The Stroop Effect Explained: Why Reading Gets in the Way of Seeing
The Stroop effect shows how automatic reading interferes with naming colours. What it reveals about attention and cognitive control.
Read the word, ignore the ink colour: easy. Name the ink colour, ignore the word: surprisingly hard, particularly when the word "RED" is printed in blue. That slowdown is the Stroop effect, and it is one of the most reproducible results in experimental psychology. It is the basis of our focus and attention test.
What Stroop found
John Ridley Stroop published the experiment in 1935. He asked participants to read colour words printed in black ink, to name the ink colour of coloured patches, and to name the ink colour of colour words printed in an incongruent colour. Reading was fast. Naming incongruent colours was much slower than naming simple colour patches. The difference is called the Stroop interference. Since then thousands of studies have used the task, and Colin MacLeod's 1991 review in Psychological Bulletin described it as a "gold standard" of attention measurement.
Why does it happen?
Several explanations exist, and they are not mutually exclusive.
- Automaticity. For skilled readers, reading is automatic. You cannot easily choose not to read a word that is in front of you. The word's meaning is activated whether you want it or not, and it competes with the response required by the task.
- Speed of processing. Reading is faster than colour naming, so the word's meaning arrives first and has to be suppressed.
- Response competition. The word and the colour each activate a different response, and the brain has to resolve the conflict before answering. Brain imaging studies show greater activity in regions linked to conflict monitoring and control, such as the anterior cingulate cortex and parts of the prefrontal cortex, on incongruent trials.
A common modern view combines these: the conflict between a strong automatic process and a weaker controlled one has to be resolved by attentional control.
Congruent, incongruent and neutral trials
The task has three typical trial types:
- Congruent: the word matches the ink (BLUE in blue). Responses are fast.
- Incongruent: the word conflicts with the ink (BLUE in green). Responses are slower and more error-prone.
- Neutral: a non-colour word or a coloured patch.
The interference effect is the extra time on incongruent trials compared with the baseline. In our test, interference is the difference between the median time on incongruent trials and on congruent ones. A smaller difference means the word disrupted you less.
Who shows a larger effect?
- Children who are learning to read show less interference at first, then more as reading becomes automatic.
- Older adults tend to show larger interference effects, in line with age-related changes in attentional control and processing speed.
- Tired or distracted people show larger effects and more errors.
- Bilinguals show Stroop interference in both languages they read fluently. Interference is weaker in a language you read slowly, which is why the test uses the page language.
Variants
- Emotional Stroop: naming the ink colour of emotionally charged words (for example threat words). People with anxiety disorders or phobias are often slower on words related to their concern, because the words capture attention.
- Counting Stroop: counting the number of items on screen when the items are digits that conflict with the count.
- Spatial Stroop and Simon tasks: conflict between the location of a stimulus and the required response.
Versions of the colour-word task are included in neuropsychological assessments (for example the Color-Word Interference test of the D-KEFS) to assess executive function.
What the effect tells you, and what it does not
The Stroop interference score is a reasonable indicator of how well you suppress an automatic response in a simple task. It is not a general "focus" score. Research on whether Stroop performance relates to real-world attention is mixed, and a single interference score in a short online test is noisy. Differences between people are also modest compared with the variation from one session to the next. Do not read too much into a difference of 20–30 ms.
Can you reduce Stroop interference?
Practice reduces interference on the Stroop task itself, but whether the gains carry over is doubtful (see does brain training work?). On the day, you can help yourself by being rested (see sleep and cognitive performance), minimising distractions, and keeping your fingers positioned on the response buttons so the motor side is easy.
Sources
- 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.
- Botvinick, M. M., Braver, T. S., Barch, D. M., Carter, C. S., & Cohen, J. D. (2001). Conflict monitoring and cognitive control. Psychological Review, 108(3), 624–652.
- Williams, J. M. G., Mathews, A., & MacLeod, C. (1996). The emotional Stroop task and psychopathology. Psychological Bulletin, 120(1), 3–24.