How Is IQ Actually Measured? The Science Behind the Number
Hard to believe, but the IQ test wasn't built to find geniuses. It was built in 1905, in France, for the exact opposite reason: identifying which children needed extra help in school.
The origin: Alfred Binet and a very concrete problem
France's Ministry of Education asked psychologist Alfred Binet and his colleague Théodore Simon for an objective way to spot children falling behind in school, without relying on each teacher's subjective judgment. The result was the Binet-Simon Scale, the first practical intelligence test in history. Binet never imagined his tool would, a century later, become a viral phenomenon with shareable "certificates" on social media.
From "mental age" to "quotient"
The idea of an intelligence quotient (IQ) came later, in 1912, courtesy of German psychologist William Stern. His original formula was simple: divide the "mental age" the test produced by the person's actual chronological age, then multiply by 100. An 8-year-old who answered like a 10-year-old scored 125 (10÷8×100).
The bell curve we use today
That formula worked for children, but made no sense for adults (is a 40-year-old "worth" less than a 60-year-old just by age?). In 1955, psychologist David Wechsler — creator of the WAIS test, still one of the most used in the world — replaced the formula with what's called "deviation IQ": instead of comparing ages, your score is compared against thousands of people in your same age bracket, on a normalized scale where the average is always 100 and the standard deviation is 15.
That means, statistically, about 68% of the population falls between 85 and 115, and roughly 95% falls between 70 and 130. Falling outside that range — in either direction — is, by definition, statistically uncommon.
What a modern test measures (and doesn't)
Today's tests — like the WAIS-IV, Raven's Progressive Matrices, or the Stanford-Binet — aren't a single test anymore, but a set of timed subtests measuring distinct abilities: verbal reasoning, visual/abstract reasoning, working memory, and processing speed. The final number is a weighted average of all of them, not one magic question.
What's left out of the number: creativity, emotional intelligence, social or practical skills. A high IQ says a lot about abstract reasoning and very little about the rest of what makes someone intelligent in real life.
Our test follows that same original logic — questions of increasing difficulty, timed, across different categories — condensed into a fast, free format. It doesn't replace a professional psychometric evaluation, but it does give you a real estimate, calculated with the same math psychologists have used for 70 years.