Stroop Test: The Stroop Effect, Experiment and Research

Updated 2026-10-25

Written by the Optics Trainer team from published research, with every source linked below.

The Stroop test asks you to name the ink color of a word while ignoring what the word says. When RED is printed in blue ink, saying "blue" takes longer than naming a plain blue patch. That delay is the Stroop effect. This guide covers the 1935 experiment, why the effect happens, how clinicians use the test, what online versions can tell you, what practice studies found, and what research reports about the Stroop test and ADHD.

What is the Stroop test?

In the Stroop test, a color word such as GREEN is printed in an ink color, and you name the ink color as fast as you can. For most adults, reading is so practiced that the word comes to mind anyway. When word and ink disagree, you have to hold back the word.

A simple Stroop effect demonstration has three parts. Most versions compare them like this:

Word reading: read color words printed in black ink. Most people find this fast.
Color naming: name the color of patches or rows of Xs. This is a little slower.
Color-word, or incongruent: name the ink color of words that name a different color, such as BLUE printed in red ink. This is the slowest, and people make more errors.
Some computer versions add congruent trials, where the word and ink agree, such as RED in red ink. These are usually answered quickly.

The Stroop experiment: what J. R. Stroop found in 1935

John Ridley Stroop published his study in the Journal of Experimental Psychology in 1935. He used five colors: red, blue, green, brown and purple. No word was printed in the color it named, and people read whole sheets aloud while he timed them with a stopwatch.

In his third experiment, Stroop screened people with the Ishihara color vision test and set aside one person's results because of red-green color vision difficulty. Any Stroop test assumes normal color vision.

Stroop's three experiments (Journal of Experimental Psychology, 1935)
Experiment	Who	Task	Result as reported
1	70 college undergraduates	Read color words printed in conflicting inks vs in black	2.3 seconds (5.6%) slower per 100 words; Stroop reported this as not reliable
2	100 students	Name the ink of conflicting color words vs name colored squares	110.3 vs 63.3 seconds per 100 items, a 74% increase
3	32 undergraduates (2 later dropped)	8 days of practice naming the ink of conflicting words, 200 responses a day	Interference fell but did not disappear; the reverse effect, ink slowing word reading, grew

Why the Stroop effect happens: the psychology

The Stroop effect is the slowdown and extra errors when the meaning of a word conflicts with the feature you are asked to report. Psychologists call the skill it draws on interference control or inhibition: holding back a habitual, automatic response to give a less automatic one.

In 1991, Colin MacLeod published a review of some 400 Stroop studies from the previous 50 years in Psychological Bulletin. He listed 18 findings that any theory had to explain. He judged that the two leading explanations at the time fell short. One said words are processed faster than colors. The other said reading is more automatic than color naming. He wrote that theories in which the word and the color are processed side by side, competing for the response, were more likely to succeed.

In a 1988 study, MacLeod and Kevin Dunbar taught people to name four new shapes with color names. After 20 hours of practice, the direction of interference reversed. They wrote that the idea of automaticity built up by training remained plausible.

How psychologists and clinicians use the Stroop test

The Stroop Color and Word Test is a widely used neuropsychological test of interference control. Not every battery uses it: the NIH Toolbox, a set of standard computer tests, measures inhibitory control with a flanker task instead. Several standard versions exist, with their own materials, timing and norms:

Golden's 1978 version, which a 2017 review called the most widely used. It counts how many items a person names in 45 seconds on each of three cards.
The Victoria Stroop Test, which uses ordinary non-color words in place of the word-reading card.
The color-word interference test in the Delis-Kaplan Executive Function System (D-KEFS), a clinical battery.

Stroop test online: what it can and cannot tell you

Many websites offer a free Stroop test online, and Stroop test games use the same conflict between word and color. They show the Stroop effect clearly, but they are not a clinical test.

Points to know about online versions:

Scoring differs. A 2017 review of Stroop norms found several scoring methods in use and wrote that none fully captures the effect. An online score usually has no norms behind it.
Timing varies by device. A 2020 study of experiment software found that online studies had more timing variability than lab systems, with considerable differences between operating systems and browsers.
Responses differ. The 1935 test used spoken answers; most online versions use taps or key presses.
Screens and color vision matter. Colors look different on different screens, and the test assumes normal color vision.
No online Stroop test can diagnose a condition.

Stroop practice and training: what studies found

People get faster at the Stroop test with practice. Studies have also asked whether Stroop or inhibition practice carries over to other skills.

Across brain training more broadly, Simons and colleagues, 2016, found extensive evidence for gains on trained tasks and little for everyday performance, and Sala and Gobet, 2019, reported a minimal effect on general cognitive skills. Our hub on the science behind the games explains near and far transfer.

Studies of Stroop and inhibition practice
Study	Design	Who	Practice	Result as reported
Stroop 1935, Exp. 3	Practice series, no control group	32 undergraduates	8 days, 200 responses a day	Interference reduced, not eliminated; reverse interference grew
MacLeod and Dunbar 1988	Three lab experiments	Adults	Up to 20 hours naming shapes with color names	Interference grew with training and reversed after 20 hours
Davidson 2003	Two experiments, spoken answers	Younger and older adults	Hundreds of single-item Stroop trials	Interference shrank in both groups; older adults kept a larger effect
Wilkinson and Yang 2012	Four groups, including no-contact control	56 adults aged 60 to 84	Six Stroop sessions, with or without feedback	Stroop performance improved; no transfer to other tasks; feedback did not change the gain
Enge 2014	Randomized, double-blind, active and passive controls	122 healthy adults	3 weeks of adaptive go/no-go and stop-signal training	No difference from active control, including on an untrained Stroop task and fluid intelligence
Thorell 2009	Active and passive controls	Preschool children	5 weeks of computerized inhibition training	Gains on 2 of 3 trained tasks; no gains on untrained inhibition, working memory or attention tasks
Kassai 2019	Meta-analysis	Children, 43 and 17 comparisons	Working memory, inhibition and flexibility training	Near transfer g = 0.44; far transfer g = 0.11, not significant

The Stroop test and ADHD: what the research reports

Because ADHD is linked with difficulty holding back responses, researchers have often compared Stroop scores between people with and without ADHD.

Training research has asked a related question. A 2015 meta-analysis of 16 randomized trials of cognitive training in 759 children with ADHD found smaller effects on symptoms when raters were probably blinded (SMD 0.20) than when raters were closest to the training and typically unblinded (SMD 0.37), and no significant effect on academic performance.

On the test itself, three meta-analyses reported different pictures, partly because of how the interference score was calculated:

Van Mourik and colleagues, 2005, pooled 17 studies with 1,395 participants aged 6 to 27. Groups with ADHD were slower on word reading (d = 0.49) and color naming (d = 0.58), with a smaller difference on the interference score (d = 0.35), and none when interference was calculated as a simple difference between the color and color-word cards. The authors wrote that the Stroop task "in standard form, does not provide strong evidence" of an interference-control deficit in ADHD.
Lansbergen and colleagues, 2007, covered all ages and Stroop versions and left out studies using Golden's scoring method. Using ratio scores across 19 studies, they found more interference in ADHD groups and concluded that interference control is "consistently compromised" in people with ADHD.
Homack and Riccio, 2004, reported that children and teens with ADHD fairly consistently scored lower than children without a diagnosis, but the test did not reliably separate ADHD from other clinical groups. They wrote that "poor performance is not sufficient for a diagnosis of ADHD."

Where Optics Trainer fits

Optics Trainer is a set of game-based vision and brain training apps for iPhone, iPad, Android, web browsers and Meta Quest. Our color-word game is modeled on the Stroop task. A color word appears in an ink color, and you tap MATCH if the ink is the color the word names or NO MATCH if it is not. Rounds last 60 seconds, and no glasses are needed.

The game keeps the conflict between word and ink that defines the Stroop task. It changes several things: you judge match or no match instead of naming the ink aloud, words appear one at a time instead of on a printed sheet, about half the words match their ink, and you score points instead of the measures a Stroop test reports. It was not tested in any study described here. It is not a Stroop test, a measure or a treatment, and Optics Trainer is not a diagnosis and is not FDA cleared. Optics Tracker, on our clinic site, is our measurement product. Optics Trainer is not a substitute for professional diagnosis or prescribed care. Consult your eye care professional for guidance about your specific condition.

The phone and tablet apps start with a 7-day free trial, and a free web account includes 3 plays a day. After that, mobile and web access is $14.99 a month or $149.99 a year, and VR access is $19.99 a month or $199 a year after a 7-day free trial.

Download Optics Trainer

Get it on iOS Get it on Android Get it on Meta Quest Play in your browser

Frequently asked questions

What is the Stroop effect in simple terms?

It is the slowdown when a word names one color but is printed in another, and you must say the ink color. Reading happens almost automatically, so the word gets in the way.

What does the Stroop test measure?

Psychologists use it to measure interference control, also called inhibition: holding back an automatic response to give a less automatic one.

What did the original Stroop experiment find?

In 1935, J. R. Stroop found that naming the ink of conflicting color words took 74% longer than naming colored squares. Reading the words was barely slowed by the ink.

Is an online Stroop test accurate?

It can show you the Stroop effect, but scores are not comparable to a clinical test. Timing and colors vary across devices, and most online versions have no norms.

Can you get better at the Stroop test with practice?

Yes. Studies from 1935 onward found that practice reduces Stroop interference without removing it. Controlled studies found little or no carry-over from Stroop or inhibition practice to other tasks.

Is the Stroop test used to diagnose ADHD?

No. Meta-analyses found that groups with ADHD often score lower on average, but a 2004 review said poor performance is not sufficient for a diagnosis. ADHD is diagnosed through a clinical evaluation.

What is a Stroop test game?

It is a game built on the conflict between a word and its ink color. Games add timers and points and use taps instead of spoken answers, so a game score is not a Stroop test score.

Sources

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