There is an entire class of argument that we see in discourse that basically relies on the idea that “physical attribute X means that Y can never be true”. Often, these arguments are built around the idea that there are fundamental attributes that are so central a person that they cannot be changed, and that they somehow define the person in some global sense.
It might seem at first that such arguments are somehow a slam-dunk. After all, who can ignore the "absolute truth" of something as fundamental as chromosomes, right? Well, not so fast - we are far more complex than such a reductive claim really acknowledges.
Chromosomes are not some deterministic piece of computer code. At best, chromosomes, and the sequences of DNA proteins that they contain are guidelines that play key roles in the development of cells in the body - but, because everything in the body is a result of series of complex biological signals in the body, you can't look at a piece of DNA and say "well, this will result in the following attribute" with absolute certainty. Instead we get a whole lot of sentences like "the following genes are associated with the following attribute" - probabilities, in other words.
Why probabilities? In part because those biological signals that influence the development of people are highly variable, and while some are generally predictable, others are responses to external stimuli. For example, we know that ingesting tobacco smoke contains a myriad of chemicals which can cause the development of cancers. It's not 100% guaranteed that a person who smokes will develop cancer, but enough do that we are quite certain that interaction with those chemicals triggers the development of cancerous cells. From a DNA perspective, those cancer cells are still very much the cells of our own bodies.
All of this is to point out the fundamental logical fallacy of trying to treat anything in the human body as an absolute. It clearly is not. If it were, things like cancer wouldn't really be a thing, would they?
Today, we are seeing a lot of arguments aimed in at invalidating transgender women in particular. Mostly these arguments boil down to "Your chromosomes are XY, therefore you are a man". Yes, there are a few variations on this, but at their core, this is what it comes down to.
It's simplistic thinking to argue that humans are simply "male or female" based on chromosomes. The reality is that we form a bimodal distribution, with the vast majority of people being phenotypically male or female, but a few outliers who exist in between for any number of reasons.
It's the outliers that are important here, because if we ignore them, we end up with an incomplete picture. The exceptions that they represent are why we cannot simply assert a simple binary, and any theory which ignores exceptions inevitably fails when those exceptions are examined more closely.
Mosaicism in genetics is an important part of the story here because it demonstrates that there are a range of conditions where chromosomes are far from clear in defining the development of the individual.
Consider for a moment a rare condition called Complete Androgen Insensitivity Syndrome (CAIS). Here, the person has a karyotype of 46X,Y but develops in a typical female phenotype. If we take it as a given that "chromosomes define the person", or even (slightly) more accurately, "chromosomes define the sex", we have a problem. We have a person with a normal female appearance, and yet their chromosomes are clearly male. So much for the essentialist claim that you can know a person's sex based on their chromosomes.
It's easy to look at these conditions and say "well, yeah, but those are so rare as to be irrelevant". Actually, no. They're very relevant because their existence demonstrates the key point of my argument, namely that biology is much more complex than any reductive argument accounts for.
Let's move on to the issue of transgender women (in particular, because they are so often the targets here).
Years ago, I read a book called "Brain Sex" by Moir and Jessel (1991). It postulated that a lot of the differences between men and women in society could be explained by structural and functional differentiation in the brain. At the time, it was an interesting hypothesis, but at best qualified as "pop science", as it really wasn't grounded in good evidence. As the 90s and early aughts rolled on, the dramatic differences between masculinized and feminized brains simply didn't pan out in the way that book anticipated.
But, over time, as neuroscience developed and we got greater insight into the structures of the brain, we started to identify places where human brains do differentiate along masculine / feminine lines. This doesn't mean that we have "male brains" or "female brains", in part because we know relatively little about how these differences might affect brain function. A difference in gross size of a region of the brain, or perhaps a difference in the density of neurones in a particular region indicates something, but even tools like Functional Magnetic Resonance Imaging (fMRI) really don't reveal much about the mechanisms at work when those regions are active.
So, as the authors of the book Brain Sex hypothesized, there is evidence for brains to differentiate along sex lines. What the book didn't predict was that those differences would be often quite small, and the precise role in differences in function are unclear.
Remember my mention of Mosaicism earlier? Well, it comes back here as a conceptual metaphor courtesy of Nguyen and colleagues (2018):
The point that this makes which is really relevant is twofold. First, that brains in particular develop along a spectrum between masculine and feminine, and second that transgender people frequently have structures that lean towards their expressed identity even before the introduction of hormone therapy.
It's this last point that I want to emphasize. Not only do we have evidence that transgender people have brain structure differentiation that leans in the direction of their expressed identity, but that those differences exist prior to the introduction of gender affirming hormones. Not only does this seriously challenge the assumptions around transgender people in general, but it raises problems with the characterization of transgender women as "a bloke wearing a dress" (which is often the kindest of the insults hurled at transgender women). It actually provides a framework from which it is much easier to understand that transgender people are a normal part of human experience.
If, as in the example of someone with CAIS, someone with 46XY chromosomes can develop apparently normal female physical features, then why would we blindly assume that there aren't those who for any number of reasons develop attributes that fall somewhere between the extremes of biologically masculine or biologically feminine?
Many in the Gender Critical world take the position that if you have (or ever had) a penis, you're simply a man and that's all there is to it. Such positions ignore the reality that human development is far more complex than that.
*Note: Links to Wikipedia are used here for accessibility. Genetics and neuroscience papers are extremely dense reading. If you want to pursue the details of a particular subtopic here, I suggest starting with the papers cited in the various Wikipedia bibliographies.
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