Stroop Effect Experiment in Psychology

Take-home Messages

  • In psychology, the Stroop effect is the delay in reaction time between automatic and controlled processing of information, in which the names of words interfere with the ability to name the color of ink used to print the words.
  • The Stroop test requires individuals to view a list of words that are printed in a different color than the meaning of the word. Participants are tasked with naming the color of the word, not the word itself, as fast as they can.
  • For example, when presented with the word “green” that is written in red ink, it is much easier to name the word that is spelled, as opposed to the color ink in which the word is written.
  • Reading, a more powerful automatic process, takes some precedence over color naming, which requires higher cognitive demands.
  • Since psychologist John Ridley Stroop first developed this paradigm back in 1935, the Stroop task has since been modified to help understand additional brain mechanisms and expanded to aid in brain damage and psychopathology research.

What is the Stroop Effect?

The Stroop effect refers to a delay in reaction times between congruent and incongruent stimuli (MacLeod, 1991).

Congruency, or agreement, occurs when the meaning of a word and its font color is the same. For example, if the word “green” is printed in the color green.

Incongruent stimuli are just the opposite. That is, the meaning of the word and the color in which it is written do not align. For example, the word “green” might be printed in red ink.

The Stroop task asks individuals to name the color of the word instead of reading the word itself.

stroop effect experiment

The delay in reaction time reveals that it is much harder to name the color of a word when the word itself spells another color (the incongruent stimuli) than it is to name the color of the word when the word itself spells that same color (the congruent stimuli).

The First Stroop Experiment

The Stroop effect was first published back in 1935 by American psychologist John Ridley Stroop, although discoveries of this phenomenon date back to the nineteenth century (Stroop, 1935).


Building off previous research, Stroop had two main aims in his groundbreaking paper:

  1. To examine how incongruency between the color of the word and the word’s content will impair the ability to name the color.
  2. To measure what effect practicing reacting to color stimuli in the presence of conflicting word stimuli would have upon the reaction times.


In order to empirically study these two major aims, Stroop ran three different experiments:

1) Experiment 1 :

Participants (70 college undergraduates) were tasked with reading the word aloud, irrespective of its color. In other words, participants must read aloud the word “green” even if written in a different color.

2) Experiment 2 :

The second experiment was the opposite of the first. Participants (100 college students) were first asked to name the color of individual squares (instead of the color of words) as a training mechanism for the subsequent task. Afterward, participants had to say the color of the word, regardless of its meaning – the opposite of the experiment 1 procedure.

3) Experiment 3 :

The third and final experiment integrated all of the previously mentioned tests with an undergraduate population of 32 participants.

The independent variable (IV) was the congruency of the font name and color.

  • Congruent (word name and font color are the same)
  • Incongruent (word name and font color are different)

The dependent variable (DV) was reaction time (ms) in reporting the letter color.


After running the three experiments, Stroop drew two main conclusions:

  1. The interference of conflicting word stimuli upon the time for naming colors caused an increase of 47.0 seconds or 74.3 percent of the normal time for naming colors printed in just squares.
  2. The interference of conflicting color stimuli upon the time for reading words caused an increase of only 2.3 seconds or 5.6 percent over the normal time for reading the same words printed in black.


These tests demonstrate a disparity in the speed of naming colors and reading the names of colors, which may be explained by a difference in training in the two activities.

The word stimulus has been associated with the specific response “to read,” while the color stimulus has been associated with various responses: “to admire,” “to name,” etc.

The observed results might reflect the fact that people have more experience consciously reading words than consciously labeling colors, illustrating a difference in the mechanisms that control these two processes.

How the Stroop Effect Works

Why does the Stroop effect occur? We can tell our brain to do lots of things – store memories, sleep, think, etc. – so why can’t we tell it to do something as easy as naming a color? Isn’t that something we learn to do at a very young age?

Researchers have analyzed this question and come up with multiple different theories that seek to explain the occurrence of the Stroop effect (Sahinoglu & Dogan, 2016).

Speed of processing theory:

The processing speed theory claims that people can read words much faster than they can name colors (i.e., word processing is much faster than color processing).

When we look at the incongruent stimuli (the word “green” printed in red, for example), our brain first reads the word, making it much more difficult to then have to name the color.

As a result, a delay occurs when trying to name the color because doing so is not our brain’s first instinct (McMahon, 2013).

Selective attention theory:

The theory of selective attention holds that recognizing colors, compared to reading words, requires more attention.

Because of this, the brain needs to use more attention when attempting to name a color, making this process take slightly longer (McMahon, 2013).


This theory argues that recognizing colors is not an automatic process, and thus there is a slight hesitancy when carrying out this action.

Automatic processing is processed in the mind that is relatively fast and requires few cognitive resources.

This type of information processing generally occurs outside of conscious awareness and is common when undertaking familiar and highly practiced tasks.

However, the brain is able to automatically understand the meaning of a word as a result of habitual reading (think back to Stroop’s initial study in 1935 – this theory explains why he wanted to test the effects of practice on the ability to name colors).

Researchers in support of this theory posit that automatic reading does not need controlled attention but still uses enough of the brain’s attentional resources to reduce the amount left for color processing (Monahan, 2001).

In a way, this parallels the brain’s dueling modes of thinking – that of “System 1” and “System 2.” Whereas the former is more automatic and instinctive, the latter is slower and more controlled (Kahneman, 2011).

This is similar to the Stroop effect, in which we see a more automatic process trying to dominate over a more deliberative one. The interference occurs when we try to use System 2 to override System 1, thus producing that delay in reaction time.

Parallel distributed processing:

The fourth and final theory proposes that when the brain completes different tasks, unique pathways are developed. Some of these pathways, such as reading words, are stronger than others, such as naming colors (Cohen et al., 1990).

Thus, interference is not an issue of processing speed, attention, or automaticity but rather a battle between the stronger and weaker neural pathways.

Additional Research

John Ridley Stroop helped lay the groundwork for an abundance of future research in this field.

Numerous studies have tried to identify the specific brain regions responsible for this phenomenon, identifying two key regions: the anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (DLFPC).

Both MRI and fMRI scans show activity in the ACC and DLPFC while completing the Stroop test or related tasks (Milham et al., 2003).

The DLPFC assists with memory and executive functioning, and its role during the task are to activate color perception and inhibit word encoding. The ACC is responsible for selecting the appropriate response and properly allocating attentional resources (Banich et al., 2000).

Countless studies that repeatedly test the Stroop effect reveal a few key recurring findings (van Maanen et al., 2009):

  1. Semantic interference: Naming the ink color of neutral stimuli (where the color is only shown in blocks, not as a written word) is faster than incongruent stimuli (where the word differs from its printed color).
  2. Semantic facilitation: Naming the ink of congruent stimuli (where the word and its printed color are in agreement) is faster than for neutral stimuli.
  3. Stroop asynchrony: The previous two findings disappear when reading the word, not naming the color, is the task at hand – supporting the claim that it is much more automatic to read words than to name colors.

Other experiments have slightly modified the original Stroop test paradigm to provide additional findings.

One study found that participants were slower to name the color of emotion words as opposed to neutral words (Larsen et al., 2006).

Another experiment examined the differences between participants with panic disorder and OCD. Even with using threat words as stimuli, they found that there was no difference among panic disorder, OCD, and neutral participants’ ability to process colors (Kampman et al., 2002).

A third experiment investigated the relationship between duration and numerosity processing instead of word and color processing.

Participants were shown two series of dots in succession and asked either (1) which series contained more dots or (2) which series lasted longer from the appearance of the first to the last dots of the series.

The incongruency occurred when fewer dots were shown on the screen for longer, and a congruent series was marked by a series with more dots that lasted longer.

The researchers found that numerical cues interfered with duration processing. That is, when fewer dots were shown for longer, it was harder for participants to figure out which set of dots appeared on the screen for longer (Dormal et al., 2006).

Thus, there is a difference between the processing of numerosity and duration. Together, these experiments illustrate not only all of the doors of research that Stroop’s initial work opened but also shed light on all of the intricate processing associations that occur in our brains.

Other Uses and Versions

The purpose of the Stroop task is to measure interference that occurs in the brain. The initial paradigm has since been adopted in several different ways to measure other forms of interference (such as duration and numerosity, as mentioned earlier).

Additional variations measure interference between picture and word processing, direction and word processing, digit and numerosity processing, and central vs. peripheral letter identification (MacLeod, 2015).

The below figure provides illustrations for these four variations:

stroop picture word  experiment

the Stroop task is also used as a mechanism for measuring selective attention, processing speed, and cognitive flexibility (Howieson et al., 2004).

Additionally, the Stroop task has been utilized to study populations with brain damage or mental disorders, such as dementia, depression, or ADHD (Lansbergen et al., 2007; Spreen & Strauss, 1998).

For individuals with depression, an emotional Stroop task (where negative words, such as “grief,” “violence,” and “pain,” are used in conjunction with more neutral words, such as “clock,” “door,” and “shoe”) has been developed.

Research reveals that individuals who struggle with depression are more likely to say the color of a negative word slower than that of a neutral word (Frings et al., 2010).

The versatility of the Stroop task paradigm lends itself to be useful in a wide variety of fields within psychology. What was once a test that only examined the relationship between word and color processing has since been expanded to investigate additional processing interferences and to contribute to the fields of psychopathology and brain damage.

The development of the Stroop task not only provides novel insights into the ways in which our brain mechanisms operate but also sheds light on the power of psychology to expand and build on past research methods as we continue to uncover more and more about ourselves.


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Saul Mcleod, PhD

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Educator, Researcher

Saul Mcleod, Ph.D., is a qualified psychology teacher with over 18 years experience of working in further and higher education.

Charlotte Ruhl

Marketing Associate Analyst

B.A., Psychology, Harvard University

Charlotte Ruhl is a recent Harvard College graduate with more than six years of research experience in clinical and social psychology.