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The “250 to 300” IQ attached to William James Sidis comes from a single 1946 book, reporting what his sister said a psychologist once told her, years after Sidis had died. No test record, and no named psychologist, has ever surfaced.
No childhood IQ test with a surviving numeric score exists for Sidis. This range reflects a genuinely exceptional, well-documented pattern of early academic acceleration — Harvard admission and a public mathematics lecture at 11 — while stopping well short of the unsupported 250–300 figure.
Set the claim aside for a moment. If someone genuinely scored in this range, here is what that would represent.
Average is 100 by construction, and about two-thirds of people fall between 85 and 115. Take the test to place your own score on this scale.
| Score range | Classification | Share of people | What it indicates |
|---|---|---|---|
| 85 – 114 | Average | ~68% | The range most people fall into. |
| 115 – 129 | Above average | ~13.6% | Roughly one person in seven. |
| 130 – 144 | Gifted | ~2% | The threshold most high-IQ societies use. |
| Above 144 | Highly gifted | ~0.1% | Figures above this range are extrapolated from the curve rather than measured. |
Percentages are approximate and describe a normal distribution. The same number means different things on tests with different standard deviations.
Thirty questions gives you a measured position on this exact scale — your score, your percentile and the ability breakdown behind it.
Find out in 30 minutes →William James Sidis was born in New York City in 1898, the son of psychologist Boris Sidis, who raised him under an intensive early-education regimen. Reports that he was reading The New York Times at eighteen months come from his father’s own accounts and were never independently verified.
He entered Harvard as a special student in 1909 at 11, the youngest in its history, and graduated cum laude in 1914 at 16. He lectured Harvard’s Mathematical Club on four-dimensional bodies in 1910, an event covered in the press at the time.
After Harvard, he taught briefly, attended Harvard Law School without finishing, was arrested at a 1919 socialist rally, and spent the rest of his life avoiding publicity in a series of ordinary clerical jobs. He died in Boston in 1944, aged 46.
By Elora Mercer, Celebrity Research WriterFigures checked by Leon Falkner, Data & Research AnalystThe estimate above rests on this record. Every entry here is a matter of public fact, which is more than can be said for the number itself.
Following a claim back to its first appearance is the part almost nobody does — and it is what most people are actually asking.
Sidis is a genuinely difficult case for this site’s method, because the real achievement record — Harvard admission at 9, formal enrolment at 11, a mathematics lecture praised by MIT professor Daniel Comstock the same year — is itself extraordinary, extraordinary enough that a number in the 250–300 range does not sound obviously absurd next to it, the way it would for most other profiles here. That is exactly why the sourcing matters more, not less.
Researchers note that Sidis’s mother and sister had a reputation for exaggerated statements about him, and the 250–300 figure is precisely the kind of claim that reputation should make readers cautious about. Amy Wallace’s 1986 biography The Prodigy repeats Sperling’s figure but is not its source — it is forty years too late for that.
The more grounded number in the record is the 254th-place ranking on a 1933 civil-service hiring list, which Sidis himself described as “not so encouraging.” It is easy to see how a rank of 254 on a list, mentioned in the same family conversations that produced the IQ claim, could calcify into “IQ 254” and then round outward to “250 to 300” — a plausible, human path from a real fact to an invented one.
How we read a number →The origin of the “250 to 300” figure, sourced to Sidis’s sister Helena’s secondhand account, two years after his death.
Corroborated by later biographical accounts, including MIT professor Daniel Comstock’s praise.
Repeats Sperling’s 1946 figure; not its original source.
No named psychologist, test date or score sheet has ever been located for the claimed test.
The figures that circulate for William James Sidis, what we were able to confirm, and where the two diverge.
An IQ of 250 to 300 — sometimes reported as the highest ever recorded — is the figure most associated with William James Sidis.
The figure comes from Abraham Sperling’s 1946 book Psychology for the Millions, which reports that Sidis’s sister, Helena, told the author that a psychologist had once tested her brother and put his IQ “between 250 and 300.” No psychologist is named, no test date is given, and no record of any such test has ever been located. Sidis had died two years before the book’s publication and never spoke of it himself.
A documented pattern of early achievement supports the lower end of this range. Nothing supports the upper end except one family member’s account, printed after Sidis was no longer alive to confirm or deny it.
Want a number that does hold up? Every figure on this page is somebody’s claim. Take the 30-question test and get a percentile you can check rather than a figure you have to trust.
No verified test score exists. The commonly cited “250 to 300” comes from a 1946 book reporting his sister’s secondhand account of an unnamed psychologist’s assessment, given after Sidis had died.
No record confirms one. The closest documented figure is his 254th-place ranking on a 1933 civil-service exam list — a rank, not a score.
He passed the entrance exams at 9 in 1907, though Harvard deferred his admission; he formally enrolled in 1909 at 11, its youngest student on record.
It is often described that way, but since no verified test produced it, it cannot be compared meaningfully to any scored result.
He avoided public life, working ordinary clerical jobs and publishing occasional academic work under pseudonyms, until his death in 1944 at 46.
Because his documented early-academic acceleration — Harvard admission and a math lecture to professors at 11 — is genuinely exceptional, even without the unsupported 250–300 claim.
You can find out in about fifteen minutes. The IQ Metrics assessment is 30 questions, scored on the same 100-mean scale used throughout this page, so your result and the estimate above are at least expressed in the same units — one measured, one inferred. Once you have a number, the leaderboard shows where it would sit, and how we score and norm it explains the benchmarking.
From the documented record rather than a test — the method is named above and set out in full under how we read a number. Compare it against the other writers profiles, starting with Voltaire.
Portrait © Matthew Yohe, CC BY-SA 3.0. The background-removed portrait above is a derivative work and is likewise offered under the same licence. · Portrait © Debbie Rowe / The Royal Society, CC BY-SA 3.0. The background-removed portrait above is a derivative work and is likewise offered under the same licence. · Portrait by Christopher Smith, U.S. Department of Health and Human Services — a work of the U.S. federal government, public domain. Background removed for the portrait above.
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