Eye color is one of the most striking and variable physical traits among humans. The three main eye colors are brown, blue, and hazel. But some rarer eye colors also exist, like green, gray, and amber. So which eye color is the rarest – black or brown? Let’s take a closer look at the prevalence and genetics behind different eye colors to find out.
Common Eye Colors
Brown eyes are the most common eye color worldwide. Approximately 55-79% of the global population has brown eyes. This wide range is due to significant geographical variation. For example, in Europe, only 29% of people have brown eyes compared to 80% in South Asia.
After brown, blue is the next most common eye color globally, with 8-10% of the world’s population having blue eyes. However, in countries like Iceland and Finland, up to 89% of the population has blue eyes.
Hazel eyes, which appear brown or green depending on lighting conditions, make up about 5% of the global eye color distribution. Green eyes are even rarer at around 2% prevalence worldwide.
Rare Eye Colors
A few very rare eye colors exist beyond the common shades of brown, blue, hazel, and green. These include amber, red/violet, gray, and heterochromia (two different colored eyes).
Amber eyes have a solid golden or copper color and make up less than 1% of the global population. This eye color results from deposition of a yellow pigment called lipochrome in the iris. Amber eyes are most common in Central and South America.
Eyes that appear red or violet also occur due to deposition of lipochrome pigments, albeit in higher concentrations. Only a handful of people worldwide have true red/violet iris coloration.
Gray eyes contain low amounts of melanin pigment in the iris. They occur in about 1% of the population, mostly among Northern and Eastern Europeans.
Heterochromia, where the two eyes are different colors, is extremely rare, thought to affect fewer than 200,000 people globally. Particularly, complete heterochromia, where each iris is a distinctly different color, is even rarer at an estimated prevalence of 1 in 150,000 people.
Is Black an Eye Color?
Contrary to popular belief, true black colored irises do not occur naturally in humans. So-called black eyes are in fact just very dark brown. The dark brown eye color results from high levels of melanin deposition in the iris during fetal development.
Melanin is the main pigment that determines eye color. Higher amounts of melanin lead to darker eyes. Brown-eyed individuals can produce large quantities of melanin, which is why darker shades of brown are possible.
But the melanocytes (melanin-producing cells) in the human iris lack the genetic ability to produce enough melanin to result in a truly black iris color. So black does not technically exist among natural eye colors.
Prevalence of Black Eyes
Very dark brown eyes are often casually referred to as “black eyes.” This eye color is most prevalent in regions like South Asia, Southeast Asia, Africa, and parts of the Middle East.
Globally, an estimated 10-15% of people have very dark brown eyes that border on black. The highest frequencies are among indigenous peoples of Africa, South Asia, and Southeast Asia, where up to 25% of some populations have extremely dark brown irises.
So while true black eyes are not possible in humans, dark brown eyes with such little distinction from black are relatively common in certain parts of the world.
Genetics Behind Dark Brown Eyes
As mentioned before, high amounts of melanin lead to darker eye colors. The melanin content in the iris is determined by a number of genes. The key ones are:
- HERC2 – regulates OCA2 expression
- OCA2 – encodes melanin producing enzyme
- SLC24A5 – regulates melanin type and density
Certain variants of these genes are associated with darker eyes. For example, the HERC2 gene has a variant that increases OCA2 expression, leading to more melanin deposition. Similarly, the SLC24A5 gene has a variant leading to production of more dense eumelanin pigment.
Inheriting these gene variants from both parents leads to extremely dark brown eyes, as the melanin content in the iris becomes very high. This inheritance pattern is common among populations where darker eyes are prevalent.
Difference in Melanin Content
The key difference between dark brown and true black eyes is subtle but important – melanin content. Here is a comparison:
| Eye Color | Melanin Content |
|---|---|
| Dark brown | High melanin |
| Black | Extremely high melanin (not attainable in humans) |
As shown above, black eyes would require an extreme amount of melanin beyond the capabilities of human genetics. So while dark brown eyes contain a lot of melanin, it falls short of the theoretical threshold for true black iris pigmentation.
Rarity of Black vs. Brown Eyes
Given that true black eyes do not occur naturally in humans, brown eyes are clearly the rarer eye color between the two.
As discussed earlier, brown eyes have an estimated global prevalence of 55-79%. Within this range, dark brown eyes make up 10-15%.
Comparatively, black eyes have an prevalence of essentially zero in human populations. The extreme melanin content required for black irises is an impossibility given the limitations of our melanin-producing genes.
So between black and brown, brown eyes are definitively the rarer eye color, with only a small percentage of the population having very dark brown irises. Black eyes remain elusive and outside the scope of human eye color.
Conclusion
In summary:
- Brown eyes are the most common worldwide, followed by blue and hazel eyes.
- Rarer eye colors like amber, red/violet, and heterochromia affect less than 1% of people.
- True black eyes do not occur naturally in humans due to genetic constraints on melanin production.
- Dark brown eyes are often colloquially called “black” but are not truly black.
- High melanin content from gene variants leads to dark brown eye color in certain populations.
- Brown eyes are definitively rarer than black eyes among humans.
So brown eyes are the rarest possible human eye color compared to black eyes, which are not genetically possible. Within the natural eye color spectrum, darker shades of brown are relatively uncommon worldwide. But they are not as rare as truly black irises, which cannot exist in humans due to limitations on melanin synthesis.
