Long before modern standardized tests, scholars like Sir Francis Galton (1822–1911) attempted to measure intelligence. Galton applied statistics to human differences and heredity, even setting up an anthropometric lab to record reaction times and other physical measures. However, these early bodily measurements proved poor predictors of academic ability. By the turn of the 20th century, psychologists shifted focus from measuring skulls or reaction times to more direct cognitive tasks. In 1905, French psychologist Alfred Binet and colleague Théodore Simon introduced the first practical intelligence test for children. Binet’s scale was designed to identify students who needed extra educational support (so they could be placed in special classes rather than labeled or institutionalized). This Binet–Simon Test became the prototype for IQ testing, predicting scholastic performance better than any previous method.

Precursors: Galton, Cattell, and Early Ideas of Intelligence

The scientific study of intelligence traces back to Galton’s work in the late 1800s. Galton, a cousin of Charles Darwin, pioneered psychometrics and emphasized heredity and statistics in human traits. He even coined the phrase “nature versus nurture”. Alongside him, American psychologist James McKeen Cattell and others attempted mental testing (e.g. reaction-time experiments), but found little correlation with school grades. Indeed, researchers eventually abandoned anthropometrics: Galton and Cattell ended their own line of research after realizing that measurements of the body “did not correlate to academic performance”. Their failure paved the way for a new approach: asking questions and puzzles rather than making physical measurements.

Alfred Binet and the First IQ Test (1905)

Alfred Binet (1857–1911) is rightly called the father of modern IQ tests. Working under a new French law mandating universal education, Binet and Simon wanted a fast way to flag “slow” learners so they could receive help. In 1905 they published the Binet–Simon Scale, a series of age-graded tasks. The test items – from naming objects and defining words to simple analogies – were designed so that a child at age X should succeed at tasks mastered by an average X-year-old. For example, one item asked children to pick which of two faces was prettier (or uglier). By determining the highest age level at which a child could answer most items correctly, Binet’s test assigned a “mental age.” Crucially, Binet emphasized the test was practical and empirical, intended to support teachers, not to label a child permanently.

The Binet–Simon test was novel and influential: it “was the first intelligence test that could be used to predict scholarly performance”. Psychologists lauded it because it assessed complex mental abilities in realistic settings. Unlike earlier attempts that tested one thing at a time (e.g. reaction time or sensory perception), Binet’s scale treated intelligence as a composite of many functions (language, memory, reasoning, etc.). These ideas fit the emerging belief that intelligence is multi-dimensional, not just a single single faculty. As Binet himself noted, intelligence involved “language proficiency, imagination, memory [and] sensory discrimination” among other abilities.

Stanford–Binet and the IQ Concept (1916 onward)

The Binet–Simon test quickly gained international attention. In the U.S., Lewis Madison Terman (1877–1956) at Stanford University revised and extended it. In 1916 Terman published the Stanford–Binet Intelligence Scale, normalizing the test on American children and coining the term “intelligence quotient (IQ)”. (The IQ formula, introduced by German psychologist William Stern, divided a child’s mental age by actual age and multiplied by 100.) Terman’s version also first applied Binet’s methods to adolescents and adults. According to Britannica, “in 1916… Lewis Terman released a revised examination that became known as the Stanford–Binet test”. This test initiated the modern field of intelligence testing: it was one of the first standardized IQ tests and even an early example of an adaptive test (administrators would skip or stop at items that were too easy or too hard for a child).

Terman and his colleagues used the Stanford–Binet for wide-ranging purposes. It played a role in educational placement, and controversially in the eugenics movement: low IQ scores were sometimes used to justify poor treatment of certain individuals or groups (a legacy widely condemned today). Terman himself famously organized the Genetic Study of Genius, a long-term project tracking hundreds of “gifted” children (sometimes called “Termites”) identified by high IQ scores. Meanwhile, by 1937 the Stanford–Binet had been published in a third revision and later evolved through five editions – remarkably, it is still in use today (now as SB-5, released in 2003).

David Wechsler and Adult Intelligence Scales (1939–1955)

While the Stanford–Binet and its kin were primarily aimed at children’s intelligence, psychologist David Wechsler (1896–1981) saw the need for an adult measure. In 1939 at Bellevue Hospital (New York) he introduced the Wechsler–Bellevue Intelligence Scale. Wechsler disliked the idea of a fixed “mental age” for adults, arguing instead that intelligence scores should reflect how someone compares to the population norm. He set the average IQ at 100 for any age group. In Wechsler’s model, an IQ of 100 always meant “average,” regardless of age. According to Britannica, Wechsler “rejected the idea that there is an ideal mental age… and he defined normal intelligence as the mean test score for all members of an age group,” assigning 100 as the standard.

Wechsler’s approach also broadened the content of IQ tests. His scales comprised multiple subtests – both verbal (vocabulary, information, comprehension) and performance (puzzles, block designs, coding tasks). This multi-faceted battery provided a profile of cognitive strengths and weaknesses, rather than one single number. The Wechsler–Bellevue test quickly became the dominant adult IQ test in the U.S., and Wechsler issued a first revision in 1942. In 1955 he published the Wechsler Adult Intelligence Scale (WAIS) – essentially an updated Wechsler–Bellevue with more representative norms (including about 10% nonwhite participants). (The earlier test had been normed on an all-white sample, reflecting the era.) He also developed a children’s version (WISC, 1949). Remarkably, Wechsler continued updating the WAIS late into life (e.g. contributing to the 1981 revision), and today the WAIS-IV remains one of the most widely used IQ tests.

David Wechsler (1896–1981), shown above in the 1950s, created the flagship Wechsler intelligence tests for adults. Wechsler’s innovations included using a deviation IQ (mean = 100) instead of a “mental age,” and testing a battery of verbal and nonverbal skills. His scales were designed for clinical use (e.g. with veterans returning from World War II) and remain influential: the modern WAIS still owes much to Wechsler’s 1939 and 1955 models.

Key Differences in Test Design and Goals

Over the century, IQ tests evolved in several key ways. Some major contrasts between early and later tests include:

  • Scoring method: Early tests like the Binet–Simon and Stanford–Binet assigned a mental age, then converted it to an IQ ratio (mental age ÷ chronological age × 100). Modern tests (like later Stanford–Binet editions and all Wechsler scales) use a deviation IQ: raw scores are normalized so that 100 is the population mean. This change avoids the outdated notion of “ideal mental age” for adults.

  • Age range: The original Binet tests targeted children, reflecting their educational mission (e.g. to keep slower learners in school). Lewis Terman’s Stanford–Binet expanded to teens and adults. Wechsler explicitly designed for adults, producing the first widely-used adult IQ scales. (He later made a children’s version, the WISC.)

  • Content and format: Binet’s items were largely practical tasks suitable for young students (e.g. picture puzzles, definitions, arithmetic). Wechsler’s tests use a diverse battery of verbal and performance subtests (vocabulary questions, arithmetic problems, pattern puzzles, memory tasks, etc.). Both the Stanford–Binet and Wechsler scales are multifactor tests covering many cognitive domains, rather than measuring only one skill.

  • Administration: Early individual tests could be lengthy. The Stanford–Binet introduced an adaptive style: items were given in order of difficulty and the exam stopped after a student failed several in a row. Later standardized tests often use fixed-item formats. In wartime, “group” Army Alpha/Beta tests were also developed to efficiently screen many recruits, though group tests are not used in mainstream IQ testing today.

  • Purpose: Binet intended his scale as a practical aid for educators, stressing that it should not label a child permanently. Over time, IQ tests came to be used in many contexts (education, clinical diagnoses, research, even employment). Notably, Wechsler’s goal was to provide a clinical tool for psychologists assessing adult patients, hence his emphasis on verbal and performance indexes.

Did You Know?

  • Personal Testing: Alfred Binet actually tested his own two daughters in early studies (1900–1902) before publishing his famous scale. He believed in studying individual differences firsthand.

  • IQ Was a Formula: The term intelligence quotient (IQ) was introduced in 1912 by William Stern, who defined IQ = (mental age ÷ chronological age) × 100. For example, a 10-year-old with a mental age of 12 would have IQ = (12/10)×100 = 120.

  • Still Evolving: Both the Stanford–Binet and Wechsler tests are regularly updated. The Stanford–Binet is now in its 5th Edition (released 2003). Wechsler’s WAIS has seen multiple revisions (WAIS-IV in 2008), but all still trace back to his 1939 and 1955 models.

  • Adaptive Testing: The Stanford–Binet was one of the first adaptive tests. Examiners would skip items far below or above a child’s level, focusing only on questions around the child’s ability. This idea of adaptivity paved the way for modern computerized adaptive IQ tests.

Throughout these developments, the definition of intelligence itself has broadened. Early researchers saw it as a practical school-readiness measure, but today it’s understood as a range of cognitive capacities. IQ tests have incorporated new theories (like multiple intelligences or fluid vs. crystallized intelligence) and new norms, while remaining rooted in Binet and Wechsler’s pioneering work. From Galton’s Victorian lab to Binet’s classroom and Wechsler’s clinic, the century-long story of IQ testing reflects changing science and society – yet the core goal remains the same: to quantify human intelligence with fairness and insight.

References & Sources

  1. Britannica – Alfred Binet
    “French psychologist who invented the first practical IQ test.”
    https://www.britannica.com/biography/Alfred-Binet

  2. Polytechnique Insights – Declining global IQ: Reality or moral panic?
    “IQ scores may not be falling, but how we measure intelligence is evolving.”
    https://www.polytechnique-insights.com/en/columns/society/declining-global-iq-reality-or-moral-panic/

  3. ViewonIQ – The NIQ-Dataset V1.3 Summary
    A comprehensive global comparison of IQ studies.
    https://viewoniq.org/?p=84

  4. APA Dictionary of Psychology – Stanford-Binet Intelligence Scales
    An overview of the test’s evolution and usage.
    https://dictionary.apa.org/stanford-binet-intelligence-scales

  5. Verywell Mind – History of IQ Testing
    Outlines contributions by Binet, Terman, and Wechsler.
    https://www.verywellmind.com/history-of-iq-testing-2795581

  6. Simply Psychology – Intelligence: Definitions and Testing
    Provides context on the development and controversies in intelligence testing.
    https://www.simplypsychology.org/intelligence.html

  7. Cambridge Encyclopedia of Child Development – Intelligence Testing
    Scholarly background on the Binet–Simon and Stanford–Binet scales.
    (Available through academic libraries or Google Books)

  8. American Psychological Association – David Wechsler
    Biographical profile and testing contributions.
    https://www.apa.org/about/governance/president/david-wechsler

  9. Frontiers in Psychology – Psychometric Origins
    An academic journal tracing the roots of modern psychometrics.
    https://www.frontiersin.org/articles/10.3389/fpsyg.2018.00463/full

  10. National Library of Medicine (PubMed Central) – The Evolution of the IQ Test
    Peer-reviewed historical and clinical summary.
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7259512/