The Origins: Alfred Binet
The first practical intelligence test was developed in 1905 by French psychologist Alfred Binet. He wasn't trying to rank humanity; he was trying to identify children who needed extra help in school. He created the concept of 'Mental Age'—the idea that a child who performs like an average 8-year-old has a mental age of 8, regardless of their actual birth age.
The IQ Formula
A few years later, German psychologist William Stern proposed the 'Intelligence Quotient' formula: (Mental Age / Chronological Age) x 100. If a 10-year-old had a mental age of 12, their IQ was 120. This formula worked well for children but became meaningless for adults, leading to the modern 'deviation IQ' system we use today.
The Stanford-Binet and World War I
Lewis Terman at Stanford University adapted Binet's work for an American audience, creating the Stanford-Binet Intelligence Scales. During WWI, the US military used 'Alpha' and 'Beta' tests to screen millions of recruits, marking the first large-scale application of IQ testing and sparking the public's fascination with 'the number.'
David Wechsler and the WAIS
In the 1930s, David Wechsler criticized the Stanford-Binet for its heavy reliance on verbal skills. He developed the Wechsler-Bellevue Intelligence Scale, which included both verbal and non-verbal (performance) tasks. This eventually evolved into the WAIS (Wechsler Adult Intelligence Scale), which is now the most widely used IQ test in the world.
The Dark Side: Eugenics
It is impossible to discuss the history of IQ without acknowledging its misuse. In the early 20th century, some proponents of eugenics used IQ scores to justify forced sterilization and restrictive immigration laws, based on the flawed belief that intelligence was a simple, inherited trait that determined a person's worth. Modern psychometrics has moved far beyond these roots, but the history serves as a reminder of the power—and responsibility—of measurement.
IQ Today
Modern tests like the WAIS-IV are incredibly sophisticated, measuring multiple domains of cognition. They are used for diagnosing learning disabilities, clinical assessments, and understanding human potential, always with the understanding that one score is just a single piece of the human puzzle.