Back to Articles
🧠
HistoryApril 21, 20263 min readIvan Ivanov

The History of IQ Testing: From Binet to Modern Assessments

The History of IQ Testing: From Binet to Modern Assessments

Measuring human intelligence has a rich and sometimes controversial history spanning more than a century. Understanding that history helps us appreciate both the power and the limits of modern IQ tests.

The Birth of Intelligence Testing (1905)

Alfred Binet and the First IQ Test

The story begins in Paris in 1905. The French government commissioned psychologist **Alfred Binet** and his colleague **Theodore Simon** to develop a way to identify students who needed additional educational support.

Binet's approach was revolutionary:

  • He created a series of tasks of increasing difficulty
  • Children were tested to determine their "mental age"
  • The idea of comparing mental age with chronological age was born

    • Binet himself warned against using his test to:

  • Label children as inherently limited
  • Measure a single, fixed intelligence
  • Rank people by intellectual worth

    • The Stanford-Binet Test (1916)

    Lewis Terman and the American Adaptation

    Stanford psychologist **Lewis Terman** adapted Binet's test for American use, creating the Stanford-Binet Intelligence Scale. Key innovations included:

  • Introduction of the **Intelligence Quotient (IQ)** formula:

    • `IQ = (Mental Age / Chronological Age) x 100`

  • Standardization for American populations
  • An expanded age range for testing

    • World War I and Mass Testing

    World War I was a turning point in the history of IQ testing. The US Army needed to evaluate millions of recruits quickly:

  • **Army Alpha Test** — for literate recruits
  • **Army Beta Test** — for illiterate or non-English-speaking recruits

    • Over 1.7 million soldiers were tested, proving that large-scale intelligence assessment was feasible.

    The Wechsler Scales (1939 to Today)

    David Wechsler's Contribution

    **David Wechsler**, a psychologist at Bellevue Hospital in New York, transformed intelligence testing with several innovations:

    **The Wechsler-Bellevue Scale (1939):**

  • Moved away from age-based scoring
  • Introduced deviation IQ (mean 100, standard deviation 15)
  • Separated verbal and performance abilities

    • **Modern Wechsler Tests:**

  • **WAIS** (Wechsler Adult Intelligence Scale)
  • **WISC** (Wechsler Intelligence Scale for Children)
  • **WPPSI** (Wechsler Preschool and Primary Scale of Intelligence)

    • Today, the Wechsler tests are the most widely used intelligence assessments in the world.

    Modern Approaches

    Raven's Progressive Matrices

  • A nonverbal test of abstract reasoning
  • Reduces cultural and linguistic bias
  • Widely used in international research

    • Cognitive Assessment System (CAS)

  • Based on the PASS theory of cognitive processing
  • Evaluates planning, attention, simultaneous and successive processing

    • Current Standard IQ Distribution

  • Mean of 100
  • Standard deviation of 15
  • Normal distribution (bell curve)
  • Regular renorming due to the Flynn effect

    • The Flynn Effect

    Researcher **James Flynn** discovered that IQ scores have been rising worldwide by about 3 points per decade. Possible explanations include:

  • Better nutrition
  • Improved education
  • Increased cognitive stimulation
  • Familiarity with test formats

    • Conclusion

    From Binet's modest beginnings to today's sophisticated assessments, IQ testing has come a long way. Although no test perfectly captures the complexity of human intelligence, modern assessments offer valuable insight into cognitive abilities and potential.

    ---

    *Curious about your cognitive abilities? Take our scientifically designed IQ test and discover your intellectual profile.*

    Ivan Ivanov
    Written by
    Ivan Ivanov
    Founder & Product Lead
    Published on April 21, 2026
    The History of IQ Testing: A Full Timeline from 1905 to Today