A gender crit on twitter tried to get me with the “biology” gotcha, and asked me to explain the science behind being trans. I didn’t have the answers immediately to hand, so I’m sure they think they won. But it’s not about winning or losing, it’s about lying and misrepresenting data, using out-of-date theories, and simplifying biology to a point where an 11-year-old can understand it, or being honest and open and presenting new information and research and admitting that our knowledge still isn’t complete.
This post is not everything I can find on the biology of trans people (there was too much). There are way too many studies for a simple blog post. It is accurate to the best of my knowledge as of February 2021.
Disclaimer: I am not a scientist
Some definitions:
Female: Noun, dictionary – a female animal or plant
So female is female.
Adjective, dictionary – of or denoting the sex that can bear offspring or produce eggs, distinguished biologically by the production of gametes (ova) which can be fertilized by male gametes.
So females can have babies. But not all cis women can have babies.
Gametes are an organism’s reproductive cells. They are also referred to as sex cells. Female gametes are called ova or egg cells, and male gametes are called sperm.
Google
Okay, trans women can’t yet get ova or egg cells. I’m not convinced it will never happen, there’s some interesting studies going into womb transplants and who knows where that will end up in a few decades, but it can’t currently happen. I have never seen a trans woman say it can happen, though, so I’m not sure where the confusion is here.
But, again, not all cis women can have babies. Some cis women are infertile, or too young or too old. Most problems with infertility are caused by ovulation. Without ovulation, there are no eggs to fertilise, So some cis women can’t create gametes and are still female.
Woman: Noun, dictionary – an adult female human being.
Okay, woman means female and female means female and female means produces gametes but some cis women don’t produce gametes, clear?
You know the problem might be the language itself because those definitions are a mess.
Okay, let’s forget the definition and concentrate on the actual biology.
Genetics
Gender critical people say that it’s basic science that female = xx gene. It is, it’s very basic science. Like first year high school science.
The very first thing that needs to be pointed out is that we don’t know everything about genetics. The human genome was mapped a century ago, but we don’t know what every gene does, and we don’t know how to do things with genes. We can’t say which genes form hearts, and there are plenty of genes doing things and we don’t know what they’re doing.
So whilst we can say xx is female and xy is male we can’t say those are the only factors determining sex, can we? Actually, we can absolutely say those aren’t the only factors determining sex because intersex people exist.
Intersex people are individuals born with any of several variations in sex characteristics including chromosomes, gonads, sex hormones or genitals that, according to the Office of the United Nations High Commissioner for Human Rights, “do not fit the typical definitions for male or female bodies”.
Wikipedia
Okay, so already, according to genetics, we have male, female, and several other variants of intersex people.
Hmm. Well, there goes the gender binary. Gender critical people have this great thing they do where they say “I accept the evidence of intersex people” and then discount it entirely. It’s fascinating to watch them do it.
Now personally, I’ve wondered if being transgender isn’t a form of being intersex. Now don’t take this out of context – I’m not saying scientists or doctors say this. I’m saying I have a sort of half-formed partial curiosity about the possibility. If being transgender isn’t part of the cisgender binary and there are biological reasons for it, it could come under the intersex umbrella, couldn’t it?
Going by the current definition of intersex, no. But I do wonder if one day, when the biological causes are more understood, the intersex definition might expand to include transgender people.
As above, definitions are somewhat circular here. We really don’t have the language to adequately express things.
What else do we know about the genetics of trans people?
Well, we know it matters, even if we don’t know how.
In 2013, a twin study combined a survey of pairs of twins where one or both had undergone, or had plans and medical approval to undergo, gender transition, with a literature review of published reports of transgender twins. The study found that one third of identical twin pairs in the sample were both transgender: 13 of 39 (33%) monozygotic or identical pairs of assigned males and 8 of 35 (22.8%) pairs of assigned females. Among dizygotic or genetically non-identical twin pairs, there was only 1 of 38 (2.6%) pairs where both twins were trans. The significant percent of identical twin pairs in which both twins are trans and the virtual absence of dizygotic twins (raised in the same family at the same time) in which both were trans would provide evidence that transgender identity is significantly influenced by genetics if both sets were raised in different families.
Wikipedia
Alright, genetics matters. But how does it matter?
Studies comparing trans women and cis men found trans women are more likely to have a longer version of a receptor gene for the sex hormone androgen and thus are less likely to bind testosterone. A variant genotype for a gene called CYP17 has been linked to trans men, but not trans women. CYP17 acts on the hormones pregnenolone and progesterone.
A 2019 meta-analysis of previous studies found:
Trans woman population exhibits significantly longer polymorphic CAG repeat sequences in the AR gene. Further studies are warranted to elucidate whether, how and to what extent multiple functional variants in sex hormone signaling genes could be associated with gender incongruence/dysphoria.
But what does any of this mean?
Foetus Development
All foetuses start off as female. In the first trimester, foetuses with a Y chromosome develop male gonads, i.e. a gamete producing organ that produces sperm. This is the only sex change development at that stage.
In the 3rd semester, hormones flood the womb and some bind to receptor genes within the foetus. In a cis male, who has developed male gonads in the first trimester, this means testosterone binds to the androgen receptors. In a trans woman, the androgen receptors are too long and the testosterone can’t bind correctly, meaning the brain doesn’t switch to being male.
Yes, I’m saying men and women have brain differences. There is evidence supporting this. I’ll get to that soon.
But it’s not just androgen receptors. A report published in February 2020 found 21 variants in 19 genes in estrogen signalling pathways critical to establishing whether the brain is male or female.
“It doesn’t matter which sex organs you have, it’s whether estrogen, or androgen, which is converted to estrogen in the brain, masculinizes the brain during this critical period,” says Dr. Lawrence C. Layman, chief of the MCG Section of Reproductive Endocrinology, Infertility and Genetics in the Department of Obstetrics and Gynecology. “We have found variants in genes that are important in some of these different areas of the brain.”
Now it does go on to say it’s too early to say if these are causes of dysphoria, but it continues with “We are saying that looking into these pathways is the approach we are going to be taking in the years ahead to explore the genetic contribution to gender dysphoria in humans.”
That there is a genetic contribution is pretty much accepted as fact at this point by scientists, researchers, and doctors. When gender critical people talk about xx and xy genes, they are clearly denying the science beyond what is taught in first year high school classes.
Brains!
In 1995 (26 years ago and we’re still arguing about this) a study found that a region of the brain called the bed nucleus of the stria terminalis (BSTc) – which is a region known for sex and anxiety responses, and which is affected by prenatal androgens – in six cadavers of trans women had female-normal BSTc sizes.
The interesting thing wasn’t that transgender women who had taken estrogen had female-normal BSTc sizes, but that cis male and cis female cadavers who had experienced hormone reversal for a variety of medical reasons had cis-normal BTSc sizes. The effects of the hormones hadn’t changed their BSTc size. (Zhou)
A followup study in 2000 counted the number of neurons in the BSTc. They found the same result: trans women’s brains matched cis women’s brains. They even managed to include a trans woman who had never taken hormones and her brain also matched cis women’s brains. (Kruijver)
In 2002 it was discovered that significant sexual dimophism in BSTc isn’t established until adulthood. It was theorized that changes in fetal hormone levels produce changes in BSTc synaptic density, neuronal activity, or neurochemical content which later lead to size and neuron count changes or, alternatively, the size of the BSTc is “affected by the generation of a gender identity inconsistent with one’s assigned sex.” (Chung)
There are differences. This is known. What exactly causes the differences is unknown. This is important.
In a 2006 review of the evidence, Gooren confirmed the earlier research as supporting the concept that transsexuality is a sexual differentiation disorder of the sex dimorphic brain. Swaab (2004) agrees.
Garcia-Falgueras and Swaab found the interstitial nucleus of the anterior hypothalamus (INAHs), which is part of the hypothalamic uncinate nucleus, in 2008. The same control methods for hormone usage was used as in the 2000 and 1995 tests and the results were even more pronounced. Cis men had 1.9 times the volume and 2.3 times the neurons as cis women, trans women were within the cis women range and trans men were within the cis men range.
But the really interesting thing here is that some of the transgender people hadn’t had any hormone treatments, which means that the brains of these transgender women had INAHs within the range of cis women even without hormones, and the brains of trans men had INAHs within the range of cis men even without hormones.
An MRI study in 2009 of trans women not yet given hormone treatments found regional grey-matter concentrations different to both cis men and cis women, but closer to cis men. However, there was a significantly larger volume of grey matter in the right putamen compared to cis men. It concluded that a distinct cerebral pattern is associated with people who are transgender. (Luders)
In 2010, Rametti’s study concluded that “compared to [cis women], [trans women] showed higher fractional anisotropy values in posterior part of the right superior longitudinal fasciculus, the forceps minor and corticospinal tract. Compared to [cis men], [trans women] showed only lower fractional anisotropy values in the corticospinal tap.” The white matter pattern in trans men was found to be shifted in the direction of cis men.
There are many, many more brain studies (Hulshoff Pol, Gizewski, another Rametti, Savic and Arver, Nawata) but this is getting long enough. They all essentially conclude the same thing: cis people’s brains and tran’s peoples brains are different.
Conclusion!
There are some obvious conclusions to make from the above data, and some not so obvious conclusions too. First, it should be noted that a lot of these studies only used a few people. It must be difficult to find enough corpses to use for the studies that require them since so few people are willing to their bodies to science. A quick Google tells me its about 600 people per year. I don’t know how many of those are trans, but it can’t be some huge number.
However, even the studies that use living people have low numbers. This suggests its hard to get funding to do huge research projects, and that means the GC theories about pharma funding and Soros bankrolling trans people is as much of an absurd conspiracy theory as everything else they believe.
Another thing about these studies is that they only seem to address binary transgender people. Non-binary people are not represented in the studies. This needs addressed.
The data itself shows many things. There is clearly some kind of genetic factor in a person being transgender, though we don’t know what. There are clearly differences between the brains of cis men and cis women, and trans women’s brains, whilst different from either, most closely align with cis women’s brains, and vice versa for trans men. However, we don’t know what causes these differences.
There are clearly biological – genetic and neurological – factors that make a person transgender, and some of these are known to come from foetal development in the womb. But there is still much more research into this needing to be carried out. We don’t have all the answers. The answers we do have clearly show “sex is male and female, XX and XY” is misleading, simplified, and absolutely not based in facts.
Lastly, this blog post may have some thinking I’m a transmedicalist or truscum. From my understanding of what that means, I’m not. At some point I will do a blog post on what transmedicalism is and why I don’t see myself as one. However, I do think there are biological reasons trans people exist – we’re born this way, we don’t grow into it. It’s nature, not nurture. And maybe some people don’t seem to fit into the “nature” thing and are still trans – I’m not saying they aren’t. I think that with further research and more knowledge we’ll understand they are transgender. I think, if you’re arguing being trans is nurture, not nature, then you’re arguing in favour of conversion therapy instead of transition.
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