Coming face to face with a probable psychopath was enough to make Dr Leanne ten Brinke rethink her career choices. Early in her 20s, while studying forensic psychology in Halifax, in the Canadian province of Nova Scotia, Ten Brinke was volunteering at a parole office, which would hold weekly group meetings for released sex offenders. “Most of the men showed contrition,” says Ten Brinke. “They really seemed to recognise the damage that they had done.” Except for one. The treatment programme seemed “like a game to him”, she says. One week, in a discussion about the impact their crimes had on victims, this rapist stared at Ten Brinke and, smiling slightly, started to say how much his victim looked like her, “and how I was ‘his type’. Clearly he was trying to scare me, and he did.”

It put her off a career working with convicted criminals, but she remained fascinated with “dark personalities” – psychopathy, mainly, but also narcissism, machiavellianism (manipulating and exploiting others) and sadism. From politics to business to the media, it wasn’t as if there was a shortage of people to study. There were selfish, callous, impulsive and manipulative people everywhere, often presenting as gregarious and charming. “It started to occur to me that these traits aren’t just confined to an underworld. These traits appear in all aspects of our lives,” she says.

Now associate professor of psychology at the University of British Columbia, Canada, Ten Brinke says these people could be in our families, or living next door. They’re the trolls online. They’re at work, at school, leading our institutions and our countries.

Instead of being specific conditions that one either has or hasn’t, psychopathy and other personality disorders are now thought to exist on a continuum, says Ten Brinke. It is estimated that 1% of the general population have clinical levels of psychopathy (scoring highly on the PCL-R, the psychopathy checklist assessment commonly used for diagnosis). Other studies have suggested that up to 18% have “elevated” levels – what we may call “dark territory”, as Ten Brinke puts it in her new book, Poisonous People: How to Resist Them and Improve Your Life. Within the prison population, the instance of clinical psychopathy is about 20%. However, these dark personalities – who are potentially the most dangerous and likely to reoffend – are particularly good at convincing parole boards to release them, probably because they can be so persuasive.