Stroop effect: why naming ink colors takes 74% longer

The Stroop effect is the delay people show when naming the ink color of a word that spells a different color: asked to say “blue” for the word RED printed in blue ink, almost everyone slows down and makes more mistakes than when word and ink match.

It is named for John Ridley Stroop, who published it in 1935 in the Journal of Experimental Psychology as “Studies of interference in serial verbal reactions,” part of his doctoral work at George Peabody College.

What Stroop actually measured

His numbers are bigger than most summaries suggest. In his second experiment, 100 students each named the colors of 100 items. Plain colored squares took an average of 63.3 seconds; color words printed in clashing ink took 110.3 seconds. That is 47 seconds, or 74%, longer.

The reverse barely happens. In his first experiment, reading 100 color words aloud took only 2.3 seconds (5.6%) longer when the ink clashed than when the words were printed in black, a gap Stroop judged unreliable. The word hijacks the color; the color hardly touches the word.

That asymmetry is the core of the standard explanation, automaticity: reading is so overlearned that it runs whether you want it to or not. The underlying speed gap was spotted long before Stroop, by James McKeen Cattell in 1886, who found people read words faster than they can name colors.

Stroop also showed that the effect bends with training. After eight days of practice naming clashing ink colors, his subjects got faster at it, and a reverse interference appeared: reading the color words in clashing ink slowed from 19.4 to 34.8 seconds, then fell back to 22.0 seconds a day later.

Nearly a century on, the task is a staple of attention research and a clinical tool, used in evaluations for ADHD, early-onset Alzheimer’s disease, dyslexia and traumatic brain injury.