The Stroop effect occurs when people attempt to name the color of words that spell out a conflicting color. The Stroop effect is named after J. R. Stroop, who first explored this phenomenon way back in the 1930s. In the Stroop task you are confronted with an interference situation: On the one hand, the word spells out a particular color, such as red; on the other hand, the color of the word is a color such as green. Because reading is so highly practiced, it is hard to suppress the spelled-out word and report the color name.

We know a great deal about the Stroop effect, but explaining the difficulty people have in this task has not proven easy. In fact, we still don’t fully understand why the effect occurs even today. As you learn more, you may be the one who cracks this ancient mystery of human thinking.