Of all the qualities that give an attractive person an edge, here's one you've likely overlooked: the limbal ring, the dark circle around iris. The limbal ring is the line that separates the colored part of the eye from the white (the sclera).

It's completely , the way we all judge others' limbal rings. In the 20 milliseconds or so it takes to assess a person's , you're factoring in the size and shade of the limbal rings. The bigger and blacker they are, the more attractive the eyes. People with the prettiest eyes 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 eighty 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.

Men thought women with the dark limbal rings were more attractive than those without, and women thought the same of men with dark limbal rings. Men and women also judged faces of the same as more attractive when the limbal rings were large.

Looking into my baby daughter's eyes, I see the blue of her iris is framed by a thick black limbal ring. The contrast makes the white of her eyes so white they look blue. The very young have the thickest, darkest limbal rings.

Which is exactly the point. The limbal ring serves as an honest signal of youth and health—desirable qualities, reproductively speaking. The ring fades with age and with medical problems. It's thickest from infancy through the early 20s. A thick, dark limbal ring may make us appear younger. It makes the whites of the eyes whiter. This might be why so many people think light eyes are so sexy: the limbal ring, when present, is more prominent.

There are so many ways to fake the appearance of youth. You can wear makeup and wigs and get tummy tucks, plastic surgery, Botox, and boob jobs.

But a fake limbal ring?

Yes, this too.

Long ago, Japanese schoolgirls discovered the edge a large limbal ring can give you by wearing "limbal ring" contact lenses. They make the eye look bigger and more defined. And while you're eyeing these contacts, you might as well buy a set that expands your pupils too. Big, dark, dilated pupils signal emotional arousal. They, too, act on the unconscious favorably.

The limbal ring is well-named. Limbis means border or edge, and it's related to limbic, meaning emotion or drives. The limbal ring, seen from inches away, is an zone. Don't until you see the whites of their eyes.

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