I think you can guess the contents from the title. Actually, it does ask a very good question - about double-blind experiments to measure the effectiveness of treatment for GID - Gender Identity Disorder. There really hasn't been as much systematic work historically as we'd like. But enough, as I've said in my reply (yet to make it through moderation)
“In over 80 qualitatively different- Pfäfflin, Friedemann & Astrid Junge -Sex Reassignment. Thirty Years of International Follow-up Studies After Sex Reassignment Surgery: A Comprehensive Review, 1961-1991
case studies and reviews from 12 countries, it has been demonstrated during the last 30 years that the treatment that includes the whole process of gender reassignment is effective.”
We know what happens without treatment too."GID, if left untreated, can result in clinically significant psychological distress, dysfunction, debilitating depression and, for some people without access to appropriate medical care and treatment, suicidality and death."- Diagnostic and Statistical Manual of Mental Disorders (4th ed.. Text revision) (2000) (“DSM-IV-TR”), pp 678-679
The reason why there have been no double-blind experiments is for the same reason there have been none on other surgical procedures, such as appendectomy: the treatment works, and if not given, we know patients die or become very ill. We also know from other attempts at treatment that no other therapy that has ever been tried has been effective.
Finally, from the World Professional Association Standards of Care, the "best practice" in treating GID:"Sex Reassignment is Effective and Medically Indicated in Severe GID. In persons diagnosed with transsexualism or profound GID, sex reassignment surgery, along with hormone therapy and real-life experience, is a treatment that has proven to be effective. Such a therapeutic regimen, when prescribed or recommended by qualified practitioners, is medically indicated and medically necessary. Sex reassignment is not "experimental," "investigational," "elective," "cosmetic," or optional in any meaningful sense. It constitutes very effective and appropriate treatment for transsexualism or profound GID. "
Not elective. Not cosmetic. And not optional.
So much for the question about the effectiveness of treatment.
Now let's get on to the biological basis. We knew that this treatment was the only effective one, but until recently, we had no theoretical basis for it, only the practical one that it had been observed to work when nothing else did.
There's a thing called "Intersex". It means being born with a body neither entirely male nor entirely female. Now TECHNICALLY it affects 1 person in 60. But only in maybe 1 in 1000 is it obvious.
It has been stated categorically by people with limited knowledge of the subject that XX chromosomes mean female, and XY chromosomes mean male.
While that is true for most people, it's not true for all.
CAH syndrome and De La Chapelle syndrome males have the 46XX chromosomes normally found only in women.
CAI syndrome and Swyer syndrome females have the 46XY chromosomes normally only found in men.
Most people with Kleinfelter Syndrome, 47XXY chromosomes, look male. But a few look female, and some have become pregnant.
Then there's those who get a "natural sex change" due to either 5ARD syndrome or 17BHDD syndrome. This can either cure or induce Transsexuality.
Transsexuality means "male brain in mostly female body" or the reverse. We can detect it from Autopsies and MRI scans, there's hundreds of medical papers on the subject now.
See the forthcoming American Psychiatric Assocn seminar:
S10. The Neurobiological Evidence for Transgenderism
1. Brain Gender Identity Sidney W. Ecker, M.D.
2. Transsexuality as an Intersex Condition Milton Diamond, Ph.D.
What makes us men or women is not between our legs, it's between our ears. A matter of biology. A Transsexual woman was always a girl, she had feminine instincts, emotions, thought processes, as other girls do. And was born that way. Her body had to be masculinised, either mostly or completely, or she'd be considered Intersexed instead if only partially.
This is a hideous condition, which, unless the body's endocrine system is modified with hormones, causes progressive neurological dysfunction. Surgery is also required in cases where the body dysmorphia caused by a cross-gendered neurology is extreme. It has the additional benefit of making the patient sexually functional - a benefit similar to restorative surgery after genital injury or amputation. I think few men would see surgical re-attachment with full functionality as "cosmetic" and "optional" should they have a severe accident.