Question
Why is the average IQ 100?
Short answer
The average IQ is 100 because tests are built that way. Publishers give a test to a large sample at each age and scale the results so each age group averages exactly 100, with a standard deviation of 15. The number 100 is a convention that makes scores easy to read, not a measured fact about people.
Want your own number? The test is 25 visual puzzles, about 12 minutes, no sign-up.
100 is a choice, not a discovery
Modern IQ tests report a deviation IQ: scores are standardized so the mean is 100 and the standard deviation is 15 within each age group (Wikipedia: Intelligence quotient). The test maker could have picked 0 and 1, or 500 and 100. They picked 100 and 15 because that is what David Wechsler used when he introduced the deviation IQ with the Wechsler-Bellevue scale in 1939 (Wikipedia: WAIS), and every major test since has kept the convention so scores stay comparable.
How the scaling works
A publisher tests a large standardization sample at each age. The average raw performance for each age group becomes 100. Performance one standard deviation above that becomes 115, two above becomes 130, and so on. Your score is where your raw performance falls on that scale for your age. See how IQ is calculated for the steps.
Try one · Logical Reasoning
Which tile completes the second pair?
Tap the option you think is right, or show the answer.
Show the answer
The missing panel shows a large outlined triangle pointing left, with small solid shapes: a triangle at the top-left corner (option 6).
How to solve it
- Compare the first two panels and name exactly what changed.
- The whole figure turns a quarter turn counterclockwise.
- The large shape becomes outlined.
- Nothing else changes.
- Apply the same changes to the third panel: a large hatched triangle pointing up, with small solid shapes: a triangle at the top-right corner.
- So the missing panel shows a large outlined triangle pointing left, with small solid shapes: a triangle at the top-left corner.
That was 1 puzzle. The full test has 25.
Why it has to be reset
Average raw performance does not stay still. Measured IQ rose through the 20th century, about 2.8 points per decade in a large meta-analysis, with gains slowing recently (Pietschnig & Voracek, 2015). If a test kept old norms, the average person today would score above 100. So tests are renormed against new samples, which resets the average to 100. Read why IQ tests are renormed.
What this means for you
A score of 100 means you did exactly as well as the typical person your age in the reference sample. It is the middle, not a pass mark. On the WAIS-IV, 90 to 109 is labeled Average (Wikipedia: IQ classification). The average IQ guide covers what counts as average in more depth, and IQ 100 shows it on the curve.
Sources
- Intelligence quotient. Wikipedia. en.wikipedia.org/wiki/Intelligence_quotient
- Wechsler Adult Intelligence Scale. Wikipedia. en.wikipedia.org/wiki/Wechsler_Adult_Intelligence_Scale
- IQ classification. Wikipedia. en.wikipedia.org/wiki/IQ_classification
- Pietschnig, J., & Voracek, M. (2015). One century of global IQ gains. Perspectives on Psychological Science, 10(3), 282-306. doi.org/10.1177/1745691615577701