do to their gender roles. Would gender roles be stronger or less stringent in a world like this? Would gender roles otherwise tend towards the same pattern as we have now, or would this somehow modify the actual role of gender?

I'm interested in both the culture and evolutionary psychology, the nature and nurture sides of the gender roles.


I remembered an interesting fact about evolution and mating strategies. In the more common mating systems, where males compete for females, the desire for male vs females depends on the 'strength' of parent/child. Since males have to compete for mates, and thus a weak male will likely never get to mate while a strong male may get dozens of mates, there is limited benefit in having a son who is going to be weaker, but massive benefit to having a son if he is likely to be particularly strong. However, since females will have no trouble finding males who wish to mate with them (it cost a male almost nothing to mate, and thus he will mate with a 'weaker' female) having a daughter who is weaker is not as disadvantageous; at the same time a 'strong' daughter doesn't benefit from her strength by having much of an increased mating opportunity, since she will likely have one child per mating-season regardless of her strength if she survives. Studies have even shown some species have very slight favoring of male or female children depending on various measurements of 'strength' or health due to this.

Pre-culture humans with conscious control of sex of the child would likely evolve to recognize this principle on some instinctual level. Younger mothers, unhealthy mothers, or mothers who had lower social status may be more prone to choosing females, while high status mothers are prone to males based off of this principle; which would likely evolve as an instinctual predisposition prior to cultural development. This would in turn likely have many interesting effects on gender roles once culture had developed. Any thoughts on what kind of effect it may have?

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    $\begingroup$ Small nit, but I think you mean "How would gender roles be affected..." Curiously, both words seem to fit your interest. $\endgroup$ – Frostfyre Aug 18 '15 at 23:55
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    $\begingroup$ Just to be sure, they can't change once they're born, right? I can see a lot of "I was supposed to be x, but you made me y" arguments, which might get very interesting. $\endgroup$ – DaaaahWhoosh Aug 19 '15 at 13:12
  • $\begingroup$ @DaaaahWhoosh no you can't change it afterwards. Yes I could see some of those arguments, though admittedly transgendered individuals (or non cis-gendered) are only a small percentage of our world, so I would imagine those arguments wouldn't be that common. Still, it would be interesting to explore a transgendered view in such a world. $\endgroup$ – dsollen Aug 19 '15 at 13:47
  • $\begingroup$ @Frostfyre would you believe I meant to do that?.....honestly? NO wait I mean..umm...Look a distraction. Now see the title says effected, just like it totally always said. Yes you clearly were imagining things, no way the title ever said anything different previously.... cough $\endgroup$ – dsollen Aug 19 '15 at 13:48
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    $\begingroup$ You misspelled "affected" in your comment. :) $\endgroup$ – Frostfyre Aug 19 '15 at 14:22

10 Answers 10


First off, this idea isn't as completely hypothetical as you might think. There have been plenty of cultures in history where infanticide was accepted practice. The two main reasons for killing a new-born were (a) seems weak, too much trouble to raise him or her if they're just going to die young anyway, and (b) wrong sex. (What else could you know about a newborn?) Today of course we can tell the sex of a baby in the womb through ultrasound, so where abortion is socially accepted people can abort children of the wrong sex. We are seeing in practice that girls are more likely to be killed by infanticide or abortion world-wide, and the ratio in China and India is measurably changing the ratio of males to females. This does not have exactly the same effects as what you describe, of course, but it gives us relevant data.

Assuming that this society was identical to human society except as affected by this one thing: There are three reasons to prefer a male child in most human societies: (a) Males are more useful in war, as @pluckedkiwi discusses. (b) You need a male heir to carry on the family line and inherit the family lands, noble titles, business, whatever. (c) To be able to retire when you get old, you need sons to support you. In most cultures, daughters will be supporting their husbands' parents.

So from the parents' point of view, males are preferred. But from the individual male's point of view, he wants lots of females around so he can easily find a wife. Many men would like multiple wives. From the individual female's point of view ... hmm. On the one hand it's to her advantage if men outnumber women, as it means she has more choices when selecting a husband. Even if it's a culture with arranged marriages, her guardians have more choices. If men are competing, presumably the rich and powerful tend to win, however the society is structured. On the other hand in a violent society she's more likely to be kidnapped, assaulted, etc.

So there are competing interests, but it's the parents who control.

So with all that preamble, here's my theory about what would happen: Cultures would develop some sort of bride price custom: A man must pay a girl's parents to be allowed to marry their daughter. Such customs have existed in human societies throughout history, so I'm not making up something crazy. This would make daughters financially valuable, and thus balance the financial value of sons. Over time, the amount paid would converge to an amount that results in parents having an equal number of boys and girls.

  • $\begingroup$ Your answer to the question "How would gender roles change?" Is basically "Assuming gender roles don't change here's what things might be like." The points you bring up (a, b, and c) are not inherent to the male and female sexes but are gender roles created by past human societies that are currently fading in the modern era in developed countries. The concept of men working and providing while women care for the home is exactly the sort of gender role that is likely to be different in this alternate reality. $\endgroup$ – Mike Nichols Jan 5 '16 at 6:06
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    $\begingroup$ @MikeNichols RE I arguably did not answer the actual question of how gender roles would change: Good point. I concede. RE not inherent: Well, I agree that tracing names and inheritance through the male is not naturally intrinsic. Males being more useful in war probably is: in physical combat, the average male is significantly stronger than the average female, and psychology, men tend to be more aggressive and prone to violence in almost every culture in history, so it's hard to argue that's some kind of coincidental learned behavior. I'm sure many points are debatable. $\endgroup$ – Jay Jan 6 '16 at 3:45

I think it might just make things a little more extreme from what we see today and have seen in the past. Some cultures will venerate women more as not only do they bring life into the world but they decide such things as your gender role in society. Of course hermaphrodites would be an interesting thing to explain. Until relatively recently many cultures including western culture believed women were responsible for such things as the sex of the child and were often blamed when they didn't produce the 'right' one. Henry VIII?

If women had control over it might be even worse for them in some places because then it will also be assumed they have control over other aspects of the gestation process and might be severely blamed, ostracized or other punishment for any unhealthy child they bring into the world.

Warlike tribes might want to produce more men to keep up their numbers.

In some, girls might be wanted to trade, making them even more like property than they have been in the past. Knowing if a child is going to be a boy or a girl could start the planning for their future even earlier, or decisions could be made even before the women gets pregnant. To provide a male heir, or a female companion for an ally...

  • $\begingroup$ As to your last paragraph, I'd think that breeding women just to sell them would only happen after first making a male heir. No point making money if you can't leave it to anyone after you die. $\endgroup$ – DaaaahWhoosh Aug 19 '15 at 13:50
  • $\begingroup$ @DaaaahWhoosh true, with an ally, you could actually plan to each have your male heir and agree to produce as a second child a mate for the others first. (then get a spare...) ;) $\endgroup$ – bowlturner Aug 19 '15 at 13:53
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    $\begingroup$ That takes arranged marriages to a whole new level... $\endgroup$ – DaaaahWhoosh Aug 19 '15 at 13:55
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    $\begingroup$ Um, I'm a little surprised about the down vote on this one. $\endgroup$ – bowlturner Aug 19 '15 at 14:53

There are far too many possibilities to cover, but a strong strategy is the development of a militaristic expansionist society where women are little more than broodmares.

Able-bodied men are highly valuable for warfare between lower technology groups. A group which focuses on only producing males has plenty of warriors, who can then overpower the neighboring groups to enslave their women. Producing only males as a strategy for expansion could give a primitive society a significant advantage so long as the rate of expansion is sufficient to bring in enough women for population growth.

Obviously this sort of strategy is not viable if everyone follows it, but the one which does go this route can quickly overpower the rest (don't count on every group doing what is best for the entire species as a whole - we certainly don't). The gender politics would likely be quite abhorrent to our sensibilities, but women would be chattel for the specific purpose of breeding. They might be individually owned, but more likely held in common by various factions (population growth would suffer if high status males had a harem back home but spent a lot of time on campaign).

Whenever the men felt there were too few women to go around, it serves as great motivation to attack the neighbors. This helps keep the population relatively balanced (for whatever gender balance becomes their preferred ratio) as men are killed off in battle and more women are enslaved. Eventually they would get large enough to need some daughters, but until warfare dies down and pressure for rapid population growth falls, and perhaps not until something akin to a modern civilization where the typical productive capacity of women becomes similar to that of men, I don't see women being emancipated.


I think that gender roles would be more strictly set in place. If you can choose the gender of your children, people would naturally need to be more aware of the differences between the two genders, so that they would be more able to make an informed decision about what gender their child will be. This awareness would be instinctive rather than learned. Because of this, there will be more segregation and expectations about people based on gender.


Though most of the modern world is patriarchal and values males more than females, that is not true in every culture. Many have been matriarchal and valued females more. This gender control would increase the chances of societies becoming female dominated. In early tribes as culture was developing it would be up to the women to decide what the tribe needed more of. More men? More women? What is the value in each? From the time this race first started communicating with each other it would be the tribes who’s women made the wisest decisions in these matters that would become dominant and spread. Since the women would be the ones making these important decisions, why would they not also make other decisions? Power and influence would steadily gather in the mother of the household. Inheritance laws would (when they finally came about) follow the mothers line not the fathers. Most of the other answers focus on patriarchal mindset. Answering the questions “What would happen in a patriarchal society if they could control the gender of their child?” However the society would evolve to be matriarchal if the species had evolved with this ability resting with the female.

Besides leadership though I suspect many other gender roles would more fluid than in modern western culture. Your gender was based upon the choice of your mother. As the mothers power grew, your role in the family would also be based upon the choice of your mother.

  • $\begingroup$ I do agree with the first statement, the presumption of male control doesn't really make sense when your looking at the evolution of the species, before culture existed neither sex dominated it. However, I think the presumption on women gaining so much power from sex choice may not be appropriate. Each women would pick the sex that suited her reproductive success, not the tribes. I don't see a tribe gaining much additional power from women collaborating on mate choice over individual choice until a high tech level, long after evolutionary psychology was already defined. $\endgroup$ – dsollen Nov 19 '15 at 18:35
  • $\begingroup$ ...Later effects, like inheritance laws, I could definately see and agree with, I just feel the answer is implying a larger effect on all society from evolutionary psychology changing, rather then specific cultureal developments later :). As to gender roles, if anything I would expect them to become more fixed. My mother chose a Male over a Female because she wanted someone to work on the fields, so I have to do that. My mother chose a female because she wanted someone to join politics so I have to do that. Parent need/expectations for you when sex chose would reinforce existing roles $\endgroup$ – dsollen Nov 19 '15 at 18:38
  • $\begingroup$ Gender Roles : You do this because your gender says so. New Race:You do this because I (mother) said so. I could see it going either way, but gender identity issues would be bigger, (I have many trans-gendered and gender fluid friends). I think that early push back would shift the race away from gender roles. $\endgroup$ – PCSgtL Nov 19 '15 at 18:48
  • $\begingroup$ Just to clarify the tribe issue. When i mentioned tribe I envisioned the first tribes. Collections of 12-50 people.(After we get above 50 most tribes split) Let us assume half are of breading age, and then half of them are women. 3-12 women. A first time mother goes to her older sister and her mother to give her advice on what to gender her first born. Something between 25%-100% of the women of the tribe where involved in the decision by default. I don't think its much of a stretch to think even more of the women in the tribe would be involved in the discussion. $\endgroup$ – PCSgtL Nov 19 '15 at 18:50
  • $\begingroup$ yes, but by the time you have sufficent communication to have a discussion on gender all the evolutionary psychology is already mostly set. Besides unless you have drastic gender dimorphism the advantage of one sex over another at a given time won't be that drastic, and due to the fisher principle they will always instinctual tend to have an even number of each sex. Does choosing to have a son this year and daughter next instead of the other have that huge an affect on the tribe success as a whole? Especially since most will still make their decisions primarily over what is best for them. $\endgroup$ – dsollen Nov 19 '15 at 18:59

You said that these people have always had this ability, but since animals don't seem to care too much about the sex of their children, I'm going to assume the use of this ability won't occur until we get a rudimentary form of communication and civilization (where mothers can explain to their daughters how to do it without having to wait a year and a half to show examples). So this society diverges from ours at the early civilization level.

At this level, men are in charge, and they will find out about this ability. Thus, just about as soon as it can be used, women will lose the right to set the sex of their children. Anyone who doesn't make the right kinds of babies will be killed, or shunned, or otherwise kept from making more babies. Similarly, tribes where the leader lets only male babies or only female babies be born will die out, and thus also not make more babies. The tribes that survive and thrive will be the ones that figure out the best ratio of men to women. I initially thought this ratio would highly favor men (since they're stronger), but after thinking about it I realized that the dangers of childbirth would probably kill enough women to warrant higher female production rates.

This ratio shouldn't be too different than what we have now(according to wikipedia, there are a few more men than women on average, but not too different); the big difference is that now it's a human-regulated ratio rather than an evolution-regulated one. And while a one-to-one or similar ratio might work out reasonably well on a macro scale, it's the micro scale where things begin to get different. Since society has been naturally selected to prefer male and female offspring, governments will want to ensure that their newest citizens are properly segregated. But since this is still a patriarchy, people are going to want male babies. Thus, to keep the balances in check, I can think of two options:

1- If you have money, you get male babies. Otherwise, pop out some girls.

2- Alternate babies. You can have a firstborn male, but your second child has to be a female (or else).

At this point, the resulting cultures diverge so much that I can no longer speak generally about them. The first case is really interesting to me, because while it splits the population into two classes, half of each generation has to switch to a different class. Men would have to fight to stay on top and get the right to have male heirs, while the fairy-tale story of a peasant girl getting married by the prince will happen all the time. Unfortunately, this option isn't really that good for sexual equality, as all women start out poor, and men rich. I don't think I want to know how that would turn out.

The second option is probably pretty close to what we do, but there are a few interesting edge cases to consider. For instance, what if your firstborn dies, and you can't get another male until you have a female first? And what if that female kills the mother in childbirth?

In both cases, it would be cool to see what happens to kids born the 'illegal' sex. Would they be killed? Rendered sexless by some horrible operation? Raised as monks or nuns or ninjas? I think it would really depend on the needs of the nation.

Anyway, I don't really think gender roles would change too much. Women will never get to choose the sex of the baby; at best, it will be a compromise. The ratio of men to women will be regulated too, so maybe even the father won't get to pick. People will still be born men or women (or anything else, but in much smaller quantities), with all the necessary bells and/or whistles, so they should still end up pretty much as men and women usually do (that second case I mentioned might change things, but I think it would only magnify what was already in place in our past). Cultures may be different, but it's hard to say how without a more specific case.

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    $\begingroup$ "otherwise kept from making more babies..." Please don't throw me in that briar patch, please! $\endgroup$ – Green Aug 20 '15 at 11:56


What determined traditional gender roles? Males are taller with more upper body strength. Females have wombs and breasts. The latter is more important.

Women have the more labor-intensive portion of reproduction. While a man spends just a short time initiating the pregnancy, the woman is tied down for about nine months. From a reproductive perspective, we have a tremendous surplus of men relative to women. If we had ten women for every man, we could produce just as many babies (about one every ten months at most).

Men tend to be given the more dangerous work because they are expendable. Lose a few men and some of the surviving men can take extra wives. Women tend to be given work that can be done even while pregnant. Safe and close to home. They have to feed infants initially (men don't produce milk). It was natural for them to wean the children by feeding them other food.

What changed?

We have seen a change in traditional gender roles. Why? Because lower infant mortality and birth control mean that large families are much less important. Women don't spend their twenties and thirties popping out kid after kid to be sure that enough survive. So women have more time to do things other than be pregnant.


To get back to the actual question, I'm not sure that gender selection would change much. Women would still have a greater role in reproduction. Men would still be stronger with more upper body strength. The roles might stay the same while their importance would be more balanced.

If food is plentiful, having more female babies will eventually lead to a larger population. If food is more limited, having more male babies could eventually lead to more hunters to get more food. When agriculture is developed, this choice could switch between increasing population or warriors. Warriors allow a territory expansion, increasing food availability.

This would encourage long term thinking. If the balance is too preoccupied with expansion and not enough with resources, people will starve. If the balance is too preoccupied with resources and not enough with expansion, they won't have enough population to utilize the resources.

I would expect the balance to shift towards women during the agricultural phase. Agriculture produces an excess of food and has fewer jobs that favor one gender. Multiple wives per husband is likely to be the norm.


You would have much more variation among cultures.

Depending on the cultural norm of a civilization many more things are possible.

In our current system we have a fairly strong culture of one to one relationships (monogamy) that is paired with an even probably of any child being male or female.

Any other cultural system (polygamy, or competition for mates) is difficult to maintain because the even gender ratio either leaves people left out in the polygamous case or needless competition in the competitive one (there are enough mates for everyone).

If the ratio can be chosen then any ration that women agree to can be easily maintained 1:10 10:1. So you could see Amazonian cultures with majority women and a few men to make children, or majority male tribes with fierce competition for the few mates. As long as the women of the group agree with the system they can maintain any ratio.


I'm new here so I can't leave a comment but part of your biological assumptions are wrong. In humans, the baby's gender is controlled by the male's contribution, ie whether they give an XX or XY gamete. If the Y chromosome is present, then the baby becomes male. In its absence, female. So the "default," in a way, is female.

I see know the questioner addressed this in his post, but stick with me.

I find it unlikely a woman could biologically assert a choice of her baby's gender. Would she have to constantly focus on the baby's gender to ensure the right hormones are released? Or just once, at some turn point in gestation, like week 5? What happens if she changes her mind, or just honestly never makes a choice? Because of the passivity inherent in pregnancy, I don't see her evolving any sort of conscious hormonal control. I can't think of any examples of animals consciously controlling hormonal release, for very good reasons.

Additionally, the actual act of procreation for women, is again, passive. So if there was to be a choice, it would have to be a choice made during an actual conscious action. For this, I would choose ejaculation. As I postulated in the comments, a man's testes could each produce male- or female-begetting gametes only. A muscle that closes only one vas deferens and allows the man to choose female or male semen seems to me far more biologically plausible than the woman.

One way I feel a woman could plausibly effect the sex of her baby is: Perhaps a male embryo releases a hormone that causes an itching sensation, and she could self-abort.


No matter who picks, a child that knows his/her gender was chosen by one parent is going to feel very beholden to that parent, especially their expectations. A son would feel very strongly that he has to live up to his father's expectations. In a less-progressive society, this would expand gender roles.

Similarly, a daughter would feel the same pressures, but the opposite way: virginity would be more highly valued, and she would respect her father's vision for her as a girl.

Of course, broad strokes here, but someone knowing their gender was selected for a reason would face enormous pressure to live up to the expectations of that gender.

  • $\begingroup$ The assumption was that all sperm contained both X and Y, that was a starting parameter. $\endgroup$ – bowlturner Nov 19 '15 at 18:21
  • $\begingroup$ Oops, you're right. $\endgroup$ – Azor Ahai Nov 19 '15 at 18:22
  • $\begingroup$ @pressTilty I'm aware how human sex is determined, trust me I know way more about the science of human reproduction then most, or indeed I ever really intended to know lol. However, I'm proposing a different species. In this one sperm encode everything for both sexes, with the mother picking via some process while carrying the child; much as how it works with alligators now (not that alligators use X or Y chromosomes). Really I probably shouldn't refer to X or Y, since the species wouldn't necessarily have sex chromosomes of this format to begin with, but I was lazy in my description :) $\endgroup$ – dsollen Nov 19 '15 at 18:24
  • $\begingroup$ I think the idea of male choosing the sex is interesting, but in all honesty it seems unlikely from an evolutionary standpoint. The male's focus is producing LOTS of sperm to win competition with other males, and for most of their life to put limited investment in the children (monogamy is relatively recent in humans, and even then isn't guaranteed). Trying to keep X and Y sperm separate and pick which to use get's confusing after the first mating, do you use depleted X or go on to use your Y? You need to put out as much sperm as possible so you alternate, but since no mating is.... $\endgroup$ – dsollen Nov 19 '15 at 18:27
  • $\begingroup$ guaranteed to be a success your be alternating every mating and ultimately be back to little mate choice, whichever you used that mating is the sex. Best to focus on just making as much sperm as possible to beat the other males, from an evolutionary standard your not planning to raise them usually. Females can pick the sex because they are caring for the fetus and thus can affect environmental affects. Though I suppose if males cared for the fetus in some way they could pick the sex. For instance if female laid an egg and male decide how warm to keep it to pick sex, like alligators. $\endgroup$ – dsollen Nov 19 '15 at 18:30

What would happen is that gender would be determined by: SUPPLY & DEMAND. Whatever gender happens to be the valued one would be produced. Then the market would be glutted with that gender, so the demand for the other gender would then spike.

I don't believe that males would be more highly valued in this situation. If a species has already gone through the horror that is too many boys in a society, they will have already begun to understand what that does to a population (as in there aren't enough women to HAVE the babies, so the population drops like a stone, creating a crisis). Girls get to chose for themselves as far as a mate and career and are more becoming more liberated in places where sex-determination is done in utero via abortive measures. Men in these societies are having to do much more to prove themselves because there are so many of them competing.

In a society where girls are chosen more often, population would explode, creating all new and different problems. Women would be less valued (as the men are in the above example) and would have to get married quickly, and the men would have the choice.

If they always had this ability, I think that population numbers would have been lower at the outset, because some families chose to keep having kids because they were looking to have a particular gender (Mom's family: 6 boys, 1 girl and the girl was the youngest.)

As to gender roles, that really depends. Because the imbalance of gender in a major way shifts the power dynamic between the sexes. What you have more of will determine what the gender roles are. For some countries, that want/need a large population over time, choosing females more often will be the norm. Because this is something we had from the start, and we are somewhat intelligent, we may figure out the correlation.


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