New research on the iris:
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.
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.