Exploring the biology and psychology behind eye color
Eyes are a “window to the soul,” and to the wonders of the world–all that we know and love, experience and discover, ponder and cherish. They are precious portals and natural marvels. Our eyes express emotions: settle in deep contemplation, shimmer with enlightenment and discover, burn in anger, blaze with passion, pool with tears, shine in admiration, soften in love and compassion, darken in grief and pain. Eyes can be organs of physical sight and spiritual blindness, or physical blindness and spiritual sight.
Physiologically, eyes are a crucial part of the visual system. They provide organisms with the ability to receive and process visual detail, as well as enabling several photo response functions independent of vision. Eyes detect and collect light from the surrounding environment, regulate its intensity, focus it through an adjustable assembly of lenses to form an image, converts it into electro-chemical impulses, and transmits these signals to neural pathways connected to the eye.
We may know how eyes work, although the optical organ is incredibly complex, but what do we know about eye color? Why do so many people have brown eyes? And why does it seem that some individuals have eyes that alter in coloration? Furthermore, what does one’s eye color reveal about his or her personality and character?
What Determines Eye Color?
We once used eye color charts to predict the eye colors of children. In the most simplified versions of these charts, brown eyes are considered dominant over blue and green eyes, while green eyes are dominant over blue. While these concepts generally hold true, the genetics of how we inherit eye colors turn out to be far more complicated. You cannot simply determine the eye colors of grandparents and parents, then calculate the odds of what color your child’s eyes will be. In fact, you can belong to a family of brown-eyed individuals and still have green or blue eyes. Contrary to popular belief, it is also possible for two blue-eyed parents to have a brown-eyed child.
The simple answer to what determines eye color is genes, but it really isn’t that easy. Other factors can affect your eye color as well, such as blood vessels, your age, disease and injury, health, and so on. In general, though, eye color depends on the amount and distribution of pigment found in the iris, as well as the frequency-dependence of light scattering by the turbid medium of the stroma. Scientists have isolated up to 16 genes that assist in some fashion with determining your eye color. Two genes, in particular, in Chromosome 15—HERC2 and OCA2–play the most significant role in eye color.
Eye color is located on the iris, a cluster of muscles in the eye that help dilate and narrow the pupil to control the amount of light entering your eye. The iris has two layers, the stroma, and the iris pigment epithelium. The amount of color you perceive when looking at someone’s eyes is a product of protein pigments.
Cells, called melanocytes, in your epithelium, contain melanosomes like eumelanin or pheomelanin. Originally, scientists thought that the amount or absence of melanocytes played a large role in determining your eye color. However, the number of melanocytes in human eyes seems to remain fairly constant. Therefore, scientists now believe that it is the
Melanin dies your hair, skin, and eyes. It is naturally brown or black, so we all really have the same color of eyes. The reason we see different colors in eyes is the same reason that the sky is blue: Rayleigh scattering.
Your eye color is controlled by the amount of melanin in your iris; more melanin means that your eyes absorb more light rather than reflect it. The OCA2 gene produces melanocytes, melanin-producing cells. The color of melanin is determined by EYCL1, EYCL2, and EYCL3, which account for brown, green, and blue eyes, but not for hazel or gray eyes. People with albinism have the OCA2 gene completely shut off.
When we are born, all our genetics are decided, but the body has not necessarily acted on all of that genetic information. When it comes to eye color, the melanin controlled by the OCA2 gene is diluted, and thus, most Caucasians have blue eyes as babies. Those with the OCA2 mutation keep their blue eyes, while for others, the OCA2 gene kicks into gear to produce more melanin and make more protein pigments, causing the eyes to change colors. As children grow up, their eye color can change to a darker blue, green, or brown. Between 9 months to 2 years after birth, the eye color tends to set; however, the process can continue until age 12. Babies of Asian, African, and Hispanic genetic heritages will likely be born with dark eyes that remain dark.
In humans, brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes the iris to absorb light of both shorter and longer wavelengths. Thus, the eyes maintain a natural brown coloration. Dark pigment of brown eyes is common in Europe, Asia, Oceania, Africa, and the Americas. The majority of people–around 50% of the population–in the world possess some shade of brown eyes. Light or medium-pigmented brown eyes are commonly found in South Europe, among the Americas, the Middle East, and South Asia.
Some research indicates that people with brown eyes are actually at a lower risk for melanoma of the eye (intraocular melanoma), perhaps due to the fact that brown eyes contain more pigmentation than blue eyes and are better able to block the harmful UV rays of the sun. Although brown eyes may filter sunlight better reducing eye cancer risks, this does not mean people with dark eyes do not need sunglasses. Some research suggests that cataracts are more common in individuals with brown eyes.
People with brown eyes are more likely to contract vitiligo than people with blue eyes. Vitiligo is a disease which causes white patches, the result of a loss of melanin, to develop on the skin. Since people with blue eyes often have less melanin in the skin, they are less likely to develop this condition. A recent study showed that 43% of participants with vitiligo had brown eyes as compared to 27% of total study participants.
Some research suggests that men with brown eyes may have more dominant facial features than those with blue eyes. In the study, researchers showed participants pictures of men with both eye colors and then asked the people to rank the men based on dominance. The researchers then presented the same exact images to another group with a slight alteration; using Photoshop, they had changed the eye colors. In both cases, participants ranked men with brown eyes as more dominant, even when the color was changed to blue. Some hypothesize that the results suggest that eye color is closely related to genes that control testosterone production, causing changes to hormone production and creating stronger facial features in male children with brown eyes.
At large, though, we often associate the following characteristics with individuals who possess brown-colored eyes:
- Are loyal, independent, and have a knack for exploring
- Often make the best leaders, exuding strength and confidence
- Have a keen sense of elegance, showing up in style and flowing with grace; very adorable
- Can cheer others up and make them laugh
Those with brown eyes often make good athletes. In fact, the extra melanin in their bodies insulates the electrical connections between brain cells, allowing for faster reaction times. Unfortunately, research conducted at the University of Pittsburgh found that women with dark-colored eyes seemed to experience more pain when giving birth compared with mothers with light-colored eyes. They are also more susceptible to depression, negative thoughts, and sleep disturbances. Chronobiology International published a study demonstrating that those with brown eyes tend to sleep two hours less than those with light-colored eyes, generally have poorer sleep cycles, and have difficulty waking up in the morning.
Black eyes contain the highest concentration of melanin in the iris. As a result, the iris absorbs all of the light waves entering the iris, causing us to perceive a very dark brown or black color. It is estimated that only 1% of the world’s population possesses brown eyes, generally individuals from Africa, Asia, and indigenous Native Americans.
Black eyes can also occur from aniridia, which means “without iris,” which provokes the absence of an actual iris in both eyes. One can inherit aniridia from his or her parents, but the disease may also appear out of nowhere. While these black eyes might look pretty, aniridia can cause the loss of vision, photophobia, genitourinary anomalies, and kidney failure.
Not surprisingly, many characteristics we associate with black eyes align closely with those attributed to individuals with brown eyes. We consider people with black eyes to:
- Be very trustworthy and reliable
- Possibly be shy in their early years
- Focus on financial and emotional security as they grow older
- Be very kind
- Prefer talking about spirituality, not gossip
- Excel in sports
- Struggle with sleeping
Scientists have hypothesized that this eye color is the result of materials found in the bloodstream due to an unstable liver—in fact, many people with this eye color do struggle with liver problems. Another theory suggests that the melanosomes produce less melanin than you would find in brown eyes. In other words, hazel eyes result from a combination of Rayleigh scattering and a moderate amount of melanin in the iris’ anterior border layer.
This color has many variations, depending on the exact amount of melanin contained in the iris and the amount of light hitting the eyeball. Hazel itself has differing definitions: some consider it synonymous with light brown or gold, others propose that it signifies a hue of green and gold.
To add to the confusion, hazel eyes often appear to shift in color from a brown to green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is why many individuals might mistake hazel eyes to be amber and vice versa. The biology behind hazel eyes can sometimes produce a multicolored iris; your eye might be light brown/amber near the pupil and charcoal or dark green on the outer part of the iris (or vice versa), especially when observed in sunlight.
Hazel eyes occur throughout Caucasoid populations, particularly in regions where blue, green and brown eyed peoples intermix.
Since hazel eyes fall into the category of ‘light eyes,’ individuals with such an ocular coloration may be more prone to develop intraocular or uveal melanoma than those with dark eyes. The same holds true regarding macular degeneration.
Much like black eyes, hazel eyes can come from a disease known as Wilson’s disease. Wilson’s disease is a genetic mutation that prevents copper from being properly processed. As a result, copper accumulates in the body’s tissues, including the eyes. The copper deposits lead to Kayser-Fleischer rings, which are dark circles around the eye, lending a hazel-like hue to the iris.
We associate both negative and positive qualities to people based on their eye color. Some characteristics attributed to individuals with hazel irises include:
- Unpredictable, spontaneous, but also fun-loving, much like their shifty eye color
- Bore easily and may not bode well in long relationships
- Possess a strong sense of adventure and passion for risk-taking
- Have a tendency to disengage when not stimulated
- Can be sharp and witty
- Avoid conflict, but when it does occur, they learn from an early age to use their wit and cunning
We have reached the last eye color we will discuss in this blog post: amber eyes.
Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This may be due to the deposition of the yellow pigment called lipochrome in the iris, found in green eyes. The iris also has a lower deposit of melatonin and a high concentration of pheomelanin. Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually comprise many other colors, including green, brown, and orange. Furthermore, hazel eyes may appear to shift in hue, be duller, and contain flecks and ripples, while amber eyes are of a solid gold hue.
Although we consider amber similar to gold, some people have russet- or copper-colored amber eyes. The iris might assume a light goldish gray as well.
Amber eyes are often confused with central heterochromia in addition to hazel eyes. Central heterochromia occurs when the iris closest to the pupil has a different color compared to the outer parts. If you truly have amber eyes, the color presents itself throughout the entire iris.
Generally, you will find individuals with amber eyes in European countries, such as Romania, France, Spain, Sweden, Italy, or Slovenia, although people from Brazil and Russia might also possess the unique hue.
When it comes to specific diseases unique to amber eyes, there are generally none. Instead, people with amber eyes fall into the category of “light eyes.” Due to the lack of melanin in the iris, they are prone to detached retinas as a result of sudden exposure to extremely bright light. The increasing incidence of carcinogenic melanomas for lighter colored eyes also stems from the lack of melanin, which acts as a protective agent in darker eyes. Keeping annual appointments with the optometrist or as recommended is crucial to maintaining healthy, luminous amber eyes.
You might have heard amber eyes called, “wolf eyes.” The association stems both from the resemblance in color and from the personality traits individuals with the eye color often possess. In addition to the following list, folklore proposes that these people get a little crazy during a full moon. They desire the wilderness, and enjoy camping. This wish, as the story goes, finds its basis in the instinctual part of the wolf which likes to howl and bark at the moon. While people do not actually howl or bark, the full moon does invigorate and propel them outdoors.
- Social, need to fit in
- Can be cunning
- Considered very appealing and most alluring eye color
- Might be a bit
reserved,but are very warm and great conversationalists
- Are wise and trustworthy
- Can hold the attention of anyone who meets them
That concludes the first part into our brief dive into the fascinating world of eye color. Hopefully, you enjoyed this post and learned something new! Stay tuned for next week’s post, Window to the Soul Pt 2.