I'm working on a story set in a world with a tri-gendered race of intelligent creatures, the protagonists in the story. I want to be able to explicitly describe sexual intercourse among other things. I'd love some suggestions on how this might work. Some questions are:

  • What are the three different types of sexual organs and how do they fit together during sex? I envision that all three genders must make love together to be able to procreate.
  • What might the sexual stereotypes be with the culture of these creatures?
  • How should I handle pronouns in the story to be able to remain clear but not be awkward.
  • How about child-bearing? Would one gender bear the child or would be handled differently?
  • What kind of societal differences would there be between this culture and our own?

I know that Isaac Asimov explored this possibility in his novel, The Gods Themselves, but I'm looking for interesting ways to explore the concept further.

I'd love any ideas you might have!

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    $\begingroup$ You could have male and female genders roughly similar to humans, and then a third acting as incubator for the fertilized egg. Easy to envision a specialized gender for child birthing being an evolutionary advantage. You can use an ungendered pronoun for this third partner (phee is my favorite). After standard intercourse between the male and female, this third phee engages in a separate session with the woman to take the fertilized egg. Or, conversely, phee engages both partners simultaneously to combine the gametes in phis own specialized womb. $\endgroup$ Commented May 17, 2015 at 4:10
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    $\begingroup$ @IsaacKotlicky: why would the "phee" gender exist, evolutionarily speaking? It's a genetic dead end unless it contributes its own genetic material. And if it does, then what's the benefit of it needing to be fertilized by two other individuals instead of just one? $\endgroup$
    – zeta
    Commented May 17, 2015 at 8:51
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    $\begingroup$ Richard Elwood, you might want to read the Wikipedia article about the evolution of sexual reproduction if you haven't already. The only part of your question's premise I find unrealistic is the requirement that all three genders be involved in order to procreate. en.wikipedia.org/wiki/Evolution_of_sexual_reproduction $\endgroup$
    – zeta
    Commented May 17, 2015 at 8:58
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    $\begingroup$ @IsaacKotlicky: Isn't that more like 5 sexes, since an OO can only be born from XO and OY parents? But my point is that for the parents, having an OO offspring won't contribute directly to their fitness. $\endgroup$
    – zeta
    Commented May 17, 2015 at 12:21
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    $\begingroup$ @A.I.Breveleri you would have more than twice as many in-laws. Not only would you have twice as many spouse as one typically does, but each spouse would come with more corresponding in-laws. You would have three conventional parents, and 6 parent in-laws, not even counting siblings. $\endgroup$
    – Neil
    Commented May 30, 2016 at 19:46

12 Answers 12


It's pretty easy to have a genetic structure shared across 3 sexes and combine across the three. You could have either one lay egg or one carry after the three sexes combine chromosomes.

However, justifying the evolution of this species is harder. We have two sexes because we need a way to trade genetics; however, once you have some way to recombine genetics there is limited use for a third sex. If two of the sexes reproduce then each of their children has half their genetics if three reproduce then each child has 1/3 of a given parent's genetics. Once you have some means to add genetic variation the extra genetic variation of a third parent does not add much more to the overall genetic health of the child compared to two parents, and the loss of your child not carrying 1/6 if your genetics it otherwise would have is significant.

There is a strong evolutionary advantage to evolve to only need two of the parents if a three-parent system existed; more importantly, it's very hard to imagine a situation where a third parent would evolve in such a situation like this. It's very hard for two parents to evolve, but it's almost certain that a two parent system would develop before a three-parent system would, and once a two parent system exists there is little reason for 3 parent system to evolve.

This is relevant because I can not answer most of your questions until I know how the evolution occurred. If I can pick from one of a number of evolutionary options I can only then describe a plausible explication for genitalia, culture, and physiology of the species in question. Thus the first question, to create a believable world, is how could this evolve. Let's see our options:

Hive Queen

Have a hive with a queen mating with males and many drones that never get to mate. In this situation, it's possible for a gender to evolve that does not benefit from spreading its own genetics, but from supporting it's 'queen' from spreading her genetics. In a situation like this one parent may share little or none of its genetics directly with the child; but it's a child, or close genetic relative, of one or both of the other parents and thus gains from helping them.

Imagine a female queen that has male drones to mate with and a 'brood' sex that carries the child produced from the male and female mating to term, but does not contribute any genetics herself, being just a uterus for carrying children to term. The brood sex will be a close genetic relative to the queen, either daughter or sister, and thus shares 50% (or possible up to 66%) of her genetics with the queen. By helping her queen produce lots of children the brood sex benefits from spreading her queen's genetics even though she herself does not spread her own.

The female may produce lots of these brood-sex children, possible many dozens. As each of these children births, one match of young the queen will summon her up to mate again with the queen and her male, to produce a new batch of children. Eventually when one of the brood sex dies the queen will choose to produce another of this sex to replace the lost one.

The big benefit of this approach would be that the queen can produce more children at once. With a traditional queen situation, a queen needs to produce lots of eggs, which requires a massive caloric intake and a huge body for processing these calories and translating them into young. With a brood sex the queen can force the brooder to put in all the caloric investment to carrying her young, and she can have dozens producing young. This makes it easier to provide nutrition to each one-the queen is spatially limited in her ability to have kids because she can only convert honey to eggs so fast, but she can always add more broods. This also means the queen can stay small, and thus mobile, instead of being a huge thing in the center of a hive that can't move. Perhaps a queen can even have multiple hives that she flies to long enough to impregnate each of her brooders, before flying back to the first hive (though this would make her vulnerable to predation; this could still work of new queens are always ready to replace the first when she doesn't come back and may add to the genetic diversity of the hive if it's forced to replace its queens on occasion?

The downside is that each of these brood children requires extra calories since multiple 'brooders' is not as efficient as one super adapted queen producing all the young. Thus this would only work in situations where producing lots of young quickly was more important than conserving resources, and it's hard to imagine this being the case often. Perhaps if the hives were mobile, one hive would set up and consume the resources in an area quickly, then break up and fly away to set up more hives. The ability for a queen to start a hive full of brooders while she travels to set up a new hive may be useful.

There are a few ways to handle sex in this format, depending on how the queen and the brooding sex differ (the standard male role would be pretty much as it already exists).

  1. The queen may serve a role closer to male and simply providing more genetics to the brooding sex, being unable to produce any children without a brooder. in which case the queen would need to be provided brood sex 'sisters' from her mother when she starts a new hive; replacing these brooders with her own daughters as they die out. This seems least likely since it means the queen and hive are worthless without brooders.

  2. Alternatively, the queen may be able to produce only brood sex children herself, but nothing else without a brooder. I could only see this if a queen could produce both brood sex and males, and is using a system where she can start a hive by herself by birthing the original ones in a Haplodiploidy system, but needs both sexes to produce new queens.

  3. Most plausible, the queen could carry any child to term herself but prefers to use brood sex whenever possible (possible in conjunction with Haplodiploidy system above). The queen's first mating may have her birthing some males and brood sex children herself, then from that point her she uses her brood sex children for all matings.

The reason I consider the third option to be the most possible is that it's the most likely to evolve. As I said a 3 sex system is hard to justify, but in this case, the 3 sex system is an obvious offshoot of the traditional 2 sex system. The brood sex may be genetically very close to queens, except sterile by themselves. They evolved as queens that were intentionally kept sterile by their mother and exploited, eventually evolution causing them to diverge as more unique sex. The queens retain the ability to birth young (be it egg or live birth) the same way the brood does, but the brood traded ability to generate eggs for an ability to carry more eggs to term etc.

To answer your questions in this case the brood would likely be referred to either as it or with female pronouns like the queen. Since the brooders would not be given much attention, and likely wouldn't be intelligent, it will not be confusing to use the same pronouns as you use for a queen, any more than calling both your wife and your dog 'she' would be confusing.

Both queen and brood sex would have very similar genitalia. I imagine the females (both sexes) would have an ovipositor that can be inverted to act as an oviduct. TO give a rough analogy something like a 'vagina' that can be pushed out to act as a 'penis' for ejecting eggs into the broodmare. Both the queen and broodmare would have very similar structures, even if the broodmare never ejects eggs into others.

Culturally the broodmare doesn't count as anything of importance. You have females and males, and biological things that their child is growing in.

Shared parental investment

Have three independent sexes, which all invest equal effort into raising the child. It's important that they all contribute equal effort, if they do not it becomes hard to explain how this is an evolutionary advantage, since one sex gets to spread their genetics with less expense; which works in our world only because the advantage of sexual intercourse is so high that females are willing to pay a disproportionate expense to have sexual reproduction; once you already have at least two sexes reproducing sexually there is no reason for allowing a third sex to contribute less effort but get the seam genetic benefit; that would be evolved away on the grounds that you can do two-person sex without the free-loader.

There are many ways to get an equal genetic contribution, though most approaches favor R select species which are unlikely to evolve sapience. Here are the key ones I can think of.

  1. With R species all three work together to produce a large batch of eggs somehow. The energy investment for each sex is about equal in producing the eggs. and the eggs are then left alone to produce lots of young without further parental investment.

  2. Close to one, each sex takes turns in raising the young through a phase. sex X produces the original eggs which it transfers to sex Y, likely through some form of internal fertilization, at a minimum to an egg pouch similar to what seahorses have. sex Y fertilizes the raw eggs and carries them partially to term, long enough for the eggs to develop a hard shell and be safely 'layable', ie able to survive outside of the body of X or Y. Y then lays the eggs so that Z can fertilize them. Z then protects the laid eggs from predation, sitting on them, protecting the clutch etc, until the eggs hatch. At this point, the children are mostly forced to fend for themselves.

  3. In a more K select species, one sex produces large, caloric expensive eggs, that it passes off to the other two sexes. The other two sexes fertilize the egg and then work together to raise the young. The put little caloric investment in going from embryo to born child, but the invest the effort in raising that child to reproductive success. Though this seems least likely, it's the one most likely to create sapient species.

  4. All species 'conceive' after intercourse. Perhaps three 'eggs' (very underdeveloped) are produced from intercourse and each sex will take and care for one of the produced young, with the primary investment being parental care raising the egg to adulthood, not in the production of the egg itself. Why you would have 3 sexes instead of one (see below) in this situation is hard for me to explain though. In terms of intercourse, I would probably do something like what I had in 2 though. Sex X and Y have internal fertilization/intercourse to produce 4 large 'layable' eggs, X and Y both take two of these eggs. they both find a sex Z and have intercourse with it to fertilize their two eggs again, and both sexes walk away with one fully fertilized egg to care for.

Your notice in all these situations I suggest that the sexes have intercourse at separate times, and that sex X may never know the sex Z that fertilized their egg, this is intentional. It seems quite difficult to justify an evolution where 3 sexes have to agree on each other as a mate, what if the X doesn't like the Z but likes the Y? how do per-sapient sexes handle mate selection and decision making? It's more plausible if each sex needs only pick a single mate at a time to have intercourse with, even if another's DNA will be added later. This also justifies the only evolutionary advantage I can think of for 3 sexes if there is a reason to delay the development of the full child. Perhaps environmental factors can cause embryos to fail to form early, so having a phase of partial-development (after X and Y mate) allows one to see if the children will successfully develop or die due to undesirable mutation/chromosomal damage, prior to final fertilization and the completion of a child developing.

It's possible that in sapient species sex X Y and Z will all know each other, but the original mating habits likely evolved around X never knowing Z, and it's only with later sapience that interest in knowing the full genetic investment in the child lead to X and Z meeting.

In terms of pronouns, I think you would simply have to create new ones for almost all of these, such as the way I said sex X Y and Z.

In terms of genitalia, I would imagine all have very similar genitalia, with only minor differences. They should all look similar since they have to all evolve from a single format. I can't say more without knowing which of the above options were used.

These systems would likely lead to R strategies where parental investment didn't exist, and thus no sapience. For the ones that lead to parental investment originally I would imagine each would raise their own child, but eventually, cultures may move towards all three sexes living as a family unit.

Hermaphrodite single-sex species

Finally, there is a third option which I consider far more likely from an evolutionary standpoint, which may still meet your needs. Have only one sex!

Instead of multiple sexes, which as I said are really hard for me to see evolving, have one sex, but which is capable of mating with multiple individuals of the same sex and producing a single child that shares DNA with three, or more, parents

With a system like this, you could claim that DNA can be split and recombined safely in more then than just binary manner. Perhaps each chromosome can be received from a different mate, or even individual chromosomes are broken into smaller gene sequences which can be split and recombined to combine genetics of multiple mates. This is not an impossible system, I think even our current DNA could evolve to an approach like this without too much hassle (potentially a minutely higher chance of birth defects, but the genetic fitness of multiple mates could more than make up for that), and a system that encodes genetics differently then DNA could definitely evolve for an alien species where this makes sense.

There is a genetic advantage to having your child contain DNA from multiple parents, the more genetically diverse a child is the more 'fit' he is on average, but the advantage is only so large. For the mother to benefit she must make sure she is still ensuring that half of her DNA is inherited from each mating she performed, otherwise she loses from the mating. There are a few options for handling this:

  1. Your child always caries half your DNA. When mating with others you recombine their DNA to form the other 50% of your child's DNA. Since the mother always provides half the DNA it never hurts her to mate with more mates, and she actually gains more genetic diversity the more she mates with others. In addition any mating the mother has allows her to provide DNA to the female she is mating with, potentially contributing some DNA to her mate's child as well. There would thus be major motivation for females to mate with multiple partners, expect a very 'free love' society.

    While it's possible to limit the mother to being able to only mate with 2 mates per child, with a system like this it feels more natural to allow many mates to all contribute DNA to the child, Once the mother has the ability to recombine DNA from multiple 'fathers' why should she stop at two?

    However, since each 'father' may very well only contribute as little as 1/10 or less of DNA to a resulting child the father would not be inclined to provide much, if any, parental investment into the children of a mother. Thus this race would likely consist of all single parents.

    Insects already have a variant of this, in which they produce many young with each young having potentially different fathers, so look at them for mating adaptations. Most noticeable it's likely that each mother will have some degree of control over how much, or if any, of a mate's DNA contributes to that of the child. This control may be conscious or subconscious. It may be that a mother attraction or pleasure with a mate will result in that mate's DNA being a larger percentage of the child's DNA; but that they mother has not direct conscious control over this factor. How much control the mother has in the percentage of DNA shared will effect mating behaviors.

    In systems like this sexual mating may be traded socially. Look at the bonobo, who use sex as a social feature. A mother may mate with lots of others to please them. If she has strong control over who's DNA is used for the child she may mate with everyone to obscure her mate choice. If she has less control (ie any mate will likely contribute at least a little DNA to the child) she may be more selective with mates, but may be willing to mate socially with someone who has earned the right, through gifts or favors. In any case since any father only contributes so much DNA it's safe for her to be less selective about mates, the less control and the more DNA the child will potentially posses from any given mate the more selective she will be.

Pronoun wise you would have mother and daughter, and that's it. You may use 'mate' to refer to those that mated with your mother, but since the child of a mother may have little if any DNA of a given mate the concept of father doesn't really mean much. sisters will exist, but only if they share a mother.

Genitalia wise you have your standard penis and vagina. Or possibly an organ that combines the two together to allow simultaneous transfer of sperm to both sides.

  1. Mother only provides 1/(mates + 1) dna to the child (limit to only 2 mates)

    This gets back to the situation where it's less advantageous for the mother to mate. If the first mate produces a child with 1/2 of your DNA, and mating again produces a child with 1/3 of your DNA then there is little incentive to mate a second time and have a child with less DNA, usually. One easy way to fix this is to force mating to be mutual. If a mother mates with another female they are both guaranteed to conceive a child that possesses the mates DNA.

    This works because each subsequent mating allows the mother to spread more of her genetics to the next generation, even if the child she is carrying possesses less of her DNA. Say after the first mating she will conceive a child with 1/2 of her DNA, and she chooses to mate with another mate. Her child will now possess only 1/3 of her DNA, however, her mate's child will possess wither 1/2, or 1/3 (if her mate has mated with another) of her DNA as well, meaning that The subsequent mating has resulted in her child having 1/6 less of her DNA, but another mate's child having 1/3 of her DNA; that's a clear benefit to the mother.

    However, if the mother can mate with an infinite number of other mates she reaches an odd point, where she doesn't care about her own child. Once she has mated with 9 mates her child only possesses 1/10 of her DNA, and thus is barely close enough to warrant caring at all; she is more interested in the other 9 children who also carry 1/10 (or so) of her DNA. This makes for a rather odd, and eventually unstable, mating solution.

    The easy way to fix this is to limit the number of mates a mother can have, in fact a system like this would almost have to place a limit on total number of mates; otherwise matings become so common that there is little intensive to care for your own child and things would evolve to an R select strategy of having dozens of young and tossing them into the world to fend for themselves.

    So, say the mothers can only mate with a total of 2 other mates, this is as much as DNA can viably combine. Now each mother benefits from having two mates. IF the mother only mates once she will have one child with 1/2 her DNA and one with 1/3 (most often, if her mate has a second mate) or at most 1/2 her dna. A mother who mates twice passes on either 100% or 5/6 of her DNA to the next generation, depending on her mates mating habits. A mother who mates with three mates has at least 3 children that all cary 1/3 of her DNA, so she at minimum passes on 100% of her DNA, and there is always a small chance of passing on more if one of her mates doesn't find a second mate.

    In addition the 2 mate option means more genetic diversity over one mate, meaning 2 mates is still beneficial over mating with a single mate who will not mate with anyone else. Plus, since you can never be sure your mate won't try to mate with someone else later making sure you mate twice means you come out ahead no matter what your other mate does.

    This system is, again, plausible to evolve. Though a 3 mate system (meaning that DNA is split into 4, which is more likely due to it's being divisible by 2) may be more likely then a 2 mate system. In this system a mother may be a single mother, or one or both of the mates may stay to raise their children as a family. Occasionally two mates may choose to have a child together without involving a third mate as well, far more common once the species reaches sapience.

    This system would lead to mothers being slightly less sexually selective then females in most mating situations of today, but still nearly as selective.

    For this system to work there must be a pretty strong promise of conception occurring with all mates, which adds a potential complication. Females would have to be able to conceive very soon after mating, with very clearly defined mating 'seasons' during which they will conceive if mated. More importantly, and difficult to evolve, a female would need a way to advertise once she has conceived, say by swelling going down, as soon as she accepted her third mate. If this didn't occur then a female could mate with other females after accepting her third mate and impregnate them, spreading her genetics, while effectively ignoring their DNA offered to her since she already conceived.

    There would be very strong evolutionary advantage for a female to evolve a way to avoid conception from a mating, or simply to mate after she has conceived but is not yet showing. To 'trick' other females into agreeing to carry a child with part of your DNA while you don't accept the need to carry any of their DNA allows you to mate with many females, and potentially spread far more of your genetics. This is a bit of an issue with this option, making it less likely to evolve; or more accurately to be evolved away from not too long after it's original evolution as people start finding ways to 'lie' about their mating.

    This could be fixed with a monogomous mating system, where mates all know and watch each other. Thus a mother may choose to mate with a second female only if she can watch the female and verify the female does not attempt to mate with more then two mates. If a female does manage to sneak a third mating she will benefit from spreading her genetics to more, at the expense of her first two mates (who may share less or none of their genetics if she mates with a third mate).

    This monogomy could take two forms. One is which all three mates choose each other and mate year after year. As I said above this is harder to manage in non-sapient species, but not impossible; and it's more justifiable when everyone is the same sex; making it easier to choose any other female as your third party mate. In this case the females would likely mate between themselves and raise their young as a third person family.

    Another option would be that a female will stay close enough to watch her mate, but not try to control her mates selection of mates. Effectively female A will declare she mates with B to everyone. When female C comes by B may choose to mate with C, which A will allow; but if B ever tries to mate with a third female both A and C will protest since they are aware of her two chosen mates (A would have advertised to C that she already mated with B). meanwhile A and C may choose mates of their own. B will watch both of them and ensure that A and C only choose one other mate, but does not try to prevent or control who that third mate these two choose is. This mating strategy works best when everyone mates in a nesting area (or village) where it's easy to watch your other mates. Perhaps A B and C will nest closely together and each will contribute to helping raise B child partially, while also focusing partially on raising their own and their second mate's child. This sounds complex, but I think it could evolve to work pretty well. Each female has incentive to stay near both her mates, because both mates contribute partially to raising of her young, so it's easy for those mates to watch her to ensure she doesn't mate again.

Genitalia wise you would likely have some system that allows sharing of sperm by both mates at the exact same time. I would guess some sort of 'dual plug' system, effectively the same as inserting your penis into her vagina the same time she does the same to you. The positioning of the 'plugs' would have to make mating easy though. Mating from in front would be preferable then behind (behind makes more sense when the male and female roles are separate, due to female having more of a need to 'defend' against unwanted matings then male). I imagine their organs would simply be more flexible to allow repositioning to align up with someone your facing.

Pronouns work as above, everyone uses female pronoun but mother is limited to the female carrying the child, while those females that mated with the mother are considered father's of the child the mother carries. Yes that means a female can be a mother and a father. Siblings would get complex terminology. you can have third and 2/3 siblings (probably a more convenient way of saying 2/3 sibling will evolve). You may have two words for sibling, one for "we have the same mother " and one for "we don't have same mother, but share partial DNA" as well. Thus you can have third, partial, and full Sisters (share mother), and third, partial, and full siblings (different mother).

In a system like this some degree of parental investment will occur from all mates for each child, since it's almost as useful to care for your partner's children as your own since theoretically all three Cary 1/3 of your children (though not quite as much, since you can never be 100% certain cuckoldry didn't occur with partner children).

Culturally I imagine there will be both 3 parent homes where the parents only mate with each other and raise children together, single parent homes where a parent raises their child alone without aid or investment from the other parents (less common), and shared communal homes where people work together to help raise children.

The communal homes are most interesting. Take my example above where A and C mate with B. Now imagine A also mates with D, and C mates with E, and for the sake of simplicity C and E happened to mate. Now you have a group of 5 parents that all share children with two other parents. The parents would all prefer to stay close to their other two mates, which could result in all 5 mates living together.

Now imagine if it got more complex, you could have 10 or 20 mates involved in a chain before the chain of matings 'loop back' by having someone mate with two people already in this mating chain. You may tend towards large communal living spaces where everyone tends to contribute and share parental care of children, but of course they all mostly focus on the children the sired. I think the entire species would evolve to have more of a 'community raising the child' mindset due to this bizarre spreading of genetics. How much this is true depends on how monogamous the species is.

Speaking of which, this species could be highly monogamous, or each mating season it could mate with different couples. I would imagine it would tend more towards monogamy, as it would make it easier to share parental investment of raising multiple generations of children at once, but it need not be mandatory.

This species will most likely have mating seasons or some similar concept, everyone becomes fertile at roughly the same time. That's required to make watching your mate and ensuring they only mate with one other mate possible.

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    $\begingroup$ these are the answers I love to see $\endgroup$
    – TrEs-2b
    Commented Aug 10, 2015 at 20:06
  • $\begingroup$ @majornorwal thank you, though I disagree with you lol. The formatting is horrible, and there are a number of places it could use some major cleanup to make it clearer what I was describing. Sadly, I've never been good at writing and I hate formatting :) $\endgroup$
    – dsollen
    Commented Aug 11, 2015 at 22:48

Diachronic hermaphrodites

I’ll offer a variant of @dsollen’s “hermaphrodite single-sex species” which I also think is more likely to have been evolved than three separate species.

Life on Earth started without differentiated sexes and many lifeforms still make do with just one sex (which is the same as none), but mixing the genetic code of two (or more) individuals seems to give some evolutionary advantages, especially for more complex lifeforms. Social patterns developed accordingly. Evolution didn’t have to create two distinct sexes and assign each individual to one of them once and forever. Some organisms are hermaphrodites, i.e. they have both kinds of genitals, but usually, they still require a mating partner to produce offspring. Other species can change sex once if necessary (e.g. remember the hand-wavey explanation in Jurassic Park), others even multiple times.

I’m suggesting a species that cannot change sex at will but will instead go through several sexual phases throughout its lifetime.

  1. After birth (or hatching), kids are presexual. Genitals are almost female, though.
  2. With puberty, youngsters become gynsexual ‘female’. Their ovaries fully develop and start to produce eggs. If they are mammals, their breasts grow and can produce milk after giving birth.
  3. With menopause, adults become intersexual. While they may still enjoy having intercourse, they are not fertile. They mostly care for the offspring they have born, support their family or larger parts of society (incl. being soldiers).
  4. With transition, matures become androsexual ‘male’. Their testicles and penis (or whatever) have grown and are fully functional. They can mate with youngsters. Since there are by design less matures than youngsters and their role is about to change anyway, long-term monogamy is unlikely, but they will probably somehow nourish their first and second phase offspring anyway.
  5. Finally, seniors become postsexual. They lose all their reproductive capabilities but are more like a man than a woman. Their bodies can focus on other organs, including the brain.

Please note that this sequence may seem like a hierarchy which suggests that males are worth more or higher advance than females, but I chose this order just because first, pregnancy is stress for the body and should be left to the (relatively) young and healthy, and second, the parent is then more likely to live afterwards to raise their children. I like the idea that every ‘man’ has to have gone through being a ‘woman’, so these ‘men’ are very differently socialized than us human men.

I don’t think language would develop grammatical gender distinction as we know it, but there certainly will be simple and exact words for the different phases of life and family relations, maybe also age-based honorifics. Culturally, I assume, there would be several rites of passage for everyone.

I tried to come up with estimates as to how long each phase should last, but I really can’t offer anything better than, in Earth terms: ten years each at least.

Three-sex arrangements

Most variants I can think of have probably already been presented, I lost track of ideas:

  • Mediator: Male inseminates bigenital ‘cogenitor’ (i.e. looking like a hermaphrodite). Cogenitor puts fertilized egg inside female (cuckoo-like).
    • Variant: Cogenitor puts modified sperm inside female (catalyst).
    • Variant: Male could also impregnate female directly.
    • Variant: Cogenitor could impregnate another cogenitor, i.e. act as either male or female.
    • Variant: Cogenitors could impregnate each other, i.e. act as male and female at once.
    • Male and female could have developed from the cogenitor as common origin.
    • Cogenitor could be a different, symbiotic lifeform – or a medicinal robot.
  • Breeder: Amale and bemale inseminate female, only then one of her eggs can get fertilized.
    • Variant: Amale and bemale are not different sexes, but still different male individuals.
    • Variant: Even more kinds of male required/possible.
  • Donors: Amale sperm-donor and bemale egg-donor both inseminate cemale breeder in whose womb an egg gets fertilized and grows.
  • Incubator: Male insemninates female as usual. Fostress extracts embryo from female (e.g. female lays an egg, or the baby gets born prematurely, kangaroo-like pocket).
    • Variant: Just two genders, but males have the pockets and probably breasts to feed from (seahorse-style).
  • Hermaphrodite: Male can inseminate uter or female. Uter can inseminate female (with its own, unaltered sperm).

By the way, if you just want to justify a promiscuous, polygamous two-sex society, make fertilized eggs not able to nest inside the womb of the woman whose ovaries the egg came from. Like pollen, they would attach to some pocket in a penetrating penis and remain there until they are released into a different vagina.

  • $\begingroup$ I know this is an old answer but thought I would still comment. Your logic on presexual>female>male>postsexual sex-order makes some sense. However, even though childbearing is relegated to the young, high female death rates during childbirth may result in a shortage of older males, or at least a skewed demographic. $\endgroup$ Commented Jul 12, 2018 at 13:06
  • $\begingroup$ @EveryBitHelps Indeed and I believe this species would not prefer longterm monogamous relationships, because the mother would prospectively become a man and look for a “wife” themself. People would look more after children they mothered than those they fathered. $\endgroup$
    – Crissov
    Commented Jul 12, 2018 at 13:25

When I speculated (unpublished) on aliens with 5 sexes, I figured the material being shuffled was in 5 parts, rather than 2 copies like ours. Two different nuclei each with 2 gametes, and an egg with mitochondrea and other asexual information. Late merger of eucaryote-style endosymbiosis after sex evolved kept them from merging, and the same species has different more-drastic phenotypes than our minor diamorphism.

Three is easy in comparison. You need a reason why the egg isn't also a bearer of chromosomes but the roles are kept distinct. Perhaps more (important) functions of organelles like mitochondrea and chloroplasts.

Maybe the cell level information is present in various organelles and sexual recombination takes place between individuals with different eggs, and this is important for disease resistance and metabolism that quickly adapts. Meanwhile, a nucleus is used for multi-cellular organism programming (only) and has its own two gamates, and this reassortment is used for evolution of the phenotype.

Note that recombination and reassortment are different mechanisms. Working in different ways helps keep the different steps from merging in a more streamlined manner.

Also, the first kind, cell-only information, is like bacteria that swap genes without having clear cut species as a concept. This type might be available across species in that world, which helps pick up immunity faster if long-lived species can learn from short-lived ones. The second kind of sex is specific to (and defining of) individial species in higher-order life like plants and animals.

Turns out, real biology is more varied than I realized! Tetrahymena thermophila has 7 sexes that can reproduce in 21 different combinations. Cells have two nuclei, just as in my story.

  • $\begingroup$ Species with 5 genders... 8427? $\endgroup$ Commented Jan 24, 2018 at 3:42

I wanted to come up with something that's not a variation of Niven's Puppeteers (two sentient males and a non-sentient female incubator).

So there are three genders that come together, but they don't have intercourse. Instead one of them makes an egg while the other two help form it, and then all three inject their reproductive cells into the egg.

So the cells from one parent will join with cells from either of the other two, but will be dominate with one, and recessive with the other.

Let's label the cells Red, Blue, and Green.

Red + Blue = Red
Blue + Green = Blue
Green + Red = Green

They all mix and combine, and which ever has the most cells at the end determines the gender.

Tl;Dr: So the cells roshambo (rock paper scissors) for control of the gender.

Edit: you could still have live birth with one of the genders acting as female, but I can't really think of a genital setup besides the male/female arrangement we have for delivering reproductive cells into a womb. So it would end up as male and different male, aka Puppeteer.

Edit 2: Taking Greens suggestion of a Cloaca into account, you could have live birth that way, where they have seasons where they switch from being a contributor to being a carrier. So for part of the year they are able to donate genetic material, and part of the year they receive it and give birth to the young. Then whichever they mate with would determine what the gender is at the end, while avoiding the Puppeteer problem.


Since you specifically want three genders, the problem is encoding three genders in genome in a stable and meaningful way. I think the easiest way is to make the species triploid with XXX, XXY, and XYY equivalents each being a separate gender.

The way I understand it evolutionarily triploidy is a response to unavoidable environmental exposure to radiation or toxins. It is common in immobile plants and animals bound to relatively small habitats that can become contaminated. It is also seen in otherwise diploid organisms in specialized organs that are continuously exposed to radiation, toxins, or oxidative stress.

Of these options most likely would be evolution from an amphibious species that was triploid in order to cope with possible contamination of small ponds and lakes it lived in. Further we can assume triploidy comes from dual fertilization of the egg. The other alternative (two copies from the mother or digyny) is actually more likely, but since it can't lead to three genders we can ignore that.

Having genes from two fathers gives higher genetic diversity than being fertilized twice by the same father, so it would seem reasonable to evolve a gate system that prevents double fertilization by the same father. The start would be a system that requires a certain time since the first fertilization before the second can happen. If only one male was available both fertilizations would still come from him.

The remaining problem is differentiating XYY and XXY into separate genders. This should start with the original differences. XYY would have more male hormones, while XXY would have an extra copy of the X chromosome. We can assume (no basis, but we can) that this means the XYY males are more aggressive and stronger and XXY males have longer lifespan.

We can then assume the evolution of a social structure where all three groups have separate roles. XXX would specialize in reproduction and child care. XXY with its longer life span would do food gathering from the area and later tool and shelter manufacture. XYY with more aggression, higher strength, and higher expandability would do scouting, hunting, and conflict with competitors.

If we assume live birth (to make them more like humans if nothing else) this gives rise to sexual trimorphism. XYY would be larger with more robust bones and exaggerated musculature. XXX would have wider hips to facilitate live birth. XXY would probably be smaller and more social.

Note that by default half the children would be XXY with quarter each XXX and XYY. Lack of pregnancies and lower aggression would also give lower mortality. So the majority of people would be XXY. As XXX and XYY are relatively as numerous we can assume permanent pair boding with opportunistic sex with XXY. XXY would be bonded to the larger tribal group, not to any specific XXX/XYY couple.

Since the XXY are neither aggressive nor targets of aggression nor permanently pair bonded, the most likely system for gene transfer between tribes would be with the XXY moving between groups. Triploidy should give higher resistance to inbreeding, so the level of movement could be low and different tribes might be quite different.

With the two types of males having different sexual patterns XYY with permanent pair bonding and XXY with opportunistic social sex differentiation to separate genders is practical. Since the two males are hormonally different evolving a system that restricts the first fertilization to XYY and the second to XXY would be practical. The benefit would be in binding the both types of males to the children genetically.

The two types of males would have the same male reproductive organs with the same function, the female would have normal female reproductive organs. Details would vary based on the specifics of the species. While I am assuming amphibious omnivore here, that could mean a mammal, amphibian, or reptile analogue. Or something entirely different.

I hope this answer gives a plausible, if unlikely, route for the evolution of a tri-gendered species. I think the other questions in the original question should be dealt separately in other questions with more details of the species given. Although the evolution I suggested does require specific social pattern to drive it.


First off, wikipedia is awesome for generating ideas about this. The following is an amalgamation of existing methods for sex-determination.

Most of the answers are predicated on the idea of a 2 sex gene structure/"fight for dominance." Adding in a third sex gene provides another way for this to occur. This could be either as a "third partner" in direct intercourse that is vital to the gamete formation (either adding the egg with mitochondria or a distinct type of sperm), or as a host where the gamete picks up genetic material from the womb once implanted (I lean toward this one). In this latter case, the third partner (P) takes the fertilized egg from the female to implant in their own womb. This is all dealing with a roughly humanoid/mammalian race, otherwise all bets are off. You could easily appropriate an existing non-gendered pronoun for this third gender. I like to use phe as it's audibly distinct but relateable to existing he/she pronouns.

In humans, aneuploidy in general leads to mutations, some of which are harmful or fatal to the fetus. For sex chromosomes, though, trisomy leads to Triple X for women and either Kleinfelters (XXY) or XYY for men. None of these really pose any genetic problems to reproduction. So we know trisomy isn't problematic.

So let's assume this species requires trisomy for sexual determination. Now what? We use a modified Z/W methodology (like birds) to determine which of the three genders the child is.

If the child has a majority X, they're genetically female (relevant point: those who are XXY Kleinfelter syndrome tend to have gynecomastia). If they are majority y, then they are male. These both fall under ZW. If they are either XXX OR YYY then they are the third gender (let's call it phee) —ZZ determination.

There are 8 possible combinations: XXX (P) XXY (F) XYX (F) YXX (F) XYY (M) YXY (M) YYX (M) YYY (P)

Since all partners will have X's (for an XP) or Y's (for a YP), there is a chance for all three genders out of any particular tripartite union. You'd think this is a 25/37.5/37.5 split between genders, but that's actually not true. Statistically, let's map it out:

P= either 100% X or 100% Y. Assuming there's no genetic testing or externally expressed sexual characteristics to distinguish between them, then it's 50/50.

M= 33% X/67% Y

F= 67% X/33% Y

So M/F has a 22.2% chance of contributing XX and 50% chance of P being X, yielding an 11.1% chance for PX and an equal chance for a regular F. The same split is true for YY, so the total chance of P(X or Y) = 22.2%. This is absent pre-implant genetic screening (so you can pair XX to a PX and YY to a PY), which could potentially double this to 44.4% (so your XX/YY is ALWAYS paired to a matching P). In this case, P is the determinant between (M or F)/P.

M/F XY gametes happen the remaining 55.5% of the time. In these cases, the P is the 50/50 determinant between M/F (since P is already not an option).

So our real gender splits are:

M=F=27.7%+11.1%=38.8% P(x or Y)=22.2%

After gamete genotyping becomes available, this will shift to 44.4% for P, and 27.7% for both M and F. The implications of this will be discussed below.

As you can see, P's are rarer than either M or F, while still being reproductively necessary. This will make them culturally and socially desired. How exactly this effects society depends upon the physical characteristics of the P. Assuming that the P is the gamete carrier (the M fertilizes the F and the P takes it from the F) and consequently the child birther, we would assume evolution would incentivize frames with wider hips and a more muscular lower body (for both fleeing to protect the fetus and for muscle tone in carrying/birthing the fetus). This is related to, but a bit different from, current female sexual characteristics.

With male-female pairs competing for the favor of a P to birth their young, P's would likely find themselves in positions of authority where they get to choose favorites - think judges, shamans, and the like. It's likely this would impact how families get structured, with They would become symbols of fertility. The birth of a P child would be seen as a sign of favor from the gods, and such children would be pampered above others. Due to "gender typing," the M would "socially" educate the M children, the F to the F children, and the P to the P children. This stovepiping, along with their favored status, means that P's will get more educational and cultural opportunities than their biological M & F siblings.

With the advent of gamete genotyping coupled with the cultural favoritism toward Phees, you'd expect that M/F couples would, when given the choice, choose to have a P over a M or F. This will quickly lead to demographic saturation (P's will be 44% of children born rather than 22%). What happens at that point depends on how culturally important Phees are as a whole (vs. as "special" individuals) and how their "modern" families are structured (are Phees another parent or a surrogate to a M/F pair like mentioned earlier). This could lead to a Phee master/(M&F) slave society. Or not.

Really, without having more concrete material to work with, the question of "what are the implications" is way too broad a question to answer...


To simplify this I would make it similar to human gender


The Female:

it can get pregnant.

In order to get pregnant the female needs two different DNA sperms:

From male and male.    (half each)


From male and female/male.  (half each)


 From female/male and female/male.  (half each)

The Male

It cannot get pregnant it can only give sperm. half of DNA needed. so that in order to get a female or a female/male pregnant he needs :

 his sperm plus the sperm of another male (half each)


his sperm plus the sperm of a female/male (half each)

The Female/Male

it can get pregnant but also can give the half part of sperm needed to get a female or a female/male pregnant.

in order to get pregnant the female/male needs:

a male sperm plus the sperm of another male (half from each)


a female/male sperm plus the sperm of a another female/male (half from each).

In order to get a female or female/male pregnant the female/male needs:

his/her female/male sperm plus sperm from a male (half each)

his/her female/male sperm plus sperm from another female/male (half each)

Female result:

from pregnant female with sperm from male and male


from pregnant female with sperm from male and female/male


from pregnant female with sperm from female/male and female/male


from pregnant female/male with sperm of male and male


from pregnant female/male with sperm of female/female and female/male

Male result:

from pregnant female with sperm from male and male 


from pregnant female with sperm from male and female/male


from pregnant female with sperm from female/male and female/male


from pregnant female/male with sperm of male and male


from pregnant female/male with sperm of female/female and female female

Female/Male result:

from pregnant female/male with sperm of female/male and female/male


female has one vagina and sperm donors can take turns, 
or 2 vaginas and the sperm donors can be in her at the same time.

male has one penis.

female/male, if regular female has one vagina then female/male has one
 but if female has 2 vaginas then female/male has 2 as well,
 and has one penis regardless of 1 or 2 vaginas.

In your story you might say that because to have a female/male born all 3 parents need to be female/males, the female/male gender look down to female and male gender, and think that a female/male gender is superior.

You might call them female, male and fema.


Females are the most beautiful.
Males are the strongest.
Femas are the most intelligent.

The typical scenario is that the 3 persons live together as a triplet and raise the child together, but some might "divorce" taking one person out of the triplet and might bring another person to join the couple to make it a triplet again, or they might choose to live as a couple instead which is not typical in their culture, or a single person might raise the child alone which is less typical and almost insane in their culture.

love unions of triplets is the norm, and partners of more than 3 like 5 or 9 its not bad viewed as long as they are all female/females.

  • $\begingroup$ In this situation the males and Fems get to share their DNA as males with less cost then females, making female genetically disadvantageous The Fems can play the role of female and raise as many young as females, but they can also mate with other fems. Thus the Fems contribute as much of their DNA to the next generation as femals via raising young, and contribute some of their DNA as males fathering young for other fems. More Fems DNA is spread then female DNA each generation. At this rate very quickly no pure female would exist, replaced entirely with Fems. Males too would be replaced. $\endgroup$
    – dsollen
    Commented May 18, 2015 at 19:17
  • $\begingroup$ @dsollen "At this rate very quickly no pure female would exist" Not if they were aware of the result of pregnancy before breeding according to the sex act combinations and they wanted to keep 3 genders alive which they might, because OP said they are intelligent. "female genetically disadvantageous" they are the most beautiful, femas can't be as beautiful as them, that might be a strong genetic advantage that keeps them around. to be a fema the parents need to be all 3 femas, all other results are female and male. $\endgroup$
    – adrian
    Commented May 18, 2015 at 22:40
  • $\begingroup$ @dsollen actually is harder for a fema to be born, 2 femas 1 male gives female or male. 2 femas and 1 female gives female or male. The only femass who are born are those out of 3 femas, in order for the new fema to give birth to a fema he/she needs 2 more femas. females and males will continue to exist if alt least one of the 3 parents is male or female. $\endgroup$
    – adrian
    Commented May 18, 2015 at 22:46
  • $\begingroup$ yes I looked closer at that later and saw that, but it adds it's own issues. If a for 30 generations females meted only with fema you still have a female not a fema? and yet if 30 generations of fema mate together and then once mate with a male or female they don't have a fema? There is no situation in which genetics should always prefer pure male or pure female over hybrid no matter what. So much that I assumed a more logical distribution. In either case it doesn't fix the problem. Fema are the most beautiful, because of the "sexy Fema hypothesis" (look up "sexy sons") $\endgroup$
    – dsollen
    Commented May 18, 2015 at 23:25
  • $\begingroup$ fema mothers (when carying child) will always choose to mate with fema fathers, because fema children are more likely to spread their genetics then any other combination and the parents want that. Fema when serving as males will mate with anything, but prefer fema. Meanwhile males and females will be just as likely to mate with fema as the 'pure' sex because why not. The result is fema genetics always spread and drive everything. Eventually evolution in Fema genes will allow them to produce fema from other matings and everyone will inherit it because Fema genes spread better. $\endgroup$
    – dsollen
    Commented May 18, 2015 at 23:28

If you have three sexes, and only two of the three are required for successful production of fertile children, then it's only a matter of time before mutation makes one of the three sexes vanish. The species can carry on without them, and it will. Nature does tend towards simpler designs, if they get the job done.

So you do need three sexes. One possible set-up is a race with triploid chromosomes. Each gender contributes a gamete with one-third of its chromosomes. Transferring a fertilized egg (or partially fertilized egg) is fraught with difficulties, so instead, you have two male sexes (blue male and orange male), which make their respective contributions in turn. Eggs which are only fertilized by one gender don't develop any further.


On societal differences, one consequence would be that each parent would only share one third of his/her/? genes with the child, and vice versa. It seems crude to simply state that parents and children would love each other less, but it would certainly evolve to be a different, more diffuse relationship than is usual for us. Perhaps sometimes it would be rather liberating, with less of a tendency for parents to want to control their children. (In contrast, I am working on a race that reproduces parthenogenetically and I want to posit that can sometimes make for an intense and occasionally smothering parent-child bond.)


Male, female and facilitator

Example - The TV series Alien Nation revealed that there are three genders for the Newcomers - Gannaum (male), Linnaum (female) and Binnaum - literally "third ones". The Binnaum "prepares" the female for fertilization (a process tactfully left unexplained and unshown - I am NOT going to go looking for fan-art), and the male provides the sperm. The process is treated as a ceremony, and a fertilization is often done in the presence of friends, who discreetly turn away as the Binnaum contributes to the proceedings.

Binnaum are greatly in the minority - about 1 for every 100 males. They do not take mates, and offer their services to many couples. They are respected by the community, as they are so important to the reproductive process.

Binnaum - Alien Nation wiki


Well, others seem to have handled most of the various methods for triploid reproduction; the likely effects on parental affection; and the unlikeliness of that evolutionary path. Since no one else has, though, I'll mention that

the most famous treatment of something similar

were the humans in Kurt Vonnegut's Slaughterhouse Five, whom the Tralfmadorians reveal to actually have seven distinct sexes, all necessary to successful reproduction:

There were five sexes on Tralfamadore, each of them performing a step necessary in the creation of a new individual. They looked identical to Billy–because their sex differences were all in the fourth dimension.

One of the biggest moral bombshells handed to Billy by the Tralfamadorians, incidentally had to do with sex on Earth. They said their flying-saucer crews had identified no fewer than seven sexes on Earth, each essential to reproduction. Again: Billy couldn’t possibly imagine what five of those seven sexes had to do with the making of a baby, since they were sexually active only in the fourth dimension.

The Tralfamadorians tried to give Billy clues that would help him imagine sex in the invisible dimension. They told him that there could be no Earthing babies without male homosexuals. There could be babies without female homosexuals. There couldn’t be babies without women over sixty-five years old. There could be babies without men over sixty-five. There couldn’t be babies without other babies who had lived an hour or less after birth. And so on. It was gibberish to Billy.

and, if we're sticking with three as the number of the gender,

there have been triradially symmetric organisms

in Earth's evolutionary history. The trilobozoa—including Skinnera and Tribrachidium—were marine animals during the late Ediacaran who were apparently steamrolled by the explosion of bilateral life in the Cambrian.

You could have an alt history or planet where triradially symmetric life flourished, producing tripartite sex arrangements.


One father is alien.

It has been pointed out that a mixing of DNA really requires only 2. A 3-gendered situation might be used to stabilize a scenario which is not an evolved one, or one that is not stable over the longer term: a hybrid. In this scenario, the basic stock is human and mating / gestation proceeds as usual. The difference is that there are two fathers. One is human and the other alien. The alien father contributes genetic material such that the offspring is an alien / human hybrid.


Main article: Reproductive isolation

Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents, but are often sterile, preventing gene flow between the species.[24] Sterility is often attributed to the different number of chromosomes between the two species. For example, donkeys have 62 chromosomes, horses have 64 chromosomes, and mules or hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed.

These progeny would be the result of two sperm and one egg. This is already possible for humans although nonviable offspring are the frequent result.


According to a study published in the Mar. 28 issue of Human Genetics, two sperm fertilized one egg and created the twins. The phenomenon occurs in about 1% of the population, but most embryos created in this way — called triploids because they have three sets of chromosomes — do not live. Says Dr. Mary Jane Minkin, a clinical professor of obstetrics and gynecology at Yale University: "This confirms that two sperm can get into an egg." Normally the cell dies. But Minkin makes a valid point: "Never say never in medicine and biology."

In the study, Souter and her colleagues suggest that there are two ways this could have happened, depending on when the actual twinning event took place. In the first scenario, the egg might have divided in two (without separating) and then each part fertilized by one sperm. Egg division before fertilization is very rare, says Minkin. The second, more likely possibility is that the egg fused with two sperm cells and created a triploid cell. Then, at the second-cell stage, each shed the chromosomes from each of the sperm — or did something to correct its chromosomal count, says Souter. "There are a whole host of potential mechanisms to explain this," she says, "but we really just don't know. We do know there were two genetic contributions from dad and one from mom."

In your scenario there is tech to facilitate viable offspring resulting from the 2 sperm 1 egg offspring.

The hybrids show some alien phenotypic characteristics which is the reason that this system is desirable. Perhaps the alien genes produce psychic abilities, or adaptations to an alien environments. In any case, the hybrids do not breed true. As opposed to mules, which make nonfunctional gametes, in these hybrids the gametes and some other somatic cells are 100% human (although of course in a scifi there may be exceptions to move along the narrative). Two hybrids producing offspring together will make a normal human. To make a new hybrid a pure stock alien father must be recruited.

These aliens would probably be close to human stock - perhaps like Vulcans or some other product of an ancient Homo diaspora. Reproductive mechanism would be the same. The existence of these aliens also suggests the possible existence of other 2 parent hybrids or even 3 parent hybrids with an alien mother.

The unfortunate upshot of this is that mating with 2 fathers (1 alien, 1 human or 3-parent hybrid) and 1 mother turns out to be like a porno movie. Which makes it easier to understand and keeps filming budgets low, but perhaps less engaging for scifi from our current era.

  • $\begingroup$ Your examples don't suggest any reason the third father is or would need to be an alien, who would in any case need to be actual (not 'probably... close') human stock; they are, in the end, just more-humans-but-from-space-for-some-reason. It also doesn't suggest any mechanism for it to involve a third father. The hybrid-fertilized mules and double-fertilized twins both come from a single coupling of a single couple. $\endgroup$
    – lly
    Commented Jul 11, 2018 at 15:57
  • $\begingroup$ @lly: /would need to be an alien/ hybrids have alien phenotypic characteristics; examples given in answer. Recurrent generation of hybrid is necessary because the hybrids revert to human. /mechanism for it/ this was the reason for the cite from Time with 2 sperm / 1 egg. Implied: if 2 sperms can come from 1 father then 1 can come from 2 fathers. $\endgroup$
    – Willk
    Commented Jul 11, 2018 at 17:43
  • $\begingroup$ Hybrids aren't alien or related to aliens in any sense except 'extra-special'. Your alien would have to be a close evolutionary cousin of humanity (and thus actually from Earth anyway) or have technology that could adjust their genetic material on the fly. (At which point, why not simply maintain the human genome? Because you want to advance humanity? Why bother with the other father?) $\endgroup$
    – lly
    Commented Jul 12, 2018 at 0:05
  • $\begingroup$ Similarly, the mechanism for it has no relation: the double fertilization from a single father is a complete accident. It's theoretically possible that a MMF threesome could end with a genetic chimera or double-fertilized egg, but the odds are vanishingly small and it's not a viable general strategy. You'd need thousands upon thousands of sessions to succeed a single time. Some people might be into that, but not because of the chance of such offspring. $\endgroup$
    – lly
    Commented Jul 12, 2018 at 0:08
  • $\begingroup$ @lly - you are right that naturally occurring this would be very rare. That is why answer included: /there is tech to facilitate viable offspring resulting from the 2 sperm 1 egg offspring. / $\endgroup$
    – Willk
    Commented Jul 12, 2018 at 22:41

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