But is psychopathy a disorder – or a different way of being? Anyone reading the list above will spot a few criteria familiar from people they know. On average, someone with no criminal convictions scores 5. “It’s dimensional,” says Hare. “There are people who are part-way up the scale, high enough to warrant an assessment for psychopathy, but not high enough up to cause problems. Often they’re our friends, they’re fun to be around. They might take advantage of us now and then, but usually it’s subtle and they’re able to talk their way around it.” Like autism, a condition which we think of as a spectrum, “psycho­pathy”, the diagnosis, bleeds into normalcy.

We think of psychopaths as killers, criminals, outside society. People such as Joanna Dennehy, a 31-year-old British woman who killed three men in 2013 and who the year before had been diagnosed with a psychopathic personality disorder, or Ted Bundy, the American serial killer who is believed to have murdered at least 30 people and who said of himself: “I’m the most cold-blooded son of a bitch you’ll ever meet. I just liked to kill.” But many psychopathic traits aren’t necessarily disadvantages – and might, in certain circumstances, be an advantage. For their co-authored book, “Snakes in suits: When Psychopaths go to work”, Hare and another researcher, Paul Babiak, looked at 203 corporate professionals and found about four per cent scored sufficiently highly on the PCL-R to be evaluated for psychopathy. Hare says that this wasn’t a proper random sample (claims that “10 per cent of financial executives” are psychopaths are certainly false) but it’s easy to see how a lack of moral scruples and indifference to other people’s suffering could be beneficial if you want to get ahead in business.

^{The American serial killer Ted Bundy, who is believed to have murdered at least 30 people}

“There are two kinds of empathy,” says James Fallon, a neuroscientistat the University of California and author of The Psychopath Inside: A Neuroscientist’s Personal Journey into the Dark Side of the Brain. “Cognitive empathy is the ability to know what other people are feeling, and emotional empathy is the kind where you feel what they’re feeling.” Autistic people can be very empathetic – they feel other people’s pain – but are less able to recognise the cues we read easily, the smiles and frowns that tell us what someone is thinking. Psychopaths are often the opposite: they know what you’re feeling, but don’t feel it themselves. “This all gives certain psychopaths a great advantage, because they can understand what you’re thinking, it’s just that they don’t care, so they can use you against yourself.” (Chillingly, psychopaths are particularly adept at detecting vulnerability. A 2008 study that asked participants to remember virtual characters found that those who scored highly for psychopathy had a near perfect recognition for sad, unsuccessful females, but impaired memory for other characters.)

Fallon himself is a case in point. In 2005, he was looking at brain scans of psychopathic murderers, while on another study, of Alzheimer’s, he was using scans of his own family’s brains as controls. In the latter pile, he found something strange. “You can’t tell just from a brain scan whether someone’s a psychopath,” he says, “but you can make a good guess at the personality traits they’ll have.” He describes a great loop that starts in the front of the brain including the parahippocampal gyrus and the amygdala and other regions tied to emotion and impulse control and empathy. Under certain circumstances they would light up dramatically on a normal person’s MRI scan, but would be darker on a psychopath’s.

“I saw one that was extremely abnormal, and I thought this is someone who’s way off. It looked like the murderers I’d been looking at,” he says. He broke the anonymisation code in case it had been put into the wrong pile. When he did, he discovered it was his own brain. “I kind of blew it off,” he says. “But later, some psychiatrist friends of mine went through my behaviours, and they said, actually, you’re probably a borderline psychopath.”

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