The brain scans being performed on some transsexuals have been biased because their brain chemistry had already been altered through the ingestion of hormones and so we ended up where we started.
Most of the background noise on the internet involves discussing what the different typologies actually do and how they behave but very little touches on the actual lack of science on the subject.
One of the more prominent examples is Kay Brown's site "On the Science of Changing Sex" which gives an inordinate amount of time and space to describing the differences between early and late onset transsexuals and how they differ but offers nothing about what actually causes gender dysphoria in the first place.
We know virtually nothing about why someone feels compelled to transition. The rest is just based on behavioral differences mostly centered around sexual orientation and how that affects the decision making process. After all she follows Blanchard and his two typologies which are explicitly divided by just that. Continuing to beat a dead horse around this distinction smells more to me about establishing a hierarchy and predominance of one over the other rather than trying to understand motivation which is far more important.
Brown goes to great lengths to make a clear distinction between feminine trans kids and the more masculine and less easy to understand (at least for her) older transwoman who more often than not was previously married to a woman. But that is not the more important question at hand is it?
All this static and talk leading away from what should be the root question, and on that front, we are no further ahead than when Hirschfeld coined the original term.