They say the eyes are "the window to the soul" and there's certainly much to be told about the enigmatic complexity of their varied colors.
Dr. Rupa Wong, a board-certified ophthalmologist who owns the Honolulu Eye Clinic with her ophthalmologist husband in Hawaii, told Newsweek: "Eye color is complex and is controlled by 16 different genes, not just the single gene that we all previously learned about in high school."
Eye color is determined by the amount and type of pigments in the front part of the iris (the colored portion of the eye) as well as by "the scattering of light by the turbid medium in the stroma [a fibrous layer of tissue] of the iris," Dr. Jovi Boparai, an ophthalmologist and ophthalmic surgeon, told Newsweek in February.
Wong noted: "There are many misconceptions about eye color—the main one being that it's simply recessive or dominant in terms of color."
Other common misconceptions include that "two blue-eyed parents cannot have a brown-eyed baby or that blue eyes are the most rare eye color," she added.
Here we unpack the complexities behind three of the most fascinating eye colors, including the rarest in the world, as explained by Wong.
Blue Eyes Are Not the Rarest
According to a study published in January in Scientific Reports, "the most common iris color in the world is brown, accounting for about 79 percent."
Blue eyes are actually the second-most common eye color in the world, Wong said in a video post on Instagram back in December last year.
Wong explained that everyone with blue eyes is linked to "a single common European ancestor from six to 10,000 years ago."
People with blue eyes have brown pigment underneath their iris and blue eyes tend to be more sensitive to light, the doctor says in the post.
The ophthalmologist also said that not all babies are born with blue eyes, "only 20 percent are." Wong noted that a study at Stanford University found that 40 percent of the eyes of blue-eyed babies got darker by the time they were two years old.
An August 2016 Stanford University study published in Acta Ophthalmologica found that "the birth prevalence of iris color was 63 percent brown, 20.8 percent blue, 5.7 percent green/hazel."
The 'Rarest' Eye Color
Only 2 percent of people have green eyes, "making it the rarest eye color, " Wong said in another post on Instagram in December.
Boparai told Newsweek in February that the rarity of green eyes varies across the globe. In Europe, they're found in around 8 percent of the population. However, "in other parts of the world, like Africa and East Asia, they're much less frequent, occurring in less than 1 percent of people," he added.
Wong notes in the video that "there's a village in China called Liqian, where two thirds of the people have blonde hair and green eyes."
The Liqian people live in Yongchang county in the Gansu province of northwest China, notes a June 2007 study in the Journal of Human Genetics.
"A small proportion of the Liqian people have been described as having mixed racial morphological traits," the study said, noting that they were believed to be descendants of ancient Roman legionnaires.
However, further analysis confirmed that "a Roman mercenary origin could not be accepted as true according to paternal genetic variation" and that the Liqian people are more likely to be a subgroup of the Han Chinese, according to the study.
"The genetic complexity of eye color is fascinating," Wong said in the Instagram post, explaining that since multiple genes contribute to eye color, it's why "it is possible for two green-eyed parents to have a brown-eyed baby."
Wong told Newsweek: "In addition, what gives blue and green eyes their color is not actually blue or green pigment but the reflection of light within the eyes (Rayleigh scattering) and lipofuscin pigment (yellow pigment) for green eyes."
Rayleigh scattering refers to "the elastic scattering of light" from atomic and molecular particles whose diameter is less than about a 10th of the wavelength of the light, explains the Encyclopedia of Color Science and Technology.
The 'Most Complex' Eye Color
Wong says "hazel eyes are thought to be the most complex and varied eye color," in another Instagram video post shared in December. This is down to their color being a mix of gold, brown and green.
Hazel eyes can change colors based on the lighting, the doctor notes, due to the Rayleigh effect. "In addition, bright light causes the pupil to constrict revealing more of the iris, which is lighter in color," Wong said in a caption shared with the post.
What you're wearing can also change the appearance of hazel eyes. "The reflective nature of clothes also causes hazel eyes to look either golden, green, or amber depending on what people wear," the ophthalmologist says in the caption.
Most people with hazel eyes were born with blue eyes and the eyes darkened over time as more melanin was produced, Wong explains in the post, adding that the reverse, where brown eyes lighten to a hazel shade, "usually doesn't happen."
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Uncommon Knowledge
Newsweek is committed to challenging conventional wisdom and finding connections in the search for common ground.
Newsweek is committed to challenging conventional wisdom and finding connections in the search for common ground.
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