The Stroop effect, explained

The 1935 experiment

In a paper published in the Journal of Experimental Psychology in 1935, John Ridley Stroop compared several tasks (Stroop, 1935). Reading color words printed in a conflicting ink was barely slower than reading them in black. Naming the ink of those same words, on the other hand, was much slower than naming the color of plain squares.

The asymmetry is the heart of the effect. The word disturbs the color, much more than the color disturbs the word.

Why reading wins

For an adult who reads every day, reading a word is close to automatic. You cannot look at a familiar word and not read it. Naming a color is something we do much less often. When the two answers compete, the fast automatic one arrives first and has to be held back, which costs time and sometimes produces an error.

This is why the effect is used as a window on cognitive control, the ability to follow a goal, naming the ink, against a stronger habit, reading the word.

What psychologists do with it today

The Stroop task appears in a very large number of studies on attention and control, often as the difference between the time on conflicting words and the time on matching ones, the Stroop cost. Versions exist with numbers, with emotional words, or with directions instead of colors, like the color-blind mode of the Stroop test on this site.

The reliability paradox

Here is the twist. An effect that almost everyone shows is great for an experiment and poor for measuring one person. When Hedge, Powell and Sumner retested people on the Stroop task, the Stroop cost reached a test retest reliability of only 0.60 and 0.66, across two studies (Hedge et al., 2018). The raw times were more stable than the cost itself.

Their explanation is that the effect varies little from person to person, so day to day noise is large in comparison. For Headgame, this means the Stroop game is fun and informative about the effect, and it stays outside your card. The reasoning is detailed in are online brain tests accurate.

Try it

Twenty seconds, four inks and one rule, which is to say the color and not the word. The score is your correct answers minus your errors, so speed alone does not pay.

Sources

1. Stroop J.R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology 18(6), 643 to 662. doi.org/10.1037/h0054651
2. Hedge C., Powell G., Sumner P. (2018). The reliability paradox. Behavior Research Methods 50(3), 1166 to 1186. PMC5990556. doi.org/10.3758/s13428-017-0935-1