What Is IQ?

IQ, short for intelligence quotient, is a number that summarises how someone performed on a set of reasoning tasks compared with other people of a similar age. It is useful, limited and often misunderstood.

IQ in a paragraph

On modern tests the scale is set so that the average score is 100 and about two out of three people land between 85 and 115. Roughly one person in fifty scores 130 or above, and about the same number score 70 or below. The number is not a measurement of how much brain someone has, or of how successful they will be. It describes how a person did on a particular test, on a particular day, against a particular comparison group.

A short history

• 1904. Charles Spearman noticed that people who do well on one mental task tend to do well on others, and proposed a general factor, g, behind the pattern.
• 1905. Alfred Binet and Théodore Simon published the first practical intelligence test, designed to identify French schoolchildren who needed extra help.
• 1912. William Stern proposed dividing “mental age” by actual age, giving the original “intelligence quotient”.
• 1916. Lewis Terman at Stanford revised Binet’s test into the Stanford–Binet, which made IQ a household word.
• 1939. David Wechsler introduced scoring by comparison with others of the same age, the “deviation IQ” still used today, with a mean of 100 and a standard deviation of 15.

What IQ tests try to capture

Most tests sample several kinds of reasoning: spotting patterns and rules, working with numbers, understanding relationships between words, holding information in mind while using it, and thinking through problems that are new. The result is a rough summary of general reasoning ability. This test looks at four of those strands: numerical, verbal, logical and visual.

What they leave out

Creativity, curiosity, wisdom, practical skill, emotional understanding, persistence and the knowledge you have built up over a lifetime are not what an IQ test measures, and they matter enormously in real life. Two people with the same IQ can differ hugely in what they do with it.

Why a score can move

• Practice and familiarity. People who have met similar puzzles before tend to score higher.
• State on the day. Sleep, stress, hunger and distraction all shift results.
• Test quality. A short, unsupervised online test has far more “noise” than a full test given by a psychologist.
• The comparison group. Average scores have risen in many countries over the twentieth century, a pattern called the Flynn effect, which is why tests are periodically renormed.

How to use your result

Treat an online score as a starting point for curiosity. If the estimate surprises you in either direction, that is a sign of how imprecise a quick test is, not a fact about you. If you need a formal assessment, perhaps for education, work or a diagnosis, ask a qualified psychologist.

Curious how yours compares? See the IQ score chart, read how this test is scored, or take the test.