Human eye color is a visible spectrum of human genetic diversity, ranging from brown to blue. For those with a unique, multi-toned color like hazel, curiosity about its origin and infrequency prompts a deeper look into the underlying biology. Understanding the mechanisms that produce this complex hue reveals why it is less common globally.
How Eye Color Is Determined
The color observed in the iris is determined by two main factors: the amount of melanin pigment present and how light interacts with the iris’s structure. Melanin, the dark brown pigment responsible for skin and hair color, is the sole pigment determining the eye’s shade. The pigment concentration in the front layer of the iris, called the stroma, primarily dictates the final appearance.
Eyes with a high density of melanin in the stroma absorb most incoming light, resulting in a brown color. Conversely, eyes with very little melanin allow light to interact with the collagen fibers within the stroma. This interaction causes Rayleigh scattering, which scatters shorter-wavelength light, making the stroma appear blue, much like the sky. Green eyes are an intermediate stage, where a small amount of yellowish pigment mixes with the scattered blue light to produce a greenish hue.
The Unique Mechanism That Creates Hazel
Hazel eyes result from a specific, moderate amount of melanin, placing them between the high concentration of brown eyes and the low concentration of blue eyes. This moderate pigment level is distributed unevenly throughout the iris, creating a multi-toned effect where different sections reflect and absorb light differently.
The characteristic shifting appearance of hazel eyes results from this moderate pigment combined with light scattering. Often, the central ring around the pupil contains more brown or amber pigment. The periphery of the iris has less pigment, allowing for more light scattering that produces green or gold tones. Because the color is a blend of pigment absorption and light scattering, the resulting hue can appear to change depending on the lighting conditions.
The Genetics Behind Intermediate Colors
The relative scarcity of hazel eyes is rooted in the complexity of their genetic inheritance. Eye color is a polygenic trait controlled by multiple genes working together, not just a single dominant/recessive pair. Current research suggests up to 16 genes contribute to the final color, with the most influential being \(OCA2\) and \(HERC2\) on chromosome 15, which regulate melanin production.
Hazel color requires a precise, intermediate level of genetic input. This means producing more pigment than blue or green eyes, but not enough to reach the high saturation of brown eyes. For example, the \(HERC2\) gene contains a variant that can effectively “switch off” the \(OCA2\) gene, leading to the minimal pigment found in blue eyes. Hazel eyes require a partial expression of these pigment-producing genes. Achieving this specific, finely tuned statistical balance is less probable than achieving either the full expression (brown) or the minimal expression (blue), making the intermediate color statistically less common than the extremes.
Global Prevalence of Hazel Eyes
Hazel eyes are less common than the most prevalent shades, though they are not the absolute rarest eye color globally. Approximately 5% to 8% of the world’s population has hazel eyes. This is significantly lower than the 70% to 79% of people with brown eyes and the 8% to 10% with blue eyes, making hazel the third most common eye color.
The distribution of hazel eyes is strongly tied to regions where populations with different ancestral eye colors have mixed over generations. They are most frequently found in specific areas of Europe, North Africa, and the Middle East. This reflects a history of genetic flow between traditionally darker-eyed and lighter-eyed populations. In the United States, the prevalence is notably higher, estimated around 18%, largely due to the diverse European ancestry of its population.