Most of us know whether we're right-handed or left-handed. But fewer people know that their eyes have a favorite too. When you peek through a keyhole, look through a camera, or aim at a target, one eye usually takes the lead without you even thinking about it.
Here's the short answer. Ocular dominance, sometimes called eyedness, is the tendency to prefer what one eye sees over the other. Because our two eyes see the world from slightly different angles, the brain leans on one of them, the dominant eye, for the most precise information about where things are. About 70 percent of people are right-eye dominant and about 29 percent are left-eye dominant.
Two Views, One Picture
Your eyes sit a few inches apart, so each one sees things from a slightly different position. That difference, called parallax, is part of how we judge depth. But when you need to pinpoint exactly where something is, the brain relies mainly on the dominant eye.
That's why eye dominance can matter a lot in sports that involve aiming, such as archery, darts, and shooting. Interestingly, in a 1998 study of professional baseball players, the combination of dominant hand and dominant eye didn't seem to affect batting average or pitching performance.
Not the Same as Handedness
You might expect right-handed people to be right-eyed, but that isn't always the case. Your dominant eye and dominant hand don't necessarily match. The brain's wiring explains why. Each side of the brain handles one half of your field of vision, using information from both eyes.
Another common belief is that your sharper eye must be your dominant one. Researchers have challenged that idea as lacking evidence. Dominance can even shift depending on where you're looking, the task at hand, or how tired you are.
Find Your Dominant Eye
You can test yourself in seconds with what's called the Porta test. Stretch out one arm and, with both eyes open, line up your thumb with something across the room, like a light switch or a door handle.
Now close one eye, then switch. With one eye closed, your thumb will seem to stay right on target. With the other closed, it will appear to jump to the side. Whichever eye was open when your thumb stayed put is your dominant eye. Eye doctors use several other methods too, including a card with a small hole in the middle.
There's a similar version called the Miles test. Make a small opening with both hands held together at arm's length, look at a distant object through it with both eyes open, then close one eye at a time.
Why It Matters
Eye doctors pay attention to eye dominance. It helps predict how happy people will be with certain kinds of vision correction, such as monovision contact lenses or some types of cataract and laser eye surgery.
Scientists are still exploring other links. Brain scans suggest people with different dominant eyes may use slightly different brain areas when looking at faces. Very strong dominance is sometimes caused by conditions such as a lazy eye or misaligned eyes. If you notice changes in your vision, talk to your eye doctor.
So go ahead and try the thumb test. It's a fun little experiment that reveals something your brain has quietly been doing your whole life.
