While only 1 in 5 Caucasian adults have blue eyes in the United States, most are born blue-eyed. Their irises change from blue to hazel or brown during infancy. Why?

"It has to do with the amount of melanin they're born with and how that melanin increases after birth," said Norman Saffra, Chairman of Ophthalmology at Maimonedes Medical Center in Brooklyn, N.Y.

Melanin, Saffra explained, is a pigment, and the more you have of it in your eyes, hair and skin, the darker they are, and thus the more sunlight they reflect. A small deposit of melanin in the irises — the muscular rings around the pupils — makes them appear blue, while a medium amount makes them green or hazel, and a lot of it makes the irises brown.

Babies aren't born with all the melanin they are destined to have. "The maturation process continues post-utero," Saffra told Life's Little Mysteries, a sister site of LiveScience. "Eye color isn't set until 2 years of age." He likened the gradual buildup of melanin in the irises to chicks developing feathers after birth.

Though some babies of non-white ethnicities also have blue eyes at birth which then brown over time, the effect is far less common than with Caucasian babies. "Darkly-pigmented individuals usually have brown-eyed babies, because the babies have more pigment to start out with," Saffra said.

This article was provided by Life's Little Mysteries, a sister site to LiveScience. Follow Natalie Wolchover on Twitter @nattyover