Red. Green. Red. Green. A simple pattern. Or so I thought, until I spent an hour at the Japanese elementary school my son attends. I had come in to do holiday crafts, and was showing the kids how to make a paper chain in Christmas colors. I told them to take a strip of red paper and bend it into a circle. Then take a strip of green paper and loop it through the red circle to start off a chain. Red. Green. Red. Green.

One boy couldn't get it right. His chain had three reds in a row followed by two greens. I was about to correct him when the teacher gently intervened. She separated his strips by color. "Take one from this pile, then one from that pile," she instructed. I noticed she didn't say the names of the colors. That's when I finally understood: The boy is colorblind!

Colorblindness is the inability to recognize certain colors. People with the most common form have problems distinguishing between red and green because a color detector in their eyes is either faulty or missing. Colorblindness is usually inherited, and affects males far more frequently than females. In Japan, about 5 percent of males are colorblind, which is slightly lower than the rate among men of European origin.