A History of IQ Testing: From Binet to Deviation IQ

Binet and the first intelligence scale

In 1905, French psychologist Alfred Binet, working with Théodore Simon, created the first practical intelligence scale. The French government had asked for a way to identify children who needed additional educational support, and Binet's solution was a graded series of tasks of increasing difficulty that a child worked through until the problems became too hard.

Crucially, Binet viewed his scale as a practical diagnostic tool, not a measure of a fixed, innate capacity. He cautioned against treating the number as a permanent ceiling — a caution that was frequently ignored in the decades that followed.

Binet intended his scale to identify children who needed help, not to rank human worth — a distinction often lost as the test spread worldwide.— On the origins of intelligence testing

Stern's quotient and the birth of 'IQ'

Binet's scale produced a 'mental age' — the age level whose tasks a child could complete. German psychologist William Stern proposed expressing this as a ratio: mental age divided by chronological age. Multiplied by 100, this 'intelligence quotient' gave us the abbreviation IQ, and the value 100 for a child performing exactly at their age level.

The ratio method worked reasonably for children, where mental development tracks age, but it broke down for adults — mental ability does not keep climbing in lockstep with the calendar. That flaw eventually pushed the field toward a different scoring method.

Terman, Wechsler and American testing

At Stanford University, Lewis Terman adapted and expanded Binet's scale for an American population, producing the Stanford-Binet in 1916. It became the dominant intelligence test in the United States for much of the 20th century and helped popularize the very idea of an IQ score.

David Wechsler, dissatisfied with relying on the age-based quotient and with tests built mainly for children, developed his own scales for adults and later children. His major methodological contribution was the 'deviation IQ', which scores a person relative to others of the same age rather than via a mental-age ratio — the approach nearly all modern tests now use.

The Army tests and mass testing

When the United States entered the First World War, psychologists developed the Army Alpha and Army Beta tests to screen and classify large numbers of recruits quickly. The Alpha was for literate recruits; the Beta used pictures and symbols for those who could not read English.

These tests marked the first large-scale, group administration of intelligence testing and demonstrated that testing could be scaled to populations. That legacy was double-edged: mass testing spread the technology rapidly, but it also fed misuse of results in ways later generations criticized sharply.

The Flynn effect and the modern scale

Researcher James Flynn documented a striking pattern: average raw test performance rose substantially across generations through much of the 20th century. Because tests are periodically re-normed to keep the average at 100, this 'Flynn effect' means a person scoring at the average today would have scored above average against the norms of decades ago. The causes are debated — better nutrition, schooling, health and more abstract, test-like thinking are among the leading candidates.

Today, the deviation IQ is standard. Scores are fixed to a mean of 100 and a standard deviation of 15, distributed along a bell curve, and norms are refreshed periodically to keep that average stable. The result is a tool far more rigorous than Binet's original scale — though, as Binet himself warned, still best read as one informative snapshot rather than a final verdict on a person.

1. 1905 — Binet and Simon publish the first practical intelligence scale.
2. Early 1910s — Stern's mental-age quotient gives 'IQ' its name.
3. 1916 — Terman's Stanford-Binet brings IQ testing into wide American use.
4. Late 1910s — Army Alpha and Beta introduce mass group testing.
5. Mid-20th century — Wechsler popularizes the deviation IQ on a 100/15 scale.
6. Late 20th century — Flynn documents rising scores across generations.