Is colorblindness an evolutionary advantage?

Dear Cecil:

I heard about a soldier who had been a spotter in helicopter patrols because, being colorblind, he could pick out camouflage from background foliage more easily. I vaguely remember reading about a study linking colorblindness in animals to selective pressure. Is there an evolutionary advantage to being colorblind?

Cecil replies:

This may seem counterintuitive, but the answer sometimes is yes.

Let’s put this squarely. The case can be made that acute color vision is a primitive trait that tends to disappear as organisms and societies become more advanced. In fact, we’ve already lost acute color vision once in our evolutionary history, then got a version of it back later. If you’re the feisty type, you can argue that color vision is an obstacle to progress, and that in the perfect world of the future, we’ll see everything in black and white.

OK, I’m exaggerating, but you can’t dismiss the idea out of hand. Let’s start with the claim that colorblind soldiers are better at seeing through camouflage. This notion has been kicking around in military and scientific circles since at least 1940, but despite the obvious usefulness of such an ability, for a long time no one bothered doing any formal investigation other than one unpublished military study indicating there was nothing to it.

More recent research, however, suggests maybe there is.

In a study published in 1992, scientists flashed a pattern on a video monitor: a 30-by-30 grid of small rectangles, all of them oriented vertically except within a randomly located 7-by-7 “target area” where the rectangles were horizontal. Test participants were asked to press one of four buttons to indicate what quadrant of the screen the target area had appeared in. Some test participants had normal color vision, while others, so-called dichromats (I’ll explain below), had severe colorblindness rendering them incapable of distinguishing red from green.

In the first trial, all the rectangles were the same color, and participants in both groups had little difficulty spotting the target. In the next trial, the rectangles were randomly colored red or green. This time around, those with normal color vision did poorly — all they saw in the brief time the pattern appeared (a fifth of a second) was a jumble of red and green.

The dichromats, on the other hand, kicked butt. Without color to distract them, they spotted the target as easily as with a monochrome pattern. The same held true when each target rectangle was replaced with a capital A while the background rectangles were replaced with Bs. Despite this conspicuous difference, randomly coloring the letters red or green completely flummoxed those with normal color vision. The dichromats, on the other hand, were unperturbed, picking out the target as effortlessly as before.

But that only happened when the colors were red and green. When the colors were red and blue, which looked different to the dichromats, they were just as confused as those with normal color vision.

Admittedly it’s a long leap from a lab experiment to the battlefield, and an even bigger one to say human color vision is evolving into a less sophisticated state. But consider:

  • About 8 percent of males have some form of colorblindness. Sure, we civilized softies coddle our defectives now rather than letting them die on hillsides. However, one researcher claims, 8 percent is more than can be explained by random variation. He speculates colorblindness may offer a positive reproductive advantage.
  • One UK study found that colorblindness was most common in the urbanized southeastern part of the country, which had been repeatedly overrun by invaders. It was much less so in the more rural north and west, where the inhabitants were more likely to have descended from Britain’s original primitive tribes.
  • We mammals have much less elaborate color vision than many species below us on the food chain. Most birds and fish are tetrachromats, meaning their retinas have color receptors, or cones, specializing in four different hues, enabling them to see colors in the ultraviolet range invisible to us. Mammals generally are dichromats, with only two types of cone and thus crude color vision. It’s thought that’s because most mammals are nocturnal, where the advantage is in having more rods in your retina, providing better night vision and color vision be damned.

Even today most primates are dichromats. Only a few species including humans are trichromats, with three types of cone, a trait we’re thought to have re-evolved when our ancestors took to foraging in daylight and better color vision improved their ability to find fruit.

Today if you want fruit you just go to the Safeway. Does that mean our color vision will again deteriorate, or will the need to distinguish colors to avoid car wrecks and win at video games stem the monochromatic tide? Give it a few thousand years and we’ll find out.

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