Slaves who run away from their masters must be mad. So Dr. Samuel A. Cartwright explained in a paper, Diseases and Peculiarities of the Negro Race, delivered in 1851 to the Medical Association of Louisiana.

Cartwright named the illness Drapetomania – from the Greek drapetes, “a runaway”. The diagnostic symptom, he announced, took the form of an overwhelming desire to flee the position in society which God had decreed and to which all rational beings should therefore adhere.

The most common cause of Drapetomania was the irresponsibility of slave owners who “made themselves too familiar, treating (the slaves) as equals”. The cure lay in “whipping the devil out of them” or, when prescribed by a qualified doctor, cutting off their big toes so they couldn’t run anywhere ever again.

Daft? Of course. But the paper was printed in an academic journal, The New Orleans Medical and Surgical Journal, and discussed in serious tones at medical conferences, mainly in the southern, slave-owning States. In the north, the theory never gained traction in the mainstream.

That is, the theory was accepted where it suited the status quo on a far wider basis and long after Cartwright’s pronouncements on slavery.

For a century now, the official classification of mental disorders in the US has been determined not by the Department of Health but by the American Psychological Association – a professional body which today represents more than 100,000 researchers, clinicians and students. A condition becomes recognised as an illness when it is included in the APA’s Diagnostic and Statistical Manual of Mental Disorders (DSM), which is updated every decade to reflect advances in knowledge and understanding. To a significant degree, DSM listings also reflect changes in the social and political attitudes in the circles from which psychologists, psychiatrists, etc. tend to be drawn.

The DSM listed homosexuality a mental illness until 1973. University departments ran experiments to find a “cure”; “sufferers” were referred to mental hospitals, usually against their will, to undergo what amounted to savage punishment and deterrence rather than medical care.

Change came in the 1973 edition of DSM, not because new research had been published or a deeper understanding developed but because, four years after Stonewall, gay rights campaigners just wouldn’t stand for it any more. APA events were regularly picketed and disrupted while, increasingly, gay psychiatrists and psychologists came out and, with growing self-confidence as the wider movement advanced, took up the fight within the association.

The crunch came when gay activists and their supporters threatened to break away from the APA and publish a rival journal with listings based on the actual state of medical knowledge. The threat to the APA’s lucrative monopoly in authenticating illnesses was the key factor.

The problem hasn’t gone away. Last month, the American heroine Chelsea Manning ended a hunger strike after a case taken by the American Civil Liberties Union forced the army to agree she that she could receive gender transition surgery in prison. In the previous month, the military authorities had threatened to charge her with gross misconduct for persisting in demands for surgery – which could have resulted in additional years behind bars in permanent solitary confinement.

Manning, a former army intelligence analyst, had been sentenced in 2013 to 35 years for handing Wikileaks 700,000 documents, videos, diplomatic cables and battlefield accounts, including video of US personnel whooping with glee as they riddled unarmed civilians from a helicopter, killing a dozen scrambling, terrified people, including two Reuters journalists. (None of the gunmen involved in the slaughter have been sentenced to a week, much less 35 years, in jail.)

The breakthrough in Manning’s campaign came amid controversy among medical professionals about Gender Identity Disorder (GID) – defined by the DSM as “strong and persistent cross-gender identification” and “persistent discomfort with his or her sex or sense of inappropriateness in the gender role of that sex.” The question was – is GID an illness or an expression of an individual’s refusal to accept a gender identification imposed from outside: does the “illness” lie in the individual or in the attitudes of society?

Modern, liberal-minded people might assume that the answer is obvious. But there is more to it.

Some trans people in the US mounted a campaign against declassification of GID – because if GID were not recognised as a disorder, reassignment surgery, already extremely difficult to obtain, would become virtually impossible – absolutely impossible if you are poor. All insurance forms in the US have to include a diagnostic code specifying the disorder which the procedure is intended to treat.

This disagreement remains unresolved. The point here is that, at every stage, political attitudes have intruded on diagnoses.

In a brilliant recent book, The Book of Woe, sceptical US psychotherapist Gary Greenberg describes how he was forced to use DSM codes to ensure that his patients’ treatment would be paid for, even in many cases in which he attached no validity to the diagnoses.

Greenberg suggests that psychiatrists relish the power that comes with the ability to “give a name to suffering. And that, in one way or another, is what it’s about. Down through the ages mental illness has been used and misused to shore up society’s prejudices. The most egregious example has to do with the classification of unmanageable women as suffering from ‘hysteria’.”

All in the ignorant past?