>>12613567My undergrad was under the department of medicine at a university, two years of general medical science, then I specialized in neuroscience for the rest of the undergrad. Then I did a masters in neuroscience.
From my perspective, everything about how a person thinks and feels has to do with some part of the brain. Damage to some area means the inability to form new memories, damage to another means loss of vision, another leads to aphasia, another leads to paralysis, and so on. Mental illnesses like schizophrenia, depression, adhd, ocd, bipolar disorder (and so on) have unique brain abnormalities.
Similarly, if you look at heterosexual males and females, you will find distinct differences between the brains (on average), specifically the sexually dimorphic nucleus. Similarly, homosexual men and homosexual women, if you look at their brains (specifically the sexually dimorphic nucleus), if i recall correctly, they differ from their heterosexual counterparts. Homosex men have similarities to het females, and homosex females have similarities to het men.
What causes these brain regions to develop differently in homosexuality and mental illnesses? I personally don't remember specifics beyond genetic risk factors and environmental risk factors which result in detectable brain differences.
I believe that transgender brains are similar to the differences between heterosexual brains and homosexual brains. Except instead of only part of the homosexual male brain being feminine, the entire brain is feminine in males who identify as female. Similarly, instead of only part of the homosexual females brain being masculine, the entire brain is masculine in females who identify as males.
What causes this abnormality? I suspect it is similar to androgen insensitivity, where males dont respond to testosterone, and so develop as females. An example of a male with androgen insensitivity is in this video, not memeing:
https://www.youtube.com/watch?v=f4mzA-8WV-s 
