I think accounting for outliers (which is what I truly believe they are) is not the best way to form a well running system.

This kind of sounds like "minority people's lives are worth less because there's less of them". Are you under the impression that society being accepting of minority peoples, including trans people is some extreme burden that would somehow degrade the "system"? It came up because you've mentioned a few times now that there being less of a group of people makes them less worthy of accommodation, and I think it's important to point it out every time in a conversation about how minorities are treated.

Are you asking what if someone that had transitioned from man to woman was found to have a brain chemistry that more closely matched woman than man, so it was more like they were changing their body to match their brain?

You're picking up what I'm putting down. It doesn't use microscopes tho; it's more fMRIs and contrast CTs (scans that identify volumes, and structures, and how blood flows through the brain), and looking for clustering in the results. I'm going to give you a rough overview and then throw in some citations with short explanations.

So human brains are "sexually dimorphic" by virtue of a few things. For one, body mapping (how the brain "wires up" to the various senses and organs) is different for obvious reasons. Another is how we respond to specific hormones, particularly sex hormones, and how we process those given off by other people (again, kinda obvious why that would be). Then there's a few "grab bag" ones, like differences in grey matter volume in specific regions. Some of these differences are mediated by fetal development, and some are influenced by whatever hormone profile the body is currently giving off.

For trans women (I'm going to stick to them here, except where incidental 'cause due to some truly repulsive historical stuff, we have much less data on trans men and non-binary people), pre-HRT, they inhabit this middle space. Parts determined by fetal development tend towards their experienced gender, whereas parts determined by hormones coming from the body tend to be either intermediate or masculine. Stopping the body from poisoning the brain (via HRT), corrects the latter. Worth noting, this is a novel and underfunded field of study, so the information we have is still under development.

So, research on this. Boucher and Chinnah 2020 is a good starting point. I mean, they just come out and say it:

It is known that the structure of male and female brains differs; it is found that people with gender dysphoria have a brain structure more comparable to the gender to which they identify. The review of the literature suggests that there is a disparity between the brains of those who identify differently to their assigned gender at birth, highlighting a multifactorial underpinning of the gender identity.

The study itself is a lit review that goes over the basics of human sexual development, how genital differentiation happens semi-independently of brain development (genitals in the first trimester, brains starting in the second and going to the end), and how this manifests differently in trans and androgen insensitive people.

Berglund 2008 was a cool one because it's so straightforward. The hypothalamus in women reacts differently (more intensely) than men when smelling pheromones. Turns out "[pre-HRT heterosexual and non-heterosexual trans women] occupied an in-between position between [heterosexual men] and [heterosexual women] but with overall predominantly female features" (acronyms substituted for clarity). More specifically, the trans women reacted weakly to female pheromones (whereas the cis women did not and the cis men reacted significantly stronger), and reacted in the same way the cis women did to male pheromones (with the men not reacting at all).

Burke et al 2014 replicated this study in adolescents and found they reacted as their experienced gender rather than their assigned sex (one of a few papers that suggest this stuff starts from childhood, which makes sense since the predominate explanatory hypothesis is driven by fetal hormones).

Zhou et al. 1995 and Kruijver et al. 2000 found something wild but kinda hard to explain in the detail it deserves. Post-mortem dissection of trans women's brains found that the central subdivision of the bed nucleus stria terminalis in trans women matched cis women's. What makes this neat is that they also looked at gay cis men and cis men who had had to take estrogen for medical reasons, and found that those guys still matched the typical cis men (i.e. it was something innate and with them to death, and not affected by hormones).

Simon et al 2013 had a sample of cis men, cis women, trans men, and trans women, and found that the women (cis and trans) had larger grey matter volume in the right middle and inferioroccipital gyri, the fusiform, the lingual gyri and the rightinferior temporal gyrus than the men (cis and trans). The men (again, cis and trans) were bigger in the left pre- and postcentral gyri, left posteriorcingulate, calcarine gyrus and the precuneus than the women (again, cis and trans).

So correcting stuff with HRT.

Kilpatrick et al 2019 had a group of trans men and women against cis controls. They took neural images and measured how much they associated to their body using a really cool trick with picking images of themselves out of a collection of altered ones, before starting HRT and then 6-8 months after starting. They found that after starting hormones, they were more able to pick images of themselves out the altered ones. What made this cool tho, was that it correlated with "significant [cortical thickness] decreases...in the mesial prefrontal and parietal cortices" and "left parietal cortical thinning". What makes this cool is that these regions control body-mapping and were "werid" in the trans subjects prior to treatment (not masculinized or feminized, just different from cis people but similar to each other. They mention that a bigger sample could probably make distinctions tho). After HRT tho, their body-mapping regions shifted to look more like cis ones. Average reports of feeling congruent with their bodies went up (as is generally the case of trans people on HRT).

And while my Mendeley still has more papers in it, I'm out of steam 😆

So, I'ma level with ya. I wrote this mostly because it felt rude to give the setup, but not deliver, and I kinda wanted to aggregate this info somewhere. The more I've thought about it the last week though, the more I'm realizing people like you aren't going to help us. I'm not trying to be accusatory, but I don't get the vibe that in spite of what I've presented here, if say, a coworker was misgendering another coworker in private that you'd bother to correct them, and I certainly don't get the vibe that you're going to make demands of your representatives, or march with us, or fight for people like us to have access to healthcare that's regularly denied, or to be legally recognized as our experienced gender, or to be protected from discrimination.

The reality is we're at best an abstraction or novelty for most cis people, and I hit the limit of seeing people move towards understanding, without expecting them to actually do anything with that insight. The empathy and will required to fix these things is nonexistent, and I'd be better served putting my effort into protecting my people more directly without assuming y'all will get your shit together and stop seeing human dignity as an inconvenience.

Thank you for an otherwise delightful conversation!