Transgender activism is based on the premise (or claim) that a person who feels they have been "born into the wrong body" has a right to a sex change. Through cosmetics, grooming, clothing and an official name change, such a person can adopt the opposite sex's gender role.
The link between seeking a sex change and sexual orientation is hotly disputed in the political arena of Western countries, where sex changes have had the most attention and debate. The medical field has established the mental disorder of Gender identity disorder in order to establish a diagnosis for treatment of people seeking to get a sex change. Many people use this medical diagnosis, in order to justify sex change therapies, but the treatment of a psychiatric condition by giving in to a patient, as well as treating them with physical surgery are the most controversial issues.
The medical and to some extent legal consensus is that transsexuality is the result of a form of Intersex. Autopsies carried out in the Netherlands by Professor Gooren et al showed that female transsexuals - those with masculinised bodies - had brains feminised in certain areas. Gay men did not.  In 2007, work  by the University of Essen in conjunction with the Klinik für Psychosomatik showed differences in neurological processing via Functional MRI scans.
"Radiologists can now confirm what transsexuals report - that they feel “trapped in the wrong body” - on the basis of the activation of the brain when presented with erotic stimuli. There is obviously a biological correlation with the subjective feelings."
In 2003, in the Re Kevin series of court decisions, as the result of a thorough analysis of the scientific evidence at the time, the Full Bench of the Family Court of Australia found as follows 
At paragraph : ‘The traditional analysis that they are "psychologically" transsexual does not explain how this state came about. For example, there seems to be no suggestion in the evidence that their psychological state can be explained by reference to circumstances of their upbringing. In that sense, the brain sex theory does not seem to be competing with other explanations, but rather is providing a possible explanation of what is otherwise inexplicable’.
At paragraph : ‘In other words (as I understand it) the brain of an individual may in some sense be male, for example, though the rest of the person’s body is female’.
At paragraph : ‘In my view the argument in favour of the “brain sex" view is also based on evidence about the development and experience of transsexuals and others with atypical sex-related characteristics. There is a vast literature on this, some of which is in evidence, and I can do no more than mention briefly some of the main points’.
At paragraph : ‘It seems quite wrong to think of these people as merely wishing or preferring to be of the opposite sex, or having the opinion that they are’.
At paragraph : ‘But I am satisfied that the evidence now is inconsistent with the distinction formerly drawn between biological factors, meaning genitals, chromosomes and gonads, and merely "psychological factors", and on this basis distinguishing between cases of inter-sex (incongruities among biological factors) and transsexualism (incongruities between biology and psychology)’.
At paragraph : ‘In my view the evidence demonstrates (at least on the balance of probabilities) that the characteristics of transsexuals are as much “biological” as those of people thought of as inter-sex’.
Non Mainstream Theories of Causation
Fred Ormland proposed in 1973 that transsexuality was the result of Freudian "Gender Envy". Unlike Homosexuality, it could not be cured.
Radical Lesbian Feminist Academic Janice Raymond ascribed Transsexuality to a Patriarchal Fifth Column "infiltrating womens’ space" and “raping womens’ bodies” in her book The Transsexual Empire - Making of the She Male. Her report commissioned by the Carter Administration on the topic of federal aid for transsexual people seeking rehabilitation and health services had the result of effectively eliminating federal and some states aid for indigent and imprisoned transsexuals. It had a further impact on private health insurance which followed the federal government’s lead in disallowing services to transsexual patients for any treatment remotely related to being transsexual, including breast cancer or genital cancer, as that was deemed to be a consequence of treatment for transsexuality.
J. Michael Bailey, a psychology professor and sex researcher at Northwestern University, published a book called The Man Who Would Be Queen which sharply disagrees with commonly proposed transsexual models. Dr. Bailey argued that sex changes are motivated primarily by sexual interests and not by the idea of a gender identity of one sex in the body of the other.
Dr. Alice D. Dreger wrote:
- In his work on transsexualism, Blanchard argued that there are actually two types of male-to-female transsexuals, with autogynephiles being one type and “homosexual transsexuals” being the other. ... he argued that male-to-female homosexual transsexuals who opt to undergo sex reassignment do so, in part, because being a woman makes more sense than trying to live as a very effeminate man attracted to heterosexual men.
While critical of the common transsexual model, Bailey states in his book that he believes sex changes should be performed. Homosexual transsexuals being extremely effeminate to begin with come into constant conflict against society's expectations, although after taking up the gender roles of a woman, they are then able to express themselves more naturally, and typically integrate with society better afterwards. Autogynephiles though, do not integrate better with society after changing to a woman, however the empirical evidence shows a typical positive change in the person's mental health, despite their regressing from general society.
Dr Bailey's theories do not account for Female to Male Transsexuals, who are between 25% and 50% of the total.
Dr Anne Lawrence, a strong proponent of Dr Bailey's "Autogynaphilia Theory" and post-operative transsexual herself in a survey  of Sexuality before and after Male-to-Female Sex Reassignment Surgery showed that before surgery, only 1 in 10 are attracted to men. Before surgery, 7 in 10 have sexual relations exclusively or nearly so with other women, though only just over half are attracted to other women exclusively. After surgery 4 in 10 end up in heterosexual relations, though only 1 in 3 are exclusively attracted to men. 1 in 4 continue to be Lesbian, often with their previous partner.