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What Is the Stroop Effect? — From the 1935 Paper to Today's Test
When a color name is printed in a different ink, naming the ink color slows down. What Stroop's 1935 paper measured, what it found, and how computerized tasks differ.
The Stroop effect is the phenomenon in which naming a color becomes slower and more error-prone when the meaning of a word and the color of its letters conflict. For example, if the word BLUE is printed in red ink, you have to answer “red”, but “blue” comes to mind first and your answer is delayed.
The original paper: Stroop (1935)
The name comes from the American psychologist J. Ridley Stroop. In 1935 he published “Studies of interference in serial verbal reactions” in the Journal of Experimental Psychology, volume 18, issue 6. Participants were given cards printed with 100 items, and the total time it took them to read all the items aloud or name their colors was measured. Five colors were used: red, blue, green, brown and purple.
- Experiment 1 — Compared the time to read a card of color words printed in mismatching ink with a card printed in black ink. The averages were 43.30 s versus 41.00 s, a difference of just 2.3 s. The ink color barely interfered with reading the words.
- Experiment 2 — Compared the time to name the ink colors of color words printed in mismatching ink with the time to name the colors of colored squares. The averages were 110.3 s versus 63.3 s: 47 s, or about 74%, longer. This large difference is the interference we now call the “Stroop effect”.
- Experiment 3 — After several days of practice at naming the colors of mismatching words, interference decreased but did not disappear.
In short, words strongly interfere with naming colors, but colors hardly interfere with reading words. This asymmetry became the starting point for theories that try to explain the Stroop effect.
How computerized tasks differ
The original paper measured the total time to get through a whole card, whereas modern research and this site show one word at a time and measure reaction time for each trial in milliseconds. This makes it possible to mix conditions randomly and to count errors and timeouts separately.
- Congruent — The word and the ink color match. Example: RED in red ink.
- Incongruent — The word and the ink color differ. Example: RED in blue ink.
- Neutral — A string that is not a color word (such as XXXX) is shown in colored ink.
The Stroop interference effect is usually calculated as the difference in mean reaction time between the incongruent and congruent conditions. Adding a neutral condition lets you separate the part where congruence speeds you up (facilitation) from the part where incongruence slows you down (inhibition cost).
Stroop interference (ms) = incongruent mean RT − congruent mean RT
The size depends on how you respond
In the original experiments, participants named the colors aloud. When you respond with buttons or keys, as on this site, there is no step of saying the color name, and interference is generally reported to be smaller than with vocal responses (MacLeod, 1991). Even so, the interference itself tends to remain clear, which is why keyboard tasks are widely used in research.
Why it is still used today
The Stroop task creates, in a very simple way, a situation in which “what you are supposed to do” (name the color) clashes with “what happens on its own” (reading the word). That made it a flagship task for studying attention, inhibition and cognitive control. In his review, MacLeod (1991) summarized several hundred studies accumulated over half a century, and the Stroop task has since continued to be used in modified forms in neuropsychological testing and brain imaging research.
References
Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18(6), 643–662.
MacLeod, C. M. (1991). Half a century of research on the Stroop effect: An integrative review. Psychological Bulletin, 109(2), 163–203.