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…

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