It details my thoughts as they've crystalised on the etyology of Transsexuality, what it is, what causes it, and is a decent summary of what we know so far.
I can't give you "the" picture of HBS. I have some significant differences from may others on the issue.
For example, I believe that the gender binary is merely a very good approximation. Just like it's best to treat the Earth as being flat, locally. It's not, but it works best when walking to the corner shop if you treat the journey as if it is.
I believe that gender is in the brain, and so is sex, and sexual orientation. But again, though it's really useful to make categories of "male", "female", "straight", "bi" etc, and while those definitions may be exact, not just approximate, descriptions of most people, for a large proportion they are only approximations, and for a small minority, bad approximations at that. So bad they're not just useless, but counter-productive.
I believe that it's most likely that "typically female" or "typically male" patterns of neural development, stereotypes to which individuals conform to a large degree (usually) happen as the result of the foetal hormonal environment, as modified by genetics. Atypical development may happen, and we're talking only about the lymbic nucleus here, the brainstem, hypothalamus etc , areas whose development is mostly or completely set before birth.
These lymbic patterns affect later development of the cerebral cortex (in ways we don't understand - insert much handwaving here, though not elsewhere), in particular lateralisation and connections in the corpus callosum. In addition, the typically female or typically male instincts and emotional responses dictated by the pre-natally hard-wired lymbic system later on lead to the child comparing themselves with peers and adults, and consequently developing a male or female "gender identity" based on who they most resemble.
Just to complicate things... the brain is such a complex organ, that in this case it's useful to see it as an assembly of different organs, much as the reproductive system isn't a single organ, but an assembly, where different parts can conform to different degrees to either of the standard stereotypes. And just as the reproductive system changes morphology, and is to some degree "plastic" and subject to environmentally caused change (notably at puberty), so are the higher brain structures. Yet no new structures develop, the basic pattern is set before birth.
So an individual, whether cissexual, transsexual, or intersexed, will usually have a brain that conforms strongly to a masculine stereotype in some areas, and weakly to a female stereotype in others, or the reverse. The areas will differ between individuals though, and what evidence we have from cases of 5ARD and 17BHDD is that perhaps a third of people can function in either gender role. I wish we had more to confirm this. For that matter, I wish we understood what gives rise to left-handedness or ambidexterity, areas of brain development post-birth that are equally as radical (neurologically speaking) as many of the bimodal sex differences.
In practical terms, one of the most important areas is that of body image. Most will have a strong body image conforming to one stereotype or another, but some will not. This is observable in Intersexed people, many of whom "want to be normal", but some of whom object strongly to the concept of having their genitals surgically altered, even though they may be atypical. In the worst case, an Intersexed person who "wants to be normal" has had their genitals surgically changed to make them as abnormal as they can be, to be those of the opposite sex to the stereotype given by their "body image".
Where I radically differ from the standard HBS model is that the evidence to me shows that while there is a strong correlation between stereotypes-conformity in all parts of the brain, it's not exact. As just one example, it's not just possible, it's inevitable that there will be some people with strong male identities, whose brains in general conform to a typically male stereotype, but whose body image is that of a female to a greater or lesser degree. Some men feel most comfortable having feminised genitalia, and some women feel most comfortable having masculinised genitalia. And many more are just afraid of surgery, and want to live with whatever they're lumbered with, even if it's not optimal. All-consuming desire for typical genitalia is *not* a reliable touchstone for determining gender, any more than chromosomes are a reliable touchstone for determining sex. They're just really, really good approximations.
The standard HBS model, as I interpret it, as over-simplistic. There are male brains, and female brains, period. If you have a male brain, you *NEED* (not just desire) male genitalia, and if you have a female brain, you *NEED* (not just desire) female genitalia, and that's all there is to it. Everything else is "psychological" and any ambiguity or anomaly a "mental illness". I don't agree with this, as it results in trying to coerce observations into fitting what is almost a religious theory rather than a scientific one. It's basically correct, but only as correct as saying the Earth is flat. The further you go from the local area, the less useful such a view becomes. Go far enough away, and it breaks down completely.
One more thing, which may colour my interpretations of the data. This is the Australian HBS group, and in Australia, it is the medical and legal position that what is termed "transsexuality" is a form of neurological intersex, where (parts or a part of) the brain is cross-gendered compared to the majority of the rest of the body.