In a three-year research project that involved interviewing 105 long- term offenders in Scottish prisons, she discovered that upbringing appeared to be an important factor in whether a child became a psychopath, as well as genetic make-up.
To discover which offenders were psychopaths she questioned them and compared their answers to a widely used list of 20 characteristics of a psychopath, the annual conference of the British Psychological Society's criminological and legal division were told. To be considered a psychopath, they needed to display a number of the 20 core characteristics. Ms Marshall added that people in some high-powered careers, such as stockbroking and politics - she did not rule out journalists - had enough of the 20 characteristics to be defined as psychopathic.
She said: "Successful psychopaths included people with careers such as stockbrokers, where a lot of action was happening and where they had a lot of power. "They have to be quite cold and callous. You could say a politician. [They] might be in control and have power. They are risk-takers.They have the characteristics of psychopaths but without the criminal intent."
She added that psychopaths made good fraudsters.
Of the sample of 105 inmates, 50 were psychopaths - including murderers and rapists - and the remainder were "normal" criminals.
By questioning them about their childhood and checking the answers with official records, she discovered that psychopaths had similar experiences when young. Her results showed that most of them had parents who disciplined their children poorly and were inconsistent in the way they treated them. For example, they would severely beat them for bunking off school on one day and laugh about it the next. "The children never knew where they stood," said Ms Marshall.
Parents were often quite hostile towards their children who, in return, were negative about them. Neglect was also common. School was found to be a boring and bad experience for most psychopaths.
Is the person next to you a psychopath?
Tick as appropriate.
o Remorseless user of others
o Pathological liar
o Glib and a con artist
o Lacking in remorse
o Fails to accept responsibility for actions
o Has high sense of self-worth
o Is chronically unstable
o Is anti-social
o Has socially deviant lifestyle
o Needs constant stimulation
o Has parasitic lifestyle
o Had childhood problems
o Was juvenile delinquent
o Is irresponsible
o Has unrealistic goals