Reading is a habit you can't switch off

Say the ink colour of the word RED printed in blue, and you slow down. A 1935 experiment measured by how much, and the result has held up ever since.

Here is a small test. Look at the word RED printed in blue ink and say the colour of the ink, not the word. Most people hesitate, or start to say red before they catch themselves. Do it for a whole list of such words and the stumbles add up.

The American psychologist John Ridley Stroop measured this in a paper published in 1935. He wasn't quite the first, since Erich Rudolf Jaensch had described it in Germany in 1929, but the effect carries Stroop's name. He gave one hundred students rows of coloured squares and rows of colour words printed in clashing inks, and timed how long they took to name the colours.

For 100 items, naming the colours of plain squares took an average of 63.3 seconds. Naming the ink of clashing words took 110.3 seconds. The words were not part of the task, and the students knew it, yet they could not stop reading them.

The reverse barely happened. When Stroop asked people to read the words aloud and ignore the ink, the mismatched colours slowed them by only 2.3 seconds on average, a difference he didn't consider reliable. The word gets in the way of the colour, but the colour hardly gets in the way of the word.

Why the word wins

A common explanation is that reading has become automatic for a practised adult. You have read words countless times, and the process starts by itself the moment one is in view. Naming the colour of something is slower and needs deliberate attention. Another account describes two pathways competing, a strong one for reading and a weaker one for naming colours. The details differ, but the picture is similar: when the two give different answers, the stronger one has to be held back, and that takes time.

This is one of the sturdiest findings in psychology. Stroop's paper is among the most cited in the history of experimental psychology, and researchers have varied the words, the senses involved and the emotional content in many ways without making the interference disappear. A version of the task is a standard test in neuropsychology, used to see how well a person can hold back an automatic response.

The lesson reaches beyond ink. A skill you practise enough stops waiting for permission. That is what makes it fast, and it is also what makes it hard to turn off.