What the Stroop effect is
Picture the word RED printed in blue ink, and suppose your job is to name the ink. The correct answer is “blue”, but the written word pulls you toward “red”. You can get it right, but it takes longer than it would for a row of blue Xs, and you are more likely to slip.
The effect is that extra time, or those extra errors, caused by the conflict. Researchers call a word congruent when its meaning matches its ink (RED in red), incongruent when the two clash (RED in blue) and neutral when there is no word at all, just a color patch or a string of symbols. Stroop interference is how much worse you do on incongruent items than on neutral ones. Matching words tend to give a small speed-up of their own.
The effect is lopsided. If you read the words aloud and ignore the ink, the ink hardly matters. The word wins the race almost every time.
Why it happens
For a fluent reader, recognizing a written word takes no decision. It happens whether you want it to or not. Naming an ink color is a far less practiced response and takes longer. So the word’s answer is ready first, and you have to hold it back while the color is still being worked out.
Two explanations have dominated the research (MacLeod, 1991). The relative-speed account says the word is simply processed faster than the color, so its answer arrives first and gets in the way. The automaticity account says the link between a word and its sound is so well practiced that it runs without effort, while the link between a color and its name is weaker, so the color has to win a contest.
Most current models blend the two. In the best-known one, Cohen and colleagues (1990) treated reading and color naming as two pathways of different strength, with attention to the task giving the weaker pathway a boost. Either way, naming the ink asks you to focus on one feature of what you see and to stop an automatic response from taking over. Psychologists call that second part inhibition, or interference control.
Where it comes from
John Ridley Stroop reported the experiments in 1935 in the Journal of Experimental Psychology, in a paper called “Studies of interference in serial verbal reactions”. People read color names printed in a different ink, and named the ink color of the same kind of words. Reading was barely slowed by the ink, while naming the ink was slowed a good deal by the words. That asymmetry is still the core of the finding, and the task has carried his name ever since.
What the test measures today
In clinics and labs the task usually has three parts: reading color words, naming color patches and naming the ink of conflicting words. The interference score compares your time or errors on the conflicting part with the baseline parts. A bigger gap means the conflict cost you more.
It is used as a measure of selective attention and inhibitory control. Scarpina and Tagini (2017) review its many versions and how they are scored and interpreted in neuropsychology. Researchers also use it with groups such as people with ADHD, where findings are mixed. One meta-analysis found more interference in ADHD groups once scores were corrected for baseline speed (Lansbergen et al., 2007). Another concluded that the interference effect was no larger than in matched controls once the overall slowing was taken into account (Schwartz and Verhaeghen, 2008). Part of the disagreement is simply how the score is calculated.
What it does not tell you
A Stroop result is one performance on one task, on one day. Reading speed, color vision, the language you are tested in, tiredness and the device you use all shape it. A large interference score does not say why it is large, and it does not diagnose ADHD or anything else. Clinicians read tasks like this next to a person’s history and other measures.
Questions people ask
What does the Stroop test measure?
It measures how well you can name a color while a competing word pulls you toward another answer. The score compares conflict with baseline conditions, and it is used as an indicator of selective attention and inhibitory control.
Why is the Stroop test so hard?
Because reading is automatic and naming ink is not. The word comes to mind whether you want it to or not, and you have to hold it back. The effect is very robust in fluent readers, so finding it hard is the normal experience.
Can I do the Stroop test online?
Yes. A browser version can show you the effect, with practice trials and timed parts for reading, color naming and conflict. Screen, keyboard and timing can change the scores, so do not compare a result with norms from a different version of the task.
General information, not a diagnosis
This guide explains how the Stroop effect is described and measured. It does not replace an assessment by a qualified clinician, and the sources it cites do not endorse Neuropsych.
References
- Stroop JR. Studies of interference in serial verbal reactions. Journal of Experimental Psychology. 1935;18(6):643-662. https://doi.org/10.1037/h0054651
- MacLeod CM. Half a century of research on the Stroop effect: an integrative review. Psychological Bulletin. 1991;109(2):163-203. https://doi.org/10.1037/0033-2909.109.2.163
- Cohen JD, Dunbar K, McClelland JL. On the control of automatic processes: a parallel distributed processing account of the Stroop effect. Psychological Review. 1990;97(3):332-361. https://doi.org/10.1037/0033-295X.97.3.332
- Scarpina F, Tagini S. The Stroop Color and Word Test. Frontiers in Psychology. 2017;8:557. https://doi.org/10.3389/fpsyg.2017.00557
- Lansbergen MM, Kenemans JL, van Engeland H. Stroop interference and attention-deficit/hyperactivity disorder: a review and meta-analysis. Neuropsychology. 2007;21(2):251-262. https://doi.org/10.1037/0894-4105.21.2.251
- Schwartz K, Verhaeghen P. ADHD and Stroop interference from age 9 to age 41 years: a meta-analysis of developmental effects. Psychological Medicine. 2008;38(11):1607-1616. https://doi.org/10.1017/S003329170700267X