So, in another thread, I ask the question, "Are we born this way?"
A lot of the responders said, "Yes".
I've read this really amazing hypothesis about how we could be born this way. And it's not necessarily in our genes.
The hypothesis states that it is gestational. During fetal development in the womb, the fetus goes through multiple stages of development. And, the fetal development doesn't take place in one fell swoop; it is staged over the 39 months of gestation.
Let me discuss MtF, and leave it to the reader to develop FtM. The genitals are largely complete by the beginning (14 to 17 weeks) of the second trimester. At this time, the brain is largely an undifferentiated blob. The brain is largely complete early (27 through 31 weeks) in the third trimester. With a normal cis boy, the genitals and the brain are both male. However, in a trans male, the genitals are formed as male by 17 weeks. However, somewhere late in the second trimester, something happens, and the fetus becomes doused in Estrogen. I like to refer to it as "Estrogen poisoning", but feel free to call it what you want. The Estrogen causes the brain to develop as female, leading to male genitals and female brain. If you recall my explanation of software in my previous post, these changes in the brain are in hardware. They are permanent.
There have been some studies that asserts that the basic organization of the male brain and the female brain are different.
Interestingly, there is actually a visible physical manifestation. Normally, the middle finger is longer than the ring finger. However, the hormone switch causes the ring finger to be as long as or longer than the middle finger. It has been postulated that finger length is in proportion to the Estrogen Poisoning.
For those of you who have significant gender dysphoria or have always felt like a girl, this may be the physiological process that made you that way.