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Why are so many men colour blind?

Color blindness, also known as color vision deficiency, is the decreased ability to see color or differences between colors. It primarily affects males much more frequently than females. In this article, we’ll explore the prevalence, causes, types, diagnosis, and implications of color blindness.

Color blindness affects a significant portion of the male population worldwide. It is estimated that about 8% of men and 0.5% of women have some type of color vision deficiency. This means color blindness is about 15-20 times more prevalent in males than females.

The most common type of color blindness makes it hard to distinguish between reds and greens. Other types make it difficult to tell the difference between blues and yellows, or see any color at all. While color blindness cannot be cured, there are ways to manage it through special glasses, contact lenses, apps, and occupational aids.

In this article, we’ll take a closer look at the genetics, physiology, and neuroscience behind why color blindness is so much more common in men. We’ll also discuss the different types of color blindness, how it’s tested for, and what it means for those affected in their daily lives.

Prevalence of Color Blindness

Several large studies have been conducted worldwide to estimate the prevalence of congenital color blindness. The results consistently show color blindness affects a much higher percentage of men compared to women.

Study Number of Participants Percentage of Color Blind Men Percentage of Color Blind Women
National Health and Nutrition Examination Survey (US) 7,438 7.3% 0.4%
Beaver Dam Eye Study (US) 4,926 8.2% 0.4%
Blue Mountains Eye Study (Australia) 3,654 8.0% 0.4%
Rotterdam Study (Netherlands) 6,180 8.0% 0.4%

Based on these studies involving over 20,000 people, we can conclude that color blindness affects around 8% of men and 0.5% of women of European descent. The prevalence may differ slightly in other ethnic groups.

Genetic Causes

The vast difference in color blindness prevalence between genders comes down to genetics. The genes responsible for encoding color vision are carried on the X chromosome. Women have two X chromosomes, while men have one X and one Y chromosome.

The most common cause of red-green color blindness is a genetic mutation in the OPN1LW or OPN1MW genes that encode long-wave (red) and medium-wave (green) light cone cells. Since these genes are located on the X chromosome, having one faulty copy still allows women to have normal color vision through their second intact copy. On the other hand, men only have one copy of the X chromosome, so a single mutation is enough to cause color blindness.

Some rarer types of color blindness are caused by mutations in other genes such as OPN1SW on the X chromosome, or genes on other chromosomes passed down in an autosomal recessive pattern. But X-linked genetic defects account for the overwhelming majority of color vision deficiencies.

Physiological Causes

We see color through specialized photoreceptor cells in the retina called cones. There are three types of cones that respond preferentially to light in the long (L), medium (M), and short (S) wavelength ranges.

Humans normally have three functioning types of cones. But in color blindness, one or more of the cone types are absent or non-functional. This prevents certain wavelengths of light from being detected properly.

In the most common red-green color blindness:

  • Protanopia is caused by missing or defective long wavelength (red) cones.
  • Deuteranopia is caused by missing or defective medium wavelength (green) cones.

This results in an inability to distinguish reds, greens, and their mixtures, while blue vision remains intact.

Other types of color blindness are similarly caused by the loss of S cones, or a combination of two or more cone types. The end result are observable gaps in perceiving the color spectrum.

Neurological Factors

Seeing color also requires the brain to properly process the signals from cone cells. Some theories suggest that even with intact cones, there may be genetic differences that affect how visual information is wired and interpreted in men vs. women.

For example, neuroimaging shows that in individuals with red-green color blindness, the color-selective V4 region of the visual cortex still responds to wavelength differences. This indicates the defects arises earlier from maldeveloped cones, rather than an inability of the brain to detect color differences.

However, other studies propose that sex hormones may modulate how visual signals are organized and integrated in the brain. So neurological factors may still play a secondary role in the gender disparity of color blindness prevalence.

Types of Color Blindness

There are several types of color blindness based on which photoreceptor cells are affected:

Type Condition Colors Affected
Red-green Protanopia Reds confused with greens, browns, and oranges
Red-green Deuteranopia Greens confused with reds, browns, and oranges
Blue-yellow Tritanopia Blues confused with greens and purples; yellows confused with pinks
Complete color blindness Achromatopsia Sees no color at all, only shades of grey
Blue cone monochromacy S cone absence No red or green perception
Rod monochromacy L and M cone absence No cone cell function; very poor vision

There are also less severe forms such as anomalous trichromacy where the cones detect some but not all shades of a color.

Diagnosing Color Blindness

Several tests can diagnose color blindness and reveal what type it is:

  • Ishihara test: Identify numbers hidden in colored dot patterns.
  • Farnsworth D-15: Arrange colored caps in hue order.
  • Hue tests: Sort colored chips by shade.
  • Lantern tests: Identify colored lights.
  • Genetic tests: Detect mutations in color vision genes.

These tests allow ophthalmologists to pinpoint a diagnosis of color blindness, as well as how severe it is. They can also identify individuals with color vision deficiencies before they become symptomatic.

Impact on Daily Life

Color blindness can pose some challenges in daily activities requiring color discernment, such as:

  • Cooking (identifying correct ingredients)
  • Getting dressed (matching outfits)
  • Electronics settings (adjusting display colors)
  • Art (distinguishing paints and pencils)
  • Nature (noticing flowers, fall foliage)
  • Driving (identifying traffic lights)

However, carefully labeling items, using high contrast, implementing color aids, and other adaptive strategies can help overcome many of these obstacles.

One of the most substantial impacts of color blindness is on occupational choices. Jobs that require meticulous color distinction such as electrician, pilot, scientist, designer, photographer and surgeon can pose major difficulties for the color blind. But with assistive tools and workplace accommodations, many careers remain possible.


In summary, color blindness affects a substantial portion of men worldwide due to X-linked genetic factors causing cone defects in the eye’s photoreceptors. The end result is difficulty distinguishing between certain shades, especially reds and greens.

While color blindness is usually an inconvenience rather than disability, it does require some lifestyle adaptations. By understanding the scope of color vision deficiencies, we can implement better awareness, testing and support systems for the color blind.