Your eye color depends on how much pigment sits in the front layer of your iris. Brown eyes have the most, blue eyes have the least, and green and hazel fall somewhere in between. Globally, about 79% of people have brown eyes, making it by far the most common color. Blue comes in second at 8 to 10%, hazel accounts for roughly 5%, and green is the rarest at around 2%.
Why Eyes Appear the Color They Do
The iris has two layers. The back layer is packed with dark pigment in virtually everyone, regardless of eye color. What actually varies from person to person is the amount of melanin in the front layer, called the stroma. Brown irises contain a large amount of melanin in the stroma, which absorbs most incoming light, especially shorter blue wavelengths. The result is the deep, warm color you see.
Blue eyes contain little to no melanin in the stroma. Without pigment to absorb it, longer-wavelength light (reds and yellows) passes through the stroma and gets absorbed by the dark back layer, while shorter-wavelength blue light scatters back out toward the observer. It’s the same physics that makes the sky look blue. There is no blue pigment in a blue eye.
Green and hazel irises sit in the middle. A moderate amount of melanin absorbs some light and scatters the rest, producing those in-between tones. Hazel eyes often shift in appearance depending on lighting because they contain just enough pigment to look brown in dim conditions and greenish-gold in bright ones.
The Genetics Behind Eye Color
Eye color was once taught as a simple dominant-recessive trait, with brown always winning over blue. The reality is more complex. At least 16 genes play a role, but two neighboring genes on chromosome 15 do most of the heavy lifting: OCA2 and HERC2.
OCA2 codes for a protein involved in melanin production inside the iris. HERC2 acts like a dimmer switch for OCA2. A single variation in HERC2 (a spot called rs12913832) determines how much OCA2 gets expressed. The ancestral version of this variation allows robust melanin production, resulting in darker eyes. The derived version dials it down, reducing melanin and producing lighter eyes. Because multiple genes contribute, two blue-eyed parents can occasionally have a brown-eyed child, and eye color can skip generations in unpredictable ways.
When Eye Color Develops in Babies
Most babies of European descent are born with blue or gray eyes. This isn’t their permanent color. The cells in the iris responsible for producing pigment, called melanocytes, need light exposure to ramp up production. Once a newborn’s eyes are regularly exposed to light, melanin begins accumulating in the stroma.
Noticeable changes typically start between 3 and 9 months, often around the 6-month mark. But the process isn’t always fast. Eye color can continue shifting until a child is about 3 years old before it fully stabilizes. If a baby’s eyes are going to turn brown, you’ll usually see darkening well within the first year. Eyes that remain light blue past 12 months are more likely to stay that way.
Eye Color Distribution Around the World
Brown eyes dominate globally because high melanin production is the ancestral default, and populations near the equator, where UV exposure is strongest, overwhelmingly carry genes for darker pigmentation. Blue eyes are most concentrated in Northern Europe and the Baltic region, where reduced sunlight created less evolutionary pressure to maintain heavy pigmentation. Green eyes are particularly associated with populations in Ireland, Scotland, and parts of Northern and Central Europe.
In the United States, the distribution skews lighter than the global average due to the country’s large European-descent population. About 54% of Americans report having light-colored eyes (blue, green, or hazel), and roughly 18% of the U.S. population has hazel eyes compared to just 5% worldwide.
Heterochromia: More Than One Color
Some people don’t fit neatly into a single eye-color category. Heterochromia refers to having different colors in or between your eyes, and it comes in three forms. Complete heterochromia means each eye is a distinctly different color, like one brown and one blue. Sectoral heterochromia means one iris has a wedge-shaped section of a different color, like a slice of pie. Central heterochromia produces a ring of a different color around the pupil, often with spoke-like rays extending outward into the main iris color.
Heterochromia is rare, though exact prevalence numbers aren’t well established. Most cases are genetic and completely harmless, present from birth or early childhood. In rarer instances, it can develop later in life due to eye injury, inflammation, or certain medications, which is worth mentioning to an eye doctor if the change is new.
Eye Color and UV Risk
The melanin in darker irises does more than create color. It absorbs ultraviolet light, offering a degree of built-in protection to internal eye structures. People with blue or green eyes have less of this natural shield, which may increase their risk of certain conditions. Some studies link lighter irises to a higher risk of rare eye cancers, including melanoma of the iris and uveal melanoma.
Despite this, awareness remains low. A Harris Poll conducted for the American Academy of Ophthalmology found that less than a third of light-eyed Americans knew their eye color was associated with greater risk of UV-related eye diseases. Wearing sunglasses that block both UVA and UVB light matters for everyone, but it’s especially relevant if your eyes are on the lighter end of the spectrum.