A pre-op transwoman told me that as a young boy she felt like she was living in a prison, and hated it.

Gender is a spectrum. Detailed information is provided above.

I think there are three things involved:

1. Our bodies
2. Our brains (technically, this is part of the body, but it seems to do its own thing)
3. Our minds

I believe that for most trans people, 2 and/or 3 don't match number 1.

For 2 vs. 1, the primary cause of this in trans women (born assigned male) seems to be a case of incomplete SRY activation in males. In other words, biologically incomplete sexual differentiation. This typically happens in utero. So there are outward male physical characteristics, but the brain never gets the message.

For trans men (born assigned female), the process is less well-understood, but seems to be related to atypical CYP allele distribution on chromsome 10.

That is to say, gender dysphoria is a real, physical thing with real, physical causes.

Where 3 vs. 1 is involved, the situation is much more complicated. Everyone has mixed masculine and feminine traits, and these can be reinforced or repressed by our environmental experiences.

In rare cases, individuals can be born where the sexual differentiation break-down happens on a more physical level-an XY individual that has the morphology (a vagina, and breasts) of a female, or an XX individual that has a penis (usually, but not always, under-developed). Sometimes these individuals are totally unaware of their condition until they seek treatment for infertility (which is always the case), or are otherwise genetically tested.

To further complicate matters-whether one is sexually attracted to males, or females, or both, or neither, is independent of one's gender identification. That is, transgenderism is not merely a matter of sexuality, but involves psychology and physiology.

So, the correct answer to your question is:

"All of the above", and it varies from individual to individual.

A common mistake of humanity (and this happens with more than our self-assessments, for example, we make this mistake in classifying other species, as well) is to assume that what things LOOK like, is a natural expression of what they intrinsically ARE. The insides don't always match the outsides. So, for example, two black people can have a white child. This happens. Nature is far more subtle than our simple stick-figure drawings.

It is physical, mental and all of the above.

There are other cultures that recognize more than one gender. The colonialists imposed their rigid view of gender identity & sexual deviancy upon their conquered peoples. Hawaii, India, , Native Americans all had a culture of acceptance of more than one gender, of sexual identity & orientation. One does not have to be LBGTQ to be a member of the group. You are welcome to learn there. It is a far less restrictive group than others on this site. The many trans folks I have met are basically caught in the wrong body. It is not a matter of acting in any particular gender accepted manner. It is feeling as one gender while being trapped in a body that is alien to them. So at birth doctors have had a history of assigning gender when genitalia is deformed or of having the organs of both sexes. Or even a circumcision gone horribly wrong. That is becoming outmoded as science & society adjust to the new realities of gender identity.

I remember reading an article many years ago about how only having two genders was scientifically incorrect and that doctors do get it wrong on birth certificates as there are many combinations of the x and y genes. It would seem that there are more than two possible genders and that we never want to recognise that mostly for religious reasons All of the above that you have said can be true. I have friends who feel that they are men trapped in womens bodies and vice versa and for many reasons and I believe them all - I think having the lived experience of not feeling ok with the sexual pronouncement is something those of us who feel ok in our assigned genders can't even begin to imagine and I totally go along with their right to do, say, be, whatever they want.

I have heard that there is a difference between gender identity and gender expression

Not speaking from direct personal experience the trans friends I have are always very brave & committed to living their life as fully s possible to whatever ends necessary

With my last comment being said, i believe it is extremely important to care for everyone.. survival in number.. it is too easy to hate people.. to genuinely care and open up for someone? that shit takes guts.

I don't think it applies to me. Their mind operates different than mine.
Its all cultural/sociological

At this point we're just beginning to understand how our brains operate.
Being born with a dick or vagina isn't indicative of how the accompanying brain is balanced to function. One may be at either end, or any point in between.
And, due to chemical changes, can change at any point along the way.
My totally unscientific opinion.

Firstly, I am not transgender so I am only relating second hand. My daughter was a tom boy and then some, always wished she was a boy. She is gay and now glad she is female but always dresses as a male.

Talking to another friend this morning, he was born female, was gay until about 50, then had a sex change. Very effective, only a couple of people who know him know he was once female and he is only interested in women.
One of our sword fighters was male until a few years back and completed her re-assignment surgery last year and now for all intents and purposes is a hetero female.

Then it gets more grey. A friends brother was transvestite, then had hormone therapy boob job and all. Lives in most ways as female, but is happily married to a very understanding lady.
A young friend who was gay, then had a sex change, became totally female but now prefers women to men.
Based on the above I consider the transgender issue to be almost all encompassing. I don't think there is or can be one perspective on it

Transmen and transwomen are usually born assuming they're the opposite gender of their birth genders, but transmales usually don't fully realize it until puberty and their bodies "betray" them.

For transwomen, it's the first time they choose feminine toys or clothing, and are punished by adults.

It's not something they "want" to achieve, or learned from culture. I never "wished" I could be a boy..I just knew I was, and my family and friends assumed it also.

We all knew at some level I was born a girl, but my mind had been initialized with male hormones so that's how I acted, assuming my male superiority, and people reacted to me as though to a male.

That's an excellent question. As a mom of a transgender adolescent it much, much more then gender roles. It carries it's own unique qualities. I absolutely know my child does not, nor ever will associate with his gender at the time of birth.