Why was the IQ test originally created?

The French government wanted a fair way to identify children who were struggling in school and might need special instruction, rather than relying on teachers' impressions. Binet designed his scale for that practical purpose and warned against treating the result as a fixed measure of a child's worth or potential.

What was the first IQ test in America?

Henry Goddard published an English translation of the Binet-Simon scale in 1908, but the first widely used American version was Lewis Terman's Stanford-Binet in 1916. The Army Alpha and Beta tests of 1917 were the first group intelligence tests given on a mass scale, to about 1.75 million recruits.

Are IQ tests still banned in California schools?

The original Larry P. v. Riles ruling barred California schools from using IQ tests to place Black students in the classes for the educable mentally retarded that existed at the time. An appeals court later vacated the 1986 expansion of the ban (Crawford v. Honig, 1994), but California has continued to restrict IQ testing of Black students for special-education decisions; current practice is set by state and district guidance. Families with a specific concern should ask their school district directly.

History of IQ Testing: Who Invented the IQ Test

Q: Who invented the IQ test?

Alfred Binet and Théodore Simon created the first practical intelligence test in France in 1905. William Stern coined the term intelligence quotient in 1912, and Lewis Terman turned it into the familiar number, mental age divided by actual age times 100, in the 1916 Stanford-Binet. David Wechsler introduced the modern deviation IQ in 1939.

The first practical intelligence test was created in France in 1905 by Alfred Binet and Théodore Simon, to identify schoolchildren who needed extra help. The term "IQ" came from German psychologist William Stern in 1912, Lewis Terman's 1916 Stanford-Binet made IQ testing widespread in the United States, and David Wechsler introduced the deviation IQ, the mean-100 scale still used today, in 1939.

Before Binet: measuring heads and reaction times

In the 1880s the English scientist Francis Galton ran a laboratory in London where visitors paid to have their reaction time, grip strength, and sensory sharpness measured, on the theory that quicker, keener senses reflected a better mind. In the United States, James McKeen Cattell coined the term "mental tests" in 1890 and gave similar batteries to college students.

The approach failed its first real test. In 1901 Clark Wissler, a student of Cattell's, found that these measures barely correlated with each other or with students' grades. Simple sensory tasks, it turned out, were a poor window into reasoning.

Binet and Simon, 1905: the first intelligence test

In 1904 the French Ministry of Public Instruction set up a commission on educating children who were struggling in the new system of compulsory schooling. Binet, a psychologist at the Sorbonne, and Simon, a physician, responded with a scale of 30 tasks of increasing difficulty, published in 1905 (Binet & Simon, 1905). Instead of measuring senses, the tasks sampled judgment, comprehension, memory, and reasoning: following simple commands, naming objects, defining words, completing sentences.

The 1908 revision grouped tasks by the age at which typical children could pass them, giving each child a "mental level." Binet was cautious about what this meant. He treated the scale as a practical tool for spotting children who needed help, warned against reading the result as a fixed quantity, and believed that intelligence could be improved through education (Fancher, 1985).

Who invented the IQ? Stern's quotient and the Stanford-Binet

In 1912 William Stern proposed dividing a child's mental age by their chronological age to get an "intelligence quotient." Lewis Terman at Stanford University adopted the idea, multiplied it by 100 to remove decimals, and published the Stanford Revision of the Binet-Simon scale in 1916 (Terman, 1916). A 10-year-old with a mental age of 12 scored 120. The Stanford-Binet became the standard individual intelligence test in the United States for decades, and Terman later launched a famous long-term study of high-scoring California children.

The ratio IQ had a built-in flaw. Mental age stops rising in the late teens, so dividing by a steadily growing chronological age makes adult scores fall for no real reason. Ratio IQs also spread out differently at different ages, so the same number did not mean the same rarity for a 6-year-old and a 14-year-old.

The eugenics era

Binet's caution did not travel well. Henry Goddard, who translated the scale into English in 1908 and used it at the Vineland Training School in New Jersey, treated low scores as evidence of inherited "feeble-mindedness." His 1912 book on the "Kallikak" family claimed to trace generations of disability to a single ancestor, and in 1913 he tested immigrants arriving at Ellis Island, reporting very high rates of feeble-mindedness among them on tests given through interpreters to frightened, exhausted people. Terman, too, was an outspoken supporter of eugenics.

These ideas fed policy. Many states passed laws permitting forced sterilization of people labeled "feeble-minded," and in Buck v. Bell (1927) the U.S. Supreme Court upheld Virginia's law, with Justice Oliver Wendell Holmes writing that "three generations of imbeciles are enough." Tens of thousands of Americans were sterilized under such laws during the 20th century. Test results were also cited in the debates leading up to the restrictive Immigration Act of 1924, though historians disagree about how much they actually shaped that law. This history is a central reason why IQ testing still draws suspicion, and why modern professional standards stress that scores describe current functioning, not destiny.

Army Alpha and Army Beta, 1917

When the United States entered World War I, psychologist Robert Yerkes led a committee, including Terman and Goddard, that built the first mass-administered intelligence tests. The Army Alpha was a written test for recruits who could read English; the Army Beta used pictures and demonstrations for those who could not. About 1.75 million men were tested (Fancher, 1985).

The program proved that group testing could be done at scale, and after the war such tests spread rapidly into schools and industry. It also produced a cautionary tale. Carl Brigham's 1923 book interpreting the Army data ranked immigrant groups by supposed innate ability. Critics pointed out that scores tracked years spent in the United States and familiarity with American culture, and some items asked about things like American products and sports figures. In 1930 Brigham publicly retracted his conclusions.

Wechsler and the deviation IQ

David Wechsler, chief psychologist at Bellevue Hospital in New York, wanted a test built for adults. His Wechsler-Bellevue Intelligence Scale (1939) made two lasting changes (Wechsler, 1939). It reported separate verbal and performance scores alongside the total, and it replaced the ratio with the deviation IQ: each person's score is compared with others of the same age and placed on a scale with a mean of 100 and a standard deviation of 15.

The deviation IQ fixed the ratio's problems, since a given score now means the same percentile at every age. It became the standard. Wechsler followed with the WISC for children (1949) and the WAIS for adults (1955), and the Stanford-Binet adopted deviation scores in its 1960 revision. Today the WAIS-5 (2024) and WISC-V (2014) descend directly from his tests. How the scale works is explained in IQ score ranges.

Raven's Progressive Matrices, 1938

In Britain, John C. Raven, a student of Charles Spearman, published the Standard Progressive Matrices in 1938. Each item shows a pattern of shapes with a piece missing, and the test-taker picks the piece that completes it. The test was meant to measure Spearman's general factor, the reasoning ability shared by all mental tests, which is explained in our article on the g factor.

Because it uses no words, Raven's became one of the most widely used tests in the world, especially across languages. It is sometimes called culture-free, but it is not: performance still depends on schooling and familiarity with abstract, test-style problems.

The Flynn effect

In the 1980s political scientist James Flynn documented that raw scores on IQ tests had been rising for decades. His 1987 study of 14 nations found gains of 5 to 25 points in a single generation (Flynn, 1987). A meta-analysis covering more than a century of data confirmed gains worldwide, with signs of slowing in recent decades (Pietschnig & Voracek, 2015). Gains that fast cannot be genetic; better nutrition, health, schooling, and more abstract everyday thinking are the leading explanations.

The finding changed practice. Tests must be re-normed regularly, or scores drift upward. It also matters in court: after Atkins v. Virginia (2002) barred executing people with intellectual disability, outdated norms and measurement error became central issues, and in Hall v. Florida (2014) the Supreme Court rejected a rigid cutoff of 70 that ignored the margin of error. More on this in our article on the Flynn effect.

Controversies: bias, the courts, and fairness

The sharpest legal challenge came in California. In Larry P. v. Riles (1979), federal judge Robert Peckham found that IQ tests had been used in a way that placed Black children in classes for the "educable mentally retarded" at far higher rates than other children, and he barred the state from using IQ tests to place Black children in those classes (Larry P. v. Riles, 1979). The Ninth Circuit upheld the decision in 1984, and later litigation narrowed how far the ban extended. In a similar Chicago case the following year, PASE v. Hannon (1980), a different federal judge reviewed the test items one by one and reached the opposite conclusion.

Researchers distinguish several meanings of "bias." Predictive bias asks whether a test forecasts outcomes like grades equally well for different groups; reviews, including the APA task force report, found that for native English speakers in the United States the major tests predict about as well for Black and White Americans (Neisser et al., 1996). That finding does not settle why average scores differ, which the same report described as unexplained, nor does it address cultural loading: items that depend on vocabulary, schooling, or experiences that are unevenly shared. Test familiarity, language, educational quality, and health all affect scores, and modern test development screens items statistically for bias and norms tests on samples matched to the census. Whether those steps go far enough is still debated, and scores are best read alongside a person's background. Our article on how accurate IQ tests are covers reliability and validity in more depth.

Modern CHC-based batteries

In 1993 John Carroll published a reanalysis of more than 460 datasets that showed intelligence organized as a hierarchy: many narrow abilities, about eight to ten broad ones, and a general factor at the top. Combined with Raymond Cattell and John Horn's theory of fluid and crystallized intelligence, this became the Cattell–Horn–Carroll (CHC) framework.

CHC changed test design. The Woodcock-Johnson III (2001) was built explicitly around it, and the Stanford-Binet Fifth Edition (2003) and the Kaufman Assessment Battery for Children, Second Edition (2004) drew on it as well. The WISC-V reports five primary index scores (verbal comprehension, visual-spatial, fluid reasoning, working memory, and processing speed) that map closely onto CHC broad abilities. A century after Binet, the main change is not the kinds of tasks, many of which he would recognize, but the shift from a single number toward a profile of related abilities. If you are curious how that looks in practice, you can take the free test, which samples six CHC-style domains in 12 or 60 untimed questions and compares you with its own online test-takers rather than population norms. For an overview of what the scores mean today, see what IQ is.

The History of IQ Testing: From Binet to Modern Batteries