About 1% of the world’s population has heterochromia, the condition of having different colored eyes. With roughly 8 billion people on the planet, that translates to around 80 million people. While that sounds like a large number, it makes heterochromia genuinely uncommon, and most cases are harmless.
What Heterochromia Looks Like
Not everyone with heterochromia has two completely different eye colors. The condition comes in three distinct forms, and two of them are subtle enough that you might not notice them without looking closely.
- Complete heterochromia: Each eye is a distinctly different color, like one blue eye and one brown eye. This is the most recognizable form and the one most people picture.
- Sectoral heterochromia: Part of one iris is a different color from the rest. You might see a wedge of brown in an otherwise green eye, for example.
- Central heterochromia: A ring of a different color surrounds the pupil, distinct from the outer iris color. Someone might have blue eyes with a gold or amber ring near the center. This is the most common of the three types.
When people casually count “different colored eyes,” they typically mean complete heterochromia. If you include central and sectoral forms, the actual number of people with some degree of mismatched eye color is likely higher than the 1% figure suggests, since mild cases often go unnoticed or unreported.
Why Eye Color Varies in the First Place
Eye color comes down to pigment. Cells in the front layer of the iris contain granules of melanin, and the amount and type of that pigment determine what color your eyes appear. Brown eyes have up to 70% more melanin than lighter colors. The cells themselves exist in roughly the same number regardless of eye color. What differs is how much pigment each cell produces and which form it takes: a brown-black form that creates dark eyes or a red-yellow form associated with lighter shades.
In heterochromia, this pigment distribution is uneven. One eye, or one section of an eye, ends up with a different concentration of melanin than the other. The result is two colors where you’d normally expect one.
Born With It vs. Developing It Later
Most people with heterochromia are born with it, and in these cases the condition is almost always benign. It happens when melanin distributes unevenly during development, with no underlying health issue. Babies sometimes appear to have mismatched eyes that even out as their eye color settles during the first year or two of life, so true congenital heterochromia is usually confirmed after that window.
In rare cases, heterochromia at birth is a sign of a genetic condition. Waardenburg syndrome, for instance, is a group of inherited conditions that affect pigmentation of the hair, skin, and eyes alongside hearing. People with Waardenburg syndrome often have very pale blue eyes or two different eye colors, and sometimes one eye contains segments of two different colors.
Heterochromia can also develop later in life, and that’s where it deserves more attention. Eye injuries, inflammation inside the eye, glaucoma, and certain tumors can all change iris color over time. Even some medications contribute: eye drops used to treat glaucoma and certain cosmetic lash-growth treatments have been documented to darken the iris of the treated eye. If your eye color changes as an adult, it’s worth getting checked, because it could point to an underlying condition that needs treatment.
More Common in Animals Than Humans
If heterochromia seems more familiar in pets than in people, there’s a reason. The trait is far more common in animals. Certain dog breeds carry it at notably high rates, including Siberian Huskies, Australian Shepherds, Dalmatians, and Shetland Sheepdogs. Turkish Van cats are famous for it. In these animals, the genetics of coat and eye pigmentation make mismatched eyes a relatively ordinary occurrence rather than a rarity.
In humans, there’s no known ethnic group where heterochromia is significantly more common. It appears across all populations at roughly the same low rate, which is part of why it draws so much curiosity when it does show up.
Does It Affect Vision?
On its own, heterochromia doesn’t impair eyesight. The pigment in your iris controls how much light enters the eye but has no bearing on how well you see. People with heterochromia have the same range of visual acuity as anyone else. The one practical difference is light sensitivity: a lighter-colored eye lets in slightly more light, which can make bright conditions a bit more uncomfortable on that side. Sunglasses handle that easily.
When heterochromia is linked to another condition, any vision effects come from that condition rather than the color difference itself. Glaucoma, for example, can damage vision regardless of whether it also changes your eye color.