What is IQ?

IQ stands for intelligence quotient: a score that compares how you did on a reasoning test with how other people did.

Where it came from

In 1905 the French psychologists Alfred Binet and Théodore Simon wrote a test to find schoolchildren who needed extra help. A few years later William Stern turned results into a "quotient": mental age divided by real age, times 100. Lewis Terman's Stanford-Binet test (1916) made that famous in the United States.

Dividing by age stops making sense for adults, so in 1939 David Wechsler switched to the method used today: your score shows where you rank compared with people your age.

What 100 means

Modern IQ tests are scaled so the average is 100 and the standard deviation is 15. About two thirds of people score between 85 and 115, and about 95% score between 70 and 130. A score of 130 is higher than roughly 98% of people. See the score chart for the full breakdown.

What the tests measure

Professional tests such as the WAIS mix several kinds of tasks: spotting patterns, working with numbers, vocabulary and verbal reasoning, remembering and rearranging information, and working quickly. People who do well on one kind of task tend to do well on the others. Psychologists call that shared factor g, for general intelligence, and the IQ score mostly reflects it.

What IQ isn't

It isn't fixed at birth. Schooling, health and practice all move scores, especially in childhood.
It doesn't measure creativity, wisdom, motivation, character or most practical skills.
A score from an online test, including this one, is a rough, fun estimate. Only a test given in person by a trained psychologist gives a clinical IQ.

Sources

Binet, A. & Simon, T. (1905). Méthodes nouvelles pour le diagnostic du niveau intellectuel des anormaux. L'Année Psychologique.
Wechsler, D. (1939). The Measurement of Adult Intelligence. Williams & Wilkins.
Deary, I. J. (2020). Intelligence: A Very Short Introduction (2nd ed.). Oxford University Press.