The appearance of human eye color is one of the most noticeable and varied traits across populations worldwide. This spectrum of color, ranging from the darkest brown to the lightest blue, results from complex biological mechanisms and genetic inheritance. Hazel eyes stand out for their multi-toned, often shifting appearance, leading many to wonder about their frequency. Understanding whether hazel eyes are truly rare requires exploring the physical science and the intricate genetic instructions that govern their development.
The Visual Science of Hazel Eyes
Hazel eye color is created by a moderate, specific concentration of the dark brown pigment known as melanin. All human eye colors, except for certain forms of albinism, are derived from different quantities and distributions of melanin within the iris. Brown eyes have high levels of melanin, while blue eyes have very low levels.
Hazel eyes occupy an intermediate position, containing more melanin than blue or green eyes but significantly less than brown eyes. This melanin is primarily contained in the anterior border layer of the iris, and its moderate concentration allows for a dual-color effect. The multi-hued appearance is enhanced by the way light interacts with the stroma, the fibrous tissue layer of the iris.
This interaction is governed by a phenomenon called Rayleigh scattering, which causes shorter wavelengths of light (blue and green) to scatter more efficiently than longer wavelengths. Since hazel eyes have enough pigment to absorb some light but not enough to mask the scattering completely, the light is both absorbed (creating the brown/gold tones) and scattered (creating the green/blue tones). The uneven distribution of the pigment across the iris gives hazel eyes their characteristic blend and the ability to shift color depending on the lighting conditions.
Global Prevalence and Comparative Rarity
Globally, hazel eyes are relatively uncommon, affecting an estimated 5% to 8% of the world’s population. This places them as the third most frequent eye color, following the overwhelming global dominance of brown eyes. Brown eyes are present in approximately 70% to 79% of people worldwide, while blue eyes account for 8% to 10%.
The true rarity is reserved for green eyes, which are found in only about 2% of the population. While hazel is far from the most common color, it is not the rarest, positioning it in a distinct, less-frequent category. The prevalence of hazel eyes varies significantly by geography, with rates much higher in certain populations where various ancestries have blended over time.
In the United States, the concentration of hazel eyes is notably higher, estimated at 18% of the population. This contrasts sharply with regions like East Asia and sub-Saharan Africa, where brown is nearly universal. Higher rates of hazel eyes are also found in parts of Europe, the Middle East, and Brazil, reflecting the genetic mixing that contributes to this intermediate color.
The Complex Genetics of Eye Color
The reason hazel eyes are relatively rare is rooted in the complex, polygenic nature of eye color inheritance. Early models suggesting a simple dominant/recessive relationship (brown dominant over blue) have been replaced by a system recognizing that eye color is influenced by many genes, possibly up to 16. The final color is not the result of a single switch but a cumulative effect of these genes working together.
Two genes, OCA2 and HERC2, located on chromosome 15, have the largest influence on eye color variation. The OCA2 gene provides instructions for creating the P protein, which is involved in the maturation of melanosomes (structures that produce and store melanin). The amount of functional P protein directly influences the total amount of melanin deposited in the iris.
The HERC2 gene acts as a “master switch” for OCA2, controlling how much of the P protein is actually produced. Light eye colors, such as blue, are often associated with a mutation in HERC2 that effectively reduces the expression of OCA2, leading to minimal melanin production. For a person to have hazel eyes, the genetic combination must result in an intermediate activity level of the OCA2 gene.
This intermediate level must produce enough melanin to prevent the pure scattering effect seen in blue eyes but not so much that the scattering is completely masked, as in brown eyes. Achieving this middle-ground concentration of pigment requires a precise combination of alleles across multiple genes. This makes the genetic blueprint for hazel eyes less probable than the high-melanin (brown) or very low-melanin (blue) extremes. The complexity of this genetic blending explains why hazel eyes are an uncommon, multi-hued phenotype.