The following is my attempt to summarize a good deal of what Thomas Szasz has been writing and saying for nearly half a century.
If neuroscientists discovered that mass murderers and people who claim to be Jesus had different brain chemistries from other people, most everyone would accept this as evidence that they suffered from a mental illness/brain disorder (MI/BD).
If neuroscientists discovered that homosexuals had different brain chemistries from heterosexuals, far fewer people would accept this as evidence that they suffered from a MI/BD.
If neuroscientists discovered that nuns had different brain chemistries from everyone else, very few people would accept this as evidence that they suffered from a MI/BD.
If neuroscientists discovered that married men had different brain chemistries from bachelors, no one would accept this as evidence that they suffered from a MI/BD.
Clearly, a difference in brain chemistry per se is not enough to make people believe that someone has a MI/BD. It takes more. Why, then, would a difference in one case be taken as evidence of MI/BD, while a difference in another case would not be? The obvious answer is that people, including psychiatrists, are willing to attribute behavior to mental illness/brain disorder to the extent that they disapprove of that behavior, and are unwilling to do so to the extent they approve of, or at least are willing to tolerate, that behavior. (Psychiatry once held that homosexuality was a mental illness. That position was changed, but not on the basis of scientific findings. Science had nothing to do with the initial position, either.)
In other words, the psychiatric worldview rests, not on science or medicine, as its practitioners would have us believe, but on ethics, politics, and religion. That would be objectionable only intellectually if that were as far as it went.
Unfortunately, it goes further, since the practitioners and the legal system they helped shape are empowered:
First, to involuntarily “hospitalize” and drug people “diagnosed” as mentally ill and thought possibly to be dangerous to themselves or others, and
Second, to excuse certain people of responsibility for their actions (for example, via the insanity defense).
P.S.: Everyone interested in liberty should read Szasz’s forthcoming book, Faith in Freedom: Libertarian Principles and Psychiatric Practices, now available at Laissez Faire Books.