Well, there are some studies that show that (at least some) brain structures of trans women are more similar to those of cis women than to those of cis men (and vice versa with trans men), though most of them have tiny sample sizes and often inconclusive results. There are also studies pointing to genetic causes (including twin studies). If you're interested in this stuff, there's this wikipedia article: Causes of transsexuality

Personally, I don't care about any of that. Sure, it's kinda interesting to read about as a biologist, but as a trans person, I don't need studies to "prove" I exist. I know how I feel gender-wise, I know how I like to express my gender, I don't care about gender roles. I know it makes me happy if I hear my new name, or get post addressed to that new name. I know it makes me uncomfortable if online forms make me pick between male and female, stuff like that. Also, I'm weary that any research that "proves" an explicit cause for being trans would lead to eugenics (transphobic parents aborting fetuses if you can test for it before birth).

It might be useful for you to understand that gender (identity) ≠ gender expression ≠ gender roles. Some people do use these interchangeably, but at least for discussion trans issues, differentiating between them is useful:
Gender (identity): you personal internal sense of gender, can be more or less strong (leading to some people to identify as 'cis by default' if they don't feel particularly like their assigned gender but don't care enough to identify as something else)
Gender expression: how you like to express you gender, includes clothes, makeup etc. Can in a wider sense also include stuff like your name, pronouns, terms of address, etc
Gender roles: how you are expected to act because of your gender. Varies widely by culture.
They are of course all interconnected, especially gender expression and gender roles.

So you can have for example a woman (= gender), who wears dresses, goes by she/her pronouns and gets called Mom by her kids (=gender expression), and who is a stay-at-home mom who likes to sew and knit, which is expected of her by society (=gender roles). Maybe she's cis, maybe she's trans, you can't tell from the example.

Or you can have another woman, who also wears dresses, goes by she/her pronouns but gets called Dad by her kids, and likes so play soccer with them (which can be a bit difficult in a dress ). Maybe she's trans and her kids are the only ones who are allowed to use masculine words for her, maybe she's cis and the "Dad" thing is a family joke because people always expect dads to play soccer with their kids. You can't tell from the example.

Or you can have yet another woman, who wears suits, goes by he/him pronouns, gets called Husband by his wife, and likes to go on motor bike rides. Maybe he's cis and a butch lesbian, or maybe he's a closeted trans woman. You can't tell from the example.


Did any of that help? (Or did it just lead to more questions?)