IQ 166

Taken literally, the bell curve puts 166 at 1 in 185,000: about 370 people in the UK and about 44,000 people worldwide. Treat that as an illustration of the model rather than a head count.

No test has enough people in its norm sample to check its accuracy at 166. In a 2,200-person sample the bell curve expects about 0.01 people at 166 or higher.

Some tests publish extended norms above 160 for exceptional children. These rest on very small samples and on modelling, so they are best treated as rough rankings.

If someone tells you they have a 166, the first question is where the number came from. The usual answers:

• Different scales. Some tests use a standard deviation of 24, so a 166 there is about 141 on the usual SD 15 scale.
• Historiometric estimates. Researchers such as Catharine Cox (1926) estimated IQs for famous people from biographies. Some of her adjusted estimates went above 200. They are educated guesses, not test results.
• Unnormed online quizzes. A test with no norm data can print any number. Very high outputs usually say more about the scoring than the person.

42 of the historical profiles on this site carry an estimate of 160 or more, and 14 reach 166 or higher. Isaac Newton and Leonardo da Vinci are the classic examples. Their numbers are retrospective judgements based on their work, not test scores.

Two people with identical estimates can have very different evidence behind them. Each profile explains what its estimate is based on and how confident anyone can be.

On a bell curve with a mean of 100 and a standard deviation of 15, about 0.00054% of people score 166 or higher. Applied to whole populations:

An IQ of 166 or higher is 4.4 times rarer than 161 or higher, and 171 or higher is 4.9 times rarer again. At this point on the curve, each extra point makes a score about 36% rarer.

What your 166 could really be. Even a good supervised test has a margin of error of roughly 5 points either side, and far more at this height. A true score from 161 to 171 would span rarities from 1 in 41,900 to 1 in 904,000.

Which scale was it on? Most modern tests use a standard deviation of 15, which is what this page assumes. If your 166 came from a test using an SD of 16, it is about 162 on the SD 15 scale. From an SD 24 scale, used by some Cattell-style reports, it is about 141.

Estimates this high are reserved for a small group of scientists, artists and thinkers. These sit near 166. None of these are test scores. They are estimates from biographical evidence, and each profile explains its basis.

How 166 stacks up against the scores just around it and the round numbers people usually ask about. Each score links to its own page.