The History of IQ Tests: 120 Years from Binet to Modern CHC Theory

The world's first practical intelligence test was the Binet–Simon scale, developed in 1905 by French psychologist Alfred Binet with physician Théodore Simon. In the 120 years since, IQ testing has passed through major transitions: ratio IQ → deviation IQ → CHC theory. This article traces that path turning point by turning point.

It began as educational support (1905, Binet)

Binet built his test not to select geniuses but to identify children struggling in school early so they could be supported. The idea was to line up tasks typically solvable at each age and measure a 'mental age' by the level a child could reach. Binet himself was famously cautious about reducing intelligence to a single fixed number.

The birth of 'IQ' (1912–1916)

German psychologist William Stern proposed dividing mental age by chronological age, and Stanford's Lewis Terman multiplied the ratio by 100, adopting it as the 'intelligence quotient' in the Stanford–Binet test (1916). The original definition was IQ = mental age ÷ chronological age × 100 — with the known weakness that a ratio IQ is hard to apply to adults.

Group testing and the deviation IQ (1917–1939)

The U.S. Army's group tests for recruit selection in World War I (Army Alpha and Beta) turned intelligence testing into a tool for measuring many people at once. The decisive shift came in 1939, when David Wechsler introduced the deviation IQ — expressing a score by how far it lies from the average of one's own age group. The modern system of mean 100 and standard deviation 15 starts here and carries into today's WAIS. For the mechanics, see the basics of IQ's average and distribution.

Today: CHC theory and item response theory (1993–)

The modern standard model is CHC theory, which merges Cattell and Horn's fluid–crystallized framework with the three-stratum theory Carroll presented in 1993. It views intelligence as a hierarchy with the general factor g at the top and broad abilities beneath. Scoring technology has also evolved — from simply counting correct answers to item response theory (IRT), which estimates ability while accounting for each item's difficulty.

What the history teaches

120 years of history show that IQ is a relative value, benchmarked to the population of its era. Change the reference group and the same person's number changes — indeed, average raw scores rose throughout the 20th century, the Flynn effect. To see where you stand by today's standard, try the free IQ test, which uses IRT scoring.