Intelligence testing and IQ
- Test
IQ tests compare a person's reasoning and problem-solving with others of the same age; the average score is 100.
What it is
Intelligence tests are standardized, individually administered batteries of subtests (verbal comprehension, visual-spatial reasoning, working memory, processing speed and others) that yield a full-scale IQ and index scores. The modern score is a deviation IQ: it shows where a person stands compared with same-age peers in a normal distribution with mean 100 and standard deviation 15. Separate versions exist for preschool children, school-age children and adults. Intellectual disability is now defined by deficits in both intellectual and adaptive functioning beginning in the developmental period, with severity set by adaptive functioning, not by IQ alone.
Key facts
- The old ratio IQ (mental age ÷ chronological age × 100) was replaced by the deviation IQ because ratios fail in adults.
- The first practical intelligence test was created in France in the early 1900s to find schoolchildren who needed extra support.
- Adaptive functioning has three domains: conceptual (language, reading, money, time), social (empathy, rules, friendships) and practical (self-care, work, safety).
- IQ is fairly stable from school age, but scores depend on schooling, language, culture, health and testing conditions.
- An IQ around 71–84 is sometimes called borderline intellectual functioning; it is not a disorder but can explain learning difficulties.
IQ ranges in a normal distribution (mean 100, SD 15)
| IQ range | Distance from mean | Approximate share of people | Usual description |
|---|---|---|---|
| 130 and above | +2 SD or more | About 2 % | Very high; often used as one criterion of giftedness |
| 115–129 | +1 to +2 SD | About 14 % | Above average |
| 85–114 | Within 1 SD | About 68 % | Average |
| 70–84 | −1 to −2 SD | About 14 % | Below average; borderline intellectual functioning |
| Below about 70 | −2 SD or more | About 2 % | Consistent with intellectual disability if adaptive functioning is impaired |
Clinical notes
- A child who struggles at school needs a hearing and vision check and a look at language and attention before an IQ result is interpreted.
- An IQ score alone never establishes intellectual disability; support needs follow adaptive functioning.
In the lab
- Full assessment: an individually administered intelligence scale plus a standardized adaptive behavior measure, a developmental history and hearing and vision checks.
- Testing in the person's own language, with norms close to the person's population, protects against misclassification.
Test yourself
Think of the answer, then tap the question to see it.
What are the mean and standard deviation of IQ scores?
Mean 100, standard deviation 15.
Roughly what percentage of people have an IQ between 85 and 115?
About 68 %, those within one standard deviation of the mean.
What three elements define intellectual disability?
Deficits in intellectual functioning, deficits in adaptive functioning, and onset during the developmental period.
How is the severity of intellectual disability graded today?
By adaptive functioning in conceptual, social and practical domains, not by IQ alone.
What is the most common preventable cause of intellectual disability?
Prenatal alcohol exposure (fetal alcohol spectrum disorder).
Connections
Educational content on behavior and the clinical relationship; not a diagnostic or treatment guide and no substitute for assessment by a health professional. If you or someone near you is at risk, seek help now from an emergency service or a mental-health helpline.
Sources: Intellectual disability: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025); Assessment of Specific Learning Disabilities and Intellectual Disabilities (Assessment (PubMed Central), 2023); Intellectual Disability (MSD Manual Professional Version, 2026).
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Representative model, for educational purposes only.