Psychopaths may give themselves away by showing how well they can lie, new research suggests.
People with strong psychopathic traits find lying easier to learn than more empathic individuals, scientists have discovered.
Researchers in Hong Kong tested 52 university students who classified as either having high or low levels of psychopathy.
Both groups of volunteers were shown photos of familiar and unfamiliar faces and asked to give either an honest or untruthful response when quizzed about whether or not they recognised the person in the picture.
Reaction times for each response were measured and the participants underwent functional magnetic resonance imaging (fMRI) brain scans.
When the task was repeated, the psychopathic volunteers were quicker to lie than they were at the start of the experiment. People with few psychopathic traits showed no change in their response times.
Lead scientist Dr Tatia Lee, from the University of Hong Kong, said: "The stark contrast between individuals with high and low levels of psychopathic traits in lying performance following two training sessions is remarkable, given that there were no significant differences in lying performance between the two groups prior to training."
Psychopathy is defined as a personality disorder characterised by lack of empathy or feelings of guilt, uninhibited behaviour and egotism.
It is often associated with murderers and serial killers. But a study published last year found that about one in five company executives have psychopathic tendencies - roughly the same rate as seen in the prison population.
Psychopaths might find it easier to lie because their brains do not have to work so hard at resolving moral conflicts, said Dr Lee.
She added: "During lying, the 'true' information needs to be suppressed and reversed.
"Thus, lying requires a series of processes in the brain including attention, working memory, inhibitory control and conflict resolution which we found to be reduced in individuals with high levels of psychopathic traits.
"By contrast, in individuals with low levels of psychopathic traits this lie-related brain activity increased.
"The additional 'effort' it took their brains to process untruthful responses may be one of the reasons why they didn't improve their lying speed."
The findings are reported in the journal Translational Psychiatry.