The Stroop Effect is an interesting effect first popularized in the 1930s by John Ridley Stroop. The effect is so persistent and so strong that Stroop Testing, which tests the effect in an individual, is sometimes used in psychological testing and assessment. People are often introduced to the Stroop Effect in beginning psychology classes as they learn about how their brains process information.

This effect is a demonstration of interference, in which the brain experiences slowed processing time because it is trying to sort through conflicting information. In the Stroop Effect, if someone is given a list of words for colors printed in varicolored ink and asked to name the color of the ink used to print each word, the subject will experience a delayed reaction time. This is because the subject's brain is trying to suppress the input from the printed words in order to focus on the color of the words.

Take a look at the following list of words: yellow, red, green, purple, brown, blue. Name the colors each word is displayed in as fast as you can. You may find that you hesitate or stumble, because your brain is attempting to process two conflicting pieces of information: a color, and a word naming a color. Most Stroop Testing includes a much longer list, incidentally.

Testing on the Stroop Effect has shown that when Stroop Testing is performed in a language learner or a young child who has not yet learned to read, people have no trouble accurately and quickly naming the colors in the list. Once people learn to read and understand a language, however, they have difficulty with the list, thanks to interference. In Stroop Testing, data can be collected about someone on the basis of how long it takes for that person to work through the list of colored words.

A number of psychologists have studied and documented the Stroop Effect, and some have devised additional tests which can be used to gather data about people. Stroop Testing can be used to assess mental acuity after an insult to the brain, in psychological testing, and in other types of work where people want to learn more about how someone's brain is functioning. Many people are surprised when they see the Stroop Effect in action for the first time, as a seemingly simple task is revealed as something which is actually a bit complicated.