Is sex binary? Let’s look at the biology

Are there more than two sexes? This is a question that has caused an enormous amount of social and political debate in recent years, but at its core, it is a scientific one, and I want to treat it as such. In other words, what we do with the answer to that question certainly has social and political ramifications, but the question itself is one of biology, not politics. Therefore, I am going to try to answer it in this post from a strictly scientific standpoint. I am not going to make any statements about politics, morality, religion, etc. Instead, I am going to talk only about the biology. As always, if you are going to read this, then all that I ask is that you lay aside any ideologies and views you might hold and look solely at the facts. Political and social positions must be based on facts, not the other way around. So, in this post, all I am going to do is present the facts.

Terms, definitions, and critical background information

On topics like this, it is always a good idea to define the opposing positions at the outset. In this case, there are basically two camps. One holds that sex is strictly binary and is determined by the presence or absence of a Y chromosome (sometimes stated more explicitly as XX = female, XY = male). The other position argues that sex is more complicated than this binary and follows more of a spectrum rather than a clear dichotomy. Some people misunderstand this and construct a straw man about people arguing for the existence of “third sex.” The argument is not that there is a third sex, but rather that sex cannot be adequately defined by two discrete categories because there are many people with both male and female traits. In other words, one position argues that sexes can be defined as two distinct boxes into which all individuals fit. The other argues that the situation is more complex and there are some individuals who do not fit cleanly into either box and are actually somewhere in the middle. I think part of the confusion arises over the way that we talk about this, and I fully admit that I have been guilty of this as well. We often say things like, “there are more than two sexes” as convenient shorthand, but what we really mean is that sex cannot be adequately defined using a simplistic dichotomy in which all individuals with a Y chromosome are males and all individuals without a Y chromosome are females. It is more complicated than that, and there are many intersex individuals that do not fit neatly into traditional categories of males and females.

Note: I opened with the question, “Are the more than two sexes?” simply because that is terminology that is familiar to most readers and introduces the topic.

Note for clarity: based on the comments thus far, I want to clarify that I don’t have a problem with a more nuanced position that says something to the effect of, “based on reproductive physiology most individuals are binary in that they either have the physiology for producing sperm or the physiology for producing eggs, but there are a variety of exceptions to this. So the binary classification is useful in some contexts, but we should acknowledge that there are exceptions and situations where the binary classification does not work.” Indeed, that is more or less what I am arguing (also note that I am only talking about biological sex, not gender or gender identity). Also, please see the addendums at the end before accusing me of a straw man or bringing up the “10 fingers” argument.

Next, we need to talk about how we define sexes, and before we get specifically to humans, it is really important to look at the biology of sex more broadly, because this gives us context and important background information. So, let’s start with the general definition of male and female. If you were to ask professional biologists to provide a general definition of “male” and “female,” the one answer you are not going to get is, “if a Y is present it’s a male; if Y is absent, it’s a female.” There’s a very good reason biologists don’t use that definition. Namely, because it doesn’t work for a very large number of organisms. You see, many organisms don’t have sex chromosomes; instead, male vs female is determined by some environmental factor (I’ll come back to that in a minute). Further, even for species with sex chromosomes there are lots of exceptions and atypical situations (again, more later).

Because of these problems biologists have historically defined sex based on the production of gametes (sperm and egg). The sex that produces small (usually mobile) gametes is considered to be the male, and the sex that produces the large, stationary gametes is considered to be the female. Thus, it is the production of gametes that defines sex, not the presence of a particular chromosome. To put that another way, sex is defined by gamete production, but in some cases, it is determined by chromosomes. In others, it is determined by environmental factors. This may seem like pointless semantics, but it is actually really important (as will become increasingly clear as we go), because the biological definition of sex is not about chromosomes. This already puts the “Y = male” position on shaky ground (it’s also worth noting that in many species it is the female that has two different sex chromosomes, not the male).

Having said all of that, there is a caveat that needs to be explained. Namely, the broad definition of male vs female that I have given can run into trouble at the individual level because some individuals are sterile, so by this definition, it seems like they simply shouldn’t have a sex. In reality, we define sex practically based on the physiology that would result in the production of a particular gamete under normal circumstances. This is important, because physiology is rarely binary. There aren’t, for example, two distinct groups of people with regards to metabolism: high and low. Rather, there is a whole spectrum of metabolic activity.

The next thing we need to talk about is genotype vs phenotype. The genotype is what a person is genetically. In other words, what their genes code for, whereas the phenotype is the physical characteristics of the individual. This is important to understand because different genotypes can lead to different phenotypes, but also the phenotype does not always match the genotype. This becomes particularly true when we start talking about epigenetics. An epigenetic effect occurs when something other than genetics affects the expression of the trait. In other words, the phenotype is determined not only by the genotype, but also by the environment, enzymes, etc. and in some cases, those factors can override the genotype.

Sexes in the animal kingdom

With all of that background in place, let’s look at the animal kingdom and see what sort of variation exists for the sexes, because there is a lot we can learn from this broad perspective (I promise I will talk about humans later). Even a cursory knowledge of zoology will quickly tell you that sex is complicated. There are, for example, many species that are hermaphrodites. This means that they simultaneously have the physiology to produce eggs and the physiology to produce sperm. They are not “male” or “female;” they are both.

Many other organisms can switch between the sexes, and in many cases do so obligately (i.e., all individuals start out as one sex and switch later in life). This is one of the places where epigenetics comes in. Anemonefish (aka clown fish) are a good example (Todd et al. 2016). Anemones are inhabited by a male-female pair, where the female is larger and dominant. Individuals start off life as males and pair up with a female, but if that female dies, this causes epigenetic changes in the male, resulting in it changing sexes and becoming a female. Thus, if Finding Nemo was biologically accurate, when Nemo’s mother died, Marlin (his father) should have changed sex and become Marla.

In many other species, individuals do not change sex as adults, but their sex is determined by the environment as they develop. Some (but not all) turtles provide a good example of this (as do crocodilians, some lizards, etc.). They are what we call temperature sex determined (TSD), and the temperature at which the eggs are incubated determines the sex of the offspring. I don’t want to get too technical here (and indeed there are important pieces of information that we don’t have yet), but I do want to briefly walk through some of how this works because it is instructive (see a more detailed overview here: Lance 2009). During early embryonic development, sex has not been determined (this is true in humans as well) and whether an embryo becomes a male or a female depends on the hormones present. Under many conditions, the embryo will develop as a female, and this seems to be largely driven by the hormone estradiol, which is made from testosterone via the enzyme aromatase. At certain temperatures, however, aromatase stops converting testosterone into estradiol, ultimately resulting in the development of male characteristics.

I went through all of that info on TSD because that background knowledge lets us look at some import questions. For example, what happens if we raise eggs at a male-producing temperature, but we supply them with estradiol? The answer is usually that females develop (Lance 2009). In other words, even though temperature usually determines sex we can over-ride that and produce a different sex. Further, the fun doesn’t stop there, because in at least some cases, we can take turtle species that do have sex chromosomes, paint the eggs with estradiol, and get hatchlings with female physiology even if they are genetically male (Freedberg et al. 2006)! In other words, we can make turtles that have male sex chromosomes develop female phenotypes, including the ability to lay fertile eggs. This is why I’ve been arguing that chromosomes sometimes determine sex, but they don’t define it. We can change the sex to be something other than what was determined genetically. To put that another way, even though chromosomes usually determine sex in these species, we can override that and make the estradiol treatment determine sex.

Similarly, there are some lizards that are usually genetically sex determined (i.e., sex is based on chromosomes) but at certain temperatures, there is an epigenetic effect and the temperature overrides the genetics and determines the sex of the hatchlings. In bearded dragons, for example, at high temperatures, animals that are genetically male (based on chromosomes) develop as females and produce fertile offspring (Holleley et al. 2015). So, if you want to insist that chromosomes define sex, rather than determining it (under normal circumstances), then you must claim that these lizards who are running around laying fertile eggs are actually males. This is a notion that any biologist would scoff at because, again, that’s not how we define sex. If you are going to claim that males are laying eggs, then you have invented your own definition of “male” that biologists do not accept.

Finally, you may be wondering, given all this complexity with TSD and chromosomes, can you ever get intermediates? The answer is, yes! There are situations where individuals don’t develop entirely as male or entirely as female and instead end up developing partially as both (Ewert and Nelson 1991), which makes it pretty impossible to maintain a view that sex is binary. In other words, up until this point, you could have tried to make a post hoc change to the original argument and claim that, “there are only two sexes, and it is determined by physiology,” but that doesn’t work, because some individuals have aspects of both male and female physiology.

The point that I’m trying to get at here is that sex is complicated. It is clearly not as simple as a binary state determined strictly by chromosomes, because we know that you can have reproductive “females” who are genetically “males.” We know that there is more to sex than simply the chromosomes. and we know that environmental factors can override the genetics. Now, you may protest to this because I have been using examples from non-human animals, but that counterargument misses the point. The point is that traits are more complicated than a simplistic understanding of genetics would lead you to believe, and there is no reason to think that sex is only complicated in non-human animals. Indeed, as I’ll explain in the rest of the post, sex is extremely complicated in humans. To put that another way, using non-human animals is a good way to get people to lower their biases and look at the evidence, and as you’ll see, the bizarre situations in other animals are highly analogous to what happens in humans.

Sexes in humans

Let’s being by looking just at the sex chromosomes. In humans, you have probably heard that there are two possibilities for sex chromosomes: XX and XY, but that is not correct. In reality, there are many possible combinations, and it’s not that uncommon for someone to have an atypical number or arrangement of sex chromosomes. Indeed, one large study found that 1 out of every 426 people (2.34 out of 1,000) had one of these conditions (Nielsen and Wohlert 1991).

For example, some people get extra X chromosomes. When this is associated with a Y chromosome, it is known as Klinefelter syndrome, and people with it can be XXY, XXXY, or even XXXXY. These unusual genotypes are associated with a combination of male and female phenotypes (with female traits being more prominent when more X chromosomes are present). People with this condition have male genitalia, but they are often have small testes and are sterile or have reduced sperm counts, they have less body hair and often no facial hair, they have lower testosterone levels, and in some cases they develop breasts (Visootsak and Graham 2006). So here, we have people who have two X chromosomes, but also a Y chromosome, breasts but also a penis, testes but low testosterone levels, etc. They simply don’t fit neatly into the discrete boxes of “male” and “female.”

Extra X chromosomes can also occur without the presence of a Y, and you can have someone who is XXX (sometimes called “superfemale”). People with this present mostly as normal female phenotypes, but they are taller on average, and often have learning disabilities (Tartaglia et al. 2010). Things often become more severe when there are four X chromosomes (“tetrasomy X”; XXXX). Some people with this develop normally, but others do not experience normal puberty, don’t develop a normal female phenotype, and are infertile. Beyond this, some individuals actually have a full 5 X chromosomes (XXXXX) and experience even more severe symptoms. Here again, we have atypical chromosome arrangements resulting in different phenotypes.

There can also be unusual numbers of Y chromosomes. For example, some people are XYY. These individuals have mostly normal male phenotypes and are usually fertile. Others may have XYYY or even XYYYY. These conditions are quite rare making it hard to generalize, but behavioral problems such as aggression have been reported in several cases (Abedi et al. 2018).

Additionally, there are XXYY individuals. These individuals are largely similar to XXY individuals, though there are some differences (Tartaglia et al. 2008). Like XXY individuals, they are generally sterile, and have reduced male features (e.g., small testes).

Finally, there is a condition known as an X monosomy (Turner syndrome; XO). This occurs when an individual has a single X chromosome and either no Y or sometimes a partial Y. These individuals appear female, but are generally infertile and do not have properly developed gonads (Fryns and Lukusa 2005). I want to pause here for a second to note that you can get a situation where someone has part of a Y chromosome. So if your definition of sex is based on the presence or absence of a Y, how do you define someone who has part of a Y? Are they only partially male?

By this point, it should be abundantly clear that sex in humans is far more complicated than XX vs XY, and there are lots of genotypes and lots of phenotypes. It should be obvious that chromosomes determine sex rather than defining it, but there are still more layers of complexity that we haven’t gotten to yet. What if I told you, for example, that it is possible to be born with normal female genitalia, even though you have a Y chromosome? This is a condition known as Swyer syndrome, and it’s often a result of a mutation on the SRY region (aka testis-determining factor) of the Y chromosome, but many other genes can cause it as well (Thomas and Conway 2014). These genes often play key roles in activating the right chemicals for an embryo to develop into a male (think back to the turtles earlier for an analogous situation), so when they are modified, those chemicals don’t get produced at the right amounts. As a result, people with Swyer syndrome have a predominantly female phenotype, but instead of having either testicles or ovaries, they have “streak gonads” which are undifferentiated pieces of tissue that can produce neither eggs nor sperm. People with this condition typically don’t go through puberty and require hormone treatments to develop secondary sexual characteristics such as breasts. However, people with this condition can usually carry a child and give birth if an embryo is artificially implanted. I want you to stop and think for a second about just how complex this is. Here we have people who have a Y chromosome, but also have vaginas, don’t have either testes or ovaries, but have all the other female reproductive physiology and can carry a child if implanted with it. The line between male and female is really blurred in this situation.

The inverse of Sewyer syndrome is “XX male syndrome.” This condition produces individuals with typical male genitalia despite the fact that they do not have a Y chromosome. The cause of this is usually a mutation that resulted in the SRY region ending up on an X chromosome (Anik et al. 2013). Much like Sewyer syndrome, individuals with this condition are generally sterile and often have reduced testes.

There are other situations that are even more bizarre. For example, there are documented cases of people developing “ovotestes.” These are gonads that have some of the features of a testis and some of the features of an ovary. This often occurs in people who are XX but have a mutation on the RSPO1 gene (Tomaselli et al. 2011), which results in ambiguous gonad development. Others actually have both an ovary and a testis and were historically referred to as “true hermaphrodites.” This can occur in both XX and XY individuals (though XX is more common) as well as individuals with some of the chromosome abnormalities described earlier. Further, some individuals with this condition are actually fertile and have children (this usual happens when one gonad is developed and the other is an ovotestis; Krob et al 1994). In other words, there are people who are reproducing even though they have both ovarian and testicular tissue (this is more common in mothers but there also people who are fathers despite this condition). You may remember from the beginning of this post that biologists have typically defined sex based on the physiology required for producing sperm vs eggs. So how are we supposed to classify these individuals who have both physiologies?

There are also cases of individuals who are chimeras. In other words, they have two sets of DNA, and in some cases, one of those sets is XX and the other is XY. In some cases, this has little effect on individuals, and they can reproduce, but in other cases, it results in the development of either ovotestes or other odd combinations of gonads as described earlier. Nevertheless, some of these individuals can still reproduce (Verp et al. 1992). To put that another way, there are people who have a Y chromosome, and have testicular tissue, but still produce eggs and give birth. Now, if you are going to insist that things are as simple as, “if you have a Y you are a male,” then you must argue that these people are males, even though they have mostly female phenotypes and give birth. This is, again, not something that any of the biologists I know would accept.

Beyond all of that, we know that there are epigenetic effects at play in sexual development (Gunes et al. 2016). There are, for example, epigenetic effects on the expression of the SRY region. Exactly how this plays out in developmental sex disorders (DSD) is still poorly understood because epigenetics is such a new field, but we know that there are epigenetic effects that influence the development and expression of male and female traits (phenotypes), and as this field expands, it is likely that we are going to discover that sex is even more complicated than we currently realize (we’ll have to wait and see).


As you can hopefully now see, the topic of sex is extremely complicated, and there is far more to it than simply XY = male, XX = female. There is a whole suite of genotypes and phenotypes, including individuals that are XO, XXX, XXXX, XXXXX, XXY, XXXY, XXXXY, XYY, XYYY, XYYYY, and XXYY. Further, there are individuals who are XX yet develop mostly as males, and there are individuals who are XY but develop mostly as females. There are literally people who give birth, despite having a Y chromosome. There are people who have both ovaries and testicles. There are people who only have part of a Y chromosome, etc.

So, if you are going to insist that Y = male, you are going to have to make some bizarre claims. For example, you are going to have to say that XY individuals with an SRY mutation are, in fact, males, despite the fact that they were born with vaginas, lack testicles, and, if implanted with a fertilized egg, can carry a fetus to term. You are literally going to have to say that a male can give birth. Similarly, you are going to have to say that some XX individuals are females, despite the fact that they have mostly male physiology (including penises). Those are, of course, nonsense positions that biologists don’t accept. Biologically, sex is defined by the physiology needed to produce particular gametes (eggs or sperm), not by sex chromosomes, but recent years have shown that this simply is not a binary situation. There are many individuals that have aspects of both male and female physiology, thus making it impossible to use binary categories.

Let me put that another way. Given the existence of individuals with conditions like XXY who have some female traits and some male traits, the existence of individuals who appear female despite being XY, the existence of individuals with both an ovary and a testis, the existence of people who give birth despite having a Y chromosome, etc., which of the following descriptions seems more accurate, “sex is strictly binary; if you have a =Y you are a male, if you don’t you are a female, no exceptions” or “sex is a complex trait with many genotypes and phenotypes as well as epigenetic factors. It is a spectrum of traits and cannot adequately be described using strictly binary categories.” Which of those does a better job of describing the enormous variation that I have discussed in this post?

Again, to be clear, I’m not making any political or social arguments here. What you do with this information and how it affects your views is up to you, but you must accept facts, and the facts clearly show that biologically, sex is more complicated than a simple binary dichotomy.

Rules for commenting on this post

 As explained, this post is solely about the science. If you think I am wrong about the science, feel free to explain, but I do not want the comments to divulge into endless political and social debates. As I said, for the sake of this post, I am just presenting the science. What you do with that is up to you. Comments that are not about biology or that tack political arguments onto biological ones will be deleted. Similarly, if you think I am wrong, please actually explain why rather than just saying, “no, Y = male.” Actually deal with the points I raised and evidence I presented. Also, be civil (see the Comment Rules for my more general policies).

Addendum 1 (Straw man accusation): Quite a few people have accused me of a straw man fallacy on the grounds that no one actually thinks that there are strictly two sexes. This is empirically incorrect. There are a lot of people (particularly in American evangelical circles) who argue adamantly that there are only two sexes and everyone is either male or female, no exceptions. I have many family members and former friends who I have frequently heard make this exact argument, and I have seen it made countless times by others (including in response to this article and by politicians). So I have not committed a straw man.

Addendum 2 (the “10 fingers” argument): The other frequent response is that we shouldn’t define things based on relatively uncommon variations, usually accompanied by the claim that, “there’s nothing wrong with saying that humans have 10 fingers, even though exceptions exist” (sometimes I get numbers of limbs or other analogous arguments). There are two points to make here. First, if someone insisted adamantly that there is only one number of human fingers (10) with no exceptions, they would clearly be wrong. Even so, it is clearly wrong to say that there are exactly two sexes, with no exceptions. That is the position I am arguing against. Second, as explained in the article, there is nothing wrong with saying that most individuals are male or female, but that strict dichotomy does not work in all cases, and the situation is more complicated and nuanced than a simply binary. Some individuals cannot clearly be defined as male or female. That is all that I am arguing here, and it is completely analogous to the correct position that, “most individuals have 10 fingers, but there are exceptions and the situation is more complicated than a simple rule that all individuals have 10 fingers. There are some who do not follow that.” So if your argument is that the binary criteria works in most cases, but there are individuals who cannot be clearly defined as male and female, then great! You and I agree, and you are claiming exactly what I was arguing.

Literature cited

(see this post if you have trouble accessing these for free)

  • Abedi et al. 2018. Rare 48, XYYY syndrome: case report and review of the literature. Clinical Case Reports 6:179–184.
  • Anik et al. 2013. 46,XX Male Disorder of Sexual Development: A Case Report. Journal of Clinical Research and Pediatric Endrocrinology 5:258–260.
  • Ewert and Nelson 1991. Sex determination in turtles: diverse patterns and some possible expliantions. Copeia 1991: 50–69.
  • Freedberg et al. 2006. Long-term sex reversal by oestradiol in amniotes with heteromorphic sex chromosomes. Biology Letters 2
  • Fryns and Lukusa 2005. Monosomies. Encyclopedia of Life Sciences.
  • Gunes et al. 2016. Genetic and epigenetic effects in sex determination. Birth Defects Research Part C Embryo Today Reviews 108:321–336
  • Holleley et al. 2015. Sex reversal triggers the rapid transition from genetic to temperature-dependent sex. Nature 523: 79–82.
  • Krob et al 1994. True hermaphroditism: Geographical distribution, clinical findings, chromosomes and gonadal histology. European Journal of Pediatrics 153:2–10
  •  Lance 2009. Is regulation of aromatase expression in reptiles the key to understanding temperature-dependent sex determination? Journal of Experimental Zoology 311:314–322.
  • Nielsen and Wohlert 1991. Chromosome abnormalities found among 34,910 newborn children: results from a 13-year incidence study in Arhus, Denmark. Human Genetics 87:81–83.
  • Tartaglia et al. 2008. A new look at XXYY syndrome: Medical and psychological features. American Journal of Medical Genetics A. 146A:1509–1522
  • Tartaglia et al. 2010. A review of trisomy X (47, XXX). Orphanet Journal of Rare Diseases 5
  • Thomas and Conway 2014. Swyer syndrome. Current Opinion in Endocrinology & Diabetes and Obesity 21:504–510.
  • Todd et al. 2016. Bending genders: The biology of natural sex change in fish. Sexual Development 10.
  • Tomaselli et al. 2011. Human RSPO1/R-spondin1 Is Expressed during Early Ovary Development and Augments β-Catenin Signaling. PLoS One 6:e16366
  • Verp et al. 1992. Chimerism as the etiology of a 46,XX/46,XY fertile true hermaphrodite. Fertility and Sterility 57:346–349
  • Visootsak and Graham 2006. Klinefelter syndrome and other sex chromosomal aneuploidies. Orphanet Journal of Rare Diseases 1.
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29 Responses to Is sex binary? Let’s look at the biology

  1. Fallacy Man says:

    Rules for commenting on this post
    (this is copied from the article, but people have a habit of not reading, so I am putting it here as well)

    As explained, this post is solely about the science. If you think I am wrong about the science, feel free to explain, but I do not want the comments to divulge into endless political and social debates. As I said, for the sake of this post, I am just presenting the science. What you do with that is up to you. Comments that are not about biology or that tack political arguments onto biological ones will be deleted. Similarly, if you think I am wrong, please actually explain why rather than just saying, “no, Y = male.” Actually deal with the points I raised and evidence I presented. Also, be civil (see the Comment Rules for my more general policies).

    Liked by 1 person

  2. vuurklip says:

    Very interesting. Thank you.


    • Tom says:

      Reading some comments is as expected and very amusing and at the same time disturbing to me. The article VERY clearly states it is about facts, not about generalizations, opinions about appropriate choice of words etc. Exactly what happens here ( it Should be called binary because everybody knows all the rest is insignificantly small in percentage AND not how it was meant to be) leads to politics, opinion, etc. this the same that happens in the currently well known subjects of science denial. This argumentation closes the door for scientifically correct formulation, which i consider a bad thing

      I think we should let the science stand as it is, with factually correct statements. We should use these as appropriate, and use the commonly accepted generalisations and more sloppy wording where they help.

      Liked by 1 person

  3. Tom says:

    i understand and accept science, and your article seems completely clear and logical for me.
    I hoped you would touch on a question that i have:
    As far as I see a lot of the social and political discussion going on is about people “identifying” as something they genetically are not: example XY is genetically male, looks like male, talks like male, but FEELS female and wants/gets a sex change operation.
    i do not question validity of the feelings or request but is there science that can explain this. or is this a yet unexplored ( and maybe not scientifically explainable) area


    • Fallacy Man says:

      It is a growing area of research, but several studies have found, for example, that brain scans of trans individuals are closer to “standard” brain scans of the sex they associate with than they are to scans of the sex they were born with (based on genitalia). There are also likely a whole lot of epigenetic effects, but as I said, the field is still new, so I can’t make a lot of conclusive comments at the moment. We’ll have to wait and see what the data shows.


  4. But, scientifically/mathematically, if (from your stated data) 425 out of 426 (99.8%) humans do fall into XX/XY classifications, then the dichotomy does exist for all but 0.2% of the studied population. There are fascinating variation and complexity in this population, as you have well documented, to be sure, but it is logical to understand how most people throughout history have thought of this as a binary classification.

    Liked by 1 person

    • Fallacy Man says:

      I should note first that the number I gave is just for unusual numbers/arrangements of sex chromosomes, not the various other conditions (don’t know what the number is for the grand total, but it is likely still low). Having said that, most individuals do fit well into the categories of “male” and “female,” but because of the existence of individuals that do not fit into those categories, it is still wrong to argue that sex is strictly binary. Finally, although the percentages are low, when you consider the size of the human population, we are actually talking about millions of people.


      • Ion says:

        by this logic we can’t call the color red, well.. red because some small percentage of the population is color blind. If we consider the size of the human population we are actually talking about millions of people. See what i did there? So basically now we can’t call something what we used to call it because of deviations from the norm?
        Having said that, we are considering people binary because of the utility of it. That’s how words work, we assign words to things to help us express the world in an easier way. Of course there are exceptions, but we need to acknowledge them as that.


        • Fallacy Man says:

          Your analogy makes utterly no sense. Red is defined by the wavelengths of light, not human perception.

          Let me correct your analogy. Would you agree with the statement, “all individuals can see the color red”? Obviously not, because there are “exceptions” to the norm, and it is important to acknowledge those situations, rather than pretending they don’t exist and proclaiming that all humans can perceive the color red. There are people who cannot see the color red, therefore it is simply incorrect to say that everyone can see it (even though most people can). Similarly, the science shows that a strictly binary system does not adequately explain the variation in sex. Therefore, it is incorrect to describe sex as a strictly binary trait.

          Utility is irrelevant.


          • scott says:

            > Red is defined by the wavelengths of light, not human perception.

            Perhaps you should take your own advice and consult the research. This is far from an accepted position, and in fact is generally dismissed by most modern scientists. See, for example, Byrne & Hilbert (2003), who provide a valuable overview of the state of color science and the problem of color perception more generally.

            More examples from the literature:

            Zeki (1983):
            “The results described here . . . suggest that the nervous system, rather than analyze colours, takes what information there is in the external environment, namely, the reflectance of different surfaces for different wavelengths of light, and transforms that information to construct colours, using its own algorithms to do so. In other words, it constructs something which is a property of the brain, not the world outside. (p. 764)”

            Palmer (1999):
            “[C]olor is a psychological property of our visual experiences when we look at objects and lights, not a physical property of those objects or lights. (p. 95)”

            “There may be light of different wavelengths independent of an observer, but there is no color independent of an observer, because color is a psychological phenomenon that arises only within an observer. (p. 97)”


          • Joel Schama says:

            Fallacy Man, you contend, “Red is defined by the wavelengths of light, not human perception” when the opposite is true.
            The “redness” of something is purely and subjectively defined by humans and other animals who can perceive colour as “red.” It is defined by our rods and cones and visual cortex to be the qualia of “red” presented to our consciousness.
            Red itself does not inherently exist in Reality, nor does colour at all. A wavelength of light has no innate colour, it is simply a wavelength, say, 680 nm; that is all.
            Only through the perception of such wavelengths do humans and other animals with colour vision perceive the phenomenological and purely subjective essence of a particular wavelength, 680 nm, as “red.”
            Colour does not exist outside our perception thereof.
            Having said that, I fully except the science behind the biology of sex and how it is defined, how it has been, and how it is here expressed by you.


          • Fallacy Man says:

            I will concede that my statement about color was incorrect. Sorry for that, physics is hardly my area of expertise and I should have known better than to comment on it. Nevertheless, the analogy is still fundamentally flawed and I stand by my overarching point


        • simplephysic says:

          Ion wrote: “by this logic we can’t call the color red, well.. red because some small percentage of the population is color blind”

          It depends, what you mean by color.

          If by color you mean light of a given wavelength, then you should define which wavelengths you regard as red. The spectrum is continous and there is no strict end of red colot.

          If by red color you mean something that human perceive as red, then you have much bigger problem, because there is always a bunch on infinite number of wavelenght that may be perceived as red by humans. People usually have three specialized retina cells, but there are cases where those cells (due to genetic changes) are different and e.g. some women can see 4 instead of 3 colors.

          See for example:

          And with your analogy: you can define red as a color with given wavelenght and rest of colors as “not red”. But then you will have plenty of colors, that are perceived as red, which are not red by your definition. If you try to use definition of red as a perception, then it is even worse.

          WIth sexes the problem is that some wants to have double choice and nothing more. You may want to have this, but in science you have to check how your image of the world applies to real world. And then the reality is much more complicated than you believe, how this reality should look like. The naming of colors is debatable the same as naming of sexes.

          And one more remark: as Fallacy Man stated: your comparison/analogy is flawed. Nevertheless it is also complicated – and there are genes as in sex problem.

          And one more remark: proof by analogy/comparison is always possible, but usually very difficult in science, because you have to make two steps: you have to prove the analogy is correct, and have the proof in the field you compare to. The example of proof “by analogy” is trisection of an angle. You can prove this geometric theorem using algebraic methods (reducibility of 3 degree polynomial), but you have the strict proof of analogy of those two.


  5. Matt Bowden says:

    I read this article with interest, and note that this is very much in line with the biology I learned some 20 years ago at university. I don’t have any problem with the science being presented.

    I do, however, note that the “complicated” variant genotypes and phenotypes you appear to be characterising as alternative genders were, in my day, characterised as genetic disorders. That is not meant to denigrate the lived experiences of the individuals who have these conditions. They are people outside of and beyond their mere genotype and have the right to exist in whichever gender identity (or lack of it) they feel most comfortable with.

    However none of this changes the fact that, for example, kleinfelters syndrome is a genetic error caused by the non-dysjunction of the XY pair during the production of sperm. It is not an “intentional” product of the human reproductive cycle. I feel that attempts to characterise these conditions any anything more is falling prey to interpreting the facts as you wish they should exactly as you opening quote warns us not to.

    Liked by 1 person

    • Fallacy Man says:

      First, to clarify, I’m not describing these as, “alternative genders” or, more accurately “alternative sexes.” Rather, the point is that they don’t fit neatly into the boxes of “male” and “female.” They defy the binary classification system.

      You are correct that these mostly occur due to genetic errors (though epigenetics may change our understanding, but we’ll have to wait and see), but I don’t think that changes anything that I have said. They still exist, regardless of why. That’s the point. We can’t draw this distinct line in the sand, because there are some individuals who don’t follow it. I don’t see why the fact that these intersex conditions were caused by mutations is relevant to that point.

      To put that another way, we are all the result of countless mutations. Indeed, the existence of sex chromosomes themselves is because of mutations ages ago. Mutations are ultimately the source of all genetic variation. So for any physiological trait with variation (which is pretty much all physiological traits) that variation exists because of mutations.

      Finally, to be clear, if someone was to make a statement like, “most people fit into binary sex categories based on the physiology to produce sperm or eggs, but there are exceptions and genetic conditions in which that is not true.” I wouldn’t really have any issues with that. My issue is with the insistence that Y always = male or that sex is strictly binary in all cases.

      Liked by 2 people

    • Fallacy Man says:

      I have added a note to the beginning of the post to clarify this and avoid further confusion.


    • simplephysic says:

      Matt Bowden: “That is not meant to denigrate the lived experiences of the individuals who have these conditions. They are people outside of and beyond their mere genotype and have the right to exist in whichever gender identity (or lack of it) they feel most comfortable with. ”

      That’s where you are wrong.

      The strict division: male, female is – in some countries used as an argument for creating a law. It means that people, who does not fall into male/female scheme cannot function in law system.

      I will give you an example: in Poland if you want to change your official sex, you have to sue your parents in a court. There was a case of a child, who cannot exist in Polish law system, because she has formally two mothers in its certificate of birth (issued in different country) – Polish administration will not give the child identification number unless she will cross out one of their official mothers. Many regions in Poland has procclaimed local law, where they stated that their region is “free of LGBT ideology” – so in fact they deny existence of any person, that does not fall into simple dichotomy and the right to speak about this (

      So you are wrong in one thing: this is used as argument to exclude those minority outside the whole law system in some countries.


  6. Joe Forte says:

    I echo what Bruce and Matt say, and also suggest the original framing is not necessarily accurate. That is, “one position argues that sexes can be defined as two distinct boxes into which all individuals fit. The other argues that the situation is more complex and there are some individuals who do not fit cleanly into either box and are actually somewhere in the middle.” This makes the first position fairly easy to debunk.

    An alternative framing might be that one position argues sex is overwhelmingly binary with several fascinating but rare exceptions; the other position argues that sex is a continuum, which many people visualize as a spectrum, with more-or-less equal distribution of each type. Like a rainbow of colors.

    In this framing it is the first position that is more accurate biologically. Why doesn’t this article show a graph of the probability distribution for human “sexes”? A picture is worth a 1000 words and all that.

    Again none of this is meant to discount gender identify, lived experience, the moral imperative of treating all people well wherever they fall on the probability distribution, and so on.


    • Fallacy Man says:

      I will concede that there are more nuanced positions that I did not address and I apologize if my framing was misleading (perhaps I tried to force a spectrum to be binary, though what you are describing sounds very similar to what I was arguing). To follow up on what I said to Matt, if someone wants to say, “sex is binary in most cases based on the fact that most individuals either have the physiology to produce eggs or the physiology to produce sperm, but there are exceptions, and there are millions of people that do not fit neatly into those categories because, for a variety of reasons, they have aspects of both physiology” I don’t have a problem with that. Indeed, that is essentially the very thing that I have been arguing. Nevertheless, the point still stands that there are many exceptions to the binary classification system. I’m not saying we throw it out and never use the terms “male” and “female” ever again. Rather I am simply arguing that it is more complicated than that simple binary classification can encapsulate. It’s useful in some contexts, but we need to acknowledge that it does not work all the time, and there are exceptions.


    • Fallacy Man says:

      I have added a note to the beginning of the post to clarify this and avoid further confusion.


  7. Alister says:

    I am no expert by anymean but when did we start using data of less the 0.17% (an estimate for what the number of people with Klinefelter syndrome the most common type of unusual chromosomes) to define anything about people should we start saying people have more fingers because about 0.01% of people are born with postaxial polydactyly


    • Fallacy Man says:

      I’m not sure how this finger example became so popular, because it is clearly an extremely flawed example. Consider the following two statements and tell me which one is correct:

      A. There is only one number of human fingers: 10. All humans have 10 fingers.
      B. Most humans have 10 fingers, but there are many exceptions, and it is incorrect to insist that 10 is the only number of fingers.

      Statement B is clearly the correct on. It is simply wrong to insist that all individuals have 10 fingers. Even so, it is simply wrong to insist all individuals are either male or female.

      To put that another way, since the whole point of this post was to treat the topic scientifically, if I, as a scientist, were to write a paper about digit numbers, I absolutely would not make the statement, “humans have 10 fingers” because that statement is wrong. Rather, I would say something akin to statement B. Even so, scientifically, it is wrong to say that there are only two sexes.

      Again, to be clear, the standard classifications of “male” and “female” are useful in many contexts, but they have limitations, and there are clearly exceptions. So it is simply wrong to insist that sex is strictly binary, and we need to acknowledge and discuss the exceptions. Also, note that even though being intersex is uncommon as a percentage, we are still talking about millions of people. It is disingenuous to act as if they don’t exist.


    • simplephysic says:

      Alister: “define anything about people should we start saying people have more fingers because about 0.01% of people are born with postaxial polydactyly”

      What about saying, that people have always 10 fingers?

      If a person has 11 fingers then they cannot get social security number, because they are (by definition) not human. In order to get social security number you should undergo oepration of removing one finger to have 10.


  8. Andrew says:

    This was a very interesting post. Your blog is, perhaps, the most interesting blog I have come across in terms of scientific issues. Is there any way you would consider doing a post on debunking Chiropractic treatments?


  9. Dave says:

    Regarding people with ovotestes that can have children you wrote “this usual happens when one gonad is developed and the other is an ovotestis”. Does that mean the developed gonad can be either an ovary or a testicle? As in, it doesn’t matter which one it is as long as it’s fully developed?


  10. Tom B says:

    As a pediatrician, ambiguous genitalia in a newborn is not an uncommon issue and discussions with the parents are often difficult. I try to be as accurate in those discussions as possible and hence my questions.
    Since most would agree there two large sex groupings and multiple less common points between the large groups, is it accurate to describe sex as bimodal (as opposed to binary or a continuum which both seem inaccurate to me and also cause great angst)?
    If not bimodal, is there another descriptor you would use?


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