Lieke Nentjes is in her early 30s. She’s slender and softly spoken; it’s hard to picture her spending countless hours in small rooms with incarcerated, unshackled psychopaths, including serially violent men who have committed murder.

As Nentjes talks, though, she reveals her confidence. “One time, there was this pretty big fella with wild, long hair sat across from me, and he suddenly said [raising her voice and half getting up from her chair]: ‘Aren’t you afraid of me?’ I was surprised. I didn’t see that coming. And I went [not raising her voice, but strengthening the tone]: ‘Why – are you afraid of me?’ And he sat back down. Then he explained that he was at the end of his therapy and was ‘re-socialising’ and no one would hire him because they were intimidated by him. He wasn’t really angry. He was frustrated.”

While the nature of psychopathy is still debated, psychologists generally agree that it entails, among other things, a lack of empathy or guilt, shallow emotions, and antisocial behaviour – treating other people badly and, in some cases, engaging in criminal acts.

It has certainly been suggested that the reason some psychopaths are able to torture or murder people is that they don’t process emotions properly – they don’t feel fear, for example, and they don’t recognise it in others.

Nentjes is based at the University of Amsterdam. Here in the Netherlands, if a criminal is found to have a psychological condition that relates to the crime, he or she is deemed to be only partially responsible. Such criminals might spend a few years in a regular jail before being sent to a secure treatment centre, or they may be sent straight for treatment.

Nentjes decided to assess a range of criminals from these centres and from jails to find out just how psychopathic they were (with particular attention to different aspects of psychopathy), to learn about their lives – their upbringing, and their criminal behaviour – and also to measure their interoceptive ability.

“Emotion is very central when you look at psychopathy – or rather, lack of emotion,” she says. “So could it be that psychopathic offenders just aren’t very in touch with their bodies?

In the course of the interviews, Nentjes asked questions to probe their levels of empathy, and how much remorse they felt about what they had done to their victims. “Some were just completely honest, and would say, ‘I don’t care,’” she says. “Others who were psychopathic would say, ‘Oh, but I’m very empathic.’ They had learned the lingo to describe feelings very accurately and they could talk about compassionate, empathic feelings towards others – but then when you look at the crimes they have committed…” Her sentence trails off.

“There is research finding that psychopathic offenders can describe emotions in terms of words, but they lack the inner experience of the emotion,” she adds.

Since assessing a person’s ability to detect a range of bodily signals is tricky, the most commonly used measures of interoceptive sensitivity are based on judgements of heart rate. One test involves asking participants to count their heartbeats over varying periods – 25 or 50 seconds, perhaps – multiple times. About 10 per cent of us are good at counting heartbeats, 5 to 10 per cent are very bad, and the rest fall somewhere in between.

In another test, volunteers are played a series of beeps that are either in sync or out of sync with their heartbeat, and asked which it is. On this type of task, about 10 per cent of the general population can do it really well, and 80 per cent can’t do it at all.

Nentjes brought the necessary equipment for the heartbeat sync task with her into the interview rooms, and took measures for 75 offenders. She found a clear link: the higher an offender’s score on the antisocial aspect of psychopathy, the poorer their performance on the heart-rate task. This does at least suggest that psychopaths who are poorer at detecting bodily signals feel less emotion and therefore less empathy for others.

Psychopathic criminals are sometimes divided into the ‘white-collar’ type, who tend to commit non-violent crimes such as fraud, and the violent type. In her interviews with this violent group, Nentjes was struck by one similarity, in particular, compared with the white-collar group: “That was their upbringing. Or rather lack of it. Emotional abuse. Sexual abuse. Neglect. A lot of physical abuse. I’ve heard people literally say that emotion is not of use to them. All they felt during their upbringing was fear.”