The History of
IQ.

1 The Birth of Intelligence Testing (1905)

The story of IQ testing begins in Paris, 1905, when French psychologist Alfred Binet and physician Théodore Simon were commissioned by the French Ministry of Education to develop a method for identifying children who needed educational assistance.

At the time, thousands of children were struggling in the newly mandated public schools, but there was no objective way to determine who needed special help versus who simply wasn't trying. Binet's innovation was creating a standardized series of tasks—arranged in order of difficulty—to measure cognitive abilities independently of schooling or social class.

The original Binet-Simon Scale (1905) included 30 tasks of increasing difficulty, from following a moving object with the eyes to defining abstract concepts. Binet introduced the revolutionary concept of "mental age"—comparing a child's performance to what was typical for different age groups.

Crucially, Binet explicitly warned against misusing his test. He viewed intelligence as malleable, not fixed, and designed his test for a specific educational purpose—identifying children who could benefit from additional instruction. He wrote: "With practice, training, and above all method, we manage to increase our attention, our memory, our judgment, and literally to become more intelligent than we were before."

2 The Stanford-Binet and the IQ Score (1916)

Lewis Terman of Stanford University brought Binet's test to America, substantially revising and extending it. His 1916 Stanford-Binet introduced the term "Intelligence Quotient" (IQ), using the formula developed by German psychologist William Stern:

IQ = (Mental Age ÷ Chronological Age) × 100

A 10-year-old performing like an average 12-year-old would have an IQ of 120 (12/10 × 100). This ratio method worked well for children but proved problematic for adults, since mental development doesn't continue linearly into adulthood.

Terman's version was more extensive than Binet's, standardized on American children, and included detailed administration and scoring procedures. It became the gold standard for intelligence testing for decades. The Stanford-Binet went through multiple revisions (1937, 1960, 1986, 2003) and remains in clinical use today as the Stanford-Binet 5.

Terman also initiated the famous "Terman Study of the Gifted" (1921), which tracked over 1,500 high-IQ children throughout their lives—one of the longest-running longitudinal studies in psychology, continuing until 2010.

3 World War I: Intelligence Testing Goes Mass Scale

World War I marked a turning point, demonstrating that intelligence tests could be administered to large groups efficiently. Psychologist Robert Yerkes led the development of the Army Alpha and Army Beta tests, which screened over 1.7 million military recruits between 1917-1919.

• Army Alpha: A written test for literate English speakers, including verbal analogies, number series, and following directions.
• Army Beta: A non-verbal test for illiterate recruits or non-English speakers, using picture completion, digit-symbol coding, and maze tracing.

These tests were used to assign recruits to appropriate positions and identify officer candidates. While revolutionary in scale, they were later criticized for cultural bias and misinterpretation of results (some researchers drew questionable conclusions about group differences without adequate consideration of educational and environmental factors).

Nevertheless, the Army testing program established intelligence testing as a practical, large-scale assessment tool and trained a generation of psychologists in test administration. Many of these psychologists went on to develop civilian tests after the war.

4 Spearman and the Discovery of g (1904-1927)

Charles Spearman, a British psychologist, made perhaps the most fundamental theoretical contribution to intelligence research. In 1904, he published his discovery of the general factor of intelligence (g). His work is still cited in modern research; see Schmidt & Hunter (1998) for meta-analytic validation of g's predictive validity.

Using a statistical technique he developed called factor analysis, Spearman observed that people who performed well on one cognitive test tended to perform well on most others. He called this pattern the "positive manifold" and proposed that it reflected a single underlying ability—g—that influenced all cognitive tasks.

Spearman's "two-factor theory" proposed that performance on any cognitive task reflects both:

• g: General intelligence, common to all mental tests
• s: Specific abilities unique to particular tasks

This discovery of g remains one of the most replicated findings in all of psychology. The existence of a general factor has been confirmed in hundreds of studies across cultures and continues to structure how we think about cognitive abilities.

5 Wechsler's Revolution (1939-1955)

David Wechsler, chief psychologist at Bellevue Hospital in New York, transformed intelligence testing with a series of innovations that still define modern tests:

Wechsler's key innovations included:

• Deviation IQ: Replaced the ratio method with standardized scores (mean 100, SD 15), making adult testing meaningful and scores directly comparable across ages.
• Performance Scale: Added non-verbal tasks (block design, picture completion) alongside verbal measures, providing a more complete cognitive profile.
• Subtest Analysis: Enabled examination of cognitive strengths and weaknesses, not just an overall score.
• Standardization: Careful norming on representative samples with detailed age-stratified norms.

6 Cattell, Horn, and the Gf-Gc Theory (1941-1998)

Raymond Cattell was a student of Spearman who both extended and challenged his mentor's work. In 1941, Cattell proposed that general intelligence could be divided into two distinct components. His foundational work is documented in Horn & Cattell's research:

• Fluid Intelligence (Gf): The ability to solve novel problems using reasoning, independent of acquired knowledge. Peaks in early adulthood and declines with age.
• Crystallized Intelligence (Gc): Accumulated knowledge and skills developed through education and experience. Continues to grow throughout life.

John Horn, Cattell's student, significantly expanded this theory. Through decades of factor-analytic research (1965-1998), Horn identified additional broad abilities beyond Gf and Gc:

• Visual Processing (Gv)
• Auditory Processing (Ga)
• Processing Speed (Gs)
• Short-Term Memory (Gsm)
• Long-Term Storage & Retrieval (Glr)
• Quantitative Knowledge (Gq)

The Horn-Cattell Gf-Gc model emphasized that intelligence is not a single entity but a constellation of relatively independent abilities, each with its own developmental trajectory and neural substrate.

7 Carroll's Three-Stratum Theory (1993)

John Carroll's 1993 magnum opus, "Human Cognitive Abilities: A Survey of Factor-Analytic Studies", represents one of the most ambitious and influential works in the history of psychology.

Carroll systematically reanalyzed over 460 datasets spanning nearly a century of intelligence research—virtually every published factor-analytic study of cognitive abilities. Using modern computing power and consistent methodology, he extracted the common threads across this vast literature.

His conclusion: cognitive abilities are organized in a three-stratum hierarchy:

• Stratum III: General intelligence (g) at the apex—the common factor underlying all cognitive tasks
• Stratum II: Approximately 8-10 broad abilities (similar to Horn's extensions of Gf-Gc)
• Stratum I: Over 70 narrow, specific abilities that cluster within the broad domains

Carroll's work validated both Spearman's g and the multiple abilities identified by Horn and Cattell, showing they weren't contradictory but rather different levels of the same hierarchical structure. This synthesis set the stage for CHC theory.

8 The CHC Integration (1999-Present)

In the late 1990s, psychometrician Kevin McGrew recognized that the Horn-Cattell and Carroll theories were describing essentially the same structure from different perspectives. His work formally integrating these frameworks created Cattell-Horn-Carroll (CHC) theory.

CHC theory rapidly became the dominant framework in the field because it:

• Unified decades of research into a coherent model
• Provided a common vocabulary for discussing cognitive abilities
• Offered clear guidelines for test development
• Enabled cross-battery assessment across different test publishers
• Connected theoretical constructs to practical clinical applications

By the mid-2000s, all major test publishers had aligned their instruments with CHC theory. The Woodcock-Johnson III (2001), Stanford-Binet 5 (2003), KABC-II (2004), and subsequent editions of the Wechsler scales explicitly reference CHC constructs.

CHC theory continues to evolve. Updates in 2012 distinguished Working Memory Capacity (Gwm) as separate from Short-Term Memory (Gsm). Ongoing research refines our understanding of processing speed factors and domain-specific knowledge abilities.

9 Major Historical Timeline

10 ACIS: The Next Evolution

ACIS represents the culmination of 120 years of intelligence testing science, combining the best insights from across this rich history:

• From Binet: The practical orientation—testing should serve a purpose and help people understand themselves.
• From Spearman: The primacy of g—ACIS maximizes g-loading (.99) to measure general cognitive ability with optimal precision.
• From Wechsler: The multi-domain structure—ACIS provides index scores across six cognitive domains, not just an overall IQ.
• From Cattell & Horn: The distinction between fluid and crystallized abilities—ACIS measures both extensively.
• From Carroll & CHC: The theoretical foundation—ACIS is explicitly grounded in contemporary CHC theory with 20 subtests sampling broad and narrow abilities.

What ACIS adds is accessibility. For the first time, a comprehensive, clinical-grade intelligence assessment is available online to anyone who wants deeper self-understanding. The test maintains the psychometric rigor of traditional instruments while removing barriers of cost, location, and professional gatekeeping.

With 20 subtests, a ceiling of 160, and reliability exceeding .95, ACIS demonstrates that online assessment can meet or exceed the standards of traditional proctored testing—continuing the evolution that Binet began over a century ago.

11 Key Figures in IQ History

12 Further Reading

For those wishing to explore this history in greater depth:

• Fancher, R.E. (1985): The Intelligence Men: Makers of the IQ Controversy. Norton. WorldCat.
• Gould, S.J. (1996): The Mismeasure of Man (Revised Edition). Norton. [Critical perspective] WorldCat.
• Carroll, J.B. (1993): Human Cognitive Abilities: A Survey of Factor-Analytic Studies. Cambridge University Press. WorldCat.
• Deary, I.J. (2001): Intelligence: A Very Short Introduction. Oxford University Press. WorldCat.
• Hunt, E. (2011): Human Intelligence. Cambridge University Press. WorldCat.
• Schneider, W.J. & McGrew, K.S. (2018): The Cattell-Horn-Carroll theory of cognitive abilities. In Contemporary Intellectual Assessment (4th ed.). Guilford.
• Flynn, J.R. (1987): Massive IQ gains in 14 nations: What IQ tests really measure. Psychological Bulletin, 101(2), 171-191. DOI: 10.1037/0033-2909.101.2.171.
• Neisser, U. et al. (1996): Intelligence: Knowns and Unknowns. American Psychologist, 51(2), 77-101. DOI: 10.1037/0003-066X.51.2.77.
• Nisbett, R.E. et al. (2012): Intelligence: New Findings and Theoretical Developments. American Psychologist, 67(2), 130-159. DOI: 10.1037/a0026699.
• Deary, I.J., Penke, L. & Johnson, W. (2010): The Neuroscience of Human Intelligence Differences. Nature Reviews Neuroscience, 11, 201-211. DOI: 10.1038/nrn2793.
• Plomin, R. & Deary, I.J. (2015): Genetics and intelligence differences: Five special findings. Nature Reviews Genetics, 16, 359-371. Nature.
• McGrew, K.S. (2009): CHC theory and the human cognitive abilities project. Intelligence, 37(1), 1-10. DOI: 10.1016/j.intell.2008.08.004.
• Horn, J.L. & Cattell, R.B. (1966): Refinement and test of the theory of fluid and crystallized general intelligences. Journal of Educational Psychology, 57(5), 253-270. DOI: 10.1037/h0023816.