Many people are curious about the prevalence of specific eye shades, such as hazel. The variation in human eye color, ranging from the darkest brown to the lightest blue, is determined by genetics and population data. Determining how common a particular eye color is requires understanding the underlying biology and global statistics.
Defining the Hazel Eye Color Spectrum
Hazel eyes are characterized by their multi-tonal appearance, often appearing to shift in color depending on the lighting. A hazel iris typically displays a complex mixture of brown, green, and gold flecks. The color is often described as having a central ring of amber or light brown around the pupil, transitioning to green or gold hues toward the outer edge.
This visual complexity results from the interaction between a moderate amount of the pigment melanin and the structure of the iris. The brown tones come from melanin, while the perception of green and blue hues is created by Rayleigh scattering. When light enters the eye, it scatters against the collagen fibers and melanin particles within the iris stroma. This scattering, combined with a mid-level of melanin, creates the distinct hazel color.
Global Prevalence and Ranking
Hazel eyes are neither the most common nor the rarest eye color globally. Worldwide, brown eyes are dominant, accounting for approximately 70% to 79% of the population. Blue eyes are typically the second most common, present in about 8% to 10% of people.
Hazel generally ranks as the third or fourth most common eye color globally, with an estimated prevalence of about 5% of the world’s population. This figure is significantly lower than the percentage for brown and blue eyes. However, the percentage varies dramatically by geographic region. For example, concentrations are much higher in the United States, where the rate is closer to 18%.
Hazel eyes are relatively frequent in parts of Europe, the Middle East, and North Africa, often reflecting a history of genetic blending. The global 5% prevalence positions hazel as more common than green eyes, which are found in only about 2% of people worldwide.
The Genetics of Eye Color
The formation of hazel eyes results from polygenic inheritance, meaning it is controlled by the interaction of multiple genes. The final eye color is determined by the amount, type, and distribution of melanin within the iris. Hazel eyes have a moderate concentration of melanin in the front layers of the iris. This places them on the pigmentation spectrum between dark brown eyes, which have high melanin, and light blue eyes, which have very little.
The two genes with the largest influence on eye color are OCA2 and HERC2, both located on chromosome 15. The OCA2 gene provides instructions for making a protein that contributes to melanin production. The HERC2 gene acts as a regulator or switch for the OCA2 gene, determining how much melanin is ultimately produced.
Variations in these genes create the entire spectrum of eye colors. The specific combination of gene variants results in the moderate pigmentation and uneven distribution of melanin that leads to the hazel phenotype. This genetic process explains the mid-range pigmentation that allows for both light scattering and pigment absorption, creating their unique, mixed color.