For the purposes of this article, imagine this scenario. It’s a Saturday night in a busy city-centre pub. Feeling bored, Tony picks up the empty beer bottle on the table in front of him and weighs it in each hand.

Suddenly, he smashes the bottle down on the table, gets up from his chair and walks over to the stranger who accidentally bumped into him earlier that evening, twisting the broken bottle into his face. A friend of the injured man named Pete reacts aggressively, pushing Tony and lashing out when others try to restrain him, drawing more people into the fight.

When the police arrive, Tony calmly takes charge of the situation, seemingly trying to smooth things over, but in fact blaming Pete. When the police subsequently try to arrest Pete, Pete explodes for a second time – this time punching a policeman in the face.

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Though this story is fictional, it serves to illustrate the difference between two types of violent offenders in our prisons. The characteristics that Tony shows are those of a psychopath: cold, calculating, superficially charming, and remorseless. “The violent act is thought through in advance, and the individual may derive much excitement and satisfaction from engaging in it,” says Stephen Blumenthal, a consultant clinical psychologist and psychoanalyst who works with violent offenders at the Portman Clinic, a specialist NHS outpatient psychotherapy clinic in London.

Pete, on the other hand, exhibits symptoms of antisocial personality disorder: a condition characterised by impulsivity and aggression. “The typical violence of the antisocial, non-psychopathic individual is driven by strong emotions and is impulsive or reactive,” Blumenthal says.