What Exactly Is (and Isn’t) ‘Biological Sex’?

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If you know me at all, you know I’m a huge nerd. If you don’t know me, I’m a huge nerd. I read science journals for fun and the Hebrew Bible in it’s original Hebrew. I watch indie movies about Emily Dickinson. When I’m not reading Star Wars comics and watching Wynonna Earp that is.

Anyway, I just finished reading the book Sex Itself by Sarah S. Richardson, a book about the history of the research on the so-called ‘sex chromosomes’ (the X and Y chromosomes). It’s a fascinating look at the gendering of chromosomes, how gendered language and concepts influenced research, and a take down of what many male genetics researchers call ‘feminist bias’.

I started reading it because I was raised in a pretty conservative Christian home and as a part of my rebellion personal growth as an adult, I’m unlearning a lot of toxic conceptions of gender and sexuality. Figuring out you’re bi later in life has a way of doing that to you. I’m also a linguist. So, as I moved away from the Christian concept of gender essentialism[1. Gender essentialism is the idea that there is a specific, fixed ‘essence’ to gender. That gender is (biologically) determined and unchanging and can be boiled down to specific traits and behaviors shared by all ‘males’ or ‘females’ at all times across culture and history.], I was inevitably drawn to discussions of gender and sex. Gender is a social construct, but what about sex? What about biology? How do these two concepts intersect? So, as one does (if you’re me), I bought several books about current research on sex and gender, including Richardson’s book.

I’ll say right off the bat that she does an excellent job distinguishing between gender as a social construct and personal identity and the science of chromosomes and genetics. However, I don’t believe she goes far enough in repudiating the idea of ‘biological sex’, which she ultimately locates in the gametes (sperm and egg) and what chromosomes they carry (XX or XY). That got me thinking about how I might define ‘biological sex’, which led me to wonder if it’s even a useful term at all. Short answer? No, it’s not. Long answer? Well, I’m getting to that.

Full disclosure: I’m going to be using specific genetic terminology since, for me, this is a discussion about defining a concept frequently touted as ‘scientific’ and ‘scientifically accurate’. I’ll try to include definitions in footnotes of terms that may not be familiar.

Why talk about biological sex?

In the wake of a more open, more vocal transgender community and advances in genomic and biological sciences, the conversation around sex and gender has gotten both more nuanced and more complicated. In the past, many people (both scientists and lay persons) conflated chromosomal and physical phenotypic features (which many dub ‘biological sex’) with the performance of specific roles and behaviors in society. Under this paradigm, ‘biological sex’ is equivalent to gender.

There are still some who hold to this model, typically under the assumption that both ‘biological sex’ and gender only exist as a binary: male and female, no more, no less. However, more and more people nowadays are willing to conceive of there being a difference between the two. Depending on where one is situated in the debate, more emphasis might be placed on either gender or ‘biological sex being one’s ‘true’ identity.

How is ‘biological sex’ typically defined?

From what I’ve seen, ‘biological sex’ signifies an irreducible biological binary that gives rise to two human phenotypes “male” and “female” that either be can be distinguished from or coterminous with gender, depending on one’s point of view. Some people say your ‘biological sex’ is your gender, regardless of personal preference. Others would differentiate strongly between the two. Yet so far as I can tell, both sides who utilize the concept of ‘biological sex’ think of it as a fixed binary. The only difference is where they might locate it and how strongly they would say it defines one’s ‘true’ identity. This, then, is the definition I will work with. So, what precisely may we deem ‘biological sex’?

There are four different categories of features of human biology that often get confused, conflated, or unduly emphasized when discussing human ‘biological sex’.

  1. Reproductive Gametic Dimorphism[1. Gamete is the name for the cells involved in sexual reproduction, i.e., the egg and sperm. I use ‘Gametic Dimorphism’ to refer to there being two different cells with different appearance and structures.]
    1. egg and sperm
  2. Chromosomal Dimorphism[2. As with Gametic Dimorphism, I use ‘Chromosomal Dimorphism’ to refer to there being a difference in appearance and structure between the X and Y. For more information on it, see Richardson’s book Sex Itself.]
    1. X and Y
  3. Chromosomal Karyotype[3. Karyotype is the number, shape, and size of chromosomes in a human cell. While this includes more than the X and Y chromosomes, for this discussion, I will focus solely on the X and Y part of the karyotype.]
    1. XX, XY, XXY, XXX, XYY, XO, etc.
  4. Physical Phenotypes (Commonly Called Primary and Secondary Sex Characteristics, i.e., anatomy)[4. Phenotype refers to the set of observable characteristics in an organism arising from the interaction between the genome and the environment.]
    1. Reproductive: genitalia, mammary glands
    2. Non-Reproductive: body hair, vocal patterns, laryngeal prominence, body fat, etc.

(There are also those who point to brain chemistry, but that’s not something I’m brushed up on at the moment, so I’ll stick to the genetic and physical differences, leaving brain chemistry for a future discussion.)

‘Binaries’ that are not actually binaries

Biology is clear, the second two categories (3 and 4) exist on a spectrum rather than a fixed, irreducable binary. Current population studies are inadequate to predict just how common non XX and XY karyotype are, but the existence of living phenotypes outside of the binary taught in schools ought to guard against any conflation of chromosomal karyotype and gender. One is not simply “either XX or XY”. An individual may have multiple copies of one or more of the X or Y chromosomes and have it make little to no difference on their physical phenotype.

Yes, there are combinations that lead to developmental differences and abnormalities, some of which are significant or detrimental, though not terminal. Some may not develop into a viable fetus at all. In the latter case, I would not consider that a ‘living’ phenotype and is therefore not relevant for understanding viable chromosomal karyotypic diversity. In the former case, we should be wary of excluding such cases, as doing so veers perilously close to perceiving such individuals as either sub or less ‘human’. An individual is not ‘less’ human for having Trisomy 21 (Down’s Syndrome), so even if someone with an XXY karyotype has developmental differences or abnormalities, they are not ‘less’ human either.

Moreover, given the complex cascade of genetic, epigenetic[5. Epigenetics is the study of those factors that affect gene expression rather than, say, those that can alter the code itself. For example, stress can actually affect which genes are turned off and on, traits that can then be inherited. So if your mom was stressed when she was pregnant, it could affect your genes. For more information see the website “What is Epigenetics?”. ], and hormonal factors necessary, it is possible to have a difference between chromosomal karyotype and physical reproductive features developed by the body. One may, for example be chromosomally XX but also have testes and a penis (from birth, not just due to surgery).

The existence of intersex individuals likewise reminds us that physical phenotype and the development of reproductive features is not a strict binary classification, but rather a spectrum. Non-reproductive physical differences likewise vary even among those individuals with the same or very similar reproductive features and cannot, therefore, be used to classify along a strict binary. All XX individuals look different, as do all XY, XYY, XXY, etc. Boiling it down to a few select features not only collapses the natural variety of phenotypes, it also erases those individuals who do not neatly fall into the binary box many would like to assign to physical phenotypes.

Thus, a binary classification based on chromosomal karyotype or physical phenotype is not useful. Many, if not most biologists today would speak of both of these as a spectrum.[6. This is where I diverge from Richardson, for as much as she speaks of chromosomal variety, she still wishes to localize a biological classification of ‘male’ and ‘female’ to individuals who are “homogametic” (their gametes both have the same chromosome, or XX) and “heterogametic” (their gametes have two different chromosomes, or XY). Whether she means the gametes they produce or the gametes that went into producing them is unclear for how brief her discussion is of it.] One cannot, then, localize ‘gender’ or even ‘biological sex’ (if by that one means a strict binary between ‘biologically male’ and ‘biologically female’) on either chromosomal or physical phenotype. Both exist along a spectrum, making a binary both inaccurate and unhelpful in classification.

It’s really not in your genes

Categories 1 and 2 actually are irreducibly binary. However, chromosomal dimorphism does not lead to chromosomal karyotypic dimorphism, as 3 shows. Moreover, the so-called “sex chromosomes” are not, in fact, the locus of sexual differentiation, though they do play a significant role. The Y is not the ‘male chromosome’ nor the X the ‘female chromosome’. Development of reproductive and non-reproductive characteristics commonly associated with ‘biological sex’ requires a complex cascade of genetic, epigenetic, and hormonal factors of which genes on the X and Y are a part, but not the whole or sole contributing factors (see Richardson).

One may have a Y chromosome and still develop ovaries, for example. One may have two X chromosomes and not develop ovaries, or a utuerus, or either. Furthermore, the presence of certain physical reproductive features does not mean they function optimally, or at all. Is a person with a uterus and ovaries less ‘female’ (to use the colloquial association between reproductive organs and gender) if their ovaries do not release eggs or if they cannot get pregnant? Is someone with testes less ‘male’ if they do not produce viable sperm?

That last point may seem a bit in the weeds for this topic, but it’s not. A definition of ‘biological sex’ that is based on the presence of physical reproductive features often fails see the bias it assumes regarding health, functionality, and reproductive ‘fitness’ of the organs present. It is assumed that specific organs play a central, identifying role in one’s ‘true’ personhood. Such essentialist thinking boils human beings down to a few select organs.

The other consequence of this line of thought is that if those organs do not function, are not optimally functional (as in the case of infertility), or are removed, it does damage to the personhood of the individual. If we define ‘biological sex’ in terms of the presence of physical reproductive features, then damage to or removal of said features places them outside the binary so defined. My mother, then, would be less ‘biologically female’ because she has had her uterus, cervix, both ovaries, and one of her breasts removed. Women in menopause are ‘less female’; men who get a visectomy are ‘less male’ (wait, people actually think that. HmmGee, I wonder why?)

But that’s absurdly reductive. When pressed, many people would agree based on such exceptions that physical features, therefore, ought not to be the basis of ‘biological sex’. In such cases, one must then fall back on other categories to define ‘biological sex’, like chromosomal karyotype.

The presence or absence of physical reproductive features, then, is not a useful standard for ‘biological sex’, both because it is not a simple binary (as ‘biological sex’ is typically described as) and because one’s physical reproductive features may not actually match one’s chromosomal karyotype. For, how can they be a useful method of distinguishing gender from ‘biological sex’ when these features themselves may not align? Though one might be tempted to speak of “normal” physical development based on chromosomal karyotype, once again the existence of intersex individuals warns against overly stressing the pathway from chromosomal to physical phenotype. The same applies to the diversity of features even amongst those with the same chromosomal karyotype or possessing certain chromosomes regardless of number.

But what about the X and Y chromosomes as a locus of ‘biological sex’? Is this not an irreducible binary that gives rise to two human forms, ‘male’ and ‘female’? One might be tempted by this option, however, it’s important to realize the implications of this line of thought. Specifically, that it assumes the X is the ‘female chromosome’ and the Y the ‘male chromosome’.

Having a Y chromosome doesn’t make one ‘biologically male’. For one, there is nothing inherently ‘male’ about the Y chromosome. It does contain genetic coding related to testes and sperm production, but those physical features are not inherently ‘male’ features, as defined above. To call the Y the ‘male chromosome’ simply because it carries genes necessary for testicular development assumes that testes are ‘male’, which assumes that physical features are what defines one as ‘male’. It’s a circular argument based on a phenotypic understanding of ‘biological sex’ seeking to use chromosomes to justify itself. Since we’ve taken anatomy out as the defining feature of ‘biological sex’, the argument that “Y is male, X is female” falls apart. Thus, chromosomal dimorphism is not a useful locus of ‘biological sex’.

Game(te) on

That leaves us with reproductive gametic dimorphism. Despite the plethora of possibilities in chromosomal karyotype and physical expression, every single one of these possibilities arises from the coming together of two human gametes, an egg and a sperm. We cannot escape this binary, as it is a fundamental feature of human sexual reproduction. It is both irreducible and necessary for human reproduction as a species.

Why favor this over chromosomal dimorphism? Well, because strictly speaking, the X and Y do not combine to create the plurality of physical phenotypes available. There may even be a time in the future of human genetics that the Y chromosome no longer functions; it’s role in genetic recombination being given over to other chromosomes (there are already species that lack the Y chromosome). The Y is a unit of DNA transmission, but not an inherent feature of ‘biological sex’. Other chromosomes could do the job equally well, but it would not make them any more ‘male’.

The egg and sperm, however, are inherently necessary for human reproduction in a way that the Y chromosome may or may not always be. These gametes carry the genetic information (DNA) that results in the development of all human embryos, regardless of karyotype and phenotype. Without this binary, humanity would not exist as we now know it. Thus, insofar as it refers to a static binary that gives rise to the variety of human phenotypes, this, and only this, I might be willing to call ‘biological sex’, but only in that extremely restricted sense.

And even then, it’s not entirely useful as a descriptor. Note that I have had to change the definition of ‘biological sex’ from where I started to include the fact that there are a plurality rather than strictly two human phenotypes. Note also that it could imply that the egg is somehow ‘female’ and the sperm, ‘male’. But that lands us right back into the circular reasoning present in the argument for chromosomal dimorphism.

The body which produces eggs is not ‘biologically female’ nor the body that produces sperm ‘biologically male’. As ought to be clear from above, a body capable of producing eggs may have varying chromosomal and anatomical phenotypes. It may, for example, have an XXY chromosomal karyotype and intersex external genitalia. One may not, therefore extrapolate from gametic dimorphism to a biological (whether chromosomal or anatomical) binary. Human reproductive gametes may only come as a pair, but human beings do not. These gametes give rise to a variety of chromosomal karyotypes, which then give rise to a variety of anatomical phenotypes that do not always align with binary thinking about ‘biological sex’.

So what do we do with the term ‘biological sex’?

One may expand one’s definition of ‘biological sex’ to being a spectrum rather than a binary. This has potential explanatory power for chromosomal and anatomical diversity. If one chooses to go this route, one must still be wary of then creating a one to one correlation between the ensuing ‘biological sex’ spectrum and the gender spectrum. No matter how many different ‘biological sexes’ one creates, gender will never map perfectly onto it given that gender is entirely a matter of social norms, concepts, and history (I’ll get to this, don’t worry).

The other option in light of the previous discussion of ‘biological sex’ is hold with the current definition of an irreducible binary that gives rise to human phenotypes—though I would speak of ‘multiple’ rather than two phenotypes—but restrict it entirely to reproduction. This is my preference. And even so, I prefer to speak of gametic dimorphism rather than ‘biological sex’ precisely because ‘biological sex’ is such a loaded phrase. It has too long been associated with a faulty view of chromosomal, anatomical, and gender binaries for it to useful strictly in a gametic sense. There’s too much baggage, too much potential for misunderstanding.

In the end, I’d rather just throw out the term altogether and use less loaded terminology. We may speak of gametic dimorphism in human sexual reproduction. We may speak of chromosomal dimorphism that gives rise to a spectrum of chromosomal karyotypes. We may also speak of a spectrum of anatomical phenotypes and reproductive organs. And we ought to do all of this without assigning ‘male’ or ‘female’ to any of it. Since only gametes exist in a binary, ‘male’ and ‘female’ in any other context have no explanatory power due to their restrictiveness. Since only gametes exist in a binary, describing them as ‘male’ and ‘female’ is misleading. One can simply speak of the egg and sperm without assigning words associated with an inaccurate perception of biological binarism.

So what do we do with ‘male’ and ‘female’?

Thus, I prefer to discuss male and female strictly to the realm of gender rather than biological features. As commonly understood, gender refers to a set of roles, behaviors, and activities. Historically, such a set has frequently been both binary (male/female) and associated with anatomical differences. Yet, the roles, behaviors and activities typically associated with each element of the binary system (for now ‘male’ and ‘female’) differ both cross-culturally and historically. ‘Male’ and ‘female’ genders have not always meant the same thing to all societies. Nor do they mean the same thing to every society who exists on the planet today. This is what myself and others mean when they say gender is a social construct.

Attempts to find commonalities among all the gendered systems found in human populations in order to find some ‘essential’ duality that can be mapped onto a perceived biological binary (which I pointed out above is a fallacy in and of itself), has little value or explanatory power. Those handful of things one might find that map cross culturally and historically need not arise from the aforementioned biological essentialism. Most especially because biology is not a binary.

But also because such studies frequently overlook systems of marginalization and oppression that relegate ‘females’ to positions of passivity and subservience. Our knowledge of historical cultures stems mostly from written traditions, which, for the most part, were written by ‘males’. In cultures in which patriarchy and misogyny existed and males were the keepers of written records, a common understanding of ‘females’ says more about the systems themselves than it does about some core category of ‘female’.

Additionally, not all societies had strictly binary conceptions of gender. ‘Male’ and ‘female’ could stand alongside a third, a fourth, or more categories. As with the plethora of chromosomal and anatomical phenotypes, the existence of non-binary gender systems ought to prevent the conceptualization of a gender binary as somehow inescapable in or essential to human society. That so many are binary may say something about humanity’s desire for the simplest possible categorization system (a binary being the least complex way to preserve both difference and similarity).

But it does not mean that a binary is somehow inevitable or even accurate. As pointed out above, what seems like a binary in biology is actually much more complicated, and the conceptualization with the best explanatory power is a spectrum. Likewise, in our society, the increased awareness of people who do not identify with our society’s accepted binary system of ‘male’ and ‘female’ requires a more nuanced system for understanding gender as a spectrum rather than a simple binary.

In fact, from a strictly linguistic perspective, ‘male’ and ‘female’ are arbitrary labels. If we understand gender in terms of a set of roles, behaviors, and activities unconnected to chromosomal or anatomical features, one need not use the terms ‘male’ and ‘female’ at all. The label doesn’t matter so much as the content of it, i.e., what the specific roles, behaviors, and activities are.

I admit, by talking about it this way I’m going to a bit of a linguistic extreme, but it’s to a point. Namely, that the label ‘male’ and ‘female’ only have meaning and content insofar as our society deems them to have it. They have no chromosomal or biological ‘reality’ undergirding the duality. That certain characteristics deemed ‘male’ are ascribed to persons who have a penis is entirely a matter of social convention. Even that the stream of sounds and visual symbols that make up the word ‘male’, and the word ‘male’ itself, are used is a function of linguistic and cultural history.

Thus, if a person born with specific chromosomes or anatomical features identifies more with the gender not typically associated with these features, why should that upset people? Having a vagina does not make one ‘female’, since ‘female’ is a category that describes roles and behavior. My having ovaries does not make me like crocheting or sewing or less ‘fit’ to be a CEO of a fortune 500 company. (My distaste for math and more introverted personality might, but that has nothing to do with reproductive anatomy or chromosomal karyotype.)

The point is this: someone with a penis is not, inherently, ‘biologically male’. That person may have an XY chromosomal karyotype (or XYY, or XXY, or…you get the point). That person may have other anatomical features as well (like testes or a lot of body hair, etc), or not. None of those things make that person ‘biologically male’ and ‘socially female’ (a distinction often used in transmisogynistic discourse) or even ‘biologically and socially male’. Based on my discussion above, they’re not ‘biologically male’ at all. If all we know are those aspects, we may only speak of a person with certain chromosomal and anatomical features.

You are who you say you are

Since none of the chromosomal or anatomical features represent some essential biological binary, the defining feature for that person’s identity is their self-conceptualization based upon how they internalize their society’s gender system. If the above person identifies with their society’s definition of ‘female’, that person is female, full stop. If they identify with their society’s definition of ‘male, they are male, full stop. If they identify with neither or both, they may choose to identify as agender, non-binary, trigender, or another of the valid conceptualizations gaining recognition. A given society may set the parameters for how it defines gender, but it is up to the individual to determine in what way, or if, they perceive themselves as conforming to this set of shared definitions. Note, too, that a society’s conceptualizations of gender can evolve and change, which may lead to an individual re-evaluating their preferred identity.

Some might ask about how the distinction between chromosomal/anatomical features and gender affect situations such as a transgendered persons who transitions either hormonally, surgically, or both. However, as a cis person, I am not at all qualified to speak on such matters and prefer to keep silent. This is not my story to tell. I leave that to those who have lived it. And if I have misrepresented or misunderstood their experience in the little I spoke on it, I ask for forgiveness and correction. I want to speak as sensitively and accurately as I can. My goal here is not to fully describe transgender experience or reality.

It is, rather, to point out both that the concept ‘biological sex’ has no basis in actual biology or genomics beyond a description of gametic dimorphism that in no way maps to chromosomal or anatomical binarism. And even then it’s not entirely useful because of the baggage associated with it and the potential for misunderstanding. Better to leave off with the idea/term altogether.

My goal is also to point out that gender is a social construct that cannot be mapped onto said non-existent binarism. Chromosomal and anatomical phenotypes are best understood as a spectrum. So, too, gender. Especially in our (current, Western, American) society given advances in science and technology, our socio-cultural and historical evolution, and more readily available contact with other cultures. I say ‘our (current, Western, American) society’ because I can neither speak to nor proscribe other cultures’ conceptualizations of gender. Again, that is not my story to tell.

That does not mean we cannot pursue cross cultural analysis of gender, only that using it in a proscriptive sense should be avoided. Cross cultural analysis of gender is best used as a tool to highlight just how wide and varying the conceptualizations of gender roles, activities, and behavior have been throughout history and across the globe currently. That, and a warning to avoid overly stressing something essentialist in the definitions provided by those societies most influenced by patriarchal and misogynistic assumptions.


Basically, biological sex is a useless term; the biological binary doesn’t exist (except for gametes); gender is a social construct; people can define themselves however they want vis-a-vis their society’s definitions of gender; transphobia is bullshit and so is gender essentialism.

Image of Sex Itself Courtesy of University of Chicago Press