What does IQ stand for?
IQ stands for intelligence quotient. The quotient part is a fossil: it describes a piece of arithmetic that intelligence tests stopped doing nearly a hundred years ago.
Two words, doing very unequal amounts of work.
Intelligence is the contested one. Psychologists have argued about what it means for over a century, and the tests measure something considerably narrower than the word suggests in ordinary speech.
Quotient is the interesting one, because it is a leftover.
Where the quotient came from
The first practical intelligence scale was built in 1905 by Alfred Binet and Théodore Simon, commissioned by the French government to find schoolchildren who needed extra educational support.
Binet’s scale produced a mental age: the age group whose typical performance you matched. A ten-year-old answering like a typical twelve-year-old had a mental age of twelve.
The German psychologist William Stern then proposed dividing one by the other:
IQ = (mental age divided by chronological age) x 100
That ten-year-old scores 120. The division is the quotient, and it is the whole reason for the name.
Why nobody calculates it that way any more
The ratio formula has a fatal problem. Mental age stops climbing in adulthood while chronological age does not, so dividing a forty-year-old’s mental age by forty produces nonsense.
It also inflates absurdly for precocious children, which is exactly where the eye-watering historical figures come from. A ratio IQ of 228 says a child performed like someone far older. It does not translate onto the modern scale at all, and it explains why so many vintage prodigy scores sound impossible.
David Wechsler replaced the ratio in the 1930s with the deviation IQ, which every mainstream test uses now. Your performance is compared to how people your own age actually score, and placed on a scale fixed at a mean of 100 with a standard deviation of 15.
No division happens anywhere in that process. The name survived regardless.
What an IQ score is today
A modern IQ is a position, not a quantity. It says: relative to your age peers, on this kind of reasoning task, here is roughly where you sit.
That is a more modest claim than intelligence quotient implies, and a considerably more defensible one. It also means the score depends entirely on the reference group, which is why how a test is normed matters more than how hard the questions look.
The four bits of vocabulary worth knowing:
- Mean: set to 100 on every mainstream test.
- Standard deviation: usually 15, sometimes 16 or 24 on older scales. It fixes how rare each score is.
- Percentile: the share of people you scored above, explained in IQ percentiles.
- Norming sample: the group your performance was compared against. Without it the score means nothing.
Binet’s own warning
Worth knowing, given what the test later became. Binet was explicit that his scale measured current performance rather than fixed capacity, and he objected to treating a score as a permanent statement about a child.
The test was designed to identify children who needed help so they could be given it. Much of what followed in the twentieth century, including the Stanford-Binet revision that Americanised the scale, moved a long way from that intent.
If you want a score of your own on a modern deviation scale, our free IQ test takes about ten minutes and shows the result immediately. How we score it is documented in full on our methodology page.
Key takeaways
- IQ stands for intelligence quotient.
- The quotient refers to Stern’s original formula: mental age divided by chronological age, times 100.
- That formula breaks down in adulthood and inflates wildly for gifted children.
- Wechsler replaced it in the 1930s with the deviation IQ, which compares you to your own age group.
- Modern IQ is a position in a distribution, not a quantity of intelligence.
- Binet designed the original scale to identify children needing support, and warned against reading it as fixed capacity.