Of all the attributes that provide attractive individuals with an advantage, here’s one that you’ve undoubtedly overlooked: 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 wholly subliminal, the way we discern others’ limbal rings. In the 20 milliseconds or so it takes to gauge a person’s attractiveness, you’re considering the size and shade of her limbal rings. The bigger and blacker they are, the more attractive the eyes. People with the prettiest eyes usually have the most eye-catching limbal rings.

Nevertheless, this is the take away of the latest study by Darren Peshek and his associates 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 choose which face was more attractive and to specify their preference level.

Men considered women with the dark limbal rings more attractive than those without; likewise, women considered men with dark limbal rings extremely attractive. Men and women also evaluated faces of the same sex as more attractive when the limbal rings were prominent.

When a mother gazes into her toddler’s eyes, she notices that 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. Very young children have the thickest, darkest limbal rings.

That’s exactly the point. The limbal ring constitutes a true indicator of youth and health — desirable qualities, reproductively speaking. The ring fades with age, or due to 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 pronounced.

There are so many ways to feign the guise of youth. You can wear makeup and wigs or get Tummy Tucks, Plastic Surgery, BOTOX^®, and Boob Jobs.

But a feign limbal ring? Yes, this too is possible. Years ago, Japanese schoolgirls discovered the edge a large limbal ring can give you by wearing “limbal ring” contact lenses. They make the eye seem larger and more distinct. (A word to the wise: Cosmetic contact lenses have been shown to cause eye infections and scarring — even blindness).

The nomenclature limbal ring is well-suited. Limbis refers to a border or edge, and it’s related to limbic, meaning emotion or drives. The limbal ring, viewed from inches away, is an intimacy zone. Don’t flirt until you see the whites of their eyes.