Look at the word RED printed in blue ink and try to say the ink color out loud, not the word. Most people hesitate, and some say "red" by mistake. That gap — the extra time and the occasional wrong answer — is the Stroop effect, one of the most replicated findings in psychology.
Why reading gets in the way
For a literate adult, reading a word is close to automatic: you don't choose to read it, it just registers. Naming a color, by contrast, takes a small amount of deliberate attention. When the word and the ink color disagree, the automatic process (reading) and the deliberate one (naming the color) point to two different answers at the same time, and the conflict costs you a fraction of a second.
The effect is named after J. Ridley Stroop, who published the original demonstration in 1935. His version was simple: name the ink color of a list of color words, some matching the ink and some not, and measure how much slower the mismatched list is.
What the interference actually measures
Researchers describe the slowdown on mismatched trials as Stroop interference: the difference in reaction time between a trial where the word and color match ("congruent") and one where they don't ("incongruent"). A bigger gap between the two means more interference from the automatic reading response.
This makes the task a way to study cognitive control — specifically, how well someone can follow an instruction ("name the color") over a more automatic habit ("read the word"). It is not a test of intelligence, vocabulary, or color vision, and a slower time on this specific task does not mean anything is wrong with you.
Where researchers use it
Beyond the classic color-word version, researchers have built many variants of the same conflict — direction words pointing the wrong way, numeral counting tasks, and others — to study attention and interference more broadly. The task's appeal is that it is simple to run and produces a reliable, measurable effect in almost everyone who takes it.
Performance on Stroop-style tasks can be affected by practice, fatigue, language proficiency, and the specific version used, so results are best compared within the same test rather than across different implementations.
Try related tests
This site doesn't yet host a live Stroop test, but you can measure related aspects of attention and processing speed right now with the reaction time test or the digit span test. Browse the full test library for more.
What does the Stroop effect tell us?
That reading is close to automatic for a literate adult, and that automatic processes can interfere with a task that asks you to do something different with the same information.
How is Stroop performance measured?
By comparing reaction time and accuracy between congruent trials, where the word and ink color match, and incongruent trials, where they don't. The size of that gap is the interference effect.
What are the limits of the Stroop test?
Results depend on language and reading fluency, can shift with practice, and vary by which version of the task is used — so a score only means something compared against the same test taken under similar conditions, not against a number from a different site.