Funny — you can't hide those lightening eyes

Q: Is it possible for eye color to change as we age? My aunt used to have brown eyes but now, at the age of 80, her eyes are green/grey. My eyes seem lighter than before, too. (Emily, Newport Beach, California)

Light eyes National Eye Institute, National Institutes of Health

Q: When I was born, I had dark blue eyes. As a child, my eyes became hazel brown. Now that I am much older, they have turned a bright green. Why are my eyes changing color so much? (Judy, Kenner, Louisiana)

Q: I was born with brown eyes. They changed to olive green and are now (I'm past 50) changing to light green and turquoise. Why does this happen? (Wanda, Memphis, Tennessee)

Q: What exactly are hazel eyes? I say hazel eyes change colors. I've read that eyes don't change color but how do you explain millions of people saying their eyes change? Someone please explain! (Bryan, Washington D.C.)

A: You get the idea. Changing eye color baffles people. It's the most-asked question I get.

Information from Richard A. Sturm, Institute for Molecular Bioscience, University of Queensland, Australia The basis of human eye color. The yellow "amebas" on the right depict pigment cells (with orange nuclei). The pigment cells contain brown pigment granules (shown in various intensities from very light brown to light brown to very dark brown). The lighter the pigment and the fewer the granules - the lighter the iris color. The circles on the left depict irises and the colors that result from the corresponding pigment cell. Blue irises result from minimal pigment and few pigment granules. Green-hazel irises have moderate pigment levels and number of granules. Brown irises have high pigment levels and many granules.

OK, all. Eye color can change over time because of age or, unfortunately, disease.

Eye disease is a cause of color change. So, ask a doctor to examine your eyes if you notice a slow loss of color. The change could be due to Fuch's heterochromic iridocyclitis, Horner's Syndrome, and pigmentary glaucoma, says Brian DeBroff, ophthalmology professor at Yale University.

Aging, however, is the usual cause of color change over time. So, yes, Emily. Color can change as we age. It does so for 10 to 15% of the normal Caucasian population. These people's eyes change slowly over many years after they reach adolescence.

Investigators considered Caucasians (non-East Asian, non-Native American, non-African) because only Caucasians commonly have lighter eyes.

"Some eyes become darker, but most become lighter with increasing age," says Richard A. Sturm, a Principal Research Fellow at the Institute for Molecular Bioscience at the University of Queensland in Brisbane, Australia.

And yes, Bryan, hazel eyes do change color. Hazel eyes as well as any lighter eyes usually darken with age. Hazel eyes are light brown or yellowish brown.

How and why eye color changes

Pigment in the stroma (the connective tissue of the front layer of the iris) gives the iris color. Iris color can lighten if the number of pigment granules in the stroma decreases or if the granules produce lighter pigment. (See figure.) The iris can also lose color if the stroma pigment degrades.

Eyes, unlike skin and hair, do not synthesize color pigment continuously. Instead, eyes retain the pigment granules and accumulate them in the iris stroma. So, if the pigment degrades, the eye color lightens.

Likewise, eyes can darken if the number of pigment granules increase or if the granules make darker pigment.

That's how the color changes. Why does it change? Genetics is the key as experimenters learned by studying twins. They observed the eyes and skin of identical twins and non-identical twins of American Caucasians between the ages of 3 months to 6 years.

Both sets of twins showed a "darkening with age of both the hair and eye colour," says Sturm. The identical twins changed color together, at essentially the same rate. The non-identical twins changed color but at different rates, which indicates a "strong genetic influence in the timing of these colour changes."

Eye color probably changes for the same reason we have one head instead of two: genes. Genes determine all body characteristics — including changing eye color as we age.

Further reading:

• Molecular genetics of pigmentation by Rick Sturm, Institute for Molecular Bioscience, University of Queensland

•Eye Color by OMIM, Online Mendelian Inheritance in Man by Victor A. McKusick

•Oregon State University and Hewlett Packard: The genetics of eye color

•Anthro Limited: How are human eye colors inherited?

(Answered October 8, 2004)

April Holladay, science journalist for USATODAY.com, lives in Albuquerque, New Mexico. A few years ago Holladay retired early from computer engineering to canoe the flood-swollen Mackenzie, Canada's largest river. Now she writes a column about nature and science, which appears Fridays at USATODAY.com. If you have a question for April, please e-mail us, including your hometown and your state (or, if you're writing from outside the U.S., your country). To read April's past WonderQuest columns, please check out her site.