The psychological occurrence we now call "the Stroop effect" was first described in 1935 by John Ridley Stroop (Stroop, J. R. (1935) -- Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18, 643-622.). Why has the Stroop effect continued to fascinate psychologists? Part of the answer is that the Stroop effect appears to tap into essential operations of cognition, thereby offering clues to fundamental cognitive processes.
In two classic experiments, Stroop first compared reading a list of words printed in black with reading the same list of words printed in incongruent colors. Stroop found that there was little difference in reading time for the two lists. Stroop then compared the naming of colors for a list of solid color squares with the naming of colors for a list of words printed in incongruent colors. Subjects averaged 74% longer to name ink colors of incongruent words.
The results of these two studies led Stroop to conclude that since people are more practiced at word reading than naming colors, there is less interference with word reading than with color naming.
Since the original study, the Stroop effect has been investigated in a variety of ways. One general finding is that the Stroop effect is very robust. For example, the Stroop effect extends to color-related words (e.g. sky and fire). It extends to non-words that sound like color words (e.g. wred and bloo). It occurs with the words are presented suboptimally (dim, faded, small, etc.). It is resistant to practice. In summary, the difficulty of removing the interference effect of the Stroop task has led some researchers to claim that the brain is wired to recognize words without effort. This explanation is called the "automatic word recognition hypothesis" (or automaticity hypothesis), and it is so widely accepted that it is often given, in psychology textbooks, as the only answer to the Stroop effect. According to this theory, reading is an automatic process, which cannot be turned off. In other words, people see the meaning or words without much effort or consciousness. On the other hand, naming colors is not automatic. It requires more effort than reading, thus creating interference in the Stroop task.
The "Speed of Processing" hypothesis suggests that word processing is much faster than color processing. Thus, in a situation of incongruency between words and colors, when the task is to report the color, the word information arrives at the decision process stage earlier than the color information and results in processing confusion. On the other hand, when the task is to report the word, because the color information lags behind the word information, a decision can be made before the conflicting color information arrives.
Different tasks develop different processing pathways, and practice, as well as biological wiring, create different pathway strengths. Consequently it is strength, not speed, that is basic. Additionally, the degree of automaticity is a function of the strength of each pathway. What this means for the Stroop task is that if two pathways are active simultaneously and the pathway that leads to the response is stronger (naming words), no interference occurs. However, if two pathways are active simultaneously and the pathway that leads to the response is weaker (naming the color of the word), interference results.