It may be possible to read a person's personality through their eyes, Swedish researchers have said.
They have detected patterns which show warm-heartedness and trust or neuroticism and impulsiveness.
The team from Orebro University read pits and lines in the irises of 428 people.
Experts said the study in Biological Psychology showed that at least some aspects of personality were determined by genetics.
"You would not want to arrest somebody on the basis of their iris"
Close-up pictures were taken of the study participants' irises, and they also filled out a questionnaire about their personalities.
The researchers looked at crypts (pits) and contraction furrows (lines curving around the outer edge of the iris), which are formed when pupils dilate.
It was found that those with more crypts were likely to be tender, warm and trusting, while those with more furrows were more likely to be neurotic, impulsive and give in to cravings.
'Trajectories'
The researchers suggest that a neurodevelopmental gene called PAX6 could also play a major role.
It is known to help control the development of the iris in an embryo.
Previous research has also shown that a mutation of PAX6 is linked to impulsiveness and poor social skills.
The team, led by Dr Matt Larsson a behavioural scientist, said: "These findings support the notion that people with different iris configurations tend to develop along different trajectories in regards to personality.
"Differences in the iris can be used as a biomarker that reflects differences between people."
Dr George Fieldman, principal lecturer in psychology at Buckinghamshire Chilterns University College, said: "This is very interesting. It shows that some aspects of personality have a genetic base and to identify them in the eye in this fascinating way is significant.
"It is surprising that this is possible. But it seems that the old aphorism that 'the eyes are the window to the soul' has some genetic basis."
He said it opened up the possibility that security services could one day use the technique to analyse people.
Airports, including Heathrow, Manchester and Gatwick are already testing iris scanning to identify people - but are not to check personality traits.
But Dr Fieldman added: "Security services would have to use such technologies with some caution. You would not want to arrest somebody on the basis of their iris."
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