There is enough evidence for it to not be obviously false that, at the least, there's a significant structural difference between cis people's brains and transsexual people's brains, and some of those differences may fall along sexual differentiation lines. More research must be done!

In any case, it's actually more-or-less irrelevant in arguing whether a trans person is actually the gender they identify as, and is mostly a curiosity until it leads to more focused treatment. The bit that does matter is the last question - am I who I am in my head, or am I who you think I am?

I'm sorry, your point is that you think, so therefore reality is so? You can't think yourself into a different sex, I understand that they do see themselves as male or female despite their physical characteristics, but that has no bearing on much whether we like it or not. And to answer your question, you are your configuration of you, whether anyone perceives it or not, including yourself. I guess it's turning a bit philosophical cause I'm sure you're going to come back and say the world is only what is perceived of it, but that's not what I believe.

> You can't think yourself into a different sex

No. You can think yourself into a different gender. Gender isn't sex - this has been written about since 1963, and is generally agreed on at this point - gender is a combination of personal and cultural mental attributes, while sex is generally "what your body physically is". This messes up slightly when you get to certain sex-driven gender attributes, but, in general, it's a reasonably clear line.

Sex is mostly irrelevant unless you're intending to sleep with someone or you're a doctor, and even then what you're asking about is generally a certain set of characteristics where a number of people fall between the lines. If you're a trans woman who's undergone GRS and you say "my sex is male", that's going to confuse an awful lot of people.

Edit: My specific point, in this response, is that identifying as a gender which does not match your sex is not "playing pretend" or less important somehow, as you insinuated in your original post.

Externally genitalia aren't the only physical characteristic, and if you judge by other physical characteristics that are testable now, though less obvious to a casual observer, transgender individuals are in many ways similar to their experiences gender more than their socially assigned gender. The biological differences between make and female aren't binary, treating them as if they are is an extreme simplification and treating external genitalia as the bright-line divider is increasingly evidently nonsensical given out knowledge of human biology and it's relation to psychology and behavior.

>There is enough evidence for it to not be obviously false that, at the least, there's a significant structural difference between cis people's brains and transsexual people's brains, and some of those differences may fall along sexual differentiation lines

Source?

Nice article. Unfortunately articles are not data. It mentions a 2013 study but yet again with a peer group of 32 that is much too small to tell you anything, and gives no further information. That 2013 I can't seem to find, but the p value if the article is correct on sample size would also be quite low, though slightly higher than the previously mentioned studies.

If the difference is so clear in an MRI why not run a simple blinded study where researchers look at brain scans and tell you who is transgender and who is not? Such a study would not only prove this theory outright, but would likely make a researcher's career.