The human eye displays a remarkable range of colors across the global population. From deep browns to striking blues and greens, eye color diversity is a result of intricate biological processes and genetic factors. The prevalence of these colors differs significantly, with some being far more common than others.
How Eye Color Develops
Eye color is determined by the amount and type of melanin present in the iris, the colored part of the eye. Melanin is a pigment also responsible for skin and hair color. Two main types of melanin influence eye color: eumelanin, which produces brown and black hues, and pheomelanin, which contributes to amber, green, or hazel colors. The concentration of melanin in the iris’s front layer, the stroma, directly affects the eye’s shade.
Beyond pigment, light scattering within the iris plays a significant role, especially for lighter eye colors. This phenomenon, known as Rayleigh scattering, is similar to how the sky appears blue. Blue and green eyes do not contain blue or green pigments; instead, they have low melanin concentrations, allowing light to scatter and reflect shorter, blue wavelengths. The interplay of melanin levels and light scattering creates the diverse spectrum of human eye colors.
The Spectrum of Rarity: Common to Less Common
Brown eyes are the most common eye color globally, accounting for 70% to 79% of the world’s population. This prevalence is due to a high concentration of eumelanin in the iris, which absorbs both shorter and longer wavelengths of light. Brown eyes are predominant in Africa, Asia, and the Americas; it is believed that around 10,000 years ago, all humans had brown eyes.
Blue eyes are the second most common, found in 8% to 10% of people worldwide. They are most prevalent in northern and eastern Europe, particularly around the Baltic Sea. Blue eyes result from very low melanin concentrations in the iris.
Hazel eyes, a mix of brown and green, are present in 5% of the global population. Their color arises from a moderate amount of melanin combined with Rayleigh scattering, and they can appear to shift in color depending on lighting conditions. Green eyes are rare, found in only 2% of the world’s population. They contain a low concentration of eumelanin and some pheomelanin, producing the green appearance.
Truly Rare and Unique Eye Colors
Beyond common shades, some eye colors and conditions are rare due to specific genetic factors or unique light interactions. Amber eyes, characterized by a solid yellowish, golden, or coppery tint, are uncommon. Unlike hazel eyes, amber eyes typically have a uniform hue without flecks of other colors, attributed to the presence of a yellow pigment called lipochrome.
Gray eyes are also rare, affecting less than 1% to 3% of the world’s population. They are often mistaken for blue eyes but have more collagen in the stroma and very low melanin levels, causing light to scatter, producing a steely gray appearance. Their hue can vary with lighting, sometimes appearing blue or green.
Red or violet eyes are extremely rare and are primarily associated with albinism, a genetic condition lacking melanin production. In these cases, the absence of pigment allows the blood vessels in the iris to become visible, resulting in red or pinkish tones. When combined with light scattering, this can sometimes create a violet appearance. Another unique and rare condition is heterochromia, where an individual has different colored eyes or multiple colors within a single eye. This can be congenital, often due to a harmless genetic mutation affecting melanin distribution, or acquired later in life due to injury or certain medical conditions.