The Stroop Effect, Explained
Why naming a word's ink color is so hard when the word disagrees — the 1935 experiment, what causes the interference, and what your Stroop cost says about attention.
5 min read
Published by Apex Ascend Group, LLC Methodology · Editorial policy
The most famous trip-wire in psychology
Print the word RED in blue ink and ask someone to name the ink color. The correct answer — blue — comes out slower, and sometimes wrong, because the word itself keeps shouting the competing answer. That lag is the Stroop effect, named for John Ridley Stroop, who published the experiment in 1935. His paper went on to become one of the most cited in the history of experimental psychology, and the task remains a standard tool in attention research nearly a century later.
The original experiment
Stroop's design compared naming colors with naming ink colors when a written color word supplied a competing response — GREEN printed in red, for example. The conflicting condition took longer, establishing the interference pattern that now bears his name.
The reverse barely happens: when people read the words aloud and ignore the ink, conflicting ink colors slow them hardly at all. Reading interferes with color naming, but color barely interferes with reading — and that asymmetry is the clue to what's going on.
Why your brain can't ignore the word
For a fluent reader, the word supplies a practiced competing response even when the instruction is to name the ink. The task requires selecting the ink response while ignoring the word response.
Resolving that conflict takes executive attention: the system that notices two responses competing and suppresses the wrong one. That's why the Stroop task is used as a measure of cognitive control rather than of color vision or vocabulary — the hard part isn't perceiving, it's overriding.
How big is the effect?
This browser version does not use a population norm. It calculates interference as your average response time on mismatched trials minus your average on matched trials, using the twenty trials in that run.
For example, if matched trials average 720 ms and mismatched trials average 810 ms, the displayed interference is 90 ms. Read that value beside accuracy and compare it only with runs made using the same input method and setup.
Can you shrink it?
Practice can make the key layout and task rules more familiar, which may change overall response time or the mismatch gap. Use accuracy as a constraint and avoid treating one smaller interference value as a clinical improvement. The Go/No-Go game uses a different rule — respond to green and withhold on red — so its score should be tracked separately.