Who Invented the IQ Test?

Key takeaways

Alfred Binet and Theodore Simon produced the first practical intelligence scale in France in 1905, and its stated purpose was to identify schoolchildren who needed different teaching, not to rank the general population.
Lewis Terman's 1916 American revision kept the method but widened the audience, proposing the tests for school classification, institutional placement and vocational selection.
Group testing arrived with the First World War: a committee that first met in May 1917 produced the verbal Army Alpha and the nonverbal Army Beta, and the final forms were published in January 1919.
Modern published batteries no longer compute a literal quotient. The Wechsler Adult Intelligence Scale, Fifth Edition, published by Pearson in 2024, reports index and composite scores and is sold at a restricted qualification level.
The history matters because the instrument's purpose has been renegotiated repeatedly, and several of those renegotiations caused real harm to the people being classified.

The first practical intelligence scale was published in France in 1905 by Alfred Binet, working with Theodore Simon, and it was built for one narrow job: deciding which schoolchildren needed different teaching. Binet's own statement of purpose, in the paper archived by York University's Classics in the History of Psychology, leaves no room for reinterpretation — "Our purpose is to be able to measure the intellectual capacity of a child who is brought to us in order to know whether he is normal or retarded." The clinical vocabulary there is the vocabulary of 1905, quoted as written. Almost everything a modern reader associates with IQ — a lifetime number, a bell curve, a ranking of adults, a score on a share button — was added afterwards by people who wanted the instrument to do more than its authors designed it to do.

The 1905 scale was a triage tool, not a ranking device

Binet and Simon's method was a graded series of short tasks, ordered by difficulty, administered one child at a time by an examiner. What made it practical was not any single task but the ordering: a child's performance could be compared with what children of various ages typically managed, which gave teachers something more defensible than an impression.

Two design decisions are worth noticing because both were later discarded. First, the 1905 paper explicitly restricted itself to the child's present state, setting aside cause, history and prognosis. Second, it was individually administered, which meant every result came with an examiner's direct observation of how the child worked. A scale that is deliberately silent about causes and is delivered face to face is a very different object from a number you obtain alone in a browser tab. The cornerstone article on what IQ tests actually measure covers the modern construct; this one covers how the instrument got here.

Terman's revision kept the method and widened the audience

The scale crossed the Atlantic and was revised at Stanford by Lewis Terman, whose 1916 book The Measurement of Intelligence carried the revision that became the American standard. Terman's opening chapter, also archived by York University, is candid about scope. He argued that "Every child who fails in his school work or is in danger of failing should be given a mental examination," and he looked further ahead: "The time is probably not far distant when intelligence tests will become a recognized and widely used instrument for determining vocational fitness."

That is the pivot. Binet asked a question about one child's teaching; Terman proposed a screening instrument for schools, institutions and employment. The same chapter reports the testing of inmates at a state reformatory and the reclassification of a large fraction of them under the period's term "feeble-minded" — a label that governed real decisions about confinement and, in the following decades, about sterilisation and immigration in several jurisdictions. Any honest history of this instrument has to say plainly that its early institutional use caused serious harm to people who had no standing to challenge the classification.

Terman's chapter also contains one glancing use of the phrase "intelligence quotient", in a sentence speculating about the minimum quotient needed for success in particular occupations. The term outlived the arithmetic, as the last section explains.

The war turned testing into a mass operation

Individual administration does not scale to an army. The committee that produced the first group tests met for the first time in May 1917; by mid-July it had built five alternate forms of a verbal test, the Army Alpha, and had designed a nonverbal companion, the Army Beta, for recruits who could not read English. The final forms were published in January 1919, and by the end of the war roughly two million men had been examined. Robert Yerkes, then president of the American Psychological Association, organised the effort; in his autobiography he wrote that "For nearly two years I lived in military psychology."

Historians of education have spent decades arguing about what those tests actually measured and how far the Army used the results, and that argument is not settled. What is not in dispute is the effect on the public: group testing made an intelligence score into something that could be produced quickly, cheaply and at scale for people who had not asked for one. Every later consumer product in this space, including the online quiz, descends from that shift rather than from Binet's clinic.

From quotient to deviation score

The word "quotient" was once literal: an early scoring convention divided a child's assessed developmental level by their chronological age. That arithmetic has been abandoned. Published batteries now convert raw performance into standardised scores referenced to a norm sample, so a result describes where a person sat relative to a comparison group rather than expressing a ratio of two ages.

Pearson's Wechsler Adult Intelligence Scale, Fifth Edition, published in 2024 for ages 16:0 to 90:11, is a useful illustration of what a current instrument looks like: a Full Scale IQ derived from seven subtests, five primary index scores, a General Ability Index, a Nonmotor Index and a set of ancillary indexes. It is also sold at qualification level C, meaning the publisher restricts purchase to people with specific professional credentials. A test that requires a credential to buy is not the same category of object as a free quiz, and the gap between those categories is the single most useful thing this history teaches. If you want the general version of that distinction, reliability and validity are the two questions that separate them.

Where a practice app sits in this story

IQ Test & Brain Training Games is our own app, built by Reign Creative, and it sits on the consumer side of the line this history draws. It includes an IQ-style test covering numerical reasoning, logic, patterns, vocabulary, working memory, spatial reasoning and processing speed, alongside brain, memory, logic and mental math games, a memory test and a reaction time test. Its Google Play listing states that IQ test, memory test and other results are estimates based on app activities, not medical, psychological, clinical, educational-placement, employment or professional diagnostic assessments — which rules out exactly the school-placement and selection uses this history describes.

Keeping the four categories apart is the whole point: practice, entertainment, educational activity and standardised psychological assessment are different things, and a century of this history is the story of what happens when they get confused. The same scepticism applies to product claims generally, which is why the evidence guide on whether brain training apps work is worth reading alongside this one, and why dating and sourcing arguments in how historians date events are a decent model for reading any claim about the past.

More on the statistics, the item types and the limits sits in the IQ tests and reasoning hub, and the wider education and brain category covers memory, arithmetic and practice design. The app itself is described on the IQ Test & Brain Training Games app page.