What is the biology of trans?

“Does the SRY gene prove trans-women and biological women are the same thing, and that a male can really be born in a female body and be, genetically, neurologically and epigenetically, a woman, with a brain wired as a biological woman’s brain?”

I was asked this question on Quora, and thought I’d share the response here as well.

Human beings have lots of foibles, many of them so neurobiologically engrained and predictable that they make up part of the definition of what it is to be human. A common, human mistake is trying to find one, single fact and pin a whole bunch of observations from the world on it.

So, human beings (like all sexually reproducing creatures) have primary and secondary sex characteristics.

• Primary characteristics are things like testes, ovaries, uterus, Etc.
• Secondary characteristics are things like breasts, beards, bone density, pelvic structure, differences in neurological development, Etc.

The SRY gene is predominantly only involved in creating the primary sexual characteristics. There are many other genes involved in this as well, but the SRY gene is the one that triggers the differentiation of cells (Müllerian duct and epithelial layers of the sex cords) into things like becoming testes instead of ovaries while in the womb.

It’s important to remember that — like every gene in your body — assuming a perfectly normally functioning body and a perfectly normal gene, the SRY gene is expressed in a bell curve between strongly and weakly expressed. There’s literally hundreds of factors involved from the time a gene expression is triggered, to the end protein result…meaning some people express gene “X” strongly and consistently, while others weakly and inconsistently, with the overwhelming majority of people being somewhere in the middle.

Remember this is true of every biochemical reaction in your body (not just genes) because that will be important in a minute. The takeaway here is, nothing (NOTHING) in nature is binary. Everything (EVERYTHING) in nature works on a bell curve (a spectrum.)

Secondary sex characteristics are developed at puberty, and are driven (predominantly) by the hormones created by the primary sex organs. The SRY gene is kind of out of the picture at this point.

Hormones from many organs, most strongly the gonads (testes/ovaries,) begin driving physiological changes in the body — leading to differentiation including in things like brain structure development, breasts, beards, Etc.

Again, like every biochemical function in your body, this is not an on/off switch, but a bell curve of function ranging from very strong to very weak. People with weaker hormonal responses will develop less characteristic, and less strongly identifiable changes to the physiology. People with stronger functioning, more characteristic development. Etc.

So, we could end here with the discussion of why there is no “man brain” and “women brain” — but instead brain development that falls somewhere on a spectrum between “Masculine” and “Feminine” with the overwhelming majority of the population falling in the range that would be considered typically male or typically female — but with a very significant portion of the population (about 5-20%) falling in a range that would be considered atypical (trans or otherwise LGBTQ+.)

This is true of every physiological sex characteristic.

But wait! There’s more!

There’s also no “nature versus nurture.”

Everything (again, EVERYTHING) in physiological development is a combination of both. Let’s look at neurological development. Neurological development is controlled by many factors including genetic pre-disposition, nutrition during brain development, stimulation during infancy and early childhood, Etc.* These are inarguably axioms of neurobiology and cognitive neuroscience.

What I’m saying is that brain development is affected by the childhood environment as much as by genetics. So, when a male child’s brain develops with more of the functioning we commonly associate with a traditional natal female’s structures — is this in any lesser way “having been born with it?”

So, now, we have to discuss what makes a “man” versus a “woman.”

• Is it a cumulative assessment of the secondary and primary sex characteristics (as shown in the bell curve example above?)
• Is it a single primary sex characteristic (having a penis, having a vagina) with no other considerations?
• Is it the genetic compliment? (XX versus XY)?

POP QUIZ! Is this a picture of a woman or a man?

This person has not been chemically or surgically altered in any way.

This person has a (naturally born) vagina, breasts that developed naturally at puberty, the female pelvic structure, the additional fatty deposits around the sex organs, female bone density, female musculature, Etc..

Take your time…

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SURPRISE! This person has an XY chromosome paring, with something called androgen insensitivity syndrome.

So are they male or female?

I guess it all depends on what physiological characteristics by which you define “male” and “female.”

But we’re not done yet!

Because now enters the difference between “physiological sex” and “gender.”

So, gender is a cultural construct — heavily based on assumptions by the culture of what behaviors and psychology are associated with physical sex characteristics.

I.e.

• People with breasts and vaginas tend to be nurturing. Therefore nurturing is a female trait. Females wear dresses.
• People with beards and penises like to kill things. Thus hunting/warring is a masculine trait. Males like to wear pants.

Now, we can debate how much of these cultural assumptions are based on biology (the “female brain”) and how much is enculturation (women taught to be nurturing.) But it doesn’t really matter for this particular discussion.

These assumptions are based on observation within a single culture that the majority of people with vaginas tends to have <behavior X> so that’s what the “female gender” is. But, as we have already discussed all of these associated behaviors fall on a bell curve, ranging from “person with <genitals> strongly showing this behavior” to “person with <genitals> not showing this behavior in any way.”

And, of course, different cultures have VASTLY different ideas of what biological and behavioral characteristics define “feminine” versus “masculine.”

So, now we have a differentiation between gender and biological sex.

So, to summarize — it should be obvious by now that attempting to pigeonhole anything about physiological sex, gender, or even what defines “male” versus “female” into simple, single-point-of-definition, clear-cut lines simply can’t be done. In fact, one should assume anyone attempting to do so is either deeply unstudied, or incredibly disingenuous.

What defines male versus female (in isolation) is an incredibly complex landscape of genetics, biochemistry, and developmental environment. Differentiating physiological sex from gender is a completely different, and incredibly complex, sociological/psychological/anthropological discussion.

* Before people jump on the “See! Trans people are damaged!” bandwagon — that’s not even remotely what’s being said here.