From Daily Mail:
In 2011, researchers from the University of California, Irvine conducted a study in which participants were shown a set of 80 pairs of faces and asked to rate them for attractiveness. … The researchers found that the male and female participants … overwhelmingly preferred the photos showing the darker limbal rings.
 
For the full story, please visit http://www.dailymail.co.uk/sciencetech/article-2971960/Could-pair-contacts-make-look-YOUNGER-Lenses-define-border-iris-years-face-makers-claim.html.