Dear Cecil: Is it possible to have eyes of two different colors? I scoffed when I heard this at work recently, but others said it happens all the time, and one guy even claimed to know a woman who was "bi" (colored, that is). Are these people imagining things, or is it just that I'm a wuss who never looks people in the eye? John O'Keefe, Westchester, IllinoisDear Cecil:I have an ophthalmological mystery for you to unravel. I was born with hazel eyes (it's on my birth certificate), that is, a muddy mix of green and brown. About two to four years ago, around age 31, they changed fairly quickly to light blue or blue-gray (depending on the light). I don't know exactly when it happened because I only found out when my girlfriend pointed it out to me. I told her she was nuts, of course, but checked in the mirror and she was right. No one has been able to explain this phenomenon to me, least of all my optometrist. My fraternal twin brother, however, forwarded me a biomedical journal article which described an adult change in eye color occurring in about 10 percent of the population. The study involved twins whose eye color was rated annually or so on a scale of 1 to 15, or blue to brown. None of the subjects had a change higher than three units, and a change of two units was typical among those whose eye color changed. Clearly my situation is different, since my color change must be near 15 units. Do you have any explanation or corroboration of this? Dave Stockhoff, via the Internet

Cecil replies:

Eye color is another one of those woefully unstudied fields. We’re OK in the name department, though. Having multicolored eyes is called heterochromia. If your eyes become darker/browner, that’s hyperchromia; if lighter/bluer, hypochromia. These conditions can signify one of two things: (1) some horrible disease, trauma, or other problem, or (2) nothing. So Dave, assuming you’re not in deep denial, I guess you’re one of those lucky guys in category two. Then again, while I hate to be the voice of doom, maybe it’s a question of waiting for the other shoe to drop.

Here’s what we know: Multicolored eyes are fairly rare, usually benign, and frequently genetic in origin, occurring in maybe 1 percent of the population. Alexander the Great supposedly had them. So does rock legend David Bowie (one blue eye, one hazel or gray, depending on whom you ask). If you want to see an example and you and Bowie aren’t speaking these days, heterochromia is common in some domestic animals. Check out a dalmatian or a Siamese cat. One way to get heterochromia is to have somebody punch you in the eye as a kid. That seems to be what happened to David Bowie, who got popped at age 12 in a dispute over a girl. In many such cases the iris of the popped eye gets darker. In Bowie’s case the pupils are also of noticeably and, dare I say, eerily different sizes. I don’t claim there’s any connection with his career, but you can imagine Dave looking in the mirror and thinking, This is not the face of a guy who was meant to drive a beer truck. Eye color is mainly a function of the pigment melanin. If your eyes have a lot of it, they’re brown. If they don’t, they’re blue. (Some details of this explanation are in dispute, but don’t worry about that now.) Green eyes result from yellowish flecks of fatty pigment against a dark background. Some men think a green-eyed woman is exotic. The truth is she’s got fat eyes. Some people’s eye color does change for no apparent reason. Your brother probably saw something about the Louisville Twin Study, which rated color change on a scale of 1 to 15. Researchers concluded that changes occur in 10 to 15 percent of Caucasian adults. On average the changes were slight, about two or three notches. But I guess somebody has to be at the far end of the curve. On the other hand — if, honest to God, your eyes shifted from hazel (medium brown) to light blue, that’s a radical depigmentation — maybe not 15 on the Louisville scale but possibly 7 or 8. When doctors see something like that they wonder, What other parts of this guy’s body are about to fall off? You didn’t say anything about blurry vision or eye irritation, which pretty much rules out Fuchs’s heterochromia, and you’re a little young for senile iris atrophy. I see some discussion here with words like “blastoma” in it … nah, can’t be. Sorry. Forget I even brought it up.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.