If your reaction to these tweets is “No they aren’t!” then check yourself, because thats transphobia talking. If your reaction is “That’s not how biology works”, please, stay a while and listen.

Let’s talk about biology, anatomy, and sex. I swear I had this essay already written before Indya’s tweets went viral.

What Makes a Man

Here is what we know today. If you’ve seen Jurassic Park then you may remember this scene:

This isn’t science fiction, tho it is very dumbed down. In human fetuses the gonads initially develop in a bipotential state, meaning they can become either ovaries or testes. The SRY gene on the Y chromosome releases a protein called Testis Determining Factor (TDF). This protein then starts a chain reaction which causes the gonadal cells to form into the Sertoli and Leydig cells that make up the testes. If TDF is never produced or is interfered with then the gonad cells form into the Theca cells and follicles which comprise the ovaries.

Once formed, the testes then begin producing a testosterone surge which typically starts in the 8th week of gestation and continues until the 24th week. This surge is responsible for the development of the penis and scrotum. Genitalia formation starts around week 9 and becomes identifiable in the 11th week. If the surge does not occur, or the body does not respond to it (such as in the case of Androgen Insensitivity Syndrome) then the genitalia form into the vulva, vagina and uterus instead.

If there is an interference in this process then you can end up with malformed bits, and this is the result of intersex conditions. Often times this is a partial development, where the external genitalia only partially form, but functional gonads still exist. Sometimes the child comes out with fully functional male or female genitalia, but mismatched gonads. Sometimes the TDF protein fails to release and the fetus grows completely functional female reproductive organs, despite the presence of a Y chromosome.

This is known as Swyer Syndrome, and an unknown number of women may have this condition. In 2015 an XY woman with Swyer Syndrome who was born without ovaries successfully carried and gave birth to a child via IVF. Usually Swyer Syndrome results in completely non-functional ovaries, but in 2008 a woman was found with Swyer Syndrome who had gone through puberty, menstruated normally, and had two unassisted pregnancies. Her condition went undiscovered until her daughter was was found to also have it.

The fact is, the vast majority of the population has never been tested for genetic karyotype, so we don’t know how common these cases actually are. Where does this come into affect for gender identity? Well, the exact same process that causes the external genitals to differentiate also occurs for the brain.

The prenatal brain doesn’t really start to develop until between week 12 and 24. The cerebral cortex, the thin outer layer of the brain that contains most of what we think of as consciousness, grows substantially during those periods of time. Prior to that, the structure present is more like a scaffolding, the basic parts of the nervous system necessary for bodily function. The primary sulci (the wrinkles in the cerebral cortex that allow for more surface area) start to form at week 14, well after the genitals have developed.

Boy Interrupted

It has been confirmed multiple times via MRI studies that there are small but significant differences between cis male and cis female brains, differences which align with the gender identities of trans people in the study. Note, this does not mean that anyone with those differences will have that gender, because gender identity isn’t that simple, but it provides evidence that there is a clear difference in masculine and feminine brains.

A change in the testosterone levels in the fetus after the 11th week can directly impact the masculinization of the cerebral cortex, as well as changes in other parts of the brain structure. This has been examined over and over again in studies of female assigned children with CAH (congenital adrenal hyperplasia) and CAIS (complete androgen insensitivity syndrome).

We found a significant relationship between fetal testosterone and sexually differentiated play behavior in both girls and boys.

— Fetal Testosterone Predicts Sexually Differentiated Childhood Behavior in Girls and in Boys

An excess of testosterone in the mother’s body during the second trimester can (and does) cause masculinization of the brain in an externally female fetus, and an interference in testosterone production or uptake can (and does) cause feminisation of an externally male fetus. This interference does not have to be external in origin, either. Any number of genetic traits can cause the brain to respond differently to testosterone. A fairly large study of transgender individuals released in 2018 found several key genes which were statistically more likely to be longer among trans women (longer, as in having more repeated fragments). Individually these genes may not have an impact strong enough to cause a malfunction of masculinization, but collectively they absolutely could reduce the ability for the fetal brain to masculinize. These genes are all passed from parent to child, giving credence to a tendency for trans parents to have trans children.

Gender is biological.

What Goes Around Comes Around

Now, here’s the kicker, it doesn’t have to take an external influence to cause this to happen. Last year a fairly large study of transgender individuals found several key genes which were statistically more likely to be longer among trans women (longer, as in having more repeated fragments). Individually these genes may not have an impact strong enough to cause a malfunction of masculinization, but collectively they absolutely could reduce the ability for the fetal brain to masculinize. These genes are all passed from parent to child, giving credence to a tendency for trans parents to have trans children.

There are, of course, numerous social influences which can have an impact on gender identity, but those only affect the awareness of one’s gender, not it’s form. The vast majority of trans people were not directly aware from childhood that their sex did not align with their gender. They just know something is wrong (some don’t even know that), and without the language to quantify that wrongness they have no idea the source of the problem. Or, they may know the problem, but feel like they have no options to change it. I did not know I was trans until that night in 2017 when I finally learned that it was possible.

And even still, just because a person’s brain manifests with these traits, that does not mean that their internal gender aligns, because there’s still so many other factors to the brain that medical science has no understanding of. That’s what science is, though! We observe the world, we make predictions, and we test those predictions until the results prove we’re wrong. Science changes, and 200 years ago the idea of someone taking hormones to change their sexual characteristics was so foreign that it wasn’t even fantasy!

Anatomy of a Shapeshifter

Chart of my hormone levels over the last two years.It has been 656 days since I started hormone therapy. Now, it took a month for my levels to normalize, so for practical purposes that means that I have been hormonally female for about 620 days. My testosterone fell to female normal levels almost immediately, and my estradiol has been over 100 pg/mL since my first blood test (cis female normal range is 30-400, varying across the menstrual cycle). In September of 2017 I was practically at pregnancy levels after switching to a much too strong dose of Estradiol Valerate. This has had significant effect on my secondary sexual characteristics.

Some of these are extremely obvious: breast development, fat migrations from my waist and upper torso into my hips and butt, hair and skin changes. My hands and feet have shrank. My pelvis has rotated, making me shorter by an inch and a half and increasing the curve of my back. I’ve lost so much strength, and can barely lift the packs of water bottles that we buy at the store (I used to heft two at a time). Jars have become my nemesis.

Changes that aren’t obvious: My prostate has shrank from the size of a golf ball down to roughly an almond, equivalent to its AFAB analog, the skene’s gland. This has eliminated my prostate cancer risk. I have erogenous zones now, and my libido & sexual attraction has changed significantly, manifesting now as a tightness in my abdomen and an ache in my chest, where previously it was entirely centered in the genitals. My sense of smell has dramatically improved, not always for the better (male body odor is so disgusting, and it carries way too far). I feel temperature extremes much stronger, alternating from feeling cold all the time to sweltering in my own flesh. I sweat differently now. Even my bone structure will change over time.

Medically, my body is identical to a post-hysterectomy cis woman, save for the construction of a few bits of soft tissue, and even those have feminized significantly since my orchiectomy. When I have blood work done, I am tested as a woman. For anything not having to do with my reproductive organs, I am treated just like any other woman.

The reality is that humans are not sexually dimorphic. The vast majority of traits that we associate as male or female are entirely flexible and dynamic. Biology is the physiology, behavior, and other qualities of a particular organism, and all aspects of this organism now align female.

I am biologically a woman, and that’s just science.