French psychologist Alfred Binet, working with his colleague Théodore Simon, created the first practical intelligence test in 1905. The French government had commissioned Binet to develop a way to identify schoolchildren who needed extra academic help, and the result was the Binet-Simon Scale, a series of tasks arranged by difficulty that measured reasoning, judgment, and problem-solving rather than physical traits.
Why the Test Was Created
In 1904, the French Ministry of Education asked Binet to find a reliable method for sorting children into appropriate classroom settings. France had recently passed laws requiring all children to attend school, and teachers needed a tool to identify students who would struggle with the standard curriculum. Binet’s goal was purely practical: figure out which kids needed specialized instruction so they could get it early.
Binet and Simon published their first scale in 1905, but it read more like a set of pilot studies than a polished instrument. The full version arrived three years later, in 1908, after extensive additional testing. That revision described four to eight tasks children should be able to complete at 11 different age levels, starting at age 3 and ending at 13. A final revision came in 1911, shortly before Binet’s death.
What the Test Actually Measured
The Binet-Simon Scale introduced a concept that would reshape psychology: mental age. Instead of giving a child a single numerical score, Binet compared a child’s performance to what was typical for children of various ages. If a seven-year-old completed the same tasks that most nine-year-olds could handle, that child had a mental age of nine. If a ten-year-old could only manage tasks typical of seven-year-olds, their mental age was seven.
This was a fundamentally different approach from what had come before. In the 1880s, the English scientist Francis Galton had tried to measure intelligence through physical and sensory attributes: reaction time, grip strength, the ability to distinguish between similar sounds. He tested thousands of people at his “Anthropometric Laboratory” in London, but the measurements turned out to have very little connection to actual intellectual ability. Binet’s insight was that intelligence had to be measured through mental tasks, not physical ones.
From Mental Age to the IQ Score
Binet never used the term “IQ.” That development came from two other psychologists who built on his work. German psychologist Wilhelm Stern proposed that a more useful measure would be a child’s mental age divided by their chronological age. A ten-year-old with a mental age of twelve would score 1.2, while a ten-year-old with a mental age of eight would score 0.8.
In 1916, Stanford University psychologist Lewis Terman took Stern’s ratio and multiplied it by 100 to eliminate the decimal point, creating the formula most people recognize: IQ equals mental age divided by chronological age, times 100. A perfectly average child scores 100. Terman also significantly expanded and restandardized Binet’s original test for American children, publishing it as the Stanford-Binet Scale. The name stuck, and the Stanford-Binet remains one of the most widely used intelligence tests today, now in its fifth edition.
How the Test Crossed the Atlantic
Before Terman published his revision, another American psychologist had already brought Binet’s work to the United States. Henry Goddard translated the Binet-Simon Scale into English and began using it at the Vineland Training School in New Jersey, where he worked with children who had intellectual disabilities. By 1913, Goddard was also administering the test to immigrants arriving at Ellis Island.
This is where the story takes a darker turn. Goddard used test results to argue that certain groups of people were intellectually inferior and should be prevented from having children, either through institutionalization or forced sterilization. His work became a favorite reference among supporters of the eugenics movement. The American Psychological Association has described Goddard’s views as part of “a dark chapter in American history,” one in which a tool designed to help children get better education was instead weaponized to justify discrimination and restrictive immigration policies.
Binet himself had warned against exactly this kind of misuse. He saw his scale as a diagnostic tool for educators, not a measure of fixed, inborn ability. He believed intelligence could be developed with the right support, which was the entire point of identifying struggling students in the first place.
The Modern Stanford-Binet
The test has been revised multiple times since Terman’s 1916 version. The current edition, the Stanford-Binet Fifth Edition, uses 10 subtests to assess five cognitive factors: fluid reasoning, knowledge, quantitative reasoning, visual-spatial processing, and working memory. It no longer relies on the simple mental-age-divided-by-chronological-age formula. Instead, scores are calculated by comparing a person’s performance against a large, statistically representative sample of people the same age.
The test can now be administered to people ranging from age 2 to adulthood, a far broader range than Binet’s original scale for schoolchildren. But the core philosophy remains recognizably his: measure what the mind can do with meaningful cognitive tasks, not how fast someone can react to a sound or how hard they can squeeze a handle.