Human eyes display a remarkable spectrum of colors. This diversity often sparks curiosity about the prevalence of different eye colors worldwide. Among the many shades, blue and green eyes frequently prompt questions regarding their rarity. This article explores the global distribution of eye colors, their biological processes, and other less common variations.
Global Eye Color Distribution
Brown eyes are the most common eye color globally, accounting for approximately 70% to 79% of the world’s population. Blue eyes are the second most common, found in about 8% to 10% of people worldwide. Their highest concentrations are in northern European countries like Scandinavia and the Baltic states.
Green eyes are considerably rarer than blue eyes. Only about 2% of the global population has green eyes, making them one of the least common eye colors. Higher concentrations of green eyes are found in specific regions, including Ireland, Scotland, and parts of Central and Northern Europe.
The Science Behind Eye Color
Eye color is primarily determined by the amount and type of melanin present in the iris, the colored part of the eye. There are two main types of melanin involved: eumelanin, which produces dark brown to black pigment, and pheomelanin (also known as lipochrome), which contributes to reddish-yellow hues. The concentration and distribution of these pigments within the iris’s front layer, called the stroma, dictate the perceived color.
Eyes appear blue or green due to a phenomenon known as Rayleigh scattering. When melanin levels are low, as in blue eyes, shorter wavelengths of light (blue and violet) are scattered more effectively, giving the iris a blue appearance. Green eyes have a small amount of melanin, including some yellowish pheomelanin, which combines with the scattered blue light to create the green hue. Eye color is a complex, polygenic trait, meaning multiple genes, such as OCA2 and HERC2, interact to influence melanin production and distribution.
Other Unique Eye Colors
Hazel eyes, for instance, are a blend of brown, green, and sometimes gold, resulting from moderate melanin levels that allow for a mix of light absorption and scattering. The appearance of hazel eyes can vary, sometimes showing different colors within the same iris. Grey eyes, which are less common than green eyes, result from very low melanin content and a higher amount of collagen in the stroma, leading to a distinct light scattering effect.
Amber eyes are characterized by a solid golden, coppery, or yellowish hue, often due to a higher concentration of pheomelanin and very little eumelanin. This gives them a clear, uniform appearance without the mixed flecks seen in hazel eyes. Heterochromia, affecting about 0.06% of the population, is when an individual has two different colored eyes or multiple colors within a single eye. Albinism, a genetic condition affecting melanin production, can result in very light blue, pink, or reddish eyes due to the lack of pigment allowing blood vessels to show through.