Oct 3, 2026·Special Population & Related Conditions
Down Syndrome and IQ: The Typical Range and What the Scores Can Tell You
Down syndrome IQ scores usually fall between about 30 and 70, the mild to moderate range, with wide individual variation and a distinctive cognitive profile.
Dr. Russell T. WarneChief Scientist
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People with Down syndrome usually have IQ scores between about 30 and 70, which corresponds to the mild to moderate range of intellectual disability, and scores above and below that band do occur. The full-scale number hides a characteristic profile of relative strengths and weaknesses, and in childhood it drifts downward even while skills keep growing.
This page covers the typical range and how much individuals differ, why IQ scores fall in childhood, the cognitive profile underneath the total, how mosaic Down syndrome differs, what goes wrong with standard tests near their floor, and what changes in adulthood. The diagnostic criteria for intellectual disability itself are covered in our article on low IQ and intellectual disability, so they are not repeated here.
The typical IQ range, and how wide it really is
Down syndrome is the most common genetic cause of intellectual disability, occurring in roughly 1 in 700 newborns in the United States according to MedlinePlus Genetics. MedlinePlus describes the intellectual disability as "usually mild to moderate." A 2019 systematic review of cognitive testing in adults with Down syndrome by Hamburg and colleagues puts numbers on that: people with the condition "typically have an intelligence quotient (IQ) ranging from 30 to 70, although IQs both above and below this range occur."
The averages in actual samples sit near the middle of that band. In the studies Hamburg's team reviewed, reported full-scale means included 46.6 on the Stanford-Binet Fifth Edition and 49.7 on a Portuguese version of the WAIS-III, and one Spanish sample had a median of 41. On the familiar scale where the population mean is 100 and the "standard deviation" (the typical distance of a score from the mean) is 15, an IQ of 50 sits more than three standard deviations below average.
Group means describe groups. Onnivello and colleagues tested 72 children and adolescents with Down syndrome aged 7 to 16 and found three distinct cognitive profiles rather than one. The lowest-scoring cluster showed the textbook pattern of higher nonverbal than verbal ability, an intermediate cluster showed the reverse, and the highest-scoring cluster did equally well in both domains. A label like "Down syndrome" predicts a range, and it does not tell you where in that range a particular child will land.
Mosaic Down syndrome
Most people with Down syndrome have a full extra copy of chromosome 21 in every cell. The National Institute of Child Health and Human Development estimates that in less than 5% of cases, only some cells carry the extra chromosome, a pattern called "mosaicism." Fishler and Koch compared 30 people with mosaicism against 30 with standard trisomy 21, matched for age, sex and parental socioeconomic background, and found that the mosaic group's mean IQ was significantly higher. Many in the mosaic group had better verbal abilities, and the authors suggested that a child with an IQ over 60 at age five and relatively typical speech could be a suspected case of mosaicism, to be confirmed by chromosome analysis.
Why IQ scores fall during childhood while skills keep growing
Parents are sometimes alarmed to see a child's IQ score drop between evaluations. Patterson, Rapsey and Glue's systematic review of longitudinal studies found that the studies assessing IQ in children with Down syndrome "reported a decline across time." The decline is real in the numbers and usually misread in meaning.
An IQ is a comparison with same-age peers. A child who keeps learning, but at a slower pace than typically developing children, falls further behind the age norm each year, so the standardized score goes down even as raw ability goes up. Grieco and colleagues describe the result in their lifespan review: in early childhood there are "mild deviations from neurotypically developing trajectories," and by school age "delays become pronounced," while nonverbal skills "remain on trajectory for mental age." Our article on what mental age is explains why the ratio between developmental level and age produces exactly this pattern.
The drift does not continue forever. In Janet Carr's population cohort, followed from six weeks of age, mean IQs rose slightly between ages 11 and 21, and at the 45-year follow-up mean verbal and nonverbal IQs had "changed little from age 21 to 45."
The cognitive profile underneath the full-scale score
A full-scale IQ averages over abilities that are not equally affected. Grieco and colleagues summarize the well-replicated pattern: "Nonverbal learning and memory are strengths relative to verbal skills," and "Expressive language is delayed relative to comprehension." Attention and executive function difficulties are present in childhood and become more pronounced with age.
A meta-analysis by Næss and colleagues refines the verbal side. Compared with typically developing children matched for nonverbal mental age, children with Down syndrome showed broad language deficits "not restricted to measures of expressive language," along with weaknesses in verbal short-term memory, the ability to hold heard words in mind for a few seconds.
That profile has direct consequences for testing. A verbal subtest that depends on repeating spoken material, or on answering in full sentences, can understate what a person understands. MedlinePlus also notes an increased risk of hearing and vision problems and that about 15% of people with Down syndrome have an underactive thyroid. A competent evaluator rules out sensory and medical explanations before attributing a low subtest score to cognition.
Measuring IQ near the floor of the test
Standard IQ batteries are built to measure precisely around the population average, and they lose resolution in the tails. Hamburg's review found that floor effects, where many people obtain the lowest possible score, "were common for IQ tests (particularly for standardised test scores)." Several widely used tests cannot report a full-scale IQ below about 40 to 45, so two people with quite different abilities can receive the same bottom score.
Sansone and colleagues proposed a fix that has spread through research on intellectual disability. Instead of reading scores off the norm tables, which compress everyone at the floor into the same number, they converted raw scores into deviation scores based on the general population norms. They concluded that "substantial and meaningful variation in cognitive ability" is "lost when converting raw scores to standardized scaled, index and IQ scores," and that the conventional method produces "erroneously flat profiles." Hamburg's team made a similar recommendation for tracking adults over time: raw scores lose less information than standardized ones.
None of this makes an IQ score the deciding fact about a person. Support needs are judged from everyday functioning, and our article on adaptive behavior assessment explains how that is measured and why it carries equal weight.
IQ in adulthood and the question of Alzheimer's disease
Adults with Down syndrome have a much higher risk of Alzheimer's disease than the general population, and this is where repeated cognitive testing becomes most important. In a study of 388 adults with Down syndrome, Fortea and colleagues found that prodromal Alzheimer's disease was diagnosed at a median age of 50.2 years and dementia at a median of 53.7 years, with symptomatic prevalence "reaching 90-100% in the seventh decade of life."
Cross-sectional IQ data show the same age boundary. In Ghezzo and colleagues' Italian sample of people without dementia, mean Wechsler full-scale IQ was 49.7 at ages 18 to 29, 48.8 at ages 30 to 39, and 33.2 at age 40 and older, though only five people in the oldest group completed that test, so the estimate is imprecise. The authors reported that neuropsychological and adaptive skills were lower after age 40 even after people with dementia had been excluded.
The practical lesson is about comparison. A single adult score near 40 cannot reveal decline, because it says nothing about where that person started. Change is detected by comparing a person with their own earlier results, ideally on measures that are not stuck at the test floor, which is another reason the raw-score approach matters.
Frequently asked questions
What is the average IQ of a person with Down syndrome?
Most reviews place the typical range at about 30 to 70, and sample means usually fall in the 40s. Individual scores vary widely, and some people score above 70.
Can a person with Down syndrome have a normal IQ?
It is uncommon but scores above the usual range do occur, more often in people with mosaic Down syndrome, whose mean IQ is higher than that of people with full trisomy 21.
Why did my child's IQ score go down?
Usually because skills are growing more slowly than those of same-age peers, so the age-based comparison falls even while raw ability rises. A drop between childhood evaluations rarely means a loss of skills.
Is IQ testing accurate for people with Down syndrome?
It can be, with caveats. Floor effects and uncorrected sensory problems can distort scores, so careful evaluators supplement the standard IQ with raw or deviation scores and with adaptive behavior ratings.
Who should evaluate an adult with Down syndrome for cognitive decline?
A physician working with a psychologist or neuropsychologist experienced in intellectual disability, ideally with earlier test results available for comparison.
The takeaway
The typical IQ range in Down syndrome runs from about 30 to 70, which is the mild to moderate range of intellectual disability, yet the distribution within that range is wide, and mosaicism and individual cognitive profiles both shift where a person lands. Childhood IQ scores tend to fall because development runs slower than the age norm while skills continue to accumulate, and adult scores are stable for decades before the age-related risk of Alzheimer's disease makes repeated testing important. A single full-scale score tells you less than the profile behind it and the trajectory across repeated testing. If you are curious how standardized scores are built and interpreted in the general population, you can take a full-length online IQ test and read the report alongside this page.
References
1. Hamburg, S., Lowe, B., Startin, C. M., Padilla, C., Coppus, A., Silverman, W., Fortea, J., Zaman, S., Head, E., Handen, B. L., Lott, I., Song, W., & Strydom, A. (2019). Assessing general cognitive and adaptive abilities in adults with Down syndrome: A systematic review. Journal of Neurodevelopmental Disorders, 11, 20. doi.org
2. MedlinePlus Genetics. (2020). Down syndrome. National Library of Medicine. medlineplus.gov
3. Eunice Kennedy Shriver National Institute of Child Health and Human Development. (n.d.). What causes Down syndrome?. nichd.nih.gov
4. Onnivello, S., Pulina, F., Locatelli, C., Marcolin, C., Ramacieri, G., Antonaros, F., Vione, B., Caracausi, M., & Lanfranchi, S. (2022). Cognitive profiles in children and adolescents with Down syndrome. Scientific Reports, 12, 1936. doi.org
5. Fishler, K., & Koch, R. (1991). Mental development in Down syndrome mosaicism. American Journal on Mental Retardation, 96(3), 345-351. pubmed.ncbi.nlm.nih.gov
6. Patterson, T., Rapsey, C. M., & Glue, P. (2013). Systematic review of cognitive development across childhood in Down syndrome: Implications for treatment interventions. Journal of Intellectual Disability Research, 57(4), 306-318. doi.org
7. Grieco, J., Pulsifer, M., Seligsohn, K., Skotko, B., & Schwartz, A. (2015). Down syndrome: Cognitive and behavioral functioning across the lifespan. American Journal of Medical Genetics Part C: Seminars in Medical Genetics, 169(2), 135-149. doi.org
8. Carr, J. (1988). Six weeks to twenty-one years old: A longitudinal study of children with Down's syndrome and their families. Journal of Child Psychology and Psychiatry, 29(4), 407-431. doi.org
9. Carr, J. (2012). Six weeks to 45 years: A longitudinal study of a population with Down syndrome. Journal of Applied Research in Intellectual Disabilities, 25(5), 414-422. doi.org
10. Næss, K.-A. B., Lyster, S.-A. H., Hulme, C., & Melby-Lervåg, M. (2011). Language and verbal short-term memory skills in children with Down syndrome: A meta-analytic review. Research in Developmental Disabilities, 32(6), 2225-2234. doi.org
11. Sansone, S. M., Schneider, A., Bickel, E., Berry-Kravis, E., Prescott, C., & Hessl, D. (2014). Improving IQ measurement in intellectual disabilities using true deviation from population norms. Journal of Neurodevelopmental Disorders, 6, 16. doi.org
12. Fortea, J., Vilaplana, E., Carmona-Iragui, M., Benejam, B., Videla, L., Barroeta, I., Fernández, S., Altuna, M., Pegueroles, J., Montal, V., Valldeneu, S., Giménez, S., González-Ortiz, S., Muñoz, L., Estellés, T., Illán-Gala, I., Belbin, O., Camacho, V., Wilson, L. R., ... Lleó, A. (2020). Clinical and biomarker changes of Alzheimer's disease in adults with Down syndrome: A cross-sectional study. The Lancet, 395(10242), 1988-1997. doi.org
13. Ghezzo, A., Salvioli, S., Solimando, M. C., Palmieri, A., Chiostergi, C., Scurti, M., Lomartire, L., Bedetti, F., Cocchi, G., Follo, D., Pipitone, E., Rovatti, P., Zamberletti, J., Gomiero, T., Castellani, G., & Franceschi, C. (2014). Age-related changes of adaptive and neuropsychological features in persons with Down syndrome. PLoS ONE, 9(11), e113111. doi.org
Hero image: human karyotype 47,XY,+21 (Down syndrome), micrograph by Josef Reischig, licensed CC BY-SA 3.0 (creativecommons.org/licenses/by-sa/3.0). Via Wikimedia Commons.
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