From the Huffington Post:
Of all the qualities that give an attractive person an edge, here's one that you've likely overseen: the limbal ring, the dark circle around the iris. The limbal ring is the line that separates the colored part of the eye from the white (the sclera). It's completely unconscious, the way we judge others' limbal rings. In the 20 milliseconds or so it takes to assess a person's attractiveness, you're factoring in the size and shade of her limbal rings. The bigger and blacker they are, the more attractive the eyes. People with the prettiest eyes often have the most prominent limbal rings. This, anyhow, is the upshot of a recent study by Darren Peshek and his colleagues at the department of cognitive science at the University of California at Irvine. The researchers showed volunteers 80 pairs of male and female faces. Each pair of faces was identical except the eyes: One had dark limbal rings and the other had no limbal rings. The volunteers were asked to pick which face was more attractive and to indicate their degree of preference.

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