Red-green color blindness affects roughly 1 in 12 men and 1 in 200 women worldwide, making it by far the most common type of color vision deficiency. That translates to about 8% of males and 0.5% of females with some degree of difficulty distinguishing reds, greens, yellows, and browns from one another.
Why Men Are Affected Far More Often
The genes responsible for detecting red and green light sit on the X chromosome. Men have only one X chromosome, so a single faulty copy is enough to cause color blindness. Women have two X chromosomes, meaning a working copy on the second X can compensate for a defective one on the first. For a woman to be red-green color blind, she needs to inherit the defective gene from both parents, which is statistically much less likely.
These two genes are remarkably similar to each other. One produces a pigment sensitive to yellow-orange (long-wavelength) light, and the other produces a pigment sensitive to yellow-green (medium-wavelength) light. Because they’re so alike, they occasionally swap material during reproduction, deleting key segments or creating hybrid genes that produce a pigment tuned to the wrong wavelength. This swapping is the most common cause of red-green color blindness and explains why it persists at such high rates generation after generation.
Rates Vary by Ethnicity
Not every population carries the same prevalence. A large U.S. study of preschool-aged children, the Multi-Ethnic Pediatric Eye Disease Study, tested thousands of kids and found significant differences among boys:
- Non-Hispanic white boys: 5.6%
- Asian boys: 3.1%
- Hispanic boys: 2.6%
- Black boys: 1.4%
Among girls, rates were under 0.5% across all groups. The difference between white and Black boys was statistically significant, with white children roughly four times more likely to have a color vision deficiency. Asian children fell in between and didn’t differ significantly from any single group. These variations likely reflect differences in how frequently the relevant gene variants appear in each population’s gene pool.
The Two Main Subtypes
Red-green color blindness isn’t one condition. It splits into two families based on which pigment is affected: protan defects (red-sensing pigment) and deutan defects (green-sensing pigment). Each family has a milder form, where the pigment works but is shifted to the wrong wavelength, and a more severe form, where the pigment is absent entirely.
Deutan defects are considerably more common. In studies of color vision among male children, the pattern consistently ranks from most to least common: deuteranomaly (mild green deficiency), then deuteranopia (absent green pigment), then protanomaly (mild red deficiency), then protanopia (absent red pigment). Deuteranomaly alone accounts for roughly half of all red-green color blindness cases, making it the single most prevalent form. People with deuteranomaly often don’t realize they see color differently until they’re formally tested, because the shift is subtle enough to go unnoticed in daily life.
How It Gets Diagnosed
Most people first encounter color vision testing through Ishihara plates, those dotted circles with numbers hidden inside patterns of colored dots. The Ishihara test is fast and effective at detecting red-green deficiency, but it can’t precisely classify which subtype you have or how severe it is.
For that, the gold standard is an instrument called an anomaloscope. You look through an eyepiece and adjust two colored lights until they match. The specific settings you choose reveal exactly which type of deficiency you have and how far your color perception deviates from typical vision. Other tests, like the Farnsworth-Munsell 100-Hue Test, ask you to arrange colored caps in order and score your errors to grade severity as mild, moderate, or strong. In practice, most people are diagnosed with just the Ishihara plates and never need the more detailed instruments unless a job or military application requires precise classification.
Color Blindness You Aren’t Born With
Most red-green color blindness is genetic and present from birth. But color vision can also deteriorate later in life. Eye diseases like glaucoma and age-related macular degeneration can damage the cells responsible for color perception. Neurological conditions, including multiple sclerosis and Alzheimer’s disease, sometimes affect the brain’s ability to process color signals. Certain medications, particularly hydroxychloroquine (used for rheumatoid arthritis), can alter color vision as a side effect. Cataracts, which cloud the lens of the eye, gradually shift and dull color perception as they progress.
The key difference is that inherited color blindness stays stable throughout your life and affects both eyes equally, while acquired color vision loss tends to worsen over time and may affect one eye more than the other.
Jobs That Require Color Vision
Red-green color blindness matters most in careers where misreading a color could cause a safety hazard. The strictest requirements apply to roles involving signal lights and color-coded systems:
- Pilots and air traffic roles: Must pass Ishihara screening and confirm the ability to identify red, green, and white signal lights.
- Merchant navy and navigation: Color blind applicants are generally not permitted, since misidentifying a colored signal at sea could cause collisions.
- Railways: Must pass all color vision tests. There is no accommodation for partial deficiency.
- Armed forces: Standards vary by branch. Air force roles typically require near-perfect color vision, while army and navy positions may allow milder deficiencies if applicants pass lantern tests at specified distances.
- Electrical work: Requires good color vision to safely identify wire colors.
- Police and fire services: More lenient. Police forces generally accept mild anomalous color vision and provide coping strategies for more severe cases. Fire services accept minor defects.
Medicine has no universal minimum color vision standard, though individual specialties like pathology or dermatology may present practical challenges. Many color-blind physicians and pharmacists work successfully with adaptations.
Living With It
For most people, red-green color blindness is a mild inconvenience rather than a disability. You might struggle to tell ripe from unripe fruit, confuse certain clothing colors, or have trouble with color-coded charts and maps. The severity varies enormously. Someone with mild deuteranomaly may only notice problems in poor lighting, while someone with complete protanopia sees no red at all and perceives stoplights and warning signs very differently.
One intriguing theory for why these genes remain so common is that color-blind individuals may actually have an advantage in certain visual tasks. Researchers have explored whether people with color vision deficiency are better at detecting camouflaged objects, since they rely more heavily on brightness and texture differences rather than color. This “dichromatic advantage” could have helped ancestral hunters spot prey hiding in foliage, potentially keeping the genes circulating at higher-than-expected rates in the population.