An Accidental Discovery That Changed Psychology: The Story Behind the Stroop Effect

One afternoon in 1935, in a laboratory at George Peabody College for Teachers in Tennessee, doctoral student John Ridley Stroop was worrying about his dissertation. He simply wanted to study a straightforward question: Why are some tasks effortless for us while others are incredibly difficult?

He designed what seemed like the simplest possible experiment. Have participants look at color words written in different colored inks, then name the ink color. For example, the word "blue" written in red ink — you need to say "red" not "blue."

The results astonished everyone.

The Afternoon That Changed Everything

Stroop later wrote in his recollections: "The first participant in the experiment was my roommate Jim. When I asked him to name the ink colors, he suddenly stuttered. This guy who usually had excellent speech, facing such a simple task, actually couldn't speak smoothly."

At first, Stroop thought Jim was just too nervous. But the next 70+ participants, without exception, all showed the same phenomenon. Average reaction time extended from the normal 0.63 seconds to 1.10 seconds — almost twice as slow. Even more interesting, the error rate also jumped from near zero to 4.5%.

This phenomenon was so consistent that Stroop realized he might have discovered something important.

The "Traffic Jam" in the Brain

I often use an analogy to explain this phenomenon to students. Imagine you're driving to an intersection. The traffic light is green, but there's a traffic cop standing next to it signaling you to stop. What's your first reaction? There'd definitely be a moment of confusion, right?

That's the Stroop effect at work. In our brains, reading is like that green light — it's automatic, fast, requires almost no thought. Identifying colors is like that traffic cop — requires active control, requires effort. When the two conflict, the brain experiences a brief "traffic jam."

In 2019, I used functional magnetic resonance imaging in my lab at Beijing Normal University to record this process. When participants faced color-word conflicts, the anterior cingulate cortex of the brain — what we call the brain's "conflict detector" — showed significantly increased activity. Immediately after, the prefrontal cortex started working, like a traffic cop, trying hard to suppress the automatic reading response so color identification could proceed.

The entire process happens in less than a second, but for the brain, it's already an intense neural war.

Why Are Some People Affected More?

Last year, one of my students, Xiao Wang, participated in our research. She's a literature PhD student with extremely fast reading speed, over 800 characters per minute. As a result, in the Stroop test, her reaction time extended by a full 1.5 seconds, double that of ordinary people. She smiled wryly: "My reading ability actually became a burden."

This actually makes sense. The stronger someone's reading ability, the harder this automated process is to suppress. Like a sports car — speed is an advantage, but when it needs to brake suddenly, the momentum is also greater.

Interestingly, we also found some special groups. For example, I once tested an illiterate elderly woman who was completely unaffected by the Stroop effect — because she couldn't automatically read those characters. Another time, we tested a group of Tibetan children. When tested with Tibetan script, the effect was obvious; switched to Chinese characters, the effect was much weaker.

Age is also an interesting factor. When my daughter was 5, she first took this test with almost no difficulty. But by age 7, after learning to read fluently, she suddenly "couldn't do it." And my 70+ year old advisor, though still mentally sharp, showed much more pronounced Stroop effect than when younger. He joked: "This proves my brain's traffic cop is getting old, can't direct traffic anymore."

From Laboratory to Life

You might ask, what's the use of studying this? Actually, applications of the Stroop effect are far more widespread than you imagine.

Last month, I visited Beijing Anzhen Hospital. Dr. Wang from the neurology department told me they now use the Stroop test for early screening of Alzheimer's disease. "More sensitive than traditional memory tests," he said, "because decline in executive control often precedes memory decline."

At the children's hospital, the ADHD clinic also uses this test. A mother once told me tearfully: "Finally there's an objective way to prove my child isn't deliberately naughty, he really can't control his attention."

Even at driving schools, some advanced training centers have started using the Stroop test to assess students' reaction abilities. After all, driving often requires quickly switching attention between multiple information sources — reading road signs, watching for pedestrians, listening to navigation — all requiring good executive control.

Latest Discovery: Brain Plasticity

In 2023, a research team from Fudan University published an exciting finding. They had a group of elderly people do 15 minutes of Stroop training daily. After 8 weeks of persistence, not only did test scores improve by 32%, but attention in daily life also noticeably improved.

One 76-year-old participant said: "Before, when I was watching TV and my spouse talked, I often couldn't hear. Now I can watch the news and chat with her at the same time." This shows that even in elderly people, the brain's executive control ability can still be improved through training.

Even more surprising, a Harvard Medical School study last year found that regular cognitive training similar to the Stroop test can increase white matter density in the brain — meaning connections between brain regions become more efficient. Like upgrading from country roads to highways.

The Profound Impact of a Simple Test

Going back to 1935, when Stroop published his research, the psychology community's reaction was quite cold. One reviewer even said: "This is just an interesting little trick."

But 88 years have passed, and this "little trick" has become one of the most cited experiments in cognitive psychology. Google Scholar shows over 20,000 related research papers. From diagnosing diseases to training pilots, from designing phone interfaces to helping people quit smoking, applications of the Stroop effect are everywhere.

Stroop himself probably never imagined that his simple experiment would help us understand how the brain works, and help countless people improve their cognitive abilities. As he said in a late-life interview: "I just happened to ask the right question."

Perhaps that's the charm of science — a simple question, a clever experiment, can open a window into understanding human cognition. And that window continues to show us the wonderful world of the brain.