Seeing Brown, Blue, Green, and Every Color in Between
A closer look at the science behind eye color.
Dec 10, 2018
For some people, a striking pair of uniquely-colored eyes may be the first thing others notice about them. But what causes our eyes to be different colors? And what role do genetics and ancestry play in determining eye color? Here we take a closer look at the science of eye color, and why our irises come in a variety of hues.
Behind the Science
To understand how melanin affects eye color, it helps to understand the basics of how our eyes perceive color. When we see color, different wavelengths of light corresponding to different colors on the visible light spectrum are actually being reflected into our eyes.1 The colored part of our eyes, the iris, contains a pigment called melanin which absorbs light. Light that is not absorbed by the melanin, is scattered and reflected by the iris.2 Eyes with high concentrations of melanin absorb more light than they reflect — this is what causes brown eyes to appear brown. Irises with loosely packed melanin scatter more light than they absorb and appear lighter in color — this results in hazel, green or blue eyes.3 Babies born with blue eyes can sometimes end up with brown eyes as melanin continues to be produced in their still developing irises.3
Around the World
The rarity of certain eye colors varies geographically. While brown is the most common eye color for individuals in Asia, Africa, and the Americas, in Europe, larger percentages of the population have lighter eye colors, such as blue or green.4,5 Finland has the largest percentage of blue-eyed individuals at a whopping 89 percent of the population.4 Scientists believe that the first pair of blue eyes belonged to an individual from South Eastern Europe who lived 6,000 to 10,000 years ago and all blue-eyed individuals since then are thought to be descendants of that individual.6
At the turn of the 20th century, nearly half of Americans had blue eyes – that number has since decreased to about 16 percent.7 Today, it is estimated that roughly 8 to 10 percent of the world’s population have blue eyes, making it second most common only to brown eyes at 55 to 79 percent of the global population.4 The rarest eye colors include hazel and amber, estimated at 5 percent; green at 2 percent; and gray, red, violet, and heterochromatic at less than 1 percent of the global population.
In some rare cases, individuals can be born with, or develop different colored eyes. This condition is referred to as heterochromia – “hetero” meaning different, and “chromia” meaning colors.8 When an individual is born with different colored eyes, this is known as congenital heterochromia.8 Those who develop different colored eyes later in life have what is called acquired heterochromia. In most cases, heterochromia is benign, but it can sometimes, especially in cases of acquired heterochromia, be indicative of a more serious condition.8 Causes of acquired heterochromia include eye disease or injury, and medication side effects.8
What Determines Eye Color
It’s a common misconception that comparing dominant and recessive genes alone can determine what color a baby’s eyes will be. It’s a little more complicated than brown being dominant and blue being recessive. Up to 16 genes influence eye color, which makes predicting eye color much more difficult.2 Parents with blue eyes can have children with brown eyes, an occurrence once thought to be impossible based upon the old model of eye color inheritance.2
While it is possible to temporarily change the appearance of your eye color through colored contact lenses, be sure to only use lenses prescribed to you by a licensed medical professional.