A person's eyes can reveal if they are a psychopath or not, scientists have found.
Experts discovered that people who suffer from the personality disorder have a unique reaction to horrific scenes - their pupils do not widen.
Pupils of non-psychopaths dilate when they see upsetting or distressing images as part of a natural response.
Researchers at Cardiff and Swansea Universities examined the effect of nasty images on offenders who are psychopathic and offenders who aren't and saw a marked difference in their eyes.
Lead author, Dr. Dan Burley, from Cardiff University's School of Psychology, said: 'Our findings provide physical evidence of an emotional deficit common to psychopathic offenders.
'The pupil has long been known to be an indicator of a person's arousal.
'Cardsharks have learned to look carefully at the eyes of their opponents to gauge if they have a great hand, and many an astute salesperson knows to up their price if your eyes reveal your excitement at their product.
'Likewise, the pupil usually dilates when an image shocks or scares us.
'The fact that this normal physiological response to a threat is reduced in psychopathic offenders provides us with an obvious physical marker for this condition.'
While the eyes of a psychopath behaved abnormally when looking at distressing scenes, the researchers were amazed when they saw their eyes behaved normally when looking at positive pictures.
They say this shows that psychopathy is not associated with an overall difficulty in responding to emotion, but rather a specific insensitivity to threatening information.
Professor Nicola Gray, a clinical and forensic psychologist from Swansea University, who provided clinical supervision for the project, added: 'This is one of the first times we have objective, physiological, evidence of an emotional deficit underpinning the offending behavior of psychopathic offenders that does not depend on invasive methods or expensive equipment.
'We hope to be able to develop this methodology to assist with clinical assessment and intervention in offender populations.'
Professor Robert Snowden from Cardiff University said that many psychopathic offenders appear to be bold, confident, and can act in a cold-blooded manner.
He added: 'It's much easier to act boldly if you have no feelings of fear, and to be cold-blooded if there is no emotion to get in the way of the act.'
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