IQ 66: what it means and where to find support
An IQ of 66 is higher than about 1.2% of people, so roughly 1 in 85 scores this low or lower. It is in the extremely low range. If a professional assessment has given you or your child a score like this, the most useful next step is information and practical support, and good free sources exist.
Where 66 sits on the bell curve
An IQ of 66 is 34 points below the average of 100, or 2.27 standard deviations. About 1.2% of people of the same age score lower, the shaded sliver at the far left of the curve below.
On a test with a standard deviation of 16, such as older Stanford-Binet editions, the same result would be about 64, and on the Cattell scale, which uses 24, about 46. Why scales differ.
A closer look at an IQ of 66
Different countries use different words for the same thing, which can be confusing. In the UK the usual term is "learning disability": the charity Mencap describes it as a reduced intellectual ability, usually identified early in life, that lasts a person's whole life. In the US and in DSM-5 the term is "intellectual disability". In the UK, "learning difficulty" is often used for something different, such as dyslexia, which affects one kind of learning rather than overall intellectual ability.
The NHS stresses how varied life with a learning disability is: "Lots of people who have a learning disability can work, have relationships, live alone and get qualifications", while others need more support throughout their life. Its page explains assessment and the help available, Mencap offers advice for people and families, and in the US The Arc supports people with intellectual and developmental disabilities. Links to all three are at the bottom of this page.
How an IQ of 66 is classified
Different tests and manuals use different words for the same score, and the words have changed over time. Here is where 66 falls on each of the systems you are most likely to meet.
Sources: American Psychiatric Association, DSM-5 fact sheet on intellectual disability; World Health Organization, ICD-11, 6A00 disorders of intellectual development; Peltopuro, M. and others (2014), "Borderline intellectual functioning: a systematic literature review", Intellectual and Developmental Disabilities, 52(6), 419–443. WISC-V descriptors from Pearson's interpretive reports, as on our good IQ score guide.
Is an IQ of 66 an intellectual disability?
A score of 66 on its own never makes a diagnosis. The American Psychiatric Association's DSM-5 describes intellectual disability as "approximately two standard deviations or more below the population, which equals an IQ score of about 70 or below", and requires three things together:
- Low intellectual functioning, confirmed by a clinical assessment and individually given, standardised testing.
- Difficulties with everyday adaptive skills in at least one of three domains: conceptual (language, reading, maths, memory), social (communication, judgement, friendships) and practical (personal care, money, work and school tasks).
- A start during the developmental period, in childhood or adolescence.
Severity is set by those everyday skills, not by the IQ number. That is why only about 1% of people have an intellectual disability (a 2011 meta-analysis of 52 population studies found about 10 per 1,000), even though about 2.3% score below 70 on the bell curve. Because tests have a margin of error, DSM-5 treats roughly 65 to 75 as the uncertain zone around 70.
Sources: American Psychiatric Association, DSM-5 fact sheet on intellectual disability; Maulik, P. K. and others (2011), "Prevalence of intellectual disability: a meta-analysis of population-based studies", Research in Developmental Disabilities, 32(2), 419–436; Hall v. Florida, 572 U.S. 701 (2014), which cites the DSM-5 range of 65 to 75.
Why scores near 70 matter in US courts
In Atkins v. Virginia (2002), the US Supreme Court ruled that executing a person with an intellectual disability is unconstitutional, and left states to decide who qualifies. In Hall v. Florida (2014) it struck down a rule that treated 70 as a strict cut-off, because every test has a margin of error. In Moore v. Texas (2017) it said courts must be guided by current medical standards rather than "lay stereotypes".
The practical lesson is the same outside a courtroom: a score of 66 is best read as a range, about 61 to 71, and judged alongside how a person actually manages daily life.
Sources: Atkins v. Virginia, 536 U.S. 304 (2002); Hall v. Florida, 572 U.S. 701 (2014); Moore v. Texas, 581 U.S. 1 (2017).
Could a score of 66 be wrong?
Low scores are the most fragile results on any test, because almost everything that goes wrong on the day pulls a score down. Common reasons include:
- Running out of time. On timed tests, including ours, an unanswered question counts as wrong.
- Nerves. A review of 238 studies found test anxiety consistently linked with lower scores.
- Tiredness. Sleep loss hits attention and speed hardest, which costs points against the clock.
- Language and format. Taking a test in a second language, on a small screen or in an unfamiliar format can lower a result without saying anything about reasoning.
What a 66 means on our test
The Assessivo test moves in steps of about 2.5 points, so it can't give exactly 66: the nearest results are 65 (4 correct) and 68 (5 correct). Every question has four options, so someone guessing every answer at random averages nine correct, a score of 78. Random guessing falls this low about once in 29 attempts, so it can happen by bad luck, but it more often means many questions were skipped or rushed.
If any of this sounds like your session, retake the free IQ test when you are rested and have 25 quiet minutes. A score that moves a lot between attempts was never very solid.
Sources: von der Embse, N. and others (2018), "Test anxiety effects, predictors, and correlates: a 30-year meta-analytic review", Journal of Affective Disorders, 227, 483–493; Lim, J. and Dinges, D. F. (2010), "A meta-analysis of the impact of short-term sleep deprivation on cognitive variables", Psychological Bulletin, 136(3), 375–389. Guessing figures are worked out from the binomial distribution for 36 four-option questions.
What is the mental age for an IQ of 66?
The first IQ tests, for children, described performance as a "mental age": the age whose typical performance a child matched. IQ was mental age divided by real age, times 100. Run that old formula backwards and you get the figures below, for children only.
| Child's age | Performs like a typical child of about |
|---|---|
| 6 | 4 years |
| 8 | 5 years 3 months |
| 10 | 6 years 7 months |
| 12 | 7 years 11 months |
| 14 | 9 years 3 months |
For adults the idea doesn't work, which is why David Wechsler replaced it in 1939 with today's method: compare each person with others of the same age. An adult who scores 66 has an adult's knowledge, interests and experience. The score only says how their test performance compares with other adults'.
How common is an IQ of 66?
Line up 100 people of the same age and about 1 would score 66 or lower; the orange dot marks the place. In a town of 10,000 people, that is about 117.
The bar shows the likely range on a professional test, about five points either way: 61 to 71, from the 0.47th to the 3rd percentile.
How 66 compares with nearby scores
| IQ | Percentile | Classic label | Score this low or lower |
|---|---|---|---|
| 56 | 0.17th | Extremely low | 1 in 596 |
| 61 | 0.47th | Extremely low | 1 in 215 |
| 66 | 1st | Extremely low | 1 in 85 |
| 71 | 3rd | Borderline | 1 in 38 |
| 76 | 5th | Borderline | 1 in 18 |
Questions about an IQ of 66
What percentile is an IQ of 66?
An IQ of 66 is at about the 1st percentile: it is higher than about 1.2% of people of the same age, and roughly 1 person in 85 scores this low or lower.
Is 66 a low IQ?
Yes. 66 is 34 points, or 2.27 standard deviations, below the average of 100. On the classic Wechsler labels it is extremely low. A single score is an estimate with a margin of error of about five points, so 66 is best read as roughly 61 to 71.
Is an IQ of 66 a learning disability?
Not on its own. 66 is in the range where an intellectual disability (called a learning disability in the UK) is considered, but a diagnosis also needs significant difficulties with everyday skills that began during childhood, assessed by a qualified psychologist.
What is the mental age for an IQ of 66?
Mental age is an idea from the earliest tests for children. By the old formula, a 10-year-old scoring 66 would perform like a typical child of about 6 years 7 months. For adults it has no meaning: modern IQ tests compare adults with other adults, and an adult who scores 66 is not like a child.
What is the difference between a learning disability and a learning difficulty?
In the UK, a learning disability is a reduced intellectual ability that lasts a person's whole life. "Learning difficulty" is often used for conditions such as dyslexia, which affect a specific kind of learning but not overall intellectual ability. In the US, "intellectual disability" is the usual term for the first.
Related: the IQ range chart, what counts as an average IQ, what IQ measures and misses and how to tell if an online IQ test is accurate.
A full IQ score of 66 combines several skills. Our guides explain each one: abstract reasoning, spatial reasoning, verbal reasoning, processing speed and fluid vs crystallized intelligence, or try the free pattern recognition test.
Information and support
If a professional assessment has given you, your child or someone you care for a score like 66, these organisations explain what it means in practice and what help is available.
Want a second reading?
Take the free Assessivo IQ test when you are rested and have 25 quiet minutes: 36 questions, and your estimated score and percentile as soon as you finish.
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