Is Sex Binary?

content note: discussion of biological sex and a binary classification thereof
two different gametes

Update: Maybe one point warrants a bit more clarification: I indeed believe that people use definitions as weapons. Especially definitions about sensitive topics. And this definitely is one. I also think that quite a few people who commented on this thread seemed to take this as a ‘gotcha moment’ to validate sometimes very problematic opinions regarding trans people. I do not endorse this – quite the opposite. But I believe it is important to engage with the argument, in order to understand it. The text focuses more on one side of the debate than the other – not with the intention to endorse or validate, but with the intention to understand. Ultimately I think the way we use and try to define words should be guided by what we believe facilitates honest and well-intended discourse. Sometimes that may also include thinking twice about whether we actually need to insist on one clear cut scientifically valid definition of biological sex if that excludes people from the conversation.

I came across an interesting thread by Zach Elliott about whether or not sex is binary (direct twitter link). The general point it makes is roughly this: If you define sex in a specific (allegedly correct) way, then sex is indeed binary.

Sex is the type of gamete one’s reproductive anatomy is organized to support.

Males are the sex which develop reproductive anatomy organized to support sperm, and females are the sex which develop reproductive anatomy organized to support ova.

The defining role of gametes

Gametes are germ cells (sperm and egg) that carry the parental genes. After conception, they fuse together and grow into an embryo. Wikipedia has a definition of sex that resembles the one Elliott posted (but is a bit broader). However, I cannot judge how prevalent this definition is, nor whether others would agree with it.

It certainly has some theoretical elegance: With sex defined like this, variations can be sorted into one of two categories. Some people, for example, do not have a phenotype that can be unambiguously identified. Others have e.g. a ‘female’ phenotype even though they have a Y chromosome. But both do have reproductive anatomy that is geared towards supporting either sperm cells or egg cells. But, of course, this is not the only possible definition of sex.

Elliott writes his thread in response to a different Twitter thread. This one bases its definition of sex more on a set of characteristics and argues that these characteristics exist on a spectrum. It argues that classifying people in one of two categories does not reflect biological reality. It also explains that doing so shuts out everyone who doesn’t feel comfortable to be put in one of two bins.

Elliott takes a much more functional definition of sex. Instead of looking at the characteristics we associate with ‘female’ and ‘male’, he makes the distinction according to the type of reproductive organs. And indeed, if you only talk about the biological function of sex, procreation, the case presents itself much clearer. A person’s reproductive system can ultimately support sperm or it can support ova, but not both.

Ambiguous cases

But what about those whose bodies do not support either of the two gametes? I asked that and Elliott responded:

“Organized to support” simply means that during reproductive development, the fetus goes down one of two paths, initiated by the presence or absence of the SRY gene.

Even if the path is disrupted, or if it is not completed, the fetus still develops towards the completion of one or the other system.

There are plenty of cases where certain parts of the reproductive tract do not form completely, such as in cases of Mullerian agenesis, where an XX fetus is born without a complete uterus and cervix.

Since the fetus in this case differentiated down the path without SRY activation, and there is no anti-Mullerian hormone present, the fetus still develops a system organized to support large gametes, even if the system is not complete.

Another person points to true hermaphrodites were apparently both ovaries and testes are present and gives a source as well.

There are cases where sufficient anatomy of both types exists in a single person


The gonads of human true hermaphrodites Willem A van Niekerk, Andries E Retief Human genetics 58 (1), 117-122, ’81 True hermaphroditism with bilateral ovotestis: a case report M Bergmann, G Schleicher, R Böcker, E Nieschlag International journal of andrology 12 (2), 139-147, ’89

Here is a link to the google scholar search for “ovotestis human”, where the publication cited indeed appears. I haven’t read it. But a quick search on wikipedia confirms that ovotestes exist in a rare condition called true hermaphroditism where a person has both ovarian and testicular tissue. Does this invalidate the definition supported by Elliot? Usually, one of the two tissues still dominates. So even if there is some ambiguity sometimes, the separation is clear in almost all cases. But maybe this is somewhat besides the important point anyway.

Difficult conclusions

Maybe we should not only ask: Is Elliott’s definition ‘correct’. We should probably also ask: How useful is it? For example, what useful advice does it have to offer to a person with androgen insensitivity syndrome who is trying to decide which bathroom to use? How useful is as a basis for discussions surrounding intersex? Is it helpful for public discourse? I don’t know.

I don’t even think there is a lot of controversy about the basic underlying biological facts. There is already a consensus that there is a large variety of differing phenotypes that fall on a spectrum of what we classically call ‘female’ and ‘male’. What is more at stake is control over definitions and words. I am writing Elliott’s thread because I think it helps to get a clearer picture of what words can mean. Make sure to also have a look at the thread he responds to.

Reading both improved my personal understanding about what people mean when they talk about biological sex. I have the feeling this field is somewhat of a minefield. I am afraid that whether or not sex is binary is more of a ‘culture war’ question than a scientific question and I am not willing to go there. But it is good to get a clearer map of the territory.


  1. 1. Sex, fundamentally, refers to sexual reproduction, which is distinguished from asexual reproduction. It involves gametes (sex cells) combining to form a new organism. Isogamous sexual reproduction involves gametes that have the same morphology inc size. In such species “sex” refers only to the gene mixing characteristics of sexual reproduction. It has no meaning when applied either to the gametes, or to the organisms providing the gametes, for these gametes and organisms are not divided into sexually dimorphic types. They’re all basically the same. So sex is in the first instance a type of gene mixing reproduction, not a quality of a gamete or an organism.

    2. But most multicellular animals are anisogamous, ie they have gametes of different sizes. Since the fusing of two gametes is what sexual reproduction requires, and since anisogamous sexual reproduction requires one gamete of each type (two smalls or two bigs won’t work) – in anisogamous species, the two types of gamete are given a label. Hence we get a derivative meaning of “sex”, particular to anisogamy – the labelling of the two different gamete types as male (small) and female (big).

    3. This is why the labelling of organisms by sex follows from the gamete type. Sex is about reproduction, anisogamous reproduction requires a contribution of one male gamete and one female gamete. Everything else – ie secondary sexual features is, well, secondary. If you bring the wrong kind of gamete to the party, your secondary equipment is irrelevant.

    4. As to that secondary equipment – it is different sized gametes that have driven the evolution of sexual dimorphism. Sexual dimorphism does not exist in isogamous species, because “sex” has no relevance or even meaning in isogamous species, beyond “sexual” ie gene mixing, reproduction. Sex, as a characteristic applied to organisms, is only meaningful for anisogamous species, and it would be contradictory to label the organism differently from the gametes. Because, evolutionarily, the gametes are the daddies (or mommies) of the camp following secondaries.

    5. As to whether sex is binary in anisogamous species, it depends what you mean by binary. Consider pitching. You can pitch left handed or you can pitch right handed. Some people can pitch both left handed and right handed. And some people can’t pitch at all. I don’t mean they pitch badly, I mean they can’t pitch at all. They have no arms. And you can’t pitch with your nose or your butt. So they are non pitchers. So is pitching binary ?

    6 (a) If you mean can you slot everyone in the world into two and only two pitching categories “Lefty and only lefty pitchers” and “Righty and only righty pitchers” -then the answer is no you can’t. Because some people belong in “Both” and some people belong in “Neither”. So in that sense pitching is not binary.

    6(b) But if you mean it in the sense – there are two and only two ways of pitching , lefty or righty – and there are precisely zero people who can do it another way, then that’s fine. The existence of “Both” and “Neither” pitchers is not a contradiction of the proposal that pitching is binary in the sense that there are exactly two, no more and no less, ways of doing it.

    7. Note that this is not necessatrily true as a matter of logic. There’s no logical bar to there being “middle arm” pitchers, who happen to have a third arm growing up from the back of their neck, and who use that arm to pitch. There could in theory be a third type of pitcher. There just isn’t in fact.

    8. And likewise with sexes. In theory there could be anisogamous organisms, which require THREE gametes all of different sexes to join up and contribute genetic material, in order to form a new organism of that species. Such a species would unarguably have three, not two sexes. But we have yet to observe such a species.

    1. interesting point about isogamous species – never thought about that. I’d agree that the binary classification may not be ideal in many instances – be it pitching or sex. It seems important to me to try and understand where people are coming from who say “you can either pitch with your left or your right arm and there is nothing in between”. But in many instances we may want to be more open with how we understand certain terms and concepts to represent the nuances we see in reality.

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