Who Invented the IQ Test? A Brief History of Intelligence Testing

It Began With Schoolchildren, Not Science

The IQ test was not invented by a scientist trying to measure intelligence. It was invented by a psychologist trying to solve a practical problem: in 1904, the French government commissioned Alfred Binet and his colleague Théodore Simon to develop a method for identifying schoolchildren who needed additional educational support. The goal was intervention, not ranking.

Binet and Simon's 1905 scale was the first formal intelligence test. It presented children with a series of tasks — naming objects, following instructions, repeating digits, defining words — and established norms for what children of each age could typically accomplish. A child who could complete tasks typical of a 10-year-old was said to have a "mental age" of 10, regardless of their chronological age.

From Mental Age to IQ

The concept of the Intelligence Quotient — IQ — was introduced by German psychologist William Stern in 1912. His formula was straightforward: divide mental age by chronological age and multiply by 100.

A 10-year-old with a mental age of 12 had an IQ of (12/10) × 100 = 120. A 10-year-old with a mental age of 8 had an IQ of 80. The formula was elegant and made scores comparable across ages — at least in childhood, where mental age develops predictably.

The problem emerged in adulthood. Mental age stops increasing in meaningful ways in the mid-twenties, making the ratio formula useless for adult comparison. By the 1930s, it was clear that a different approach was needed.

David Wechsler and the Modern IQ Score

David Wechsler, a Romanian-American psychologist working at Bellevue Hospital in New York, solved the adult problem in 1939 with the Wechsler–Bellevue Intelligence Scale. His innovation was to replace the ratio formula with what he called a "deviation IQ" — a score based not on a ratio of ages, but on how far a person's performance deviated from the average for their age group.

In Wechsler's system, 100 was defined as the average for each age group, and a standard deviation of 15 was set. A score of 115 means performance one standard deviation above the average for your age; 85 means one standard deviation below. This is the same system in use today on virtually every major intelligence test.

The Wechsler scales — now in their fifth editions as the WAIS-V (adults) and WISC-V (children) — remain the gold standard in clinical and educational assessment worldwide.

The Stanford–Binet and American Mass Testing

While Wechsler was developing his adult scale, American psychologist Lewis Terman at Stanford had already adapted Binet's original test into the Stanford–Binet Intelligence Scale in 1916 — adding Stern's IQ ratio formula and extending the norms. Terman was an enthusiastic proponent of intelligence testing and believed IQ scores should guide educational and vocational placement at scale.

During World War I, psychologists Robert Yerkes and Terman helped develop the Army Alpha and Beta tests — the first mass group intelligence tests, administered to approximately 1.75 million military recruits. This was the first large-scale demonstration that intelligence testing could be administered efficiently to huge populations, and it established the infrastructure and institutional legitimacy that drove the postwar expansion of IQ testing in schools and industry.

The Dark Chapter

The history of intelligence testing cannot be told without acknowledging its entanglement with the eugenics movement of the early 20th century. Terman, Yerkes, and many contemporaries believed that IQ scores reflected immutable genetic endowment and used test score differences between demographic groups to advocate for immigration restriction, forced sterilisation programmes, and racial segregation in education.

The scientific consensus today is that these conclusions massively overreached the data, ignored environmental factors (differential access to education, linguistic barriers, test bias), and reflected the prejudices of their era as much as any scientific finding. The tests themselves were psychometrically real; the ideological superstructure built on them was not.

Intelligence Testing Today

Modern intelligence tests look quite different from their predecessors. Contemporary scales like the WAIS-V assess multiple cognitive factors separately — verbal comprehension, visual-spatial ability, fluid reasoning, working memory, and processing speed — rather than collapsing everything into a single score. The full-scale IQ remains available as a composite, but clinicians increasingly work with the factor profile.

Online IQ tests, including the IQScore assessment, draw on the psychometric tradition established by this history: normed against a population distribution, mean of 100, standard deviation of 15, assessing multiple reasoning domains. The mathematical structure Wechsler established in 1939 remains the foundation.