As adults, many things that we do, we do automatically, almost without thinking. We look in the correct direction before crossing the road; we travel to work without consciously making decisions about the route. Being able to do these things automatically frees up our brain to work on other, more important issues. However, on some occasions, being able to prevent ourselves from doing things automatically, or avoid the usual routine action, is also an important human skill. For example, it is this capacity, known as inhibitory control, that enables us to be polite when we're given an unwanted present, or to avoid taking the usual route to work to avoid a burst water main. Occasionally, this skill of inhibitory control fails us and we act automatically, without thinking. We might lose our inhibitory control after a glass of wine, finding that we inadvertently offend someone by speaking our mind.

As children, almost as soon as we learn to perform actions we also begin to learn how to prevent them. A child's inhibitory control is general poorer than an adult’s. Because of this, we often think that children do things ‘without thinking’ or seem to react automatically. When a young child kicks a football onto a road we worry that they may run out, without thinking, as they automatically chase the ball. As children get older, we worry less. An older child may still want to chase the ball but is able to prevent the chasing action when they reach the kerb. In the preschool years, children have little inhibitory control, they learn new skills quickly and they learn to do things automatically and efficiently. This is a great way for young children to learn but sometimes leads them to say or do things inappropriately.

Most children learn to control their actions as they get older but some continue to have difficulties in controlling automatic actions. In extreme cases, these children continue to act without thinking, blurt out comments inappropriately, seem overactive, and are easily distracted. Amongst other symptoms, this kind of behaviour is characteristic of children with Attention Deficit Disorder or ADD.

Psychologists have developed tests that help us to assess an individual’s inhibitory control. Usually, these tests require someone to suppress an automatic response in order to perform the task correctly. A classic example of such a test is the Stroop test. The Stroop test uses word reading as an automatic process. Most of us find it hard to prevent ourselves from reading words in front of us, for example:

DON’T READ THIS!

When carrying out a classic Stroop test, we ask people to read words written in coloured ink. In order to perform the task correctly, participants are instructed to name the colour of the ink, such as:

TABLE
LEGAL
BURST

In this case, the correct responses are “red”, “blue” and “green”, the colours of the ink. In the classic Stroop test, the words themselves are names of colours but the actual colour is different to the name. The participant is asked to name the colours of the ink, not to read the words themselves, for example:

BLUE
GREEN
RED

Again, the correct responses are “red”, “blue” and “green”. This time, notice how difficult it was to ignore the written word. When performing this task, we find that the automatic word-reading process interferes with our ability to name the ink colour of the word. This interference causes us to make more errors and to be slower in performing the task.