Colorblindness is most common among Caucasian boys and least common among African-Americans.

New research published in the journal Ophthalmology measured how common color blindness, or color vision deficiency (CVD), is among U.S. preschool children. The 4,005 children studied were part of the Multi-Ethnic Pediatric Eye Disease Study, based in southern California.

The study found that 5.6 percent of non-Hispanic white boys had some form of CVD, compared to 3.1 percent of Asian boys, 2.6 percent of Hispanic boys, and 1.4 percent of black boys. Only 0-0.5 percent to girls of any ethnicity were colorblind.

CVD is the partial or complete inability to see colors. Most people with CVD have difficulty seeing red or green, so their world is made up of shades of blue, yellow, and brown. Others can’t see the color blue, and a very small number can’t see any color at all.

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CVD is usually genetic and present at birth. However, it can also be caused by certain conditions, such as diabetes or high blood pressure, as well as by some medications.

The back layer of the eye, called the retina, is covered in light-sensitive cells called rods and cones. Rods detect black-and-white light and are quick to respond to motion. Cones are slower to respond, but break white light into three parts: red, blue, and green, just like a computer screen. If someone has a mutation in the genes that code for one of the three types of cones, then the cones of that type may not function properly, or may be missing entirely.

The genes for color vision are located on the X chromosome, which means that boys only need a single mutation in order to lose some ability to see color. Girls, on the other hand, have two X chromosomes, so they need mutations on both copies in order to lose their color vision. This is why boys have much higher rates of CVD than girls, and why non-CVD women can still pass CVD on to their sons.

Most mammals are dichromats, meaning they can only see two colors: blue, and a single yellow color consisting of all red, yellow, and green light. “At some point the human eye evolved to be able to divide the middle wavelength light between red and green light receptors,” said Kathryn Albany-Ward, founder of Colour Blind Awareness, in an interview with Healthline. “They then had the evolutionary benefit of being able to identify red berries against green leaves.”

This ability is most useful in the tropics, which might explain why people of African descent are still more likely to be able see all three colors, known as trichromacy. Meanwhile, as humans migrated to northern Europe, different traits became more useful.

“Being a dichromat means a person is more easily able to spot changes in shape or camouflage and movement, which are not as noticeable to someone with trichromatic vision,” said Albany-Ward. “For this reason, people with CVD were actively recruited in the second World War to spot the enemy hidden by camouflage because they did not notice the camouflage but instead were easily able to see the outlines of the hidden soldiers by their shape and movement.”

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CVD can be difficult to detect in children because they may be embarrassed by their inability to tell colors apart. Children might use incorrect colors in pictures, such as a purple sky or green faces. They may refuse to eat green vegetables, which will look an unpalatable brown. They are often hesitant or hold back in color-related activities, such as team sports where the children wear colored jerseys, or putting toys away in color-coded bins at playtime.

“Children who are color vision deficient are not drawn in the same way to those aspects of the image which have varying colors,” said Dr. Rohit Varma, a professor of ophthalmology at the University of Southern California’s Keck School of Medicine, in an interview with Healthline. “They may be viewed as children who are not particularly with it, or non-participatory, or whose attention is not on the material. Both the child and the teacher can be mutually disappointed in what occurs in class.”

Teachers may mistake this hesitancy for slowness in school, making it important to detect CVD as early as possible so children can get the help they need. “We have advocated that children ought to get an eye exam early, even as early as six months, for various conditions of the eye,” said Varma. “However, they’re not going to be able to tell if the child is color vision deficient until about four years of age.”

Although color vision tests are available online, Varma urges parents to take their children to pediatric eye care specialists if they suspect their child might have CVD. There are many forms of partial CVD that a home test might not detect.

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Since more than 1 in 20 boys have CVD, it is likely that every classroom will have at least one child with color vision difficulties. Here are tips for parents and educators to help children with CVD:

  • Good lighting can make it easier for children with partial CVD to recognize color.
  • Label materials by color, such as pencils and paints.
  • Don’t color-code boxes for toys or books, but tag them with different shapes.
  • Avoid color-on-color books and teaching materials. Black-and-white is best for children with CVD.
  • During sports and games, make sure the child can see who is on his or her team, and which tokens to use on game boards.
  • Use high contrast colors like blue and black on whiteboards. Avoid red, green, and pastel colors.
  • Use different textures, not colors, when creating color-coded materials such as pie charts.
  • Assign a buddy for tasks that require color differentiation. This is especially important later in education, when children are expected to perform tasks like reading color-coded maps in geography, graphs in mathematics, or litmus paper in chemistry.

CVD remains a problem for many people throughout their lives. Certain professions require the ability to tell apart colors, such as an electrician selecting wires, or a pilot watching for the indicator lights in an airplane cockpit.

“All of them, if they do those tasks, would have to compensate in some way,” said Varma. But he adds, “If you have a child or a relative with this condition, there are ways to help and still be very successful in life.”

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