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Prader-Willi Syndrome and IQ

Posted on by Mahesh
Understanding IQ

Prader-Willi Syndrome and IQ: Average Scores, Subtypes and Strengths

Prader-Willi syndrome and IQ: average scores sit near 60, about 40 points below the general population, with a range from the 40s to the 90s. See how deletion and maternal UPD differ, the jigsaw-puzzle strength what a growth hormone trial found and how IQ fits into diagnosis.

By , Intelligence Research WriterPublished October 26, 2026 · 10 min readReviewed by Aarav Vardhan, Editor-in-Chief

Number line of IQ scores from 35 to 100 with the moderate (35 to 49) and mild (50 to 69) intellectual disability bands shaded and four Prader-Willi syndrome studies marked: a mean of 62.3 and range 39 to 96 in 26 youths, about 60 in a UK population sample, a median of 52 in 85 adults, and verbal IQ of 60.8 with a deletion against 69.9 with maternal UPD.

Prader-Willi syndrome and IQ: people with the syndrome typically have a mild intellectual disability, with average scores around 60, roughly 40 points below the general population and individual results ranging from the 40s to the average range. A UK population-based study found an approximately normal distribution shifted about 40 points down, a study of 26 young people found a mean of 62.3 (range 39 to 96), and adults in a French cohort had a median Full Scale IQ of 52 on the WAIS-III. The genetic subtype changes the profile more than the total: people with maternal uniparental disomy (UPD) tend to have higher verbal IQ, and those with a deletion higher performance IQ. This guide covers the genetics, the published IQ figures, the deletion-versus-UPD differences, the visual-spatial strengths (including jigsaw puzzles), what a growth hormone trial found and how IQ fits into diagnosis. It is general information, not medical advice: questions about a particular child or adult belong with their specialist team.

  • Average: about 60 in two samples (62.3 in 26 youths; roughly 40 points under the population mean in a UK population sample) and a median of 52 in French adult inpatients.
  • Genetics: loss of paternally expressed genes on chromosome 15q11.2-q13: a deletion in 65 to 75% of cases, maternal UPD in 20 to 30% and an imprinting defect in 1 to 3%.
  • Subtypes: a meta-analysis of 744 cases found full-scale IQ 2.7 points lower with a deletion, verbal IQ 7.5 lower and performance IQ 4.0 higher than with UPD.
  • Strength: visual-spatial tasks. Children with the syndrome placed more than twice as many jigsaw pieces as typically developing children in one study.
Number line of IQ scores from 35 to 100 with the moderate (35 to 49) and mild (50 to 69) intellectual disability bands shaded and four Prader-Willi syndrome studies marked: a mean of 62.3 and range 39 to 96 in 26 youths, about 60 in a UK population sample, a median of 52 in 85 adults, and verbal IQ of 60.8 with a deletion against 69.9 with maternal UPD.
Number line of IQ scores from 35 to 100 with the moderate (35 to 49) and mild (50 to 69) intellectual disability bands shaded and four Prader-Willi syndrome studies marked: a mean of 62.3 and range 39 to 96 in 26 youths, about 60 in a UK population sample, a median of 52 in 85 adults, and verbal IQ of 60.8 with a deletion against 69.9 with maternal UPD.

What is Prader-Willi syndrome?

Prader-Willi syndrome (PWS) is a rare genetic disorder caused by the lack of expression of genes on the paternally inherited copy of chromosome 15q11.2-q13. There are three molecular routes to it: a deletion of that region on the father’s chromosome (65 to 75% of cases), two copies of the mother’s chromosome 15 with none from the father, called maternal uniparental disomy (20 to 30%), and an imprinting defect (1 to 3%). Newborns are floppy and feed poorly. Later come short stature, an insatiable appetite (hyperphagia), delayed development, cognitive disability and behavioural difficulties, probably because of hypothalamic dysfunction. Population studies put the birth incidence between about 1 in 15,830 (Victoria, Australia, 2003 to 2012) and 1 in 26,676 (Flanders, 1993 to 2001), with 1 in 22,000 in a UK population-based study. The same region on the mother’s chromosome is the one that goes missing in Angelman syndrome, its sister disorder, which is usually too severe for a standard IQ test.

Cognitive disability is one feature among many, alongside hunger, weight and health. That context matters for how IQ is read: the score is one input into a picture that also includes adaptive skills, health and behaviour. Compare the patterns in Down syndrome and IQ, Williams syndrome and IQ, fragile X and IQ, Klinefelter syndrome and IQ and Turner syndrome and IQ.

What is the average IQ in Prader-Willi syndrome?

Four studies give the clearest picture. They used different samples and tests, so the figures are not interchangeable, but they agree on the centre of the distribution.

Published IQ figures in Prader-Willi syndrome
Study Sample Finding
Curfs et al. (1991) 26 youngsters Mean IQ 62.3, range 39 to 96
Whittington et al. (2004) People over age 3 found through a UK health region, plus others IQ roughly normally distributed, mean about 40 points below the general population
Copet et al. (2010) 99 adult inpatients, 85 able to complete the WAIS-III Median Full Scale IQ 52 (middle half 46 to 60), Verbal 53, Performance 52.5
Roof et al. (2000) 38 people (24 with a deletion, 14 with UPD) Verbal IQ 60.8 with a deletion, 69.9 with UPD

Against the ICD-10 bands, a score near 60 sits in the mild range and a score near 52 sits just inside it, at the border of moderate. A small share of people with PWS score in the borderline or average range: the top of the youth sample was 96. The UK study found the whole distribution shifted, not a separate low group, which is why individuals vary so much around the average.

ICD-10 bands for intellectual disability (WHO)
Band IQ range Adult mental age
ICD-10 F70 mild 50 to 69 Adult mental age 9 to under 12 years
ICD-10 F71 moderate 35 to 49 Adult mental age 6 to under 9 years

Two cautions on the figures. The adult median of 52 is lower than the younger samples, and its authors noted that their scores were slightly lower than most other studies, in a cohort of inpatients on a specialist unit. And the published numbers describe people who could take the test: in the French adult cohort 14 of 99 could not cope with the test situation, so they are not in the 52. See what an IQ of 65 means and IQ and mental age for what scores in these bands imply day to day, and low IQ vs learning disability for how the terms differ. Scores at either extreme of the curve are also less precise than scores near 100, because a test has few items at the ends of its range; the ceiling problem at the top is set out in the guide to an IQ of 200.

Does the genetic subtype change IQ?

The two main subtypes, paternal deletion and maternal UPD, are the best-studied split. The totals are close; the profiles differ. A meta-analysis pooling 13 studies (423 people with a deletion and 318 with UPD) found that the deletion group had a full-scale IQ 2.69 points lower (95% interval 0.52 to 4.86), a verbal IQ 7.5 points lower, and a performance IQ 4.02 points higher. Individual studies agree on the direction.

Deletion versus maternal UPD: what four sources found
Source Sample Verbal IQ Performance IQ Full-scale IQ
Roof et al. (2000) 24 deletion, 14 UPD UPD higher by 9.1 points (69.9 vs 60.8); 50% of UPD and 17% of deletion cases at 70 or above No difference –
Copet et al. (2010) 57 deletion, 27 non-deletion No significant difference Deletion higher No significant difference
Yang et al. (2013), meta-analysis 13 studies, 744 cases Deletion lower by 7.5 Deletion higher by 4.0 Deletion lower by 2.7
Whittington et al. (2004) Population-based sample UPD better verbal ability Some deletion cases strong visuospatial; UPD weaker coding –

A gap of about three points in full-scale IQ is smaller than the margin of error of a single score (see the margin-of-error explainer), so the subtype tells you little about the total and more about the shape. A related split is the size of the deletion. In 96 people (49 with UPD and 47 with a deletion), Milner and colleagues found few cognitive differences between the larger type I and smaller type II deletions, though ability tended to be lower with type I. UPD cases showed more autistic-like social difficulty. For how an uneven profile shows up on a test, see verbal vs non-verbal IQ scores.

Strengths and weaknesses: the cognitive profile

The profile is uneven, and the strongest finding is visual-spatial. In the 26-youth study, the highest scores came on performance tasks, with Block Design the best subtest in nine children and Coding or Mazes in five, and the group did better on visual-motor discrimination than on auditory skills. Dykens (2002) tested the old clinical impression that people with PWS are unusually good at jigsaws. Children with PWS scored higher than age- and IQ-matched peers with mixed intellectual disability on standard visual-spatial tasks (Object Assembly, Triangles and a visual-motor integration test), though below typically developing children of average IQ. On jigsaw puzzles they placed more than twice as many pieces as the typically developing group, and scored on par with them on word searches. Puzzle skill was not predicted by IQ, age, gender, obesity or obsessive-compulsive symptoms, but by genetic subtype.

A later study of 26 people with PWS and 26 typically developing children matched for mental age found that the PWS group solved puzzles by piece shape while the comparison children used the picture. The pattern is the reverse of Williams syndrome, where jigsaws are a weakness. The practical point for IQ is that a single Full Scale score hides a spiky profile: a performance strength can sit beside a verbal weakness, and which pattern appears depends on the subtype.

Does growth hormone change IQ in Prader-Willi syndrome?

One randomised trial looked directly. Siemensma and colleagues followed 50 prepubertal children aged 3.5 to 14, first in a two-year randomised trial and then for four years of treatment, testing cognition every two years with short forms of the WPPSI-R or WISC-R. Over the two-year trial, the treated children’s estimated total IQ stayed level while that of untreated controls fell, significantly so on the Similarities and Vocabulary subtests. After four years of treatment, Similarities and Block Design scores were above baseline, total IQ was unchanged from baseline, and children with the larger baseline deficits gained most. The authors concluded that treatment prevented deterioration of some skills and improved abstract reasoning and visuospatial skills. That is a finding about change over time in a trial, not advice: treatment decisions belong to the child’s endocrinologist and family.

Your own number

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Take the IIF-certified assessment and get your score with the scale it was measured on, the percentile it corresponds to and the confidence range around it — the three figures most online tests leave out.

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Does IQ predict behaviour or health in Prader-Willi syndrome?

Less than people expect. In 25 children and 61 adults with PWS, Dykens and Cassidy (1995) found no difference in maladaptive behaviour between higher and lower IQ groups, so the score did not sort people by how they behaved. A diagnosis of intellectual disability needs weaknesses in everyday adaptive skills, not a score alone. That is why two people with the same score can need very different support, and why how to read an IQ test report recommends reading the subscores and not the headline.

How is IQ tested in Prader-Willi syndrome?

  • Test choice. Research uses the Wechsler family: the WAIS-III in the French adult cohort and short forms of the WPPSI-R and WISC-R in the growth hormone trial. Clinical teams choose by age and ability (see the WPPSI-IV and the KBIT-2).
  • Who can sit it. Some adults cannot complete a full Wechsler test, so published scores are weighted toward those who can.
  • Reading the result. Look at the verbal and performance split and the subtests, and at the confidence interval, not only the Full Scale number.
  • Re-testing. Scores shift with age, treatment and test version, so compare like with like.

Quick answers

  • What is the average IQ of someone with Prader-Willi syndrome? About 60 in two studies (a mean of 62.3 in 26 youths and roughly 40 points under the general mean in a UK sample); a median of 52 in French adult inpatients.
  • Can a person with Prader-Willi syndrome have a normal IQ? A few score in the borderline to average range (the top of one sample was 96), but the whole distribution is shifted down, so it is uncommon.
  • Is Prader-Willi syndrome an intellectual disability? Most people with it have a mild intellectual disability. Diagnosis also requires deficits in adaptive functioning.
  • Which subtype has the higher IQ? Maternal UPD is about three points higher on full-scale IQ and seven higher on verbal IQ in a meta-analysis; a deletion is about four points higher on performance IQ.
  • Does growth hormone raise IQ? In a randomised trial it prevented a decline in untreated controls and improved some reasoning and visuospatial scores, with total IQ unchanged after four years.

The bottom line

Prader-Willi syndrome and IQ come down to a distribution shifted about 40 points below average, centred in the mild intellectual disability range, with enough spread that a few people score near average and some score in the moderate range. The subtype shapes the profile (verbal strengths with UPD, performance and visual-spatial strengths with a deletion) more than the total, and a growth hormone trial suggests scores can be protected. Read any single number alongside adaptive skills and the person’s own strengths, and to see how a score of your own is built, start with the IQ Metrics IQ test and the IQ tests hub.

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Chart of the cognitive profile often seen in Down syndrome: relative strengths in visual and social skills, relative weaknesses in verbal memory, expressive language and executive function.Understanding IQ

Down Syndrome and IQ

Most people with Down syndrome score in the mild to moderate range of intellectual disability, with an average IQ of around 50 and a typical range of about 30 to 70. The number hides a wide spread, a consistent profile of strengths, and test limits that matter more here than almost anywhere else.

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Bars showing anxiety in 45 children with Williams syndrome followed over time: 60 percent had an anxiety diagnosis at the first assessment, 82.2 percent at some point in the study and 51.1 percent had chronic, persistent anxiety.Understanding IQ

Williams Syndrome and IQ

Williams syndrome and IQ: mean full-scale IQ was 61 in 62 adults, and 292 children averaged in the borderline range on the KBIT-2, stable from age 4 to 17. Language is a relative strength and visuospatial construction an extreme weakness. Here is what the studies show.

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Bell curve of IQ scores with mean 100 and standard deviation 15, shaded up to 70: 1.0 percent of people score 65 or lower and 1.3 percent score 65 to 69, which together make the 2.3 percent below 70, with 65 marked inside the ICD-10 mild range of 50 to 69.Scores & Scales

What Does an IQ of 65 Mean?

An IQ of 65 is the 1st percentile: about 1 person in 102 scores it or lower. It sits inside the ICD-10 mild range, is labelled Extremely Low on Wechsler tests and has a likely true score of about 60 to 73. See its mental age, what it means for daily life and how courts treat it.

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Tagged chromosome 15, genetic syndromes and iq, growth hormone and iq, intellectual disability, IQ Scores, is prader-willi syndrome a learning disability, jigsaw puzzle skills, mild intellectual disability, prader willi iq level, prader-willi deletion vs upd, prader-willi syndrome and iq, prader-willi syndrome average iq, prader-willi syndrome iq, verbal iq vs performance iq, wechsler scales

Average IQ in the UK vs the US

Posted on by Mahesh
Scores & Scales

Average IQ in the UK vs the US: What the Data Shows

The average IQ in the UK vs the US is 100 in both, because each country’s tests are normed on its own people. See what can be compared: PISA 2025 maths (UK 488, US 463), adult skills (266 vs 251), online samples, the Flynn effect in each country and where the real gaps sit.

By , Data & Research AnalystPublished October 26, 2026 · 10 min readReviewed by Aarav Vardhan, Editor-in-Chief

Paired dot chart of UK and US PISA 2025 scores: maths 488 against 463 (a clear gap), reading 494 against 490 and science 511 against 502 (within error), with bars showing 12.0 percent of UK and 17.5 percent of US students low in all three subjects, beside the line 100 equals 100 for average IQ.

The average IQ is 100 in both the UK and the US, because every IQ test is standardised on its own national sample, so no IQ test can say which country scores higher. What can be compared are other measures, and the average IQ in the UK vs the US looks different on each. On the OECD’s PISA 2025 school tests the UK averages 498 across maths, reading and science against 485 for the US; in the 2023 Survey of Adult Skills England scores 266 against 251 for the US; and an online Raven’s-style test taken by thousands of people in 2025 puts the two countries within half an IQ point of each other, 101.6 and 101.0, which is well inside its own noise. This guide covers why both averages are 100, what each comparison measures, how large the gaps are against their error margins, how the Flynn effect is behaving in each country and where the real differences sit inside each one.

  • By construction: 100 in both countries. The Wechsler tests are published in separate UK and US editions, each normed on its own population.
  • School skills (PISA 2025): UK 498, US 485. The UK ranks 8th and the US 15th of 86 systems; the maths gap (25 points) is statistically clear, the reading and science gaps are not.
  • Adult skills (PIAAC 2023): England 266, US 251, ranked 12th and 21st of 31 entries.
  • Online IQ samples (2025): UK 101.6, US 101.0, from self-selected volunteers, so a difference this small carries no meaning.
Paired dot chart of UK and US PISA 2025 scores: maths 488 against 463 (a clear gap), reading 494 against 490 and science 511 against 502 (within error), with bars showing 12.0 percent of UK and 17.5 percent of US students low in all three subjects, beside the line 100 equals 100 for average IQ.
Paired dot chart of UK and US PISA 2025 scores: maths 488 against 463 (a clear gap), reading 494 against 490 and science 511 against 502 (within error), with bars showing 12.0 percent of UK and 17.5 percent of US students low in all three subjects, beside the line 100 equals 100 for average IQ.

Which country has the higher average IQ, the UK or the US?

Neither, on IQ tests; the UK is slightly ahead on school and adult skills tests, mainly in maths. The scoreboard puts the measures side by side, with the size of each gap read against its error margin.

UK versus US on every comparable measure
Measure UK US Is the gap meaningful?
IQ tests, each normed on its own country 100 100 None by construction
Online Raven’s-style sample, 2025 test-takers 101.6 101.0 0.5 points: within noise
PISA 2025 maths, age 15 488 463 25 points: clear gap
PISA 2025 reading, age 15 494 490 5 points: within error
PISA 2025 science, age 15 511 502 10 points: within error
Adult skills 2023, average of three (England for the UK) 266 251 15 points: sizeable

The pattern is consistent but modest. Where a clear gap exists it is in maths, and it is a gap in what 15-year-olds and adults have been taught and practised, not a difference in mental ability. The sections below take each row in turn.

Why the average IQ is 100 in both countries

An IQ score is a position, not a quantity. A publisher gives a test to a representative sample of a population, sets the sample’s average to 100 and its spread (standard deviation) to 15, and reports every later score against that sample. The UK and US editions of the Wechsler scales are standardised separately, so a 100 on the UK edition means “average for the UK norm sample” and a 100 on the US edition means “average for the US norm sample”. See what an IQ of 100 means and what the average IQ test score is.

That has two consequences. First, asking which country has the higher average IQ on a nationally normed test has a built-in answer, a tie. Second, a fair comparison needs one test, one scoring rule and comparable samples in both countries, and almost nothing published meets that standard. Norms also age (see below), so two editions normed in different years can sit a few points apart for that reason alone. The IQ percentile calculator places a score on a norm sample’s curve, and standard deviation 15 versus 16 covers the scale mechanics. A position score also loses meaning far from the average, which is why an IQ of 200 cannot be certified and why scores near 60, as in Prader-Willi syndrome, are read together with adaptive skills.

What do country IQ tables say about the UK and the US?

Lists of average IQ by country exist, and they are where most searches for this question end up. The 2026 edition of one online table, built from people who took a Raven’s-style test in 2025, gives the UK 101.57 (14th, from 7,310 test-takers) and the US 101.04 (18th, from 22,605). Its previous edition had 99.68 and 99.74. The 0.53-point gap between the two is far smaller than the swing between editions of the same table. Our review of that ranking adds two more problems: the country rows, weighted by number of takers, average 98.48 and not 100, and country scores track the logarithm of sample size (r = 0.60), which is what a self-selected sample looks like.

Older national-IQ compilations such as Lynn and Vanhanen’s 2002 book work differently: they average and adjust small studies of mixed quality, and for 104 of the 185 countries in the book no study was available, so values were estimated from neighbours. Neither kind of table measures the same thing in both countries with the same instrument, which is why country IQ rankings are weaker than the charts suggest, and why our average IQ by country page labels the source of every number.

School skills: PISA 2025, UK versus US

PISA tests 15-year-olds in maths, reading and science every three years, and the 2025 round covered 91 school systems. It is a test of what students have learned, not an IQ test (see PISA test vs IQ test and what the 2025 results do not show about IQ). Using the OECD’s published means and standard errors, which the Smartest Countries Index compiles, the UK and the US compare like this.

PISA 2025: United Kingdom versus United States (OECD means; z compares the gap with its standard error)
Measure UK US Gap Is the gap clear?
Maths 488 463 25 Clear (z = 4.2)
Reading 494 490 5 Within error (z = 0.8)
Science 511 502 10 Within error (z = 1.6)
Average of the three 498 485 13 Ranked 8th vs 15th of 86
Top performers in at least one subject (%) 18.3 17.8 0.4 Level
Low performers in all three subjects (%) 12.0 17.5 5.5 UK lower

The maths gap is large against its error, about four standard errors. The reading gap (under one standard error) and the science gap (about 1.6) are not statistically clear, so the honest reading is that the UK is ahead in maths, and level or slightly ahead elsewhere. The two countries have almost the same share of top performers (18.3% and 17.8%), while the US has more students at the bottom: 17.5% score low in all three subjects, against 12.0% in the UK. The OECD marks the US data with a sampling caution, so US figures are less certain than the UK’s. Over time, the three-subject average has fallen 4.7 points in the UK and 7.5 in the US since 2012, and between the 2022 and 2025 rounds it rose 3.4 points in the UK and fell 4.8 in the US. In our own index, which blends school skills (70%) with research output (30%), the UK ranks 9th and the US 18th of 86.

Adult skills: the 2023 Survey of Adult Skills

The OECD’s Survey of Adult Skills (PIAAC) tests adults aged 16 to 65 in literacy, numeracy and adaptive problem solving. The UK took part through England, which stands for the UK in the table; the US took part as a whole.

Survey of Adult Skills 2023: England (UK) versus United States, mean scores
Skill England (UK) US Gap
Literacy 272 258 13
Numeracy 268 249 20
Adaptive problem solving 259 247 11
Average of the three 266 251 15

England ranks 12th of 31 entries and the US 21st. The gap is widest in numeracy. Adult skills are learned, depend on schooling, language background and the age mix of the population, and correlate with IQ without being IQ. They show the same direction as PISA: a modest UK lead, largest in maths.

The Flynn effect in the UK and the US

Norm samples go stale because raw scores have risen about three IQ points a decade for much of the 20th century (see why IQ norms expire). Flynn’s 2009 analysis found that British children aged 5 to 15 gained about 14 points on Raven’s matrices between 1942 and 2008, with gains slowing in the later decades. For the US, Dworak, Revelle and Condon (2023) analysed 394,378 adults who took an online cognitive battery between 2006 and 2018 and found a decline in composite scores and in matrix reasoning, letter and number series and verbal reasoning, with the steepest drop among people aged 18 to 22 and those with less education. Three-dimensional rotation scores rose. The authors did not test causes, and an online sample is not a representative one.

For comparing the two countries the lesson is practical: a score is only comparable with scores on the same norm date. A US test normed in one year and a UK test normed in another are measuring against two different yardsticks, and either may be a few points off today.

Your own number

Where would your own score land?

Take the IIF-certified assessment and get your score with the scale it was measured on, the percentile it corresponds to and the confidence range around it — the three figures most online tests leave out.

Find your IQ score now! →

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The gaps inside each country are larger than the gap between them

Differences between places inside the UK, and between US states, are much larger than the UK-US gap, and they follow schooling and qualifications. Our Smartest Places in the UK 2026 ranks England’s 294 council areas: Richmond upon Thames is first and Fenland last, and in Fenland 19.0% of adults hold degree-level qualifications against 33.9% across England. The Smartest States 2026 ranks all 50 US states on NAEP 2024 test scores and college degrees, with Massachusetts first and West Virginia last. See also the smartest cities in America and average IQ by US state. Within a country, who you compare matters far more than which flag they carry, and socioeconomic status and IQ covers the strongest driver.

Does the average IQ in the UK change with age?

By definition, no. IQ norms are built in age bands, so the average is 100 at every age: a 70-year-old is compared with other 70-year-olds, not with 25-year-olds. What changes with age is raw performance. Speed and fluid reasoning peak in early adulthood and decline later, while vocabulary and knowledge keep rising for decades (see peak brain age and the IQ scale by age). A UK and a US adult of the same age have the same average on their own norms, for the same reason a UK and a US adult do overall.

How to compare two countries fairly

  • Use one instrument. A comparison needs the same test, translated and scored the same way, in both samples.
  • Ask who was sampled. A random school sample (PISA) is not an online volunteer sample, and the difference can exceed the gap you are trying to measure.
  • Read the gap against its error. A 25-point maths gap with standard errors of 2 and 5 is clear; a 5-point reading gap with the same errors is not.
  • Match the norm date. Scores from tests normed years apart are not on the same yardstick.
  • Separate skills from ability. School and adult skills tests measure learning, which depends on schooling and opportunity (see cultural bias in IQ tests). To see where your own score sits, start with the IQ Metrics IQ test.

Quick answers

  • What is the average IQ in the UK? 100 on UK-normed tests. Online samples report about 100 to 102.
  • What is the average IQ in the US? 100 on US-normed tests. The same online table reports about 101.
  • Is the UK smarter than the US? IQ tests cannot say, because each is normed on its own population. School and adult skills tests favour the UK modestly, mostly in maths.
  • What is the average IQ in the UK in 2026? Still 100 by construction. The 2025 online sample gives 101.6.
  • Why do sites list different numbers? They use different tests, samples and adjustments, and several adjust small studies or estimate from neighbouring countries.
  • Does the average IQ differ by age in the UK? No, norms are age-banded. Raw performance does change with age.

The bottom line

The average IQ in the UK vs the US is 100 versus 100, because each country’s IQ tests are normed on its own people. The measures that can be compared put the UK modestly ahead: 488 against 463 in PISA 2025 maths (a clear gap), 266 against 251 in adult skills, and a half-point difference in online IQ samples that means nothing. Treat any single “UK IQ” or “US IQ” number as a label for a particular test, sample and date, and use the IQ tests hub to see how a score is built.

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What is Average IQ Test Score?Scores & Scales

What is Average IQ Test Score?

The average IQ test score is 100 by definition. About half of people score 90 to 109 and about 68% score 85 to 115. See the average range on each chart, why the average stays at 100, how scales differ and where to find averages for groups.

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A comparison card contrasts PISA and an individual IQ test. PISA surveys a sample of 15-year-olds in each country and school system, reports no individual score, and rotates test booklets. An IQ test is given one to one and reports one diagnostic score to the person tested.Understanding IQ

PISA Test vs IQ Test

Is PISA an IQ test? No. PISA surveys applied school literacy across whole education systems and returns no individual score, while an IQ test diagnoses one person. See the new PISA 2025 results, published September 2026, and the real debate over using PISA as a national IQ proxy.

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IQ Scale By AgeScores & Scales

IQ Scale By Age

The IQ scale is the same at every age, with an average of 100, but the test changes: the WPPSI-IV covers ages 2.5 to 7.5, the WISC-V ages 6 to 16 and the WAIS-IV and WAIS-5 ages 16 to 90. See which test fits each age, how a child’s IQ was once calculated and how stable IQ is over a lifetime.

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Tagged adult skills piaac, average american iq, average british iq, average iq by age, average iq by country, average iq in the uk, average iq in the us, average iq uk 2026, average iq uk vs us, Flynn Effect, iq norms, IQ Scores, national iq, pisa 2025, uk vs us iq

Philosophers Ranked by IQ

Posted on by Mahesh
Research & Evidence

Philosophers Ranked by IQ: What Cox’s 1926 Study Estimated for 22 Thinkers

Philosophers ranked by IQ: no philosopher has a measured score, but Catharine Cox’s 1926 study estimated 22 of them from biographies. See the full table from Leibniz (205) and Mill (200) down, why the gaps sit inside the error, who is missing and what the GRE shows for philosophy students.

By , News & Research WriterPublished October 26, 2026 · 10 min readReviewed by Aarav Vardhan, Editor-in-Chief

Dot chart of 12 of the 22 philosophers in Cox's 1926 study, each showing the rating Cox obtained and the higher corrected estimate: Leibniz is highest at 205, then Mill at 200, and the rest run from 190 down to 160.

No philosopher has an IQ on record, so every list of philosophers ranked by IQ rests on one source: Catharine Cox’s 1926 study, which estimated childhood and youth IQs from biographies for 301 historical figures, 22 of them philosophers. On her corrected figures Leibniz comes first at 205 and John Stuart Mill second at 200 for his first 17 years, with Kant at 160 to 175 in the middle of the table. Plato, Aristotle, Socrates, Nietzsche and Russell are not in the study at all, so the numbers attached to them online are guesses. This guide gives the full table of 22, explains how the estimates were made and why the gaps between neighbours are smaller than the method’s error, lists what circulates for the philosophers Cox left out, and sets beside it the one thing that has actually been measured in living philosophy students: their GRE scores.

  • The source: Cox’s The Early Mental Traits of Three Hundred Geniuses (1926), which in fact rates 301 people born between 1450 and 1850.
  • The top of the table: Leibniz 205, Mill 200, then Arnauld, Berkeley and Schelling at 190.
  • The field average: before any correction, her philosophers averaged 156 for the period to age 26, the highest of any field in the study. Her soldiers averaged 125.
  • The catch: the figures are retrospective, rounded to the nearest five, and corrected upward by a formula that Cox called a crude approximation.
Dot chart of 12 of the 22 philosophers in Cox's 1926 study, each showing the rating Cox obtained and the higher corrected estimate: Leibniz is highest at 205, then Mill at 200, and the rest run from 190 down to 160.
Dot chart of 12 of the 22 philosophers in Cox’s 1926 study, each showing the rating Cox obtained and the higher corrected estimate: Leibniz is highest at 205, then Mill at 200, and the rest run from 190 down to 160.

How did Cox estimate IQ for people who never took a test?

Cox, a student of Lewis Terman, collected biographical records of the first 26 years of life for each of her subjects and wrote each up as a case study. Two or more qualified judges then read the material and assigned an IQ, using the logic of the early Stanford-Binet: what the person did, and at what age, implies a mental age, and mental age over real age gives a ratio IQ (see IQ and mental age and deviation IQ versus ratio IQ). Each person got two ratings. The first, called AI, rests on the first 17 years. The second, AII, adds ages 17 to 26. She also graded how reliable each record was, from .11 for a thinly documented life to .82 for the best, and the philosophers’ records averaged .60 for the longer period against .53 across the whole study.

Because records are incomplete, raw ratings are pulled toward the middle. Cox corrected them with a formula: corrected = obtained + (field average − 100) × (1 − reliability), rounded to the nearest five. For Leibniz that is 190 + 56 × (1 − .75) = 204, printed as 205. For Kant it is 145 + 56 × (1 − .43) = 177, printed as 175. Cox described the result as a crude approximation that indicates a point below which the true IQ probably did not fall. The corrected figures are the ones that travel: the 205 on popular lists for Leibniz and the 175 for Kant are her corrected AII values, not scores. The method is the same one behind many genius IQ lists, and it is why an IQ of 200 appears for historical figures at all.

Philosophers ranked by IQ: all 22 in Cox’s study

The table ranks the 22 philosophers by the higher of their two corrected estimates, breaking ties by the second figure, and shows the ratings she obtained before correction beside them. Ties are marked with an equals sign.

The 22 philosophers in Cox’s study, ranked by their higher corrected IQ estimate (Cox 1926, Table 12A)
Rank Philosopher (lived) Obtained, to age 17 / 26 Corrected, to age 17 / 26 Record reliability, to age 26
1 Gottfried Leibniz (1646–1716) 185 / 190 195 / 205 .75
2 John Stuart Mill (1806–1873) 190 / 170 200 / 180 .82
3= Antoine Arnauld (1612–1694) 135 / 160 175 / 190 .43
3= Friedrich Schelling (1775–1854) 175 / 180 180 / 190 .82
3= George Berkeley (1685–1753) 150 / 175 170 / 190 .75
6= Auguste Comte (1798–1857) 150 / 170 170 / 185 .75
6= Tommaso Campanella (1568–1639) 145 / 155 170 / 185 .43
8= David Hume (1711–1776) 155 / 160 175 / 180 .60
8= Francis Bacon (1561–1626) 145 / 155 165 / 180 .53
8= Marquis de Condorcet (1743–1794) 155 / 165 175 / 180 .75
8= Nicolas Malebranche (1638–1715) 125 / 135 165 / 180 .20
8= René Descartes (1596–1650) 150 / 160 170 / 180 .60
8= Victor Cousin (1792–1867) 135 / 165 155 / 180 .75
14= Baruch Spinoza (1632–1677) 130 / 145 170 / 175 .43
14= Immanuel Kant (1724–1804) 135 / 145 160 / 175 .43
16= Johann Gottlieb Fichte (1762–1814) 145 / 155 165 / 170 .75
16= Sir William Hamilton (1788–1856) 140 / 160 150 / 170 .82
16= G. W. F. Hegel (1770–1831) 150 / 145 170 / 165 .60
19= Henri de Saint-Simon (1760–1825) 130 / 135 155 / 165 .43
19= John Locke (1632–1704) 125 / 135 150 / 165 .43
19= Thomas Hobbes (1588–1679) 140 / 135 165 / 165 .43
22 Friedrich Schleiermacher (1768–1834) 150 / 145 160 / 160 .75

Read the table as tiers, not as a league. Cox rounded to the nearest five and the method’s own error is larger than that, so the order inside a band means nothing. Two philosophers reach 200 or more (Leibniz and Mill), three sit at 190 (Arnauld, Berkeley and Schelling), eight at 180 to 185, eight at 165 to 175 (Kant and Spinoza at 175, Hegel at 170, Locke at 165) and one at 160. A philosopher who tops the table on one record may drop on another: four of the 22 rated lower on the longer record than on the first, Mill by 20 points and Hegel, Hobbes and Schleiermacher by five.

What the top of the table shows

Leibniz leads on the longer rating: 185 to age 17 and 190 to age 26, on records graded .75, corrected to 195 and 205. Cox notes that he had taken a bachelor’s degree and written a treatise before his 17th birthday. Our Leibniz profile puts his estimated IQ at 180 to 195, because the 205 is a corrected figure, not a score.

John Stuart Mill has the best-documented childhood in the study. His record is graded .82, and Cox wrote that no other record of an eminent man’s early development is more complete. The three ratings of his first 17 years averaged 190, corrected to 200, and she cites his conversation at five with Lady Spencer on the merits of Marlborough and Wellington. Yet his second rating is 170 (corrected 180), because she judged his later achievement less striking than that of many other youths. Mill is therefore first among her philosophers on the first 17 years and joint seventh on the longer window. That is a method result, not a fact about Mill.

Kant sits mid-table: 135 to age 17 and 145 to age 26, on records graded only .43, corrected to 160 and 175. Because the reliability is low, the correction adds 25 to 30 points. Our Kant profile puts the documented record at 150 to 170. The extremes show the same effect: Nicolas Malebranche was rated 135 on records graded .20, and the formula lifted him to 180, level with Descartes and Hume. A low-reliability record produces a big correction, which is why the corrected column is flattering to the least-documented lives.

Which fields scored highest in Cox’s study?

Cox also reported the average estimate for each field before correction (her Tables 20A and 20B). Philosophers lead, ahead of scientists, and soldiers are lowest. The two groups of writers (149 and 148 to age 26) and the revolutionary statesmen (144) fall between scientists and statesmen.

Average obtained IQ estimate by field in Cox’s study (Group A, before correction)
Field People Average, to age 17 Average, to age 26
Philosophers 22 147 156
Scientists 39 135 152
Religious leaders 23 132 145
Statesmen 43 135 142
Musicians 11 130 140
Artists 13 122 135
Soldiers 27 115 125

The order matches the idea behind the whole exercise, that fields which reward abstract reasoning and verbal ability should rate high, but it is also partly built into the sample. Cox rated people who were already famous, judges knew who they were rating, and the philosophers’ records were better documented than the soldiers’, which raises their reliability and so their corrected scores. Pascal (195 corrected), Voltaire (190) and Goethe (210) are often called philosophers, but Cox classed them as a scientist and two writers. Our Pascal, Voltaire and Goethe profiles give each one’s source.

Which philosophers are missing, and what circulates for them?

Cox drew her 301 subjects from a list of the thousand most eminent people, kept only those born between 1450 and 1850, and excluded hereditary aristocrats. The ancient philosophers fall outside the window. Nietzsche, born in 1844, was inside it but is not among the 301, and neither are Schopenhauer or Russell (born 1872). That leaves a set of famous names whose IQs online come from nowhere in particular.

Philosophers outside the study: the number that circulates and what the evidence is
Philosopher Number that circulates What the evidence is
Socrates 140 to 180 No test and nothing written by him. Our profile estimates 135 to 165.
Plato 146 and 180 No test. The two circulating numbers do not even agree with each other.
Aristotle 148 to 200 No test. Our profile estimates 150 to 170 from his documented career.
Friedrich Nietzsche 165 and 190 No test. A scholarship at Schulpforta and a Basel chair at 24 support an estimate of 150 to 165.
Bertrand Russell 180 No source at all, and born too late for Cox’s survey.
Christopher Langan 195 A real test, but its own creator’s later renorming puts the conversion about thirty points lower.

A fair rule from these rows: if a number attached to a philosopher cites neither a test nor Cox’s table, it is a guess with a confident tone. Where celebrity IQ numbers come from explains how the same pattern plays out across the whole celebrity IQ section.

Your own number

Where would your own score land?

Take the IIF-certified assessment and get your score with the scale it was measured on, the percentile it corresponds to and the confidence range around it — the three figures most online tests leave out.

Find your IQ score now! →

Secure & encryptedInstant results10–20 minutes

What has been measured: GRE scores of philosophy students

The one measurement we have is of living students, and it is not an IQ test. The American Philosophical Association summarised Educational Testing Service data on GRE takers from August 2011 to April 2014, grouped by the field they intended to study. Prospective philosophy graduate students had the highest mean Verbal Reasoning score of any field and the highest Analytical Writing score (4.4, ahead of English at 4.2), and ranked 19th on Quantitative Reasoning at 153.

Mean GRE Verbal Reasoning score by intended graduate field, August 2011 to April 2014 (130 to 170 scale)
Intended field Mean Verbal Reasoning
Philosophy 160
English 157
Physics 156
History 156
Chemistry 153
Psychology 152
Finance 151

Three cautions apply. The GRE measures learned verbal and quantitative skills, so it is not an IQ test (see GRE and LSAT scores to IQ). The sample is people who chose to apply to graduate school in each field, a self-selected group, which is the same caveat that applies to average IQ by college major. And the data are a decade old, on a revised test. The result is still the nearest thing to a measured comparison, and it points the same way as Cox’s field averages.

Why historical IQ rankings should be read with care

  • They are retrospective. Judges rated records of people whose fame was already known, which can colour the ratings.
  • Record quality drives the number. The philosophers’ records ranged from .20 to .82 reliable, and a low reliability triggers a large upward correction.
  • They are rounded and tied. Nineteen of the 22 share a rank with someone else; the real uncertainty is wider than five points.
  • They select on success. Only people famous enough to be written about are rated, so the table says nothing about philosophers as a group.
  • They are childhood ratio IQs. They estimate precocity, not adult reasoning, and they are not on the deviation scale of a modern test. IQ and creativity covers why a high score does not predict philosophical originality.

Quick answers

  • Who is the smartest philosopher by IQ? No philosopher has a measured IQ. On Cox’s corrected estimates Leibniz ranks first at 205 and John Stuart Mill second at 200.
  • What was Kant’s IQ? Never tested. Cox obtained 135 and 145 on records graded .43 reliable and corrected them to 160 and 175.
  • What was Nietzsche’s IQ? Never tested, and he is not in Cox’s study. The 165 and 190 that circulate cite no test.
  • What was Plato’s IQ? Unknown. The numbers 146 and 180 both circulate and contradict each other.
  • Do philosophers have high IQs? There is no IQ data on philosophers as a group. Prospective philosophy graduate students had the top GRE Verbal and Analytical Writing means among all fields in 2011 to 2014.

The bottom line

Philosophers ranked by IQ means philosophers ranked by Cox’s 1926 estimates: Leibniz at 205 and Mill at 200 lead 22 thinkers whose figures are corrected, rounded and uncertain, with Kant at 175 and Locke at 165 further down. Everyone outside her window, from Socrates to Russell, has a number with no source. Treat the table as a record of how one 1920s method rated documented childhoods, and for a score of your own, start with the IQ Metrics IQ test and the IQ tests hub.

Sources

  • Cox, C.M. (1926): The Early Mental Traits of Three Hundred Geniuses (Genetic Studies of Genius, Volume II), Stanford University Press: Table 12A (obtained and corrected ratings, record reliability), Tables 20A and 20B (field averages), the correction formula and her comments on Mill, Leibniz and the correction. Transcribed and cross-checked by IQ Metrics against the book’s own Tables 6, 7, 20A and 20B.
  • Terman, L.M. et al. (1925): Genetic Studies of Genius, Volume I, summarised in our Terman study page.
  • American Philosophical Association (2014): Philosophy Student Performance on the Graduate Record Examinations (GRE), summarising Educational Testing Service data for tests taken 1 August 2011 to 30 April 2014 (ETS GRE guide).
  • The circulating numbers and estimates for Socrates, Plato, Aristotle, Nietzsche, Russell and Langan come from our own celebrity profiles linked above, each of which names its sources.

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Tagged average iq of a philosopher, catharine cox, do philosophers have high iq, estimated iq of geniuses, gre scores by major, highest iq philosophers, historical figures iq, IQ Scores, john stuart mill iq, kant iq, leibniz iq, nietzsche iq, philosophers iq, philosophers ranked by iq, smartest philosophers

What Does an IQ of 200 Mean?

Posted on by Mahesh
Scores & Scales

What Does an IQ of 200 Mean? Is It Possible?

An IQ of 200 is 6.67 standard deviations above average, about 1 in 76 billion, and no adult test can measure it. See why a child’s ratio IQ of 200 is possible, how test ceilings stop it by about age 11, what Terman and Cox found near that line and where the famous 200s come from.

By , Data & Research AnalystPublished October 26, 2026 · 12 min readReviewed by Aarav Vardhan, Editor-in-Chief

Column chart of the 643 children in Terman's main gifted group by corrected IQ band from 130 to 204: most fall between 140 and 154, and a single girl stands alone in the 200 to 204 band.

An IQ of 200 is 6.67 standard deviations above the average of 100 on the standard Wechsler scale, a score that about 1 person in 76 billion would reach, so the bell curve predicts roughly 0.1 of a person among the 8.2 billion alive today and about 1.5 among the roughly 117 billion humans ever born. No adult test can print it as a measured rank, and nobody has been shown to have scored it. The 200s that do appear come from three places: a young child’s ratio IQ (mental age divided by age), a score on a wide-spread scale or a high-range test, and retrospective estimates for people who died long before testing existed. This guide covers the arithmetic of 200 on each scale, why a child’s 200 is possible but an adult’s is not, what Lewis Terman’s gifted children and Catharine Cox’s historical geniuses show near that line, and how to read a claim of 200 when you meet one.

  • Rarity: 1 in 76 billion on a standard deviation (SD) 15 scale, 1 in 4.9 billion on SD 16 and 1 in 64,700 on Cattell’s SD 24, where a printed 200 matches a Wechsler 162.
  • Ceiling: the WAIS-IV and Stanford-Binet 5 stop near 160. Pearson’s WISC-V extended norms reach 210, for children only.
  • The one 200 in the record: among 643 gifted California children in Terman’s main group, one girl scored 200 to 204 on a corrected ratio IQ, and the three highest scores were all girls’.
  • Possible? As a young child’s ratio IQ, yes. As a certified rank among adults, no.
Column chart of the 643 children in Terman's main gifted group by corrected IQ band from 130 to 204: most fall between 140 and 154, and a single girl stands alone in the 200 to 204 band.
Column chart of the 643 children in Terman’s main gifted group by corrected IQ band from 130 to 204: most fall between 140 and 154, and a single girl stands alone in the 200 to 204 band.

How rare is an IQ of 200?

On a normal curve with a mean of 100 and a standard deviation of 15, a score of 200 sits at z = +6.67. The share of people at or above it is about 0.0000000013%, or 1 in 76 billion. That is more than nine times the number of people alive, so the curve expects about a tenth of a person at 200 today. At this distance the bell curve is a model, not a measurement: a standardisation sample of 3,000 people holds about 0.00000004 of a person at 200 or above, which is why every norm table stops long before it (see the IQ bell curve and the IQ ceiling effect). The IQ percentile calculator places any score on the curve.

A printed 200 on three common IQ scales (normal-curve arithmetic)
Scale Standard deviation z score for 200 A score of 200 is about Wechsler score with the same rarity People alive (of 8.2 billion)
Wechsler tests, Stanford-Binet 5 15 6.67 1 in 76.4 billion 200 0.1
Stanford-Binet Form L-M (older edition) 16 6.25 1 in 4.9 billion 194 2
Cattell scales 24 4.17 1 in 64,700 162 127,000

So a Cattell 200 is roughly a 162 on the Wechsler scale and an L-M 200 is roughly a 194. That is one reason 200 turns up in older and high-range literature: it was quoted on a wide scale or as a child’s ratio score. Standard deviation 15 versus 16 covers the mechanics, and the IQ score converter moves a score between scales. Even counting every human who has ever lived, the SD 15 curve implies 1.5 people and the SD 16 curve about 24. That is arithmetic, not evidence: nobody could confirm one of them, and our guides to an IQ of 180 and an IQ of 170 show the same limit one rung down.

Is an IQ of 200 possible?

It depends on what is meant by IQ. Three different things carry the same number, and only one of them is a rank.

  • As a rank among adults (a deviation IQ). Not measurable. The main adult tests cap their Full Scale IQ near 160, and a norm sample of a few thousand cannot contain anyone this rare, so a printed 200 would be an extrapolation.
  • As a young child’s ratio IQ. Yes, arithmetically. Under the original formula a child with a mental age twice their real age scores 200. Such children are real but very rare: Leta Hollingworth found only twelve above 180 in 23 years of searching New York schools, as the IQ of 180 guide reports.
  • As a number a test prints. Yes. Pearson’s WISC-V extended norms list composite scores up to 210 for children, and an unsupervised online quiz or a high-range test can print any number it likes. A printed score is not the same as a certified one.

So the honest answer is that 200 is possible as a number and as a young child’s ratio score, and unproven as a rank. Nobody has produced a test record showing an adult 200 against a normed sample.

The ratio IQ route: what a child needs for 200

The first IQ formula, used by the 1916 Stanford-Binet, was mental age divided by chronological age, multiplied by 100 (see deviation IQ versus ratio IQ and IQ and mental age). A 200 therefore means a mental age of exactly twice the real age. The table shows what that demands.

Mental age a child needs for a ratio IQ of 200
Real age Mental age needed What the child must pass
4 years 8 years Items an average 8-year-old can pass
5 years 10 years Items an average 10-year-old can pass
6 years 12 years Items an average 12-year-old can pass
7 years 14 years Items an average 14-year-old can pass
8 years 16 years Items written for a 16-year-old, close to adult level
9 years 18 years Adult-level items
10 years 20 years Adult-level items

The arithmetic hits a wall quickly, because a test has a last item. The 1937 Stanford-Binet Form L-M reached a top mental age of 22 years 10 months in one published description (another gives 22 years 11 months, so we use the lower). Dividing that ceiling by a child’s age gives the highest ratio IQ the test could ever report at that age.

Highest ratio IQ a test with a top mental age of 22 years 10 months can report (our arithmetic)
Real age Highest possible ratio IQ Can it reach 200?
6 years 381 Yes
8 years 285 Yes
10 years 228 Yes
11 years 208 Yes
12 years 190 No
14 years 163 No
16 years 143 No

Two things follow. Past 11 years and 5 months no child can reach 200 on such a test, however able, because the mental-age scale has run out. And the table explains the best-known big number: 228 is exactly what a ten-year-old who reached the top of the scale would score. We have not found a published test record behind the famous 228, and the highest IQ ever recorded covers why Guinness dropped the category. The point here is structural: a ratio IQ above 200 is a statement about the test running out of items, not about a bigger mind. What is the highest IQ score a test can report? sets out the modern limits.

What did Terman find near 200?

The best evidence on children at the top comes from a study that began in 1921. Lewis Terman searched the public schools of Los Angeles, San Francisco, Oakland, Berkeley and Alameda, about 168,000 pupils, and followed 1,528 children who scored in the top 1% (see our data study of the Termites). Of the 1,070 tested on the individual Stanford-Binet, the range ran from 135 to 200 and 77 scored 170 or more. In the main group of 643 children, Terman’s corrected scores put 15 at 180 or above and 3 at 190 or above, and exactly one in the 200 to 204 band.

Terman main group (643 children): the top bands of corrected IQ
Score band (corrected ratio IQ) Boys Girls Children
200 to 204 0 1 1
195 to 199 0 0 0
190 to 194 0 2 2
185 to 189 1 1 2
180 to 184 8 2 10
175 to 179 2 6 8
170 to 174 11 9 20

The bands are Terman’s own corrected scores: for older children who hit the top of the test he applied a correction, which makes the highest rows partly estimates by design. One child at 200 among about 168,000 searched is a striking result against the curve, which predicts 0.000003 of a pupil at 200 on SD 15 and 0.00004 on SD 16. Two things explain the gap. The 1916 scale gave ratio IQs, which stretch in the tails, and the group was hand-picked from teacher nominations, not a random draw. So the one 200 is not a count of people who would score 200 on today’s tests. It is the closest thing to a documented example of what the number looked like in 1920s California, and the three highest scores belonged to girls.

Where do the 200s in books and lists come from?

Almost every 200 on a famous-people list is one of four things: a retrospective estimate, a childhood ratio IQ, a high-range test score or a second-hand claim. The table sorts the best-known ones.

Well-known figures at or near 200 and what each number is
Person Figure What the number is
Johann Wolfgang von Goethe 210 A 1926 retrospective estimate from biographies (Cox), corrected upward from 185 for his first 17 years and 200 to age 26
Gottfried Leibniz 205 Cox’s corrected estimate; the rating she obtained was 190
John Stuart Mill 200 Cox’s corrected estimate for his first 17 years; the rating she obtained was 190
Marilyn vos Savant 228 A child’s ratio IQ; Guinness listed it from 1986 to 1989, then retired the category in 1990
William James Sidis 250 to 300 A second-hand report in a 1946 book; no test record has surfaced
Christopher Langan 195 A high-range test score; its own creator’s later renorming puts the conversion about thirty points lower

The first three rows come from Catharine Cox’s 1926 study of 301 historical figures, which estimated childhood and youth IQs from biographies. The highest rating she obtained before any correction was Goethe’s 200 for the period to age 26, on records she graded .82 reliable. Five of the 301 have a corrected estimate of 200 or more in at least one of her two columns: Goethe, Leibniz, Mill, Hugo Grotius and Thomas Wolsey. Wolsey’s shows how fragile the correction is: his record was graded only .11 reliable, so the formula pulled his obtained 165 up to 200. Cox called the correction a crude approximation. Philosophers ranked by IQ sets out her method and the full table for the 22 philosophers in her study, and what is a genius IQ score covers how the word “genius” got attached to numbers like these.

Your own number

Where would your own score land?

Take the IIF-certified assessment and get your score with the scale it was measured on, the percentile it corresponds to and the confidence range around it — the three figures most online tests leave out.

Find your IQ score now! →

Secure & encryptedInstant results10–20 minutes

Can any test report an IQ of 200?

For adults, no. The WAIS-IV and the Stanford-Binet Fifth Edition cap their Full Scale IQ near 160, so a report that prints 160 means “at or above the top of what this test can measure”. For children there is one genuine route. Pearson’s WISC-V Technical Report 6 (Extended Norms, 2019) raises the top of the composite scales to 210, and says the extension is most useful for younger children because older age bands leave less room above them. That makes 200 a printable number on one children’s test, and an extrapolated one: it rests on a few children at the top of the standardisation sample, not on a count of people at 200. The IQ of 170 guide explains how the extension works.

High-range tests are different again. They are untimed, usually unsupervised, and normed on volunteers who already thought they would do well, so their top scores say more about the scoring table than about rarity. A result above about 145 from an online quiz says more about its scoring formula than about the test-taker, and high-IQ societies explain why their lines are best read as percentiles.

Is an IQ of 200 twice as smart as an IQ of 100?

No. An IQ score is a position on a distribution, not a quantity with a true zero, so “twice” has no meaning for it, in the same way that 20 degrees Celsius is not twice as warm as 10. The only place a ratio is literal is the old mental-age formula, where 200 meant a mental age double the real age. The IQ of zero guide covers the zero point, and the IQ comparison page shows what a gap between two scores means in percentile terms. At the top, the precision is also poor. Applying Kelley’s formula to an observed 200 gives an expected true score below it, because extreme scores are partly luck.

What an observed 200 implies (Kelley formula, normal errors)
Test reliability Expected true score Likely range (95%)
0.90 (a short or unsupervised test) 190 181 to 199
0.95 (a good group-administered test) 195 189 to 201
0.98 (a full individually administered battery) 198 194 to 202

Error also grows toward the tails and no test has enough hard items at this level, so the table is optimistic. The margin-of-error explainer shows the same arithmetic nearer the middle of the scale.

If you have been told you scored 200

  • Find the scale. Ask which test it was and whether its spread was 15, 16 or 24. A Cattell 200 is closer to a Wechsler 162.
  • Ask whether it is a ratio score. A child’s 200 on an old Stanford-Binet is mental age over age, and it cannot exist past about 11 years old on that test.
  • Check who gave it. An individually administered test from a psychologist carries weight; an unsupervised web quiz or a high-range test does not.
  • Look for the ceiling. If the test stops at 160, a 200 cannot come from it.
  • Ask for a percentile or a range. “Far into the top fraction of a percent” is honest; “exactly 200” is not. To see how your own result sits on the curve, start with the IQ Metrics IQ test.

Quick answers

  • Is an IQ of 200 possible? As a young child’s ratio IQ and as a printed number, yes. As a certified adult score against a normed sample, no.
  • What percentile is an IQ of 200? Effectively the top 0.0000000013% on SD 15, about the 99.9999999987th percentile.
  • How many people have an IQ of 200? None verified. The SD 15 curve predicts about 0.1 of a person among 8.2 billion.
  • Who has an IQ of 200? No one with a verified adult score. The names that circulate are ratio IQs, retrospective estimates or second-hand claims.
  • Is 200 the highest IQ ever? No. Larger numbers such as 228 and 250 to 300 circulate, and none is a verified deviation IQ.
  • Can an online test give an IQ of 200? It can print one. It cannot certify one.

The bottom line

An IQ of 200 means 6.67 standard deviations above average on the Wechsler scale, about 1 person in 76 billion, far beyond the ceiling of any adult test. The 200s in circulation are children’s ratio IQs, which run out of items by about age 11, wide-scale or high-range scores, or biographical estimates such as Cox’s. Terman’s one 200 in 643 gifted children shows what the number looked like in practice, and the arithmetic shows why it cannot be a rank. Read the scale, the test and the date before taking any 200 at face value, and start from a percentile with the IQ tests hub.

Sources

  • Terman, L.M. et al. (1925): Genetic Studies of Genius, Volume I: Mental and Physical Traits of a Thousand Gifted Children, Stanford University Press (Table 5 for the corrected score bands), and the range of 135 to 200 and 77 children at 170 or more as tabulated in our Terman study page.
  • Cox, C.M. (1926): The Early Mental Traits of Three Hundred Geniuses (Genetic Studies of Genius, Volume II), Stanford University Press: the obtained and corrected ratings in Table 12A and the correction formula, as transcribed and cross-checked by IQ Metrics against the book’s own summary tables.
  • Pearson (2019): WISC-V Technical Report 6, Extended Norms (Raiford et al.).
  • The top mental age of the 1937 Stanford-Binet Form L-M (22 years 10 months to 22 years 11 months) as given in reference works on the scale; the ratio-ceiling table is our own arithmetic. Population Reference Bureau: estimate of about 117 billion humans ever born.
  • Rarity, percentile and Kelley figures are computed from the normal distribution. The scale facts and the vos Savant, Sidis and Hollingworth background follow our guides to the highest IQ ever recorded, an IQ of 180 and the ceiling effect.

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Bars on a logarithmic scale showing how rare a printed IQ of 180 is on three scales: about 1 in 20.7 million on a standard deviation 15 scale, 1 in 3.5 million on standard deviation 16 and 1 in 2,330 on Cattell's standard deviation 24.Scores & Scales

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Turner Syndrome and IQ

Posted on by Mahesh
Understanding IQ

Turner Syndrome and IQ: What the Research Shows About 45,X

Most girls and women with Turner syndrome have an IQ in the normal range, with verbal skills above non-verbal, visuospatial and maths skills. See why published averages differ, how mosaic and ring X karyotypes change the picture and what a referral clinic found.

By , Intelligence Research WriterPublished October 23, 2026 · 11 min readReviewed by Aarav Vardhan, Editor-in-Chief

Three findings about Turner syndrome (45,X) as cards: 77 percent of 30 adult women showed a clinically significant split with verbal IQ above non-verbal IQ; in a referral clinic the median full-scale IQ was 76 with a 45,X karyotype and 95 with other karyotypes, a gap that was not significant after correction; and 6 of 23 people with a ring X chromosome had an intellectual developmental disorder.

Most girls and women with Turner syndrome (45,X) have an IQ in the normal range, but the profile is uneven: verbal skills tend to sit above non-verbal, visuospatial and number skills, and intellectual disability is uncommon outside rarer karyotypes such as a ring X chromosome. Published averages differ because clinic samples run lower than community ones, so no single figure fits everyone. This guide covers Turner syndrome and IQ: what the studies found, why their numbers disagree, how mosaic and ring X karyotypes differ, the maths and attention pattern, whether the parent the X came from matters, what hormone therapy adds, and how it compares with other genetic conditions such as Klinefelter and Down syndrome.

  • How common: the 2024 international guideline gives about 50 per 100,000 females, roughly 1 in 2,000.
  • IQ: usually in the normal range. A classic 1997 study of 80 females with a single X called intelligence “usually normal”; a 2024 referral clinic found a median full-scale IQ of 76 with 45,X and 95 with other karyotypes, a gap that did not survive correction for multiple comparisons.
  • Profile: verbal IQ above performance IQ. In 30 adult women, 77% showed a clinically significant split.
  • Learning and attention: in the clinic sample, 18% had a mathematics learning disorder and 26% ADHD, 26% anxiety and 16% autism; screening of 30 adults found far lower attention and autism rates.
  • Higher-risk group: with a ring X chromosome, 6 of 23 had an intellectual developmental disorder.
Three findings about Turner syndrome (45,X) as cards: 77 percent of 30 adult women showed a clinically significant split with verbal IQ above non-verbal IQ; in a referral clinic the median full-scale IQ was 76 with a 45,X karyotype and 95 with other karyotypes, a gap that was not significant after correction; and 6 of 23 people with a ring X chromosome had an intellectual developmental disorder.
Three findings about Turner syndrome (45,X) as cards: 77 percent of 30 adult women showed a clinically significant split with verbal IQ above non-verbal IQ; in a referral clinic the median full-scale IQ was 76 with a 45,X karyotype and 95 with other karyotypes, a gap that was not significant after correction; and 6 of 23 people with a ring X chromosome had an intellectual developmental disorder.

What is Turner syndrome?

Turner syndrome is a chromosome condition in females in which all or part of one X chromosome is missing. It is usually described by karyotype: classic 45,X, mosaic forms such as 45,X/46,XX in which some cells have two X chromosomes, and structural changes such as an isochromosome Xq, a deletion or a ring X. It brings short stature and early loss of ovarian function, and it can affect the heart, kidneys, hearing and thyroid, which is why the 2024 international guideline runs to about 100 printed pages. Alongside diagnosis, growth, puberty, fertility and co-morbidities, its topics include neurocognition and neuropsychology.

What is the average IQ in Turner syndrome?

In most studies the group average falls in the normal range, with performance below verbal. The table sets out six studies, and the differences between them are the lesson.

What six studies of Turner syndrome and IQ found
Study Who was tested What they found
Skuse and colleagues, 1997 80 females with a single X chromosome Intelligence was “usually normal”; the 25 with a paternally inherited X had better verbal and executive skills than the 55 with a maternal X
Downey and colleagues, 1991 23 adult women, compared with 23 women of short stature without Turner syndrome No difference in verbal IQ; lower performance IQ and full-scale IQ, mainly from spatial and mathematical items
Gawlik and colleagues, 2002 29 girls tested on a Wechsler scale Normal full-scale and verbal IQ; lower non-verbal subtest scores; no link to karyotype
Bjorlin Avdic and colleagues, 2023 30 adult women in Sweden A verbal above non-verbal split in 77%; ADHD thresholds reached by 2 and autism thresholds by 1
Fezza and colleagues, 2024 38 assessed youth in one referral clinic (of 114 patients) Median full-scale IQ 76 with 45,X and 95 with other karyotypes; verbal above performance in both groups
Cicione and colleagues, 2025 104 people with 45,X, mosaic 45,X or a ring X chromosome Intellectual developmental disorder in 6 of 23 (26%) with a ring X; a higher risk than in the other two groups

Three points stand out. First, the verbal-performance split is the most consistent result. The 2024 guideline says the features of the childhood profile, persistent visual-spatial differences, arithmetic difficulty and executive-function differences, continue in adolescence and sit behind a lasting gap between verbal and performance IQ. Second, the full-scale total is pulled down only by the performance side: in the older adult study the verbal scores matched the comparison group, and the gaps appeared on the spatial and mathematical items. Third, the spread is wide. In the 2024 clinic, the middle half of children with other karyotypes scored between 75 and 103, and the middle half with 45,X between 51 and 88.

Why do published averages disagree?

Because of who gets tested. The 2024 clinic sample is a referral sample: only 38 of 114 patients had a neuropsychological assessment, at a median age of 11.3, and children are often sent for assessment because of concerns about learning or behaviour. That selection pushes the average down and the problem rates up, exactly as in the Klinefelter literature (see Klinefelter syndrome and IQ). The same study found the 45,X versus other-karyotype gap in full-scale IQ (76 versus 95, p = 0.027), but it did not meet the stricter threshold set to allow for many comparisons, so it is a signal, not a settled difference. Community samples such as the 1997 and 2002 studies report normal verbal and full-scale scores.

The same gap shows in other rates. In the clinic, 26% had ADHD and 26% an anxiety disorder, 18% a specific mathematics learning disorder and 16% autism spectrum disorder. In the Swedish adult sample, screening reached ADHD thresholds in 2 of 30 (7%) and autism thresholds in 1 (3%), and a Bayesian analysis found no association between the IQ split and either. Neither figure is a prevalence for all women with Turner syndrome; both depend on how the group was found. The full-scale IQ versus index scores explainer shows why a total can hide this kind of unevenness.

Does mosaic Turner syndrome affect IQ differently?

Generally less. In a 2023 multicentre chart review of 631 patients from three paediatric centres, neuropsychological or mental-health concerns were documented for 48.2%, and a mosaic karyotype was associated with a lower risk of both. The picture is the reverse for a ring X chromosome. The 2024 guideline notes that with a ring X and functional loss of the XIST gene a more severe cognitive phenotype may be seen, and a 2025 cohort of 104 people found an intellectual developmental disorder in 6 of 23 (26%) with a ring X, a higher risk than in the 45,X and mosaic groups but lower than earlier reports had suggested. Karyotype is a risk factor, not a verdict: the 2002 study found no relation between karyotype and IQ in 29 girls, and the 2024 clinic found a gap that did not survive correction.

Why do maths and spatial skills stand out?

The weak spot is spatial and numerical reasoning, not language. In 2008 Murphy and Mazzocco compared 18 girls with Turner syndrome with 89 comparison girls matched on full-scale IQ. The Turner group did not differ on untimed arithmetic calculation or on accuracy in checking sums, but they had limited mastery of counting skills and took longer to verify answers, and they differed from the matched girls on visual-spatial skills, working memory and reading. That is the signature of an uneven profile rather than a lower overall level: with IQ held equal, the specific difficulties remain. Dyscalculia and IQ covers how number difficulties and IQ relate more generally, and spatial IQ tests show what the visual-spatial side measures.

On a current Wechsler scale this shows up in the index scores. The WISC-V no longer reports verbal and performance IQ; it reports a Verbal Comprehension Index, a Visual Spatial Index, a Fluid Reasoning Index, a Working Memory Index and a Processing Speed Index, so a visuospatial difficulty would show as a lower Visual Spatial Index than Verbal Comprehension Index. WISC-V index scores explained walks through them, and verbal versus non-verbal IQ scores covers the older split.

Does the parent the X chromosome came from matter?

It may. In 1997 Skuse and colleagues studied 80 females with a single X chromosome, 55 with the X from the mother (45,X[m]) and 25 with the X from the father (45,X[p]). Those with the paternal X were significantly better adjusted, with superior verbal and higher-order executive-function skills, which the authors linked to a gene or genes for social cognition that is imprinted and not expressed from the maternal X. Their reading was that the same locus could help explain why 46,XY males, whose single X is always maternal, are more vulnerable to language and social-cognition conditions such as autism. It is an influential finding and an old one; treat it as a hypothesis the field has kept testing, not as a rule for any one person.

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Does hormone therapy change IQ?

Possibly at the margin. The 2024 guideline summarises the evidence that women with Turner syndrome who received hormone replacement therapy performed better than those who did not on measures of overall IQ, expressive vocabulary and visuospatial processing, while stressing that this does not remove the characteristic neurocognitive profile in some women. Hormone therapy is given for pubertal development, bone and heart health, not for intelligence, so the cognitive findings are a secondary note rather than a reason to start or stop it; that decision belongs with the treating team.

How does it compare with other genetic conditions?

Turner syndrome sits at the milder end. Most people with it score in the normal range with a clear verbal-performance pattern, while some other chromosome conditions shift the whole distribution lower:

IQ in five genetic conditions, as covered in our guides
Condition Typical IQ picture Guide
Turner syndrome (45,X) Usually the normal range; verbal above non-verbal; higher risk with a ring X chromosome This guide
Klinefelter syndrome (47,XXY) Normal range, average about 93; intellectual disability rare Klinefelter syndrome and IQ
Down syndrome Average about 50, typical range about 30 to 70 Down syndrome and IQ
Williams syndrome Mild to moderate disability; mean full-scale IQ 61 in 62 adults Williams syndrome and IQ
Fragile X syndrome Males moderate to severe; females vary far more Fragile X and IQ

For the idea of what a score means at the low end of the scale, see what an IQ of 65 means, and for how the label relates to everyday skills, low IQ versus learning disability.

What to do if a child or woman has Turner syndrome

  • Ask for a neuropsychological assessment at key transitions. The guidelines recommend screening for neuropsychological concerns at key stages of development, and a Wechsler index profile tells you more than a total.
  • Look at the profile, not the total. A verbal strength can lift a full-scale score over the average line while spatial, number and working-memory skills still need support.
  • Plan for maths early. Counting, estimation and spatial tasks benefit from explicit, concrete teaching before gaps compound.
  • Screen for attention and anxiety. Both are reported more often in clinic samples, and both can lower a test score without lowering ability; see ADHD, autism and dyslexia and IQ test scores.
  • Use scores to plan, not to predict. The spread inside every group is wide, and a single figure says little about school, work or life. For what a score can and cannot tell you, see how to read an IQ test report.

Quick answers

  • Does Turner syndrome affect IQ? Often a little, mostly on the non-verbal side. Overall intelligence is usually in the normal range.
  • What is the average IQ in Turner syndrome? It depends on who was studied. Community samples report normal verbal and full-scale scores; a referral clinic reported medians of 76 for 45,X and 95 for other karyotypes.
  • Is Turner syndrome an intellectual disability? Not usually. Risk is higher with a ring X chromosome (6 of 23 in one cohort).
  • Does mosaic Turner syndrome have a different IQ? Mosaic karyotypes carry a lower risk of neuropsychological concerns in a 631-patient review.
  • Can a girl with Turner syndrome have a high IQ? Yes. Scores cover the normal range, and in the clinic sample the middle half of children with other karyotypes scored 75 to 103.
  • Is Turner syndrome the same as Klinefelter syndrome? No. Turner syndrome is a missing X chromosome in a female; Klinefelter is an extra X in a male. Both show an uneven profile with a typically normal-range IQ.

The bottom line

Turner syndrome and IQ: the typical person scores in the normal range, with verbal skills above non-verbal, visuospatial and number skills, and the totals reported in the literature depend heavily on who was tested. A ring X chromosome carries a higher risk of intellectual disability, mosaic karyotypes a lower risk of concerns, and hormone therapy may help at the margin. The score that matters most is the profile: a full-scale IQ can look average while one index sits much lower, which is why an assessment should report all of them. For where a score sits on the standard scale, try the IQ Metrics IQ test and the IQ percentile calculator.

Sources

  • Gravholt, C.H. et al. (2024): Clinical practice guidelines for the care of girls and women with Turner syndrome, European Journal of Endocrinology, 190, G53-G151.
  • Skuse, D.H. et al. (1997): Evidence from Turner’s syndrome of an imprinted X-linked locus affecting cognitive function, Nature, 387, 705-708.
  • Fezza, J.I. et al. (2024): Spectrum of neuropsychological challenges in Turner syndrome, Frontiers in Endocrinology, 15, 1461103.
  • Cicione, M.E. et al. (2025): Revisiting the neuropsychological and clinical profile of mosaic Turner syndrome with a ring X chromosome, American Journal of Medical Genetics Part C.
  • Kremen, J. et al. (2023): Neuropsychological and mental health concerns in a multicenter clinical sample of youth with Turner syndrome, American Journal of Medical Genetics Part A.
  • Bjorlin Avdic, H. et al. (2023): Cognitive profile in adult women with Turner syndrome: IQ split and associations with ADHD and ASD, Cognitive Neuropsychiatry, 28(3), 207-225.
  • Downey, J. et al. (1991): Cognitive ability and everyday functioning in women with Turner syndrome, Journal of Learning Disabilities, 24(1), 32-39. And Murphy, M.M. and Mazzocco, M.M.M. (2008): Mathematics learning disabilities in girls with fragile X or Turner syndrome during late elementary school, Journal of Learning Disabilities, 41(1), 29-46.
  • Gawlik, A. et al. (2002), PMID 12818125: a Polish-language study of intellectual functioning in 29 girls with Turner syndrome (abstract read). The fragile X, Williams, Down and Klinefelter figures in the comparison table follow our own guides.

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