Pigment determines eye color, and a number of genes influence pigment. Brown is the most common eye color worldwide by a large majority.
As many as 16 genes influence eye color by determining the amount of melanin inside the specialized cells of the iris. Melanin is the pigment responsible for eye color.
In this article, we explain what eye color is and what causes it. We also take a look at the breakdown of the proportion of people around the world with each eye color.
The iris is the name of the colored part of the eye.
The iris surrounds the pupil, which is the small black hole in the middle of the eye that helps control how much light enters it.
Scientists used to think that eye color linked to just one gene and that brown eyes were dominant over blue eyes.
They now know that what determines eye color is more complex.
As many as 16 genes influence eye color. Most of these genes play a role in the production, transportation, or storage of melanin.
Melanin is a brown pigment that determines the color not only of someone’s eyes but also of their hair and skin.
Eyes that are not brown do not have different color pigments. Instead, they absorb less light because they have less melanin. As a result, they scatter more light, reflecting it along the spectrum of light color.
An iris with the least melanin will appear blue. Those with a little more melanin will appear green or hazel, for example.
The vast majority of people in the world have brown eyes.
The second most common color is blue, but people can also have green, gray, amber, or red eyes.
Some people have eyes that are different colors than each other.
According to estimates, 70–79% of the world’s population have brown eyes, making it the most common eye color worldwide.
In fact, the American Academy of Ophthalmology (AAO) state that everyone on Earth had brown eyes around 10,000 years ago.
Nowadays, the AAO note that about half of those living in the United States, and a higher proportion of people in Africa and Asia, have brown eyes.
Brown-eyed people are, however, more at risk of cataracts as they get older.
Blue is the second most common eye color globally, with estimates suggesting that 8–10% of people have blue eyes.
In the U.S., that proportion is higher, at about 27%.
Scientists believe that it is possible to trace all blue-eyed people back to a common ancestor, who likely had a genetic mutation that reduced the amount of melanin in the iris.
Most people with blue eyes are of European descent.
Hazel eyes are more common in North Africa, the Middle East, and Brazil, as well as in people of Spanish heritage.
Amber eyes, which have slightly more melanin than hazel eyes but not as much as brown eyes, account for about 5% of the world’s population.
People of Asian, Spanish, South American, and South African descent are most likely to have amber eyes.
An estimated 2% of the world’s population have green eyes, making them very rare overall.
However, green eyes are very common in some parts of the world, including Ireland and Scotland.
In the U.S., where many people descend from ancestors from Ireland and Scotland, about 9% of people have green eyes.
Close to 3% of the world’s population have gray eyes.
People with gray eyes have little or no melanin in their irises, but they have more collagen in a part of the eye called the stroma.
The light scatters off the collagen in a way that makes the eyes appear gray.
Red or violet
People with albinism or ocular albinism usually have little or no melanin in the iris. This lack of pigment causes red or violet eyes.
As eye pigmentation is important for vision, people with ocular albinism often have problems with their eyesight.
A person with ocular albinism may have very blurry vision or poor depth perception. They may experience rapid, involuntary eye movements, have higher light sensitivity, or find that their eyes look in two different directions.
The two eyes might be completely different from one another, or one part of the iris might be different than the rest.
The AAO explain that in people with central heterochromia, the iris has two rings of different colors, while in those with complete heterochromia, one iris is a different color than the other.
Some people are born with heterochromia. In others, an eye injury or health problem might cause it.
Many babies born with blue eyes will experience a change in eye color as more pigment builds up over their first few months.
A baby’s eyes might become less blue or even turn brown, but change tends to stop by the age of 1 year.
For cosmetic reasons, some people choose to wear colored contact lenses to change the color of their eyes.
People can develop heterochromia at any point in their life. Possible causes include:
- eye injury
- swelling due to iritis or uveitis
- Fuchs’ heterochromic cyclitis
- acquired Horner’s syndrome
- ocular melanosis
- iris tumor
- Posner-Schlossman syndrome
- Chediak-Higashi syndrome
The AAO advise anyone who notices a color change in one or both eyes to speak to a doctor.
At least 16 genes play a part in determining someone’s eye color.
Brown eyes have the highest amount of the pigment melanin in the iris.
Lighter color eyes have less melanin. This reduced pigmentation affects how they reflect light and can make them appear blue, green, amber, or hazel.
People with red or violet eyes have little or no melanin in their irises.
Globally, brown is the most common eye color. Scientists believe that it was the only eye color up until about 10,000 years ago.