Myths and famous claims
What was William James Sidis's IQ?
What is documented
No documented IQ test score exists for William James Sidis. What is documented is that he qualified for Harvard at 9, entered at 11 and graduated at 16, in 1914. The famous figure of 250 to 300 is an unverified later estimate, which one well-known column called conjectural.
Want your own number? The test is 25 visual puzzles, about 12 minutes, no sign-up.
What is documented
William James Sidis was a real and remarkable prodigy. He qualified for admission to Harvard at age 9, entered at 11, and graduated at 16 in 1914 (NPR 2011). That record is solid. It tells you he was doing university work at an age when most children are in elementary school.
What is not documented is a test. The sources we checked name no intelligence test he took, no date and no score.
Where the famous number comes from
The figure usually quoted for Sidis is an IQ of 250 to 300. It is a later estimate, and the syndicated Straight Dope column called it conjectural (Straight Dope, Sidis). NPR's 2011 profile repeated a related claim, that his IQ "was estimated" at 50 to 100 points above Einstein's, without naming any test, test date or source (NPR 2011). That comparison has a second problem: Einstein's own IQ is not known either. Psychologist Russell Warne found no credible figure for him (Warne 2023). A number defined as being above an unknown number is not a measurement.
Try one · Quantitative Reasoning
Which number comes next?
5, 7, 12, 20, 31, ?
Tap the option you think is right, or show the answer.
Show the answer
The next number is 45 (option 5).
How to solve it
- Find the gaps between neighbors: 2, 5, 8, 11.
- The gaps between the numbers grow by 3 each time.
- So the next gap is 14, and 31 + 14 = 45.
That was 1 puzzle. The full test has 25.
Why child prodigies attract huge numbers
Early IQ scores were ratios: mental age divided by chronological age, times 100, so a 10-year-old performing like an average 12-year-old scored 120 (Britannica, IQ). By the same arithmetic, a 6-year-old performing like an average 12-year-old would score 200. Under that formula, a very young child doing very advanced work produces very large numbers. Modern tests no longer work this way, and few still compute mental ages.
Estimates from biographies could also run high. Before Catharine Cox's 1926 study, Lewis Terman estimated Francis Galton's childhood IQ at close to 200 in 1917, using only a biography (Simonton 2020). We do not know how the Sidis figure was produced, but numbers of this size have a history of coming from methods like these rather than from a test.
How 250 compares with what tests can measure
Modern IQ tests have a ceiling. The WISC-V full-scale IQ ranges from 40 to 160 (Pearson: WISC-V UK sample report), and the Stanford-Binet 5 composite also ranges from 40 to 160 (PRO-ED: SB5). On a scale with a standard deviation of 15, a score of 250 would be ten standard deviations above the mean, far beyond the range any of these tests can report. Even 160 is already about 1 in 32,000 (see IQ 160). Guinness stopped naming a highest-IQ record holder in 1990 because it no longer trusted tests to be uniform or reliable enough for the job (Knight, Financial Times 2009).
How to judge a famous IQ claim
- Is a test named, with a date and an age? For Sidis, no.
- Is the score within the range real tests report?
- Is it compared with another unknown number, such as Einstein's?
- Was it a ratio score or a biography-based estimate, and does the claim say so?
For more on record claims, read the highest IQ ever recorded. For what high ability looks like in children, see giftedness, and for the scale itself, the highest possible IQ score.
Sources
- NPR Staff (2011, January 23). Meet William James Sidis: The Smartest Guy Ever? All Things Considered www.npr.org/2011/01/23/132737060/meet-william-james-sidis-the-smartest-guy-ever
- Adams, C. Why did William Sidis, the world's (2nd) smartest human, achieve so little in life? The Straight Dope (syndicated newspaper column; date not shown on page) www.straightdope.com/21342513/why-did-william-sidis-the-world-s-2nd-smartest-human-achieve-so-little-in-life
- Warne, R. T. (2023, September 14). The search for Albert Einstein's IQ. russellwarne.com (blog of a research psychologist) russellwarne.com/2023/09/14/the-search-for-albert-einsteins-iq/
- Encyclopaedia Britannica. IQ, revised by Jeannette L. Nolen (read via Wayback Machine, December 2024) www.britannica.com/science/IQ
- Simonton, D. K. (2020). Galton, Terman, Cox: The Distinctive Volume II in Genetic Studies of Genius. Gifted Child Quarterly. doi:10.1177/0016986220921360 doi.org/10.1177/0016986220921360
- Pearson Clinical UK. WISC-V UK sample interpretive report (Q-global). 2016. www.pearsonclinical.co.uk/content/dam/school/global/clinical/uk-clinical/files/wisc5-sample-interpretive-report.pdf
- Roid, G. H. Stanford-Binet Intelligence Scales, Fifth Edition (SB5), PRO-ED product page. 2003. proedinc.com/products-13290.html
- Knight, S. (2009, April 10). Is a high IQ a burden as much as a blessing? Financial Times (read via Wayback Machine) www.ft.com/cms/s/0/4add9230-23d5-11de-996a-00144feabdc0.html