"But you can't change your chromosomes."
Every Person of Gender [*] has probably heard somebody say this. "You can't change your chromosomes," says the paragon of the respectful, tolerant, liberal cis[**] person -- let's call them "Casey" for gender-neutrality, though I've noticed this person is more likely to be a man than a woman -- plaintively. "No matter what you change in your body, you'll always have XX chromosomes if you're a trans man, or XY chromosomes if you're a trans woman."
One might wonder just how it is that Casey has such an intimate knowledge of the contents of one's genome when they were actually only introduced to one perhaps 15 minutes ago. Let's suppose for the moment, though, that one is a person for whom the hegemonically-presumed relationship between external genitalia and genetics holds. Again, one may wonder why Casey is declaiming on either when they have seen neither. Putting that aside; if pressed further, Casey likely protests further that they're not a transphobe. No, of course not. How could you accuse them of being transphobic? (How could you accuse anyone of being transphobic?) They're not trying to deny that you can present as the gender you identify as (and they are wont to choose words like "present as" and "identify as" in order to avoid acknowledging that you, in fact, are the gender you say you are). No, not at all! They're simply concerned that you might forget that chromosomes can't be changed, and out of her concern for your welfare, wants to save you all the time you might spend at bogus chromosome repair clinics otherwise.
If asked why any of this is germane, Casey might say that they're trying to elucidate the relationship between sex and gender. Sex, one may have been told in one's Introduction to Women's Studies class, refers to the "biological aspects of being male or female" (quotation marks placed in order to draw attention to the ambiguous meaning of most of those individual words as well as their combination), while gender refers to the "social aspects of being a man or a woman". While the sex/gender dichotomy is oversimplified (do you really believe that biology doesn't affect behavior, or vice versa?), it's also beside the point.
I'm not an expert on human biology, which is actually just fine for the purposes of this essay, because statements like "But you can't change your chromosomes" are grounded in a political position, not a scientific one. Such a statement may appear to be about science, and Casey may sincerely believe they're only citing the simple scientific facts that everyone learns in sixth grade. But the context in which the statement frequently appears grants it the ability to do political work: specifically, to lend an aura of scientific legitimacy to power structures, and to rhetorically shore up the superior political position enjoyed by those who bear identity documents that match the gender inside their heads.
For one thing, when someone reduces sex differentiation in humans to a claim that everyone is born with either XX chromosomes and ovaries or with XY chromosomes and testicles, you can tell they haven't learned much since sixth-grade science class. While it is fine to be ignorant of a particular topic, scientific honesty requires admitting exactly when you are and aren't certain, and the excessive certainty inherent in attributing undue weight to chromosomes is a sure sign that somebody is disguising a political assertion as a scientific one. In fact, not every human has a karyotype that falls into the category of XX or XY; not everyone who appears to have a penis and testicles is XY; not everyone who appears to have a vulva and clitoris is XX; and all of these variations (many of which are grouped under the label "intersex") are more common than you think (unless, of course, you're already well-versed in human diversity). The XY woman who gave birth to an XY daughter is just one example out of many. (This particular woman had mosaicism -- a combination of XX and XY cells -- but her existence still points to the fact that dividing humans into objectively-defined classes of "biological males" and "biological females" isn't as simple as you think.) If you want to know what it looks like when a scientifically informed person responds to the "XX=girl, XY=boy" trope, this post from a biologist friend of mine is an example. You know you're not reading the words of an informed and scientifically honest person when you see oversimplifications and excessive certainty, and attribution to "nature" of human norms and intentionality (the notions of "normal" and "disordered" people).
You might protest that you ought not to be asked to discard a cherished rule (the rule that there are people called 'biological males' and 'biological females', and that they can be identified based on karyotype, which is always either XX or XY) in order to acknowledge the existence of a few people who diverge from it. But if you take the view that every human is just as good, just as normal, and just as typical an example of a human being as any other human is, then you can't countenance a rule that says that there is a default human state -- having a binary sex -- and that humans who are intersex can only be explained as subordinate people, in terms of their divergence from the norm. While I would be going beyond my expertise if I said much more on this point, I recommend _Fixing Sex_ by Katrina Karzakis and _Evolution's Rainbow_ by Joan Roughgarden if you want to know about diversity that goes beyond the sex binary.
So, as a thought experiment, I'll pretend that there really are only two chromosomal sexes and that it's always obvious from examining the outside of an infant at birth what set of chromosomes it came with. What I really want to ask in the rest of this essay is why cis people bring up chromosomes when the subject of trans people comes up, especially when talking about what it means for a trans person to transition.
Everybody -- we'll pretend -- has either XX or XY chromosomes. You can't see chromosomes directly, but rather, you have to take a blood sample from the person whose chromosomes you want to test, and analyze it in a lab (or have someone else do the same). Everybody has reproductive organs, some of which can be seen directly and some of which can't. And everybody has a set of genitalia, which can be seen directly (if you ask nicely). Finally, everybody has a mental map of their body: it's what tells you how far you need to move your hand to get to the bag of chips next to the computer, or how hard you need to kick someone who says they know what your gender is better than you do. That mental map also represents your genitals, usually as either a penis and testicles or a vulva and clitoris; perhaps some people also have mental maps that are flexible enough to adapt to whatever set of genitalia your body actually has, and others have maps that don't correspond to either. When you get turned on, if you're not asexual, your brain uses the mental map to figure out what you might like to do in order to relieve that situation. You can't see or measure this mental map (or, at least we don't know how to do that yet), and most people probably never stop to contemplate that they have one. There's no reason to stop and contemplate your mental map unless you have one that doesn't match the rest of your body. In the same way in which you imagine that you see the world directly rather than in a way that's mediated through a tangle of optical and neural processing mechanisms, you imagine that you experience your body directly and not in a way that's mediated by your brain's mental map of it (analogously to the way that device drivers mediate between a CPU and a hardware peripheral, if you're a nerd). Needless to say, like any mental event or state, the map has a physical representation in your brain (though as far as I know, it's not well-understood just how that representation works), and so is part of your body in the same way that your toes, heart, or genitals are. When we talk about someone's gender identity, their internal body map seems to be part of that, although identity may not match mental map either, in the same way it may not match the externally observable qualities of one's body. But there does seem to be a correlation, if not a causal relationship, between having a brain that tells you that you ought to have boy bits and needing to be able to recognize yourself as a man when you look in the mirror and to be recognized that way by others.
When well-meaning Casey tells us that it's important to distinguish the category of people who identify as women, and the category of "biological females", what they mean is that for any given person "Jamie", Jamie's identity (which they cast as subjective, non-biological, non-physical, and unreliable) may correlate with a different gender than Jamie's body (objective, biological, physical, unambiguous); so, therefore, a person can have a different "biological sex" than the one that's commonly associated with their gender. Perhaps this is exactly Casey's definition of what it means to be a "transgender" person. Casey goes on to reassure us that they respect how transgender people self-identify and are even magnanimous enough to use the right pronouns (53% of the time, anyway), but it's still always important to remember that a transgender person has a biological sex, which is different from their gender.
If we take this last belief of Casey's as a given, and recall that they were talking about chromosomes a lot, note the choice that these two facts, taken together, reveal. If you believe in a "biological sex", a function from person to sex category that you can compute without asking the input person, what physical evidence do you use to make the determination? (We haven't yet defined what "biological sex" is, but Casey usually doesn't, either.) We could choose evidence that's observable to anyone, without invasive procedures or special equipment (but possibly with a request to doff one's clothes): examining someone's genitals, chest, location and amount of body hair, and so on. The problem for people like Casey, who are invested in the idea of an immutable mapping from person to "biological sex", is that almost all of a person's observable sexed (or gendered) characteristics are mutable. (There are exceptions, like height, but in that case, there is so much variation within sexes that knowing someone's height doesn't contribute much data if you're trying to guess their gender with high accuracy.)
If Casey has to admit that all of the criteria they can use to determine someone's biological sex are mutable, they would have to admit that there is no such thing as a biological sex that someone is born with and that never changes. Oh, no! This won't do. So Casey has to look for something else. Chromosomes are what they find. Chromosomes are such a useful device for maintaining the discourse of objective sex categories that if they didn't exist, we would have to invent them.
But why differentiate based on chromosomes? If you're moving on to unobservable characteristics (strange territory already, since people usually consider themselves to be good at attributing sex based solely on observable characteristics -- characteristics that can be seen in a fully dressed person, even), why not use the mental maps I was talking about earlier? Defining sex in terms of the map your brain maintains of the rest of your body would fit one of Casey's needs, insofar as the mental map is, or seems to be, immutable; if there were a way to change it, people whose mental maps differ from their bodies would surely seize the opportunity to change their minds rather than pursuing the expensive and socially stigmatized procedures that change the body to match the map.
I'll let you ponder that question for a bit; once you've pondered it, we can consider another facet of the mystery of why people like to talk about chromosomes while knowing so little about them. Consider, again, the statement "You can't change your chromosomes." Is it possible Casey has a misconception about just what it is that a trans person sets out to change?
Susan Stryker defined "transgender" as follows: you're transgender if, at any time, you depart from or challenge the assessment of your gender that an observer made when you were born. Nothing about this definition implies that anyone changes their gender, whether they're trans or not. But trans people sometimes describe themselves as "transitioning": if they don't change their gender (or sex), then what do they transition from and what do they transition to? I don't speak for all trans people, but to me, it seems like a trans person who modifies their body chemically (and not all do) typically does so in order to provide their brain with the mixture of hormones that's best suited to the person's individual psychological well-being, or to bring the rest of their body closer to what their mental map says it is, or both. And a trans person who modifies their body with a scalpel (and not all do) does that for the latter purpose, as well. The reason why a person would do all this is the gender that's in their brain. And please understand: I'm not talking about "brain sex" or "brain gender" in the way that people like Larry Summers do, to provide pseudo-scientific justifications for social inequalities that limit certain opportunities to people depending on perceived gender. Rather, I'm simply referring to the idea that some people's brains seem wired to expect a particular hormone mixture and body shape. Is it wrong to use the word "wired"? Is it possible that you could learn to have a brain that expects to be estrogen-based, or a brain that gets excited about sexual situations in which you're doing things with your (possibly nonexistent) penis, or acquire such a brain due to early childhood events? Maybe, but given that nobody is out there trying to create little trans children, it seems unlikely; it wouldn't serve anyone's interests to create conditions that encourage people's brain maps of the rest of their bodies to diverge from the body itself. And it seems exceedingly uncommon for a person's brain sex, in the way I'm using the term, to change over the course of their life. It also seems difficult to impossible for a person to change their brain sex -- and for some people, that's not for lack of trying.
I'm arguing that gender and sex are most sensibly calculated from the state of your brain rather than the shape of your genitals or reproductive organs. Why? For one thing, we know that if a cis person has to lose their breasts or testicles due to cancer, or gets their penis shot off in a war, they don't typically decide to start expressing a different gender as a result. As for chromosomes, while there certainly seems to be a statistical correlation between being XX and having a "female" body and mental map, and between being XY and having a "male" body and mental map, clearly chromosomes do not determine the sex of the body mapped out inside one's head, because there are so many examples of people for whom the chromosomes don't match the map. A person whose chromosomes don't match their map is far more likely to be happy living in a way that manifests the sex or gender that their mental map dictates than in a way that reflects the sex that their chromosomes are usually thought to suggest. So, if you're such a person, what are chromosomes to you besides a biological accident that -- if you're a person who wants to reproduce -- could make it harder for you to reproduce in a way that would feel right for you? Chromosomes are a red herring. A red herring, that is, in discussions of how we ought to talk and think about sex and gender categories in regular social life. They're certainly not a red herring for people who seriously study genetics and sex differentation, and I'm certainly not suggesting that scientific researchers ought not to discuss their work.
So the statement "you can't change your chromosomes" is irrelevant in most contexts in which Casey would offer it, because a trans person like Jamie seeks to change neither their gender nor their sex. Rather, Jamie strives to change some of their sexed physical characteristics in order to increase their subjective level of psychological stability and well-being, because being a woman who developed with a lot of typically-male characteristics can be an uncomfortable state to be in, and the same is true for a man who developed with a lot of typically-female characteristics. Medical interventions are a set of coping strategies, not a way to change yourself into something different from the person you were born as.
I hope this suggests why undue attention to supposedly-objective sex differentiators, as well as terms like "biologically male" and "born female", are problematic: their meaning is unclear, they are predicated on the unreality of other people's subjective experience, and they help reinforce the social norm that it's OK to deny the reality of someone else's subjective experience when they're a member of a subordinate social group. It is apparently very important to some people that there be a distinct notion of biological sex, which they can measure objectively without having to trust the self-reporting of the person whose sex needs (why?) to be determined, and which never changes from the time a person is born. For some people, maybe it is important because they were taught in school that male humans invariably have XY chromosomes and female humans have XX chromosomes, and they simply don't want to admit to believing something that is wrong or incomplete. For other people, perhaps it's just very important to believe in their own assessments of strangers' genders, and locating these assessments in biology rather than in their own set of personal, subjective, psychological beliefs lends some cognitive authority to what would otherwise seem like a capricious set of prejudices. What for cis people is a matter of not having to admit mistakes can be for trans people a matter of life and death; when you're seen as a person "lying" or "pretending" to be a different gender, the consequences can be deadly. And the whole idea that having a brain sex that differs from other sexed or gendered characteristics in your body could be a form of "deception" is rooted in the idea of an objective, observable biological sex.
I hope I've shown that the rhetoric of chromosomes is an appropriation of scientific language to reinforce a particular ideology, not a manifestation of scientific thinking -- that is, the kind of thinking based on examining evidence and drawing logical conclusions from it. Anyone who does science will tell you that part of the job is to make models: abstractions that explain natural phenomena while inevitably leaving out some details. All sex and gender categories are models. This is why we say that sex is a social construct, not just gender: while individuals in all their natural splendor are observable phenomena, the idea of sex -- an abstraction that clusters concrete individuals into one of two absract categories -- is a human creation that exists only inside the minds of people who know about it. While sex and gender classification systems, like any other models, are useful tools, they are only useful when we remember that they exist to help us understand the world in all its complexity, not to hinder us in that understanding. Models make the world harder to understand when we discard or deny data that doesn't fit the model, rather than extending the model. To argue that a person with XY chromosomes can't really be female because your model says that people with XY chromosomes are always male is to confuse the map with the territory.
Like the XX/XY classification system, the notions of "man" and "woman" also constitute a model -- a social-scientific one rather than a biological one. When you get right down to it, there is, in fact, no such thing as a walking, flesh-and-blood man or woman; to say that any individual dude coincides with the abstract notion of a person with some qualities we associate with manhood, or that any individual chick is an exemplar of the abstract notion of a person with some qualities we associate with womanhood, is also to confuse the map with the territory. Calling people "men" and "women" is a convenient abuse of notation, in the mathematical sense. I'm not an essentialist, though, even if what I've written so far sounds essentialist at first blush. For every person, either "man" or "woman" is an imperfect approximation to the impossible task of characterizing their gendered (or un-gendered) identity in words. (And for some people, neither "man" or "woman" is enough.) Rather than making an essentializing statement that people are born in either the "man" or "woman" box, I'm simply arguing that female-assigned-at-birth (FAAB) women deserve no special access to the tool of labelling oneself as "woman" that male-assigned-at-birth (MAAB) women lack, and that MAAB men have neither more nor less of a right to appropriate the label "man" for themselves than do FAAB men.
Ultimately, people who cling to imperfect models rather than extending them aren't full of desire to obtain more perfect scientific knowledge. Rather, they wish to retain privilege: the higher status associated with being a person whose subjective gender matches the objective sex that results from the sex deterministic criteria they're positing. But being a particular gender doesn't require one to pass a test or be certified by an external authority. The only division of people into male or female that matters in everyday social life is the division arising from each person's conception of themselves as male or female (or neither, or both); think about how even when no trans people are involved, cis people interact with each other based on the gendered cues most people present (like clothes, makeup or lack thereof, or hair) in order to communicate their internal gender identities. Nobody demands proof; the only time when objective standards of gender generally get brought up is when trans people ask for the same privileges that cis people take for granted (mostly, the privilege of being trusted when you say what your own gender is). Even when Casey isn't claiming that trans people should be denied any specific privileges, reminding any nearby trans person that "you can't change their chromosomes" really just means reminding that person that they are subordinate.
To remind me that you believe that my chromosomes suggest my sex is different than what my subjective experience indicates is to privilege knowledge you (theoretically) have access to -- "objective" measurements like blood tests -- over knowledge I have access to -- my own experience. Yet the data that tell me that my mental map is of a male body are no less real for the mode in which I've collected them. Perhaps more to the point, when you remind me of that, you're applying a different standard to me because you know that I'm trans than you would apply to a person who you believe is cis. Normally, we trust that a given person's gender presentation, which is an expression of their subjective sex (except, of course, in situations where someone doesn't feel safe contradicting others' beliefs about their sex), reflects their actual sex or gender. It's only when we believe someone is trans that we have a reason to enumerate different ways to determine their gender (chromosomes, genitalia, and so on; everything but the brain). The rhetoric of chromosomes implies that there is a higher standard of evidence for gender when it comes to a person believed to be trans; that there ought to be any standard of evidence for gender at all. It's both reflective of bad science and of a desire to evade the imperative to respect each individual's subjective experience of being a member of a particular gender. (By the way, why would someone lie about their subjective sex? There are zero social privileges to be gained by voluntarily assuming the social position of a person who's perceived as being trans.) So it's time to relegate chromosomes to the same bin as blood types and cholesterol levels: data that are interesting to specialists and relevant in some particular medical contexts, but irrelevant to determining whether someone has the right to be treated in a manner consistent with their needs.
[*] (a phrase coined by imfallingup, meaning anybody who regularly has to explain what their gender is)
[**] A cis (short for cisgender) person is a person whose gender as assessed by others consistently matches the gender they feel themselves to be; you'll also see the more specific word "cissexual", referring to a person whose brain is well-tuned for the rest of their body, without requiring bodily adjustments to live comfortably.