The Science Behind Brown and Hazel Eyes: What Makes Them Different?

Did you know? Brown eyes account for 70-80% of the world’s population, while hazel eyes account for less than 5%. This striking difference is more than just a numerical difference. Many people mistakenly believe that hazel eyes are just a variation of light brown. Still, in fact, the two eye colors are fundamentally different.
Let’s explore the science behind brown and hazel eyes and uncover their differences, from genetics to optical properties.

Color composition 

Brown Eyes

• Color Composition: Mainly determined by the high levels of melanin in the iris. Melanin is a dark pigment that absorbs most of the light, giving the eyes a single hue from light brown to dark brown or even almost black.
• Pigment Content: Relatively high levels of melanin.

Hazel Eyes

• Color Composition: A more complex color, it is the result of the combined effects of melanin and lipochrome. There is less melanin in the iris, and lipochrome is present (which is usually associated with green and yellow tones). This combination results in a variety of colors such as brown, green, golden yellow, and even amber.
• Pigment Content: Relatively low levels of melanin, and contains lipochrome.

Color performance

Brown Eyes: Usually a single shade of brown, ranging from light brown, medium brown, to dark brown or even a dark brown close to black. The depth of its color depends mainly on the amount of melanin in the iris.

Hazel Eyes: A complex and variable color, usually a brown, green, and golden (or amber) mixture. These colors may appear as spots, stripes, or rings on the iris.

There are significant differences in color expression between brown eyes and hazel eyes. Brown eyes appear a uniform brown tone because the dense melanin in the iris absorbs most of the incident light, creating a profound and steady visual effect. In contrast, hazel eyes are more lively.

Color changes under light

Brown eyes:

They are relatively stable under light. Since brown eyes contain a higher concentration of melanin, they absorb most of the light. Hence, the eye color maintains a consistent brown tone under different lighting conditions, angles, or environmental colors, and does not show significant color changes.

Hazel eyes:

They have the characteristic of “changing color” under light, and their color expression is stronger. In bright light, they show multiple layers of colors such as green, gold, and light brown, making the eyes look more lively. The color tends to be dark brown in dim light, and the green and gold tones are weakened.

Genetic mechanisms

The formation of brown eyes is mainly determined by the strong expression of OCA2 and HERC2 genes, which promote the production of a large amount of eumelanin in the iris, making the eyes appear relatively uniform in color from light brown to dark brown, and even nearly black.
Hazel eyes are the result of the combined action of multiple genes. In addition to OCA2 and HERC2, genes such as SLC24A4 and IRF4 that regulate pigment distribution are also involved. The pigment content is moderate and unevenly distributed, forming iris characteristics with mixed colors like brown, green, and gold.

Genetic mechanism

The inheritance of brown eyes is relatively simple, mainly controlled by the OCA2 and HERC2 genes, and is a single-gene dominant inheritance. The mechanism is that HERC2 regulates the high expression of OCA2, produces a large amount of eumelanin, and forms a stable brown hue. This inheritance is highly predictable. If parents carry dominant genes, their children will basically have brown eyes.

The inheritance of hazel eyes is much more complicated and belongs to a multi-gene codominant inheritance. In addition to the specific heterozygous type of the OCA2 gene, multiple modifying genes such as IRF4 and SLC24A4 are required to work together. This mechanism causes uneven distribution of pigments, resulting in unique mixed tones, incomplete penetrance, and diversity in its inheritance. When both parents are hazel, children may have multiple eye colors, explaining the rarity of hazel eyes and the relative prevalence of hazel eyes in mixed-race populations.

Geographical and ethnic distribution

Brown eyes

Brown is the most common eye color in the world, accounting for about 70% to 80% of the world’s population. This eye color shows pronounced differences in regional and racial distribution:

• Geographic distribution: Brown eyes dominate in Africa, Asia (especially East Asia, Southeast Asia, South Asia, and West Asia), and most of the Americas. In these regions, dark brown eyes are ubiquitous. In Europe, West Asia, and the Americas, brown eyes are also common. Still, the proportion of light brown tones may be higher.
• Racial differences: Brown eyes are mainstream among Africans, Asians (including East Asian, Southeast Asian, South Asian, and West Asian descent), Latin Americans, and Native American descendants. This is related to the evolutionary protection mechanism that people living in areas with intense sunlight have higher melanin in their eyes to resist ultraviolet rays.

Hazel eyes

Hazel eyes account for about 5% of the global population. It is less common than brown eyes, but more common than blue or green eyes. Its regional and racial distribution also shows a specific concentration:

• Regional distribution: Hazel eyes are more common in North African, Middle Eastern, Brazilian, and Hispanic populations. There is also a considerable proportion of hazel eyes in Europe; for example, in countries such as France, Germany, and the United Kingdom, the proportion of hazel eyes (often classified as “intermediate”) is not low.
• Racial differences: Although hazel eyes can appear in people of any ethnic background, they are more common in people of European and Middle Eastern descent. This reflects the complex genetic mixture and multiple combinations of pigment genes in these regions.

Medical characteristics

Brown eyes

• Stronger photoprotection:
  Due to their higher concentration of eumelanin, brown eyes absorb and filter ultraviolet (UV) and bright light more effectively, providing natural protection against strong sunlight.
• Lower photosensitivity:
  Dark irises make brown eyes less susceptible to glare in the sun. They are less likely to suffer from photosensitive eye diseases (such as photoinduced headaches and photokeratitis).
• May be more susceptible to pigment-related inflammation:
  Some studies have shown that dark irises may be more susceptible to certain types of pigmentary uveitis or anterior iris inflammation. However, the risk difference is not significant.

Hazel eyes

• More sensitive to light:
  Hazel eyes have lower pigment content, which makes light more easily reflected after entering the eye, which may lead to higher light sensitivity and glare reactions.
• Potentially higher risk of UV damage:
  Less pigment means a weaker ability to filter UV rays, and prolonged exposure to bright light areas may increase the long-term risk of cataracts or macular degeneration (compared to brown-eyed people).
• Less associated with specific eye diseases:
  Although hazel eyes are less often “linked” to particular diseases in medical literature, they retain a certain degree of protection compared to lighter eye colors (such as blue and green).