Why Is 100 the Average IQ?
Quick Answer
100 is average by design, not by measurement. When an IQ test is developed, it's given to a large representative sample, and the median raw performance is defined as 100. Scores are then spread on a bell curve with a standard deviation of 15. Every re-norming re-centers the average back to 100.
How Test Norming Works
IQ tests are 'normed' before release: publishers administer the test to thousands of people chosen to mirror the population by age, region, and education. Whatever raw score lands in the exact middle of that sample becomes 100. The rest of the scale is built outward in standard deviations of 15 points — 115 is one deviation above average, 85 one below.
This means IQ is inherently relative. If everyone in the world got smarter overnight, the average IQ would still be 100 after the next norming — the bar would simply have moved.
The Flynn Effect: The Moving Average
Raw performance on IQ tests rose steadily through the 20th century — roughly 3 points per decade — a phenomenon called the Flynn effect. Better nutrition, schooling, and familiarity with abstract problem-solving all contributed. Test publishers periodically re-norm to pull the average back to 100, which is why a score from a 1970s test isn't directly comparable to one from today.
In some developed countries the Flynn effect has slowed or slightly reversed since the 1990s, an active area of research.
Frequently Asked Questions
Has the average IQ always been 100?
Yes, by construction — every properly normed IQ test defines its sample's median as 100. What changes over time is the raw performance required to score 100, which rose throughout the 20th century (the Flynn effect).
What percentage of people have an IQ of exactly 100?
About 50% of people score within 90–110, the 'average' band. Scoring exactly 100 places you at the 50th percentile — the precise middle of the distribution.
Is the average IQ different in different countries?
Published national estimates vary, but they're heavily affected by which tests were used, when they were normed, and sampling quality. Within any country that norms its own tests, the average is 100 by definition.
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MyIQScores Editorial Team
Researchers in cognitive psychology, psychometrics & educational science
Last updated
May 10, 2026
All content on MyIQScores is reviewed for scientific accuracy against peer-reviewed research in cognitive psychology and psychometrics. Our editorial team cross-references each article with published literature before publication and updates pages whenever new research warrants a revision.