What Is IQ? A Clear Guide to the Intelligence Quotient

Where the idea of IQ came from

The modern intelligence test began in Paris in 1905. The French government asked psychologist Alfred Binet and his colleague Théodore Simon to find a practical way of identifying children who needed extra support at school. Binet deliberately avoided measuring head size or reaction times, which earlier researchers had tried. Instead he gave children short tasks — defining words, repeating digits, spotting what was missing in a picture — and noted which tasks a typical child of each age could manage.

From this came the idea of a mental age. A seven-year-old who solved problems like a typical nine-year-old had a mental age of nine. The German psychologist William Stern later proposed dividing mental age by actual age and multiplying by 100. That ratio was the original intelligence quotient, and it is why the term still contains the word quotient.

In the United States, Lewis Terman at Stanford adapted Binet's work into the Stanford–Binet test in 1916. During the First World War the US Army tested more than a million recruits, which brought group testing into the mainstream — along with heavy criticism, because many of those early tests were biased toward people with formal schooling and fluent English.

How David Wechsler changed the scoring

The ratio formula worked poorly for adults: mental age stops climbing in the late teens, so a forty-year-old's ratio would be meaningless. In 1939 David Wechsler introduced the deviation IQ. Rather than dividing ages, your raw performance is compared with a large, representative sample of people your own age, then converted to a scale with a mean of 100 and a standard deviation of 15.

Every major professional test now uses this approach, including the Wechsler Adult Intelligence Scale (WAIS), the Wechsler Intelligence Scale for Children (WISC) and the current Stanford–Binet. If you want the numbers behind the bands, our guide to the IQ scale walks through each range.

What IQ tests actually measure

Most researchers describe intelligence as a set of related abilities. Scores on very different tasks tend to rise and fall together, a pattern Charles Spearman called the general factor, or g. Beneath g sit broader abilities, described in the Cattell–Horn–Carroll model, such as:

• Fluid reasoning — solving unfamiliar problems, the skill Raven's Progressive Matrices targets.
• Crystallized knowledge — vocabulary and learned facts.
• Working memory — holding and manipulating information over a few seconds.
• Processing speed — doing simple mental tasks quickly and accurately.
• Visual–spatial processing — imagining and rotating shapes.

What an IQ score does not tell you

IQ predicts school grades and, more modestly, job performance across many occupations. Yet it leaves out a great deal. Motivation, curiosity, emotional skill, creativity and practical know-how all matter in real life, and they are only loosely related to test scores. Our comparison of IQ and emotional intelligence explores that gap in more depth.

Scores also carry measurement error. Even a professionally administered test has a margin of roughly five points either side, and a short online test like ours is less precise still. Treat any single result as an estimate, not a label. You can read exactly how our test is built and scored on the methodology page.

Common questions