If you can't stop gazing into your new sweetheart's eyes, or if a smouldering look leaves you weak in the knees, there's a scientific reason for it—and it might not be what you'd expect.
A recent study by researchers at the University of California at Irvine, published in the journal "Evolutionary Psychology," found that the darker your limbal ring—the border between the colored part of your eye (the iris) and the white part (the sclera)—the more attractive you are to others.
The researchers showed volunteers 80 pairs of male and female faces, each of which were the same except for the eyes; one face had very dark limbal rings, and the other had none. The participants had to choose which faces were more attractive and by how much. The results? "For male and female observers, both male and female faces with a dark and distinct limbal ring are rated as more attractive than otherwise identical faces with no limbal ring," the researchers wrote. The results were the same regardless of gender.
How does the limbal ring make you more attractive? According to "Psychology Today," it all has to do with the fact that we're hard-wired for reproduction. "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 twenties. A thick, dark limbal ring may make us appear younger." Also, it's something you have to be close-up to see, which makes just looking for it feel more intimate and arousing.
As with all of the other main indicators of virility and youth—think dark lips, perky breasts, lustrous hair, even height and strength—there are ways, nowadays, to fake a darker limbal ring. Some contact lenses come with a dark limbal ring etched right on (though if you look closely, you can tell that they're not the real deal).