The IQ test is barely more than a century old. It started as a practical tool to help schoolchildren and grew into one of the most famous measurements in psychology. Here is how it happened.
Timeline
The first intelligence test. French psychologist Alfred Binet and his colleague Théodore Simon created the Binet-Simon scale to spot children who needed extra help at school. It compared a child's “mental age” with their real age.
The word “IQ” is born. German psychologist William Stern proposed dividing mental age by actual age, creating the idea of an intelligence quotient.
The Stanford-Binet. Lewis Terman at Stanford University adapted Binet's test for Americans and multiplied the quotient by 100. The Stanford-Binet made “IQ” a household word, and updated versions are still used today.
Testing goes big. During World War I the U.S. Army gave the Army Alpha and Army Beta tests to more than a million recruits, the first mass intelligence testing in history.
Raven's Progressive Matrices. British psychologist John C. Raven created a test made entirely of visual pattern puzzles, so it does not depend on language or culture. The pattern-grid puzzles on our test follow this style.
Wechsler changes the math. David Wechsler published the Wechsler-Bellevue scale, and later the Wechsler Adult Intelligence Scale (WAIS). Instead of mental age, he compared each person with others their own age, setting the average at 100 with a standard deviation of 15. That is the system used today.
Mensa is founded. Roland Berrill and Lancelot Ware founded Mensa in England as a society for people who score in the top 2% on an approved intelligence test.
The Flynn effect. Researcher James Flynn showed that average IQ scores rose by about 3 points per decade through much of the 20th century, which is why tests have to be re-normed regularly.
Still evolving. Updated editions of the Wechsler and Stanford-Binet tests are used by psychologists worldwide, and online IQ tests let anyone check their reasoning skills in minutes.
The bell curve, explained
IQ scores follow a bell curve. Most people cluster around the average of 100. Each 15 points is one standard deviation, so a score of 130 is two standard deviations above average, higher than about 98% of people.
What an IQ test measures
IQ tests measure reasoning: spotting patterns, solving new problems, working with numbers and words, picturing shapes in your head, and holding information in short-term memory. They do not measure creativity, character, or how hard you work, which is why plenty of successful people have average scores and plenty of high scorers never become famous.
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