Oct 4, 2026·Special Population & Related Conditions
Prader-Willi Syndrome and IQ: Typical Scores, Genetic Subtypes, and the Jigsaw Puzzle Finding
Prader-Willi IQ scores average around 60, about 40 points below the general population, with verbal and visual strengths that differ by genetic subtype.
Dr. Russell T. WarneChief Scientist
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People with Prader-Willi syndrome usually have IQ scores in the mild to moderate intellectual disability range, and a population-based UK study found a mean about 40 points below the general population average of 100. Scores still follow a bell-shaped spread around that lower mean, so a minority of people score in the borderline range or above, and the shape of a person's abilities depends partly on which genetic mechanism caused the condition.
This page covers the typical Prader-Willi IQ range, the cognitive profiles of the two main genetic subtypes, the jigsaw puzzle finding, why school achievement lags behind IQ, and growth hormone research. The diagnostic criteria for intellectual disability are explained in our article on low IQ and intellectual disability and are not repeated here.
The typical Prader-Willi IQ range
Prader-Willi syndrome affects an estimated 1 in 10,000 to 30,000 people, according to MedlinePlus Genetics. Babies have weak muscle tone and feeding difficulties, and from childhood onward most develop an extreme, persistent hunger. The cause is a loss of function of a group of genes on chromosome 15 that are normally active only on the copy inherited from the father. About 70% of cases result from a deletion on the father's chromosome 15, and about 25% from "maternal uniparental disomy," meaning the person inherited both copies of chromosome 15 from the mother.
MedlinePlus describes the typical level as mild to moderate intellectual impairment. The clearest population figure comes from Whittington and colleagues, who tried to identify every person with the syndrome living in one UK health region and tested everyone over age three who consented. The IQ distribution they found was roughly normal in shape, with a mean about 40 points below that of the general population. The authors interpreted this as a global effect of the Prader-Willi genes that shifts the whole bell curve downward.
In a later US study of 96 children and young people by Dykens and colleagues, the mean composite IQ on the Kaufman Brief Intelligence Test was 62 among those who had never received growth hormone and 75 among those who had (the treatment question is discussed below).
Family resemblance depends on the subtype. In a follow-up, Whittington's team correlated the IQs of people with Prader-Willi syndrome with those of their brothers and sisters. The correlation was about 0.5 for people with maternal disomy, the usual figure for ordinary siblings, but close to zero (−0.07) for people with a deletion. The authors read this as another sign that the two subtypes differ biologically. Our article on whether IQ is genetic explains how family resemblance is used to study genetic influence in the general population.
Deletion versus maternal disomy: two cognitive profiles
The two main subtypes have similar overall IQs but different strengths. Roof and colleagues tested 38 people with Prader-Willi syndrome and found that those with maternal disomy had verbal IQs about 9 points higher than those with a deletion (69.9 against 60.8). Half of the disomy group had a verbal IQ of 70 or above, compared with 17% of the deletion group, while performance IQ did not differ between the subtypes.
Whittington's population study found a matching pattern. People with disomy had better verbal abilities and a specific weakness on the coding subtest, a timed task that involves copying symbols quickly. Some people with deletions had strong visual-spatial skills.
A 2013 meta-analysis by Yang and colleagues pooled 744 cases from 13 studies. Compared with people who had maternal disomy, people with a deletion averaged 2.7 points lower in full-scale IQ, 7.5 points lower in verbal IQ and 4.0 points higher in performance IQ. The same analysis found a much higher risk of psychiatric illness in the disomy group. In practice, a single full-scale score can hide a meaningful verbal-nonverbal split, and the expected direction of that split depends on the subtype.
Jigsaw puzzles, word searches and uneven skills
In 2002 Elisabeth Dykens tested a long-standing clinical impression that people with the syndrome are unusually good at jigsaw puzzles. Children with the syndrome scored higher than IQ-matched peers with other causes of intellectual disability on standard visual-spatial subtests, though still below typically developing children of the same age. On word searches they scored on par with typically developing peers, and on jigsaw puzzles they placed more than twice as many pieces as the typically developing group. Puzzle skill was not predicted by age, IQ or sex, but it was related to genetic subtype, and later reviews such as Rosenberg and colleagues' report that people with a deletion have the stronger puzzle skills.
The skill appears to rest on a particular strategy. Verdine and colleagues found that people with Prader-Willi syndrome relied on the shape of the pieces, whereas typically developing children matched on mental age used the picture. The Prader-Willi group did better on a blank interlocking puzzle, the two groups did equally well on picture puzzles, and, as a later review by Dykens, Lee and Roof summarized, the Prader-Willi group performed relatively poorly on formal spatial tests. A striking puzzle skill therefore does not mean that general spatial reasoning is intact.
School achievement often falls short of what IQ predicts. In Whittington's population sample, reading, writing and arithmetic scores were commonly below the level expected from full-scale IQ, and the strongest predictor of that shortfall was the share of time spent in special schools. The authors suggested that immature social behavior and behavioral problems may mask intellectual ability. A measure of everyday skills, described in our article on adaptive behavior assessment, adds information that neither IQ nor achievement scores capture on their own. Compulsive behaviors and temper outbursts, which MedlinePlus lists as common, are also worth recording alongside test results.
Growth hormone and cognition
Growth hormone is widely used in Prader-Willi syndrome for growth and body composition. The cognitive findings below are descriptive, and decisions about treatment belong with the person's endocrinologist.
In a randomized trial by Siemensma and colleagues, 50 prepubertal children were assigned to growth hormone or no treatment for two years. Scores in the treated group stayed stable, while the untreated group declined, significantly on the similarities and vocabulary subtests. After four years of treatment, similarities and block design scores were higher than at baseline. IQ in that study was estimated from just two subtests, which limits precision.
Dykens' US study compared children who had or had not received growth hormone, controlling for family income and maternal education. Treated children had higher verbal IQ (81.6 against 67.5) and composite IQ (74.6 against 62.3), and those who started before 12 months of age had higher nonverbal and composite scores than those who started between one and five years. Children followed consistently on or off treatment for up to five years both showed stable IQ, so the gap was present at the start and did not widen. The study was not randomized, and the authors acknowledge that unmeasured factors could contribute.
Frequently asked questions
What is the average IQ of a person with Prader-Willi syndrome?
Around 60 in most studies, roughly 40 points below the population average, with a bell-shaped spread that reaches the borderline range and above.
Does the genetic subtype affect IQ?
It affects the profile more than the total. People with maternal disomy tend to have higher verbal IQ, while people with a deletion tend to have higher performance IQ and stronger jigsaw puzzle skills.
Are people with Prader-Willi syndrome really good at jigsaw puzzles?
Many are. In Dykens' study children with the syndrome placed more than twice as many pieces as typically developing peers, using a strategy based on piece shape.
Does growth hormone raise IQ in Prader-Willi syndrome?
Studies link treatment with stable or higher scores on some measures, especially when started in infancy, but the evidence comes from one small randomized trial and observational comparisons.
Who should assess a person with Prader-Willi syndrome?
A psychologist or neuropsychologist experienced with genetic syndromes, working with the person's Prader-Willi clinic or endocrinologist and including a measure of adaptive behavior.
The takeaway
The typical Prader-Willi IQ sits in the mild to moderate intellectual disability range, around 60 on average, with a bell-shaped spread that reaches the borderline range and above. The genetic subtype shapes the profile: maternal disomy tends to favor verbal skills, a deletion tends to favor visual and puzzle skills, and the full-scale score can hide either pattern. School achievement often lags behind measured ability, and growth hormone has been linked to better cognitive scores. To see how a standardized score is built for the general population, you can take the RIOT IQ test.
References
1. MedlinePlus Genetics. (2022). Prader-Willi syndrome. National Library of Medicine. [medlineplus.gov](. medlineplus.gov
2. Whittington, J., Holland, A., Webb, T., Butler, J., Clarke, D., & Boer, H. (2004). Cognitive abilities and genotype in a population-based sample of people with Prader-Willi syndrome. Journal of Intellectual Disability Research, 48(2), 172-187. [doi.org/10.1111/j.1365-2788.2004.00556.x](. doi.org
3. Dykens, E. M., Roof, E., & Hunt-Hawkins, H. (2017). Cognitive and adaptive advantages of growth hormone treatment in children with Prader-Willi syndrome. Journal of Child Psychology and Psychiatry, 58(1), 64-74. [doi.org/10.1111/jcpp.12601](. doi.org
4. Whittington, J., Holland, A., & Webb, T. (2009). Relationship between the IQ of people with Prader-Willi syndrome and that of their siblings: Evidence for imprinted gene effects. Journal of Intellectual Disability Research, 53(5), 411-418. [doi.org/10.1111/j.1365-2788.2009.01157.x](. doi.org
5. Roof, E., Stone, W., MacLean, W., Feurer, I. D., Thompson, T., & Butler, M. G. (2000). Intellectual characteristics of Prader-Willi syndrome: Comparison of genetic subtypes. Journal of Intellectual Disability Research, 44(1), 25-30. [doi.org/10.1046/j.1365-2788.2000.00250.x](. doi.org
6. Yang, L., Zhan, G. D., Ding, J. J., Wang, H. J., Ma, D., Huang, G. Y., & Zhou, W. H. (2013). Psychiatric illness and intellectual disability in the Prader-Willi syndrome with different molecular defects: A meta analysis. PLoS ONE, 8(8), e72640. [doi.org/10.1371/journal.pone.0072640](. doi.org
7. Dykens, E. M. (2002). Are jigsaw puzzle skills "spared" in persons with Prader-Willi syndrome? Journal of Child Psychology and Psychiatry, 43(3), 343-352. [doi.org/10.1111/1469-7610.00025](. doi.org
8. Rosenberg, A. G. W., Wellink, C. M., Tellez Garcia, J. M., Pellikaan, K., Van Abswoude, D. H., Davidse, K., Van Zutven, L. J. C. M., Brüggenwirth, H. T., Resnick, J. L., Van der Lely, A. J., & De Graaff, L. C. G. (2022). Health problems in adults with Prader-Willi syndrome of different genetic subtypes: Cohort study, meta-analysis and review of the literature. Journal of Clinical Medicine, 11(14), 4033. [doi.org/10.3390/jcm11144033](. doi.org
9. Verdine, B. N., Troseth, G. L., Hodapp, R. M., & Dykens, E. M. (2008). Strategies and correlates of jigsaw puzzle and visuospatial performance by persons with Prader-Willi syndrome. American Journal on Mental Retardation, 113(5), 343-355. [doi.org/10.1352/2008.113:342-355](. doi.org
10. Dykens, E. M., Lee, E., & Roof, E. (2011). Prader-Willi syndrome and autism spectrum disorders: An evolving story. Journal of Neurodevelopmental Disorders, 3(3), 225-237. [doi.org/10.1007/s11689-011-9092-5](. doi.org
11. Whittington, J., Holland, A., Webb, T., Butler, J., Clarke, D., & Boer, H. (2004). Academic underachievement by people with Prader-Willi syndrome. Journal of Intellectual Disability Research, 48(2), 188-200. [doi.org/10.1111/j.1365-2788.2004.00473.x](. doi.org
12. Siemensma, E. P. C., Tummers-de Lind van Wijngaarden, R. F. A., Festen, D. A. M., Troeman, Z. C. E., van Alfen-van der Velden, A. A. E. M., Otten, B. J., ... Hokken-Koelega, A. C. S. (2012). Beneficial effects of growth hormone treatment on cognition in children with Prader-Willi syndrome: A randomized controlled trial and longitudinal study. Journal of Clinical Endocrinology & Metabolism, 97(7), 2307-2314. [doi.org/10.1210/jc.2012-1182](. doi.org
Hero image: Sky puzzle, by Jared Tarbell, licensed CC BY 2.0 (creativecommons.org/licenses/by/2.0). Via Wikimedia Commons.
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