Yes, science has discovered that MOST Male and female brains have specific differences. However, SOME do not! There are males who have 'female' brains, yet are perfectly normal males. And women who have 'male' brains who are normal females. So much for that theory. So, there's more going on there. We still don't know exactly what.
The most interesting thing that I see on these forums, is the wide variety of people who are transsexual, and the wide variety of their past lives experiences. Some are born, then know that something's wrong as soon as they become self aware. For others, it takes years, or decades. Why? Maybe there is something genetic there, but it doesn't always get triggered. Perhaps an infectious disease makes some gene become active. Or makes one gene act on another to cause it. Or even a life experience, because as we now know, our experiences make changes in our brains, as new thoughts and memories do make physical changes in our brains at the molecular level. So basically, for transsexuals, there are lots of different ways to get there just as there are lots of different ways to become a crossdresser. It's like, how do you get from New York to LA? There are lots of different ways. Some people will have some things in common, like some will all take cars, planes or trains. Others will all travel an night. And others will all stop over in Chicago. But we all get there with an ever so slight different combination of ways. Same with where we wind up identifying our gender, at whatever age. It's a combination of everything we started with, along with everything we've experienced, psychologically and physically, along the way.
The most difficult thing is to be able to examine yourself honestly, without letting wishful thinking get in the way. All too often I wind up reading about someone's transgender tale, and the thing that sticks out the most is that they want so much to be female....just because. Without much self introspection at all. Just that they really believe that they feel like a female. When, exactly, how could they know that? They've never been a female. What I hear most often is stereotypical beliefs of what men think females feel. Which, if you ask any real woman, is usually very wrong. So that brings up the question; Why do they think they're feeling like females? The answer is, we really don't know. Usually, anyway. I'm one of the fortunate ones who managed to figure out where my gender dysphoria came from.