What IQ stands for and where it came from
The first practical intelligence test was published in France in 1905 by Alfred Binet and Théodore Simon. Its purpose was modest: to find schoolchildren who needed extra help. Binet scored children by "mental age", the age at which an average child could pass the same items.
In 1912 the German psychologist William Stern proposed dividing mental age by chronological age, which gave the Intelligenzquotient, or intelligence quotient. Lewis Terman at Stanford University adapted Binet's test for the United States in 1916 as the Stanford-Binet and multiplied the quotient by 100. A 10-year-old performing like an average 12-year-old scored 12 ÷ 10 × 100 = 120. That ratio is where the name comes from, even though no modern test calculates IQ this way.
The ratio worked tolerably for children but broke down for adults, because mental age stops rising in the late teens. A 40-year-old cannot meaningfully have a "mental age" of 50. It also produced different spreads of scores at different ages, so a 130 at one age was not equally rare at another. Both problems led to the scoring method used today.
What an IQ test measures
A modern IQ test is a battery of short subtests. Each targets a different ability, and the scores are combined into a full-scale IQ. The Wechsler adult test, the most widely used clinical IQ test, groups its subtests into five areas:
- Verbal comprehension: vocabulary, explaining how two ideas are alike, general knowledge.
- Visual-spatial ability: assembling designs, judging how shapes fit together.
- Fluid reasoning: finding the rule in a pattern, such as matrix reasoning items.
- Working memory: holding and manipulating digits or images in your head.
- Processing speed: doing simple visual tasks quickly and accurately.
People who do well in one area tend to do well in the others. Psychologists call this shared factor g, for general intelligence, and the full-scale IQ is mainly an estimate of it. Our page on fluid and crystallized intelligence explains the two broad abilities underneath.
How a modern IQ score is calculated
In 1939 David Wechsler replaced the mental-age ratio with the deviation IQ. Your raw score is compared with a large norm sample of people your age, and your distance from their average is expressed on a scale with a mean of 100 and a standard deviation of 15. An IQ of 115 means you scored one standard deviation above the average for your age, ahead of about 84% of them. An IQ of 85 means one standard deviation below, ahead of about 16%. Because the scale is the same at every age, scores can be compared across age groups even though raw ability differs. The full method, including error margins, is on how IQ is calculated.
What IQ tests are used for today
- Clinical and educational assessment. Psychologists use tests such as the WAIS-5, the WISC-V and the Stanford-Binet 5 as one part of diagnosing intellectual disability, specific learning difficulties and giftedness, and to track change after a brain injury.
- Neuropsychology and ageing. IQ batteries give a baseline against which memory or attention problems can be judged; short screens such as the MoCA are used for suspected cognitive decline.
- Research. IQ is one of the most studied measures in psychology, linked to education, health and income across large populations.
- Selection. Employers and the military use short cognitive ability tests, such as the Wonderlic, that correlate with IQ.
- Self-knowledge. Online tests such as our IQ test give adults an estimate of their band; they are not diagnostic.
What IQ does not capture
IQ is a good summary of reasoning and learning ability, and a poor summary of a person. It does not measure creativity, practical judgement, emotional skill, motivation or expertise built over years. Two people with the same score can have very different strengths, which is why clinical reports focus on the profile across areas rather than the single number.
Scores also depend on the conditions of testing. Fatigue, anxiety, unfamiliarity with the format and a weak command of the test language all lower results. A single score is best read as a band with a margin of a few points either side.
Sources
Binet, A. and Simon, T. "Méthodes nouvelles pour le diagnostic du niveau intellectuel des anormaux". L'Année Psychologique, 1905.
Terman, L. M. The Measurement of Intelligence. Houghton Mifflin, 1916.
Wechsler, D. WAIS-5 Technical and Interpretive Manual. Pearson, 2024.
Neisser, U. et al. "Intelligence: knowns and unknowns". American Psychologist, 1996.
Frequently asked questions
What does IQ stand for?
Intelligence quotient. The term comes from William Stern's 1912 proposal to divide mental age by chronological age; Lewis Terman multiplied the result by 100 for the 1916 Stanford-Binet.
What is IQ?
A standardised score that shows how your reasoning and problem-solving compare with people of your age. The average is 100 and the standard deviation 15, so 68% of people score between 85 and 115.
What does IQ mean?
In everyday use it means general intelligence. Technically it is a rank: an IQ of 115 means you scored higher than about 84% of people your age on that test.
What does IQ actually measure?
Mainly general reasoning ability, the shared factor behind vocabulary, pattern finding, working memory and processing speed. It does not measure creativity, character or practical wisdom.
What are IQ tests used for today?
Diagnosing intellectual disability and learning difficulties, identifying gifted children, assessing the effects of brain injury, research, and in short forms, employee selection. Online tests are used for personal interest.
What is an IQ test?
A set of timed and untimed tasks, such as matrices, number series, vocabulary and memory items, scored against a norm sample of people the same age. Clinical versions take roughly 45 to 90 minutes with a psychologist.
Does IQ mean anything?
Yes, within limits. IQ predicts school results and job performance better than most single measures, but the correlations are moderate, so it leaves plenty of room for effort, personality and luck.