A barrister friend described a case to me. A gory crime was witnessed by multiple bystanders. “The defence is going for diminished responsibility. Maybe insanity,” he said, rolling his eyes. “But is the murderer mad?” I asked. My friend shrugged. “Presumably.” Diminished responsibility will reduce your sentence, but a plea of insanity might get you off altogether.

So, for a jury to look kindly on your claim of insanity, you have to be in an actual psychotic state during the crime? But surely someone who plans a murder, knows what he’s doing and is delighted by the result, though not psychotic, is hardly mentally well. I spoke to a forensic psychiatrist. “I think some people do commit violent crimes just because they are badass. They want to get rid of the drug dealing competition, so they kill. It’s unlikely they’re mentally ill.”

Elaine Whitfield Sharp, an American defence attorney, disagrees. She has publicly stated her belief that murderers are mentally ill, saying; “Their brains don’t work like the rest of ours do,” and that the boundary they overstep is, in itself, evidence of illness.

Jonathan Pincus, a Georgetown University neurologist, and psychiatrist Dorothy Lewis of New York University believe that childhood abuse, neurological disturbances and psychiatric illness are the factors that lead to murder. In the United States, between 60 and 80 per cent of prisoners are known to have suffered serious physical and/or sexual abuse prior to their crimes. However, they might not be openly psychotic at the time of their crimes.

So, what kind of madness is forgivable and what isn’t? “It’s not about forgiveness, it’s about appropriate handling. Trying to get real justice and place people where they will be safe,” professor Pamela Taylor of Cardiff University tells me. But jury members are deciding how they feel about the defendant, aren’t they? In Common Law countries, yes, she says—in Europe the judicial system is inquisitorial and expert-led.

There is the notoriously emotional case of Peter Sutcliffe. Despite four psychiatrists testifying to his paranoid schizophrenia, the judge (influenced by his wife who sat beside him throughout the trial) ignored that evidence and sentenced Sutcliffe to 20 consecutive life sentences in prison. Sutcliffe was later sectioned and transferred to a secure psychiatric unit.

This kind of emotional sentencing is rare, says Jeremy Dein QC: “The system is scrupulous to try to work out precisely what the mental state of the offender was—these cases are highly complex and sad—all participants do their best in difficult circumstances.”

The insanity plea is based on the case of Daniel M’Naughten, the Scottish woodturner who, in a psychotic state, killed the prime minister’s secretary on Whitehall in 1843. He was found not guilty by reason of insanity, later defined as follows: “Insanity was to be a defence to criminal charges only if at the time of the committing of the act, the party accused was labouring under such a defect of reason, from a disease of the mind, as not to know the nature and quality of the act he was doing; or, if he did know it, that he did not know he was doing what was wrong.”

Anyone who works in mental health would consider that pretty narrow. Taylor agrees that everyone charged with murder should be psychiatrically assessed, as used to be the case. Mistakes are made—there are people in prison who should be in hospital, including those who refuse to plead insanity because they believe, erroneously, that they are sane.

Psychoanalysis has a less concrete approach. One psychoanalyst told me: “The psychotic patient who kills usually has a history, like the man who murdered the journalist at Finsbury Park station, after being rejected from 12 mental health institutions. Or a murderer I saw with a history of early childhood abuse who, on his way to murdering a builder who had ripped him off, popped into a restaurant and started mending a leaking toilet. A waiter kicked him out. I always say if the waiter had been able to see that it was the man who was leaking, not the toilet, the murder wouldn’t have happened.”

This psychoanalyst sees all murders as an enactment of psychotic concrete thinking. “The madness seems offset by conscious criteria, like planning. So, a gangster is killing for conscious motives but the reason he has become a psychopath is to avoid psychotic anxiety. Also terrorists and lone attackers like Westminster Bridge are mad but it’s sanctioned madness.”

So, should all prisons be hospitals? A forensic psychiatrist who has worked on dozens of murders believes that all prisons should have some therapeutic milieu, like Broadmoor. Back to my barrister friend; “Given that diminished responsibility can often be fought out in court with one psychiatrist arguing the defendant is mad and another arguing he is sane, yes, I would say the law is absurdly out of date with modern psychiatry and needs to change.”