On this planet there is a species similar to humans with two sexes having differing reproductive organs. The concept of gender is non-existent on this planet. I'm not writing this for any political reason, it just fits with other themes (imagine Vulcans not seeing the "point" in genders, for example). The people have varied hair styles and clothing and perfumes and jobs etc, but these are not associated with a person's sex. You would have no idea if someone has a K or a Z in their trousers by looking at them.

How do individuals on this world efficiently identify potential mates? I'm trying to think of an easy method that doesn't seem ridiculous at face value. I don't like the idea of people straight up asking each other all the time, or having a sign over their heads or a band on their arm, or a specific type of squeal they let out. Pheromones of some kind are a reasonable idea I am toying with, but I'm open to suggestions!

EDIT: a lot of people are bringing up important issues relating to monogamy. These are good ideas, but don’t address the issue of identifying partners to mate with. I’d like to assume monogamy for the purpose of this question so the answer isn’t “people don’t need to tell the sex” or similar.

EDIT2: There are so many excellent answers and suggestions here I wish I could accept them all. I will wait longer and accept the one that I think best answers the question with some ideas about identification.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Jun 27 '19 at 2:25
  • $\begingroup$ The odd way that the question is framed " How can I tell the physical sex...." etc is perhaps leading to a bias in answers. Here we need questions to be objectively answerable. This question doesn't really lie in that category. (From review.) Consequently voting to close as too broad. $\endgroup$ – BLT-Bub Jul 3 '19 at 6:08
  • $\begingroup$ @Confoundedbybeigefish.Only the title is worded that way, the question in the body is clearer: "How do individuals on this world efficiently identify potential mates?" Maybe just editing the title would be more sensible than suggesting closure of a question which has received such fascinating responses? $\endgroup$ – IMSoP Jul 6 '19 at 22:20
  • $\begingroup$ In this world, I cannot visually identify potential mate either. I can tell that this one is a women, but I have no idea if she'd fancy having a ball with me. The solution is not to ask straight up, but to talk and learn to know her $\endgroup$ – Madlozoz Nov 22 '19 at 14:43

23 Answers 23


How do individuals on this world efficiently identify potential mates?

For an "efficient" identification there ought to be some outward difference between sexes. This difference might not lie within the human sensorium (for example, a difference in body smell that humans can't pick up, a different skin color that only shows in ultraviolet or infrared and so on).

Anatomical differences could be enough to either be easily spotted (sexual dimorphism, or simply noticeable bulges in different places) or lead to different behaviours, not necessarily "squealing". Skin color, hair pattern, bone structure.

All these could be even unnoticeable one by one or to the untrained eye, but immediately obvious to your aliens (akin as how humans can be trained to sex pullets from cockerels by eye, but they don't know how they do it - it becomes obvious to them).

Fashion is another possibility. If the difficulty you noted was also awkward for them, they'd probably evolve means to circumvent it and make mate selection more efficient again. In some Earth cultures, specific items of apparel were reserved to indicate social status, marital status, or even sexual availability - fans, sashes, garlands, shoes ("And now you've gay bracelets and bright feathers three! / "Yes: that's how we dress when we're ruined," said she -- Thomas Hardy, The ruined maid)

Body language is another useful possibility.

Or they might use a sexed language, so after exchanging a few words everybody would know where everybody else stands (some Earth languages have different registers or modes to address inferior-to-superior, male, female, young-to-old and vice versa). Lots of possibilities there. I remember once a young English friend visited and "taught himself" some Italian words, without noticing that Romance languages are gendered while English isn't - I am happy to see you has two different forms for happy depending on whether the speaker is male (contento) or female (contenta). Gleaning words from his female cousin, the poor boy started talking as a girl would.

really avoiding gender

But say we don't want to have any outward expression of sex - of gender. Can we still efficiently select for mates? Apparently we can't.

How do we cheat?

Ursula K. LeGuin created a world, Gethen, where humans are hermaphrodites - they have only one sex (at most), but which one depends on how they experience kemmer, a periodic hormonal imbalance that can make them either male or female. This characteristic is shared by the regul, aliens in C. J. Cherryh's Faded Sun trilogy, but the sex choice (which happens once and lasts for the remainder of a regul's life) may be influenced by pheromones.

So, your society is genderless and normally people do not even have (external) sexual organs (this is the Gethen norm). Any two persons may meet and fall in love or feel attraction for one another. When they do, and stay together for a sufficiently long time in a sufficiently intimate context, their bodies start reacting to one another (this is the Regul mechanism), choosing two complementary sexes either at random or based on their physiochemical characteristics - who knows, maybe the one healthier, larger or with more body fat becomes a female, to maximize chances of carrying out a pregnancy. The more sexed the one becomes, the more sexed the other (but with opposite gender).

Should the language need to reflect this, it would need male, female and neuter mode, like the Stsho of Lyene (another C. J. Cherryh's creation, in the Chanur series) do. They might then maybe say something like "When I-neuter saw it, I-male asked her out" to emphatically describe love at first sight.

The maximum efficiency is then attained: whoever your potential mate is, will always be sexually compatible because it's your potential mate.

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    $\begingroup$ +1 for the language difference, became important to differentiate between the sexes once the close came on, so their language would be sure to include that. I am also wondering if the OP has thought about the usage of pronouns in this story. $\endgroup$ – katatahito Jun 26 '19 at 7:13
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    $\begingroup$ This is an excellent idea. It could be a simple as there being a male and female "I" when people speak about themselves. $\endgroup$ – Behacad Jun 26 '19 at 13:40
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    $\begingroup$ "Quite obviously there should be some outward difference between sexes" Why? There are many animals with very little sexual dimorphism in real life where determining the sex is quite difficult for people. There are even animals that are capable of changing their sex after adulthood due to environmental pressure and so the only real difference is in what kind of gametes they produce like clown fish. $\endgroup$ – Shufflepants Jun 26 '19 at 14:32
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    $\begingroup$ The trouble with social/language differences is they are functionally indistinguishable from gender. Merely saying "they have no concept of gender" and then depicting they have social/language differences associated with genitals is like saying aliens have no disease but then depicting that they have microorganisms that invade their bodies and cause uncomfortable or threatening symptoms. $\endgroup$ – called2voyage Jun 26 '19 at 15:31
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    $\begingroup$ @Shufflepants But those animals don't have difficulty distinguishing between the sexes of their own kind; evolution is not kind to sexually-reproductive species that can't get to square one. $\endgroup$ – chrylis -on strike- Jun 26 '19 at 22:53

This may not translate well to other languages where sex and gender may not be different structures in the same way.

You can't

Because gender is the social indication of what's in your pants.

So by your requirement to not have gender, you're equally not allowed to indicate what sex you are. Any act that would do so would class as a gendered indication, action, or language. Hence why Pratchett referred to this as "the tricky part" of Dwarven courtship, as he represents the dwarves as a strictly ungendered society with no sexual dimorphism.


The question doesn't really make sense to me if you're restricting yourself to two-sex reproduction. If, biologically, this species reproduces sexually via pairing, then almost by definition there has to be a way for the species 'in the wild' to figure out which one has the sword and which one's got the sheath. Pheromones, secondary sexual characteristics, highly visible sex organs - something. The fact that they therefore aren't actually identical makes it near impossible for their cultures to have evolved to make it irrelevant. On the other hand, there's plenty of other reproductive systems.

If they're hermaphrodites, then nothing's the problem and it makes good sense from a cultural perspective - gender doesn't matter because it basically doesn't properly exist.

If they're like ants, then males literally do not have fathers - a female can decide to have a kid at any time and if she doesn't bother with the guy, the child is a boy. Gender wouldn't exist because they'd likely think of themselves as almost two different species, or else two halves of a single whole.

If they're like, say, clownfish, then it wouldn't so much be irrelevant as totally flexible. Clownfish are basically a transgender person's dream, since they swap their sex almost at will. That's not actually how they work, of course, but I see no reason why you couldn't do that for your made-up species. It would be workable, biologically, and would blur the lines pretty hard between male and female from a cultural perspective. You'd pair up with someone you liked, and one of you would basically just go female (or male) if you weren't already sexually compatible. Weird, but probably pretty unique. I don't think I've seen that version very often in a way that wasn't clearly politically motivated.

Of course you could just go super extreme and have them reproduce via sporing or seeding or post-egg feritilization or something that just doesn't give a damn about finding Z or K at all, you just drop your net and hope there's fish today. I'm not totally sure this is compatible with higher life, mind you, but I can't think of any specific reason it shouldn't be if the species has strong intraspecies cohesion and views anyone of their species as part of a supertribe.

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    $\begingroup$ Your point about being able to identify partners in the wild makes sense, but they are no longer in the wild, so things are different now. Just like with us. Remove obvious sexual characteristics and gender and things get a lot more complicated. $\endgroup$ – Behacad Jun 26 '19 at 13:39
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    $\begingroup$ @Behacad In the wild, you might find that "clothes" are removed alongside "obvious sexual characteristics" - so, the genitals can be clearly seen, and gender identified. $\endgroup$ – Chronocidal Jun 26 '19 at 14:47
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    $\begingroup$ You also don't necessarily need every couple to be able to reproduce to propagate the species. If pairing in this society is entirely at random with regards to reproductive compatibility then only 1/2 of couples will be able to reproduce. That's actually totally fine if each couple that does reproduce averages 4 kids or more, and since half of couples won't be able to have their own kids, there may be greater demand for adoption or surrogate parents $\endgroup$ – Kevin Wells Jun 26 '19 at 17:34
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    $\begingroup$ Or perhaps the child of two W's is a Z, the child of two Z's is a W, and the child of a Z and a W is a coin flip. Weirder systems than this show up in nature. $\endgroup$ – Draconis Jun 26 '19 at 17:54
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    $\begingroup$ in iain banks culture series members of the culture have the ability to change their gender at will. without any political motivation (i don't actually get what you mean by that). gender change takes a while (half a year?) so it doesn't happen on a whim, but it's fast enough for a couple that decides that they want children. $\endgroup$ – eMBee Jun 27 '19 at 7:49

Matchmaker, matchmaker, make me a match....

On your world, there is a system of matchmakers who take care of tracking and matching appropriate partners. This may be government-run (with access to health records that would reveal sex), arranged by families, or otherwise performed by "the system" for the person. With this, most of your inhabitants simply don't worry about identifying potential partners, because they know that they will receive an appropriate one when the time is right (they have achieved physical maturity, completed mandatory military service, passed a literacy test, etc.). Dating, therefore, isn't really a thing on your world, or if it is, "swinging" and "mixing" very much aren't. This sort of system can give your protagonist their own struggle or quest if it turns out that they don't like their match, leading to hilarious hi-jinks as they realize that they don't know any other way to find a mate, or if they do, can't figure out a way to do it in a socially acceptable way.

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    $\begingroup$ Damn, ninja'd. Good point, though, have a +1. $\endgroup$ – Ruther Rendommeleigh Jun 26 '19 at 13:12
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    $\begingroup$ this is the first thing I thought of. I suspect Mothers will do the match making and "sell off" their non-child bearing children to other families who want a mate with specific traits.This is because only the mothers would know what kind of genitals people have. $\endgroup$ – Reed Jun 26 '19 at 14:34
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    $\begingroup$ Or if that seems too archaic, then there could be dating agencies people use to find a mate. They know what kind of genitalia you have because you said so when you signed up on Alien-Tinder :) $\endgroup$ – Philipp Jun 26 '19 at 14:40
  • $\begingroup$ @Philipp exactly. The exact means is flexible, the point is that there is a system in place. In our own world, some conservative religious groups have developed this into organized practices, where young people will either register with a matchmaker or their parents will do so for them. $\endgroup$ – Robert Columbia Nov 22 '19 at 13:37

Gender is existent, but irrelevant.

First, let me start by challenging "non-existent". I think "non-existent" is a very strong word though: the fact that there are more than one gender is still a biological reality in the society you describe, so if you allow me I will replace it by "irrelevant", in the sense that though the biological reality is there, society stopped giving gender (and all things that come with it) the importance that we humans give to that.

Of all the things we think about selecting a partner, such as (but not limited to) personality compatibility, affluence, society relevance, similar goals, etc... gender is just one more. Of course, for our society, gender tends to be one very important parameter, as there is a lot of pressure on the "reproduce" imperative.

So, for a genderless society, this reproduction imperative may have been downplayed a lot. I would suggest "outsourcing" the reproduction, a bit like McMaster Bujold does in the Vorkosigan Saga (TLDR: artificial wombs and genetic tech that allows two parents regardless of gender to have a child), but reproduction partners and even for-hire wombs can be alternatives: this will depend a lot on how you want to base the rest of your society.

That probably means the society will have another goal, and that goal will become the first imperative to select a partner. Is money-oriented? invention-oriented? art-oriented? This will become the priority above gender.

In a society like this, searching for a partner based on gender will be viewed as either a kink or a sort-of-backwards-mentality, as they will be, in a sense, playing against society expectations of choosing a partner for something that most people don't care. Depending of the maturity of the society, this can be viewed as something endearing, or something dangerous, and either way there will be individuals who will react the opposite the majority of the society thinks (so, they will be always haters and supporters).

The best bet for people like this will be closed communities (not necessarily secret, think... Amish, for example, or internet forums) where like-minded people can interact without fear of being censored for that.

Edit: based on the replies, I modified the text to be more clear about the irrelevance of gender, and downplay a bit the sexuality thing. Previous text will still be available in edit history.

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    $\begingroup$ This. Also worth noting that as long as the number of children produced by the sexually reproductive couples is high enough you don’t even need artificial means. That works exceptionally well in communal or extended family groups where reproductive couples and their children are supported by related non-reproductive couples. Hell, one branch of evolutionary biology holds that boys are more likely to be born gay in large families in order to help the family (and the reproductive sons) reproduce as a whole. $\endgroup$ – Joe Bloggs Jun 26 '19 at 9:09
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    $\begingroup$ Exactly! Another way that makes sense is that there’s two genders, but genetics work differently: E.g. reproductive cells from multiple matings form a chimera, and it’s unknown which male ends up contributing the cells that become genitals. Therefore people have sex with as many as possible, uncoupled from the people they’re emotionally close with. $\endgroup$ – flying sheep Jun 26 '19 at 12:56
  • $\begingroup$ Based on your comments I polished the reply adding what I think is a better explanation. Thanks for your feedback. $\endgroup$ – Stormbolter Jun 26 '19 at 13:16
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    $\begingroup$ A good example of such society is Ann Leckie Ancillary trilogy. The people in the book are aware there are two different sexes, but it's irrelevant for social interactions and gender roles - and bringing someone's sex to attention is rude and barbaric. "Outsourcing" reproduction is an awesome solutio for this. $\endgroup$ – Cumehtar Jun 26 '19 at 15:36
  • $\begingroup$ @Cumehtar I also thought of that book series, but I never got a clear picture on how they solved the issue, while in Vorkosigan Saga it is one of the main themes so there are more chances to extrapolate info :) $\endgroup$ – Stormbolter Jun 26 '19 at 15:43

1. Their mating is only when "in heat" or "in musth".

Unlike humans who have sex for fun and can do it any day of the week or month of the year, perhaps your aliens are only periodically receptive to mating. There would be clear biological signals of this: lots of pheremones, the swollen genitals of female chimpanzees or, in some bird species,moulting their feathers to grow breeding plumage.

So you could say that some secondary sexual characteristics are temporary: alien men only grow beards when they are in heat, or that alien women only have noticeable breasts when they are in heat (or breast feeding). Or invent some visual signals! Perhaps their foreheads turn blue, or their hands go scaly, or their eyes turn purple. Or a crest on their head which is normally hidden by hair stands up and gives them a zebra mane or cock's comb.

The mating doesn't have to be the whole population at once. Use elephants or gorillas as a model, when only one female at a time will be in heat in a small population. Though obviously that would be hundreds or thousands of people a day in a city like London.

Elephant musth isn't really a mating heat for males, but you could adapt it as such and merge it with something like the Pon Farr of the Vulcans

Also your aliens could still have sex for fun. But outside of heat/musth, there is zero chance of pregnancy. Plus the mechanics might be a bit different if the reproductive organs shrink or change shape when they are not in heat. For example some hamster species grow and shrink their testicles seasonally.

2. There is a subtle but permanent signal which humans may not notice.

They have sexual dimorphism, but it is really subtle and humans can't tell the difference between the sexes, but to the aliens it is obvious. Consider all the bird species where the males and females are - to our eyes - identical in size and plumage and call. Here's an article on how to tell the sex of parrots. It says:

The only way to definitively tell the sex of a parrot is to wait and see if it lays eggs

However, parrots have no problem pairing up and reproducing, so there is obviously something about being male or female which they can detect that we can't!

How you deal with this will depend on the needs of your story. Either:

  • Humans just can't tell the Ks from the Zs without the application of science. The aliens can see ultraviolet, so the social signal is markings on their skin which are invisible to humans. It's scent, but humans have a rubbish sense of smell. It's the infrasound/ultrasound frequencies in their voice which humans can't hear.
  • Or humans can see the signal, but it is very subtle and they tend to overlook it, because it is irrelevant in human social interactions with other humans. All the Ks have dark brown eyes, all the Zs have a subtly lighter shade of eyes. Zs have noses which are a tad wider. Ks have attached earlobes or dimples or put their left thumb on top when they clasp hands (see human examples

If, to borrow an irrelevant human term, everyone is 50/50 bisexual, then statistically, 50% of relationships will be between couples who can have biological children. If there's a cultural expectation that those who can have biological children have around 4 children instead of 2, and adoption is more common, then you'll still have a stable population with most people who want to raise children able to.

But, honestly, I don't know that this is that much of an issue even if everyone wants their own biological children. Queer communities have been having children for a long time, it's just a matter of negotiating other people and finding a sperm or egg donor. And in a society where same-sex relationships are completely normalized, you'll have a lot more resources to help with that process.

It does mean families end up being more complex than the nuclear family you're probably thinking of, but that has been the norm for cultures all over the world throughout history. You're already making a big cultural change here, this would just be one of the side effects.

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    $\begingroup$ As a variant, even if they are having children naturally, without artificial insemination and artificial womb, sex for the purposes of reproduction and for the social purposes can be completely separated. Even in a pretty archaic society we can imagine a religious duty to go to the temple of the goddess of fertility and lay with a person the priests assign to you once a year (so that priests keep track of real sex of each person and their descent, but it's irrelevant to the other aspects of life). $\endgroup$ – Cumehtar Jun 26 '19 at 16:35
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    $\begingroup$ I actually think 50% of relationships will be heterosexual if the relationships are formed blindly. I can see why you'd come up with 33% on the theory that same-sex relationships are sort of double-counted, but I think that's the wrong intuition. Can't say I've formalized my intuitions properly though. $\endgroup$ – Ben Millwood Jun 26 '19 at 17:04
  • $\begingroup$ @BenMillwood I wrote 50% at first, but backed off to 33%. It's either "there's 3 options, so 33%", or "each Z can either get with a Z or an X, so 50%", and I'm not sure which is the correct interpretation. It also doesn't really capture all the possibilities in reality, so it's a simplification regardless. $\endgroup$ – Tesset Jun 26 '19 at 17:10
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    $\begingroup$ I think there's four options, ZZ ZK KZ KK. Or to put it another way: it must be the case, by symmetry, that at any given time the singles pool is about 50/50. So if you pair off a random two of them, by randomly selecting one and then another, the second is about 50/50 to be the same as the first. $\endgroup$ – Ben Millwood Jun 26 '19 at 17:14
  • $\begingroup$ (I agree this isn't hugely important to the answer, but it does change whether the average couple should have 4 or 6 children, absent any surrogacy / etc.) $\endgroup$ – Ben Millwood Jun 26 '19 at 17:15

You're sort of applying gendered social norms to a non-gendered world. Here on earth, attraction is largely based on gender, not sexual anatomy. A person might be attracted to men, or women, or non-binary people, or some combination of the above. People aren't attracted to penises or vaginas which is why, for example, most men who date and fall in love with trans women consider themselves heterosexual regardless of their partner's anatomy (as it should be, I might add). So, it would stand to reason that, in a world without gender, people would just be attracted to each other regardless of anatomy. If everyone looks the same, one would be just as likely to be attracted to a K or a Z. It's likely that 50% of ones sexual partners would be K's, the other half Z's, because the attraction would occur before the knowledge of the anatomy. One would develop sexual practices to accommodate both. Any time you're dating someone, its a coin toss, but a low-stakes one because it doesn't have the weight of identity that it has here; no one can tell a homosexual pair from a heterosexual one, including the people in it at the beginning, so a K who finds themselves in love with another K doesn't need to adjust their identity the way a man from earth would if he found himself in love with and attracted to another man. Without the weight of gender roles and heteronormativity, the cultural importance of sexual anatomy would plummet.

This doesn't have to lead to the breakdown of monogamous pairing. If two individuals got together and found that that their anatomy was not compatible in a way that would lead to offspring, they would do what homosexual and other non-fertile couples do here on earth: adopt or find a surrogate. Perhaps surrogacy is extremely commonplace in this culture and one would think nothing of, for example, carrying a child for one's sibling, who is a K who fell in love with another K. You might consider giving this species some kind of non-gestational reproduction, like egg-laying. This would lower the burden of surrogacy, which might explain how such a system would emerge and the species was more easily able to shed the concept of gender.

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    $\begingroup$ As well as being egg-layers, you could make them marsupials and both sexes have pouches (like the yapok does) but both with milk producing nipples inside . That would also mean a surrogate only does a couple of weeks of pregnancy, and then hands over the newborn joey to develop in the adoptive parent's pouch. $\endgroup$ – DrBob Jun 27 '19 at 10:09

A Couple Options:

Nude Social Tradition

You could have some sort of social practice where undressing is non-sexual, see Japanese Mixed Onsen bathing practices before opening to the West. Also similar could be Finnish Suana's. These present an opportunity to get a peak at an acquaintance's genitals... could be pervy, could not.

Z seeking K

These people could have a very different structure pre-dating before humans do. They do not proposition people they don't know very well for dates anywhere, instead there are specific settings (like speed dating) where the K's are set up on one side and the M's on another side - or inner ring/outer ring. Something like that


Assuming that one of the sexes menstruates similarly to how human women menstruate, then they must have to do certain things that the other sex does not do. Therefore seeing someone pull a tampon-like thing out of their pocket/bag might be an indicator one way or another. The menstruating sex could also exhibit certain characteristics while menstruating that the other does not (but is able to pick up on) - smell, redness, hot flashes, something else.

Stand up to Pee

Another way to tell could be to notice their bathroom habits. Assuming that one sex is not able to effectively stand while peeing, if they notice someone that does use a urinal (either by going into the urinals only bathroom or bc they are also in the bathroom) then they can confirm they are of the opposite sex.

In general, if you make it hard to assume someone's sex by eliminating the outward gender differences and most secondary sex characteristics... you also make it hard to figure out who to mate with. I am also not sure if you are implying that this race is exclusively "straight", which I would question if you are getting rid of gender differences.

The main problem I see is that it becomes hard for Person Z to determine that Person K is K, while Person Z determines that Person K knows Person Z is Z, and vice versa. If that made any sense. The last two don't really address this too well but I think the first two do.

I am not sure you can avoid pansexualism and avoid talking about overtly about sex, all while making the sexual-dimorphism unable to be sensed.

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    $\begingroup$ I really tried to include the concept that gay people find a suitable "mate" even though they can't always tell if someone is a top or bottom (this is less important for verse people) by talking about it... but I didn't think it would help the answer too much $\endgroup$ – katatahito Jun 26 '19 at 6:35
  • $\begingroup$ +1. I would also add which bathroom they go into in the last section. $\endgroup$ – Alexandre Aubrey Jun 26 '19 at 18:15
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    $\begingroup$ Alexander, I doubt they would have separate bathrooms in such a world. $\endgroup$ – Max A. Jun 26 '19 at 20:54

Social gatherings

Most modern societies have (more or less strict) rules about where and when looking for potential partners is appropriate. You don't usually flirt at work, on public transport or in the supermarket. You do at, say, parties, festivals and many social functions, which, conveniently, come with their own sets of rules and expectations. You could easily expand these to have a "gendered" element, e.g.:

  • different styles of dancing
  • differently decorated drinks or desserts
  • sex based musical elements
  • dating games

Friends and relatives

Parents, grandparents or older siblings could play a role in "matchmaking", getting in contact with other families and suggesting potential mates. Finding a partner would become less of a solo endeavour and slightly more of a community thing. This could, in a stricter society, also lead to arranged marriages.

Just show the goods

If people frequently need to know a person's sex, there's a good chance that showing or touching genitals isn't taboo in your society. That could mean that nudity is the norm in certain social settings, it could mean that being fully or partially naked is a sign of availability, or perhaps pyhsical contact is just more common.

Combine it with other indicators

In our world, sex is not the only thing you'll want to know about someone before asking them out. Some important details, for example whether they already are in a monogamous relationship, are not as obvious. You've expressed a dislike for using jewelry or items of clothing to signal gender, but if you expand the idea to include other details, it might become more practical and potentially tell your audience a lot about the culture.


The aliens don't know themselves.

Perhaps this is also what dwarven do, too, in Pratchett's universe, so this is really "tricky".

In other words, if two people get together, they just try it out. If it fails, they retry with someone else. For each try they have 50% chance of success (or less because of other failure modes of mating).

This means that monogamous pairings are difficult to achieve. So perhaps you should have a different social structure than monogamy and families. To give you two examples:

  1. The Mosuo in China have a "walking" marriage where the male goes to the female the night for mating, and the offspring stays in the mother's house.

  2. The cheela of Forward's Dragon's Egg have a clan hatchery, where the female drops off the inseminated egg. Both males and females enter something like a menopause and have he urge to parent the hatchlings.

Of course if a female has been successful in bearing a child she will know that she is female.

If you find it fascinating that people don't know which sex they have, you could posit having the gender randomly changing every few months.


It's already the case in our world that even if someone fits your genital preferences (whatever they are), that doesn't mean they are sexually compatible with you, and even if you have the complementary sets necessary for having children, that doesn't mean that you will share the same intentions regarding children, and it doesn't even mean biology is guaranteed to be on your side.

Your hypothetical society can just feel those effects more strongly. Sexual relationships that don't intend to rear children are largely not picky about what genitalia are involved: everyone knows how to have fun with all of them, though individuals might get on more or less well with particular ones. After all, everyone knows (and I wish more people in the real world did!) there's a lot more to physical intimacy than which genitals go where. Sexual relationships that aim to bear children, on the other hand, have to be more picky, and it may be necessary to go through several to find one that's the right fit for both people involved.


Just have your species date without regard to gender at all. This happens among bisexuals in our world already. If they happen to end up with someone who they can't physically have a child with, they can adopt or use technologies to have a child.

The scenario of ending up with someone who one can't physically have a child with will be very common in your world, since - unlike in our world - everyone is effectively bisexual, so alternative ways of having children will also be common and probably seen as "just something you do shrug". As well, not having children at all will be a common option too.

  • $\begingroup$ The logical consequence of this is that in the roughly half of couples that are capable of bearing children, four or five offspring average would barely achieve replacement levels (depending on other causes of infertility, and the prepubescent mortality rate). Being an only child would be rare, and having 6-10+ children (to help make up for those couples who have less than 5) would not be at all uncommon. [In gamer terms, think "2d6 - 2" as a first approximation of the frequency distribution]. $\endgroup$ – Monty Harder Jun 27 '19 at 19:59
  • $\begingroup$ @MontyHarder all that only makes it more plausible $\endgroup$ – Max A. Jul 10 '19 at 12:45

Not much different than figuring out if someone's single

Is there a way to tell if someone's single just by looking at them? (Really, please let me know in the comments if you have one; keep in mind lack of a ring isn't a good indicator under a certain age and for certain cultures).

Flirting could involve mentioning your sex, allowing the other party to respond "Sorry, can't work, I'm also a K".

It just comes up in conversation

I don't have the ability to tell which state/province someone was born in just by looking at them, or how many kids they have, even less the name of those kids... yet somehow after knowing someone for a few months it's something that I probably know about them.

Public showers and changing rooms

I'm thinking of something along the lines of what we find here in gyms. You could make it so that your species needs/wants to clean themselves more often, making public showers and changing rooms more common than it is on Earth.

If it's the norm that both sexes use the same facilities, and they have different sex organs, it will be easily seen who's got what between their legs.

If it's the norm that those facilities are sex-specific, then just look at which facility the other goes in.


similar to the above point, if bathrooms are sex-specific, just look at which one they go in.

Clothing will inevitably give some clues

Different shaped sex organs will require different shape clothing to be worn comfortably. It may be hidden by other clothes on top of it (similar to how humans wear shirts over bras, pants over panties etc.) but some activities are easier done when wearing minimal clothing (swimming comes to mind) at which point the shape of the clothes will indicate the sex.

Online dating/Organized dating/Dating apps

It used to be that case that finding a mate was hard, but now that Dinder is so popular all the Z's and K's make a profile, enter their sex and look at sexy singles in their area.

Have sex

At the very least when you and your potential mate get undressed you'll be able to figure out whether or not you interlock.

  • 1
    $\begingroup$ Rings aren't even guaranteed in cultures which use them to find someone who's single. You could be, say, in a long-term, monogamous relationship, but still not married. So... yes, your point about there being no real, good indicators is very correct. $\endgroup$ – Fund Monica's Lawsuit Jun 26 '19 at 18:52

The Diskworld Dwarves exhibit most of the traits and behaviours you want by for different reasons. Male and Female dwarves do exist and they are from male-female pairings. However, the sexes appear very similar in day to day live.

As it is hard to tell the genders they are not a visible part of everyday life. As they work in harsh environments, they wear heavy clothing which further masks what little physical difference there might be. We are given to understand that the short, wide, powerful build of dwarves means they all have wide hips, large pectorals and muscular build. I dare say that a pregnant dwarve may not even show until near term.

Now as things happen to be hard to see their society took an interesting but not uncommon turn. They are very, very, very prudish about gender and sex. They just do not mention it or bring it up (and as a result differentiate much). This has actually made them very progressive with everything being equal. A dwarf is a dwarf. They only have the term "king" but it really means monarch as a king is either sex (which is only sometimes known publically if a king gives birth but even whether the king or a partner did the birthing might not be pubic knowledge). They do not have much problem with individuals sexuality as it is very private business so if that pair happens to be a same-sex couple or not is not your business or concern. It is often rarely known if there is, in fact, a couple as publically little affection is displayed (and I cannot recall if they have the idea of marriage...)

The fun part is when dwarves do decide to start courting (which again is quite a private thing, not much public display) it can take a long time before they are close enough to discuss what may or may not be hiding under their respective beards and armour. Following that, we must assume there is a further discussion (in the fullness of time) about whether the couple would like to engage in any recreational acts or have children (directly or adopted).


It's very easy for humans to be unable to tell. Here on Earth, even experienced ornithologists have trouble telling the male and female apart for some species of bird, such as the (British) robin. It's not a problem for the bird, though.

The clue could be pheromonal. It could be a subtlety of voice pitch or tone.

From the non-gendered species' POV I Can think of various reasons why biological male/female does not have any social implications. Maybe, they come into heat at a particular short time of year or in response to protracted and mutually agreeable courtship. At all other times they are asexual. Or maybe they are hermaphrodites, both male and female at the same time, all the time. (It might be important for them to be able to distinguish left- and right-gendered individuals, if love is ever to be requited!).

Moving into the more alien, then maybe, like Moties, they change sex from time to time. Or that sex and egg-laying is something carried out by non-sentient entities that grow as buds from their bodies which detach and roam around as near-brainlessly as insects. (Probably, in this context, there would be no nurturing of infants. They would have to grow up from tiny almost brainless "seeds" into large creatures with the intelligence of, say, a cat, before they would start to be nurtured and helped to progress onwards to adulthood.) Or even, that they are all biologically female, and that a male is a tiny brainless thing that takes up residence within a female body to fertilize eggs -- and once there, whatever ability for living independantly it ever possessed withers away, and it becomes nothing more than a conveyor of genes via sperm.

By the way, this last does exist here on Earth, in some species of fish!


Scent and sexual behavior

The problem of sexual differentiation had long been solved by nature. While some species have very strong sexual dimorphism, other have virtually none, and we humans have to run a DNA test to tell whether we are dealing with a male or a female. However, the species themselves need a way to tell with certainty which gender they are dealing with. Leaving it to chance would make it very detrimental to species' survival.

Thus, even if there are no outward sign of the sex, species may emit a specific scent which not only tells the others of one's sex, but can inform them if he/she is ready to mate. For the species that have no sense of smell, like birds, sexual behavior is coming to help. Males and females behaving differently when they are around each other. Pigeons may be a good example here.


When they are specifically seeking mates they could simply leave their genitals uncovered. This would serve both purposes of identifying one's sex as well as one's desire to find a mate.


There are so many interesting aspects to this question! To build a functional world like this, aspects of gender, anatomy, fertility, orientation, courtship, and intent to reproduce all come together.

Even in our own culture, there are obstacles to determining whether someone is an appropriate match for intercourse and/or reproduction. By drawing on some of these for inspiration, you can imagine how genderless societies might solve similar problems:

  1. Courtship rituals can involve sexual signaling that does not extend to other areas of life. So where gender is a kind of all-encompassing presentation that we often read as shorthand for sex (and sexual orientation), courtship rituals can involve temporary ways of looking and behaving for the purpose of intercourse or reproduction.

    Imagine people who go to a particular place, or attend a particular meeting, or style themselves a certain way, or highlight certain physical characteristics, or use a certain kind of vernacular, or seek out a matchmaker, etc. to indicate intent to reproduce and the role they're prepared to play in it.

    Humans face courtship problems like these when trying to judge, for example, (a) someone's orientation, or (b) interest in having children. As far as I know, there are some signals in each case, but accurate judging is difficult. Even where possible, mere inspection of anatomy is insufficient.

  2. Concealed ovulation is a lack of perceptible change to indicate that a person is currently fertile. As a species, humans exhibit concealed ovulation—although you can e.g. estimate your own fertility level by tracking your cycles, there are apparently no obvious cues that you or your potential reproductive partners can use to judge your fertility.

    Humans are nonetheless able to reproduce quite well. Very surprisingly, lack of fertility signaling is not the dealbreaker it seems to be. Perhaps for similar reasons, lack of gender signaling is not the dealbreaker it seems to be. You could even imagine that people in your society might not only have lack of cultural gender signals, but also indistinguishable external anatomy, making it difficult for everyday people to judge reproductive compatibility except by trial and error. This tracks with the human solution, which is some combination of subtle subconscious behavioral/pheromone cues and persistence.


The way many cultures on Earth do it today.

There are many things that affect whether two people are a suitable match. In some cultures you need to be from the same caste or tribe. Sometimes there are lineage restrictions; for example, a child of a forbidden relationship can also be "tainted" and that cuts out some marriage options. Even beyond these matters, sometimes people have economic restrictions. Then there are the personal-interest factors -- do these people have the same broad life goals, aspirations regarding children, etc? What about preferences on how they spend their leisure time? What about dietary restrictions -- will the vegan be happy with a barbecue-fanatic mate?

At the societal level, you sometimes see "filtering" and matchmaking done by people who know all the pertinent information, whether that's parents arranging introductions or professional matchmakers or friends making suggestions/introductions. At the individual level, while you can do some preliminary research (e.g. looking at social media), at some point people actually talk to each other about things that matter in a potential relationship.

Requirements for particular anatomy don't need to be any different than factors the some communities and individuals care about today. Prior art can help you here.


A while ago I had a related idea involving an alien species who believed they had two genders/sexes, but actually had three sexes. The two sets of gametes were both provided by subsets of what they thought to be "males" - who were visually indistinguishable - whilst the "females" played no genetic role in parenthood but only provided physical nurturing.

Traditionally this had not arisen as a problem because mating was highly promiscuous. However, a recent uplift into galactic society had resulted in the aristocracy attempting to establish patrilineal bloodlines, and wondering why they suddenly weren't having any children despite amassing substantial harems. Outside scientific troubleshooters were then called in to sort out this seeming epidemic of infertility.

The proletariat were completely unaffected, because they were still promiscuous. Tri-gendered species existed elsewhere in galactic society, but with sufficient trimorphism to easily tell themselves apart. Because these particular aliens believed themselves to be bi-gendered, they had not even considered the possibility of the truth being otherwise.

It would later transpire that the "females" could actually tell the difference between the "male" subsets by scent, but since they were considered of lesser social status, nobody had bothered asking them. In fact, details of the females' biology allowed them to choose which males' gametes to combine, but they still needed one of each subset to work with.


There's tons of physical differences between men and women that have nothing to do with genitals. Adults are very different based on their sex.

Women are usually shorter, their muscles are less noticeable, voice has a high pitch and female face is more oval or heart shaped with less noticeable jaw, slightly bigger eyes and smaller eyebrows. Not to mention breasts and waist/hips.

Men have more square face with stronger jaws and "sharper" characteristics, they are usually taller with more noticeable muscles (even with skinny men), their voices deepen in puberty, and they have chest and facial hair. Men also have Adam's apple.

Position of genitals and shape of hips dictate very different way of walking between men and women. Most men have bigger hands and feet than women too.

Sex hormones affect your body (and mind) in all kind of ways.

Now, this doesn't apply to kids, and there's always exceptions to the rule with adults But these are things that happen on average.

It might be that your people decide to purposely look as androginous as possible, doing all kinds of stuff from breast binding, shaving, wearing gender neutral clothes and hairstules, using makeup... Differences can still slip through, especially if you look for them.


In birds males are often times more brightly colored than females, so perhaps the males naturally have brightly colored skin, hair, and eyes, while the females have skin hair, and eyes that is naturally dull in terms of color. Alternatively both sexes could be naturally bright colored but with different colors on their bodies. Given that these colors would be their natural hair, skin, and eye colors, they wouldn't quality as different styles, and so would be only associated with sex, and not gender.


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