What IQ is, and what it isn't

A scale built to help children in school

In 1904, France's Ministry of Public Instruction set up a commission to identify children who needed special education. The psychologist Alfred Binet was one of its members and, together with the physician Théodore Simon, he published a scale of short tasks in 1905, ordered from easiest to hardest: repeating digits, comparing lengths, defining words, completing sentences.

The central idea was mental age. If an eight-year-old could solve what most ten-year-olds could solve, that child's mental age was ten. Binet insisted that the scale was meant to guide teaching, and that the result was not a fixed label stuck on the child forever.

Stern's quotient

In 1912, the German psychologist William Stern proposed dividing mental age by chronological age. That is where the name comes from: intelligence quotient. A few years later, Lewis Terman of Stanford University adapted Binet's scale for the United States and multiplied the quotient by 100 to get rid of the decimals. A ten-year-old with a mental age of twelve scored 120.

The math works poorly for adults. Performance on this kind of task stops growing in late adolescence, and dividing by an age that keeps going up would make everyone lose points with every birthday. A different ruler was needed.

Wechsler and the deviation IQ

In 1939, David Wechsler published a battery for adults with the solution still in use today. Instead of mental age, the result became the person's position relative to others in the same age group. The group average becomes 100, and each standard deviation is worth 15 points.

That is why an IQ today indicates a position, not a quantity measured like pounds on a scale. A 115 means one standard deviation above the average of your age group, and about 2 in 3 people fall between 85 and 115. The shape of that distribution is explained in What each score means.

The g factor

Also in 1904, the British psychologist Charles Spearman noticed that children's grades in very different subjects moved together: those who did well in one tended to do well in the others. He called the common factor g, for general intelligence. More than a century later, the finding still holds. People who do well on visual matrices also tend to do well on number series, verbal reasoning and short-term memory, even though the relationship is far from perfect.

Raymond Cattell later separated two faces of this factor. Fluid intelligence is reasoning in the face of new problems; crystallized intelligence is accumulated knowledge, such as vocabulary and general information. Reasoning tests like IQNorth's lean toward the fluid side: of the 37 questions that count, 20 involve visual patterns, series and figure rotation, tasks that depend little on what a person has studied.

What the number predicts, on average

IQ is among the most studied numbers in psychology, and it does predict things. In a review commissioned by the American Psychological Association, Neisser and colleagues (1996) reported a correlation of around 0.5 between IQ and school grades, and smaller associations with years of schooling and job performance. Studies that follow people for decades, such as the one that tracked Scottish children tested in 1932, have even found a link with health in old age (Deary, 2001).

The phrase that matters is on average. A correlation of 0.5 is strong for psychology, and it still leaves plenty of room for exceptions. In any group of people with the same IQ, some do very well in school and some do poorly, for reasons the test can't see, such as interest, effort, support at home or health.

What it doesn't say about you

A reasoning test records how you solved certain kinds of problems, on one day, in one format. It doesn't measure character, honesty, kindness, courage or sense of humor. Creativity is left out too: research finds some relationship between the two, but people with similar IQs vary widely in their ability to come up with original ideas.

Nor does the number say how much a person is worth. That confusion has an ugly history: in the early 20th century, intelligence test results were used as an argument in favor of sterilization laws and immigration restrictions in the United States. Binet, who created the first scale, already warned against treating the result as destiny.

IQ describes a position on one type of task, compared with people your age. The rest of who you are falls outside that ruler.

If you want to see how IQNorth's estimate is calculated, with its margin of error and stated limits, read the method or take the test.

Read next: Does a high IQ guarantee success?