IQ & Intelligence Glossary
39 key terms used in IQ and intelligence testing (IQ, deviation IQ, standard deviation, percentile, g factor, fluid intelligence, CHC theory, WAIS, WISC, item response theory, Flynn effect, giftedness and more), each defined in one sentence with a link to an in-depth article.
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- IQ (intelligence quotient)
- IQ is a score that expresses performance on an intelligence test relative to people of the same age; on today's major tests it is scaled to a mean of 100 and a standard deviation of 15. Learn more →
- Deviation IQ (DIQ)
- A deviation IQ expresses how far a score lies from the mean of same-age peers in standard-deviation units, rescaled to a mean of 100 and SD of 15; modern tests such as the Wechsler scales use it. Learn more →
- Ratio IQ
- Ratio IQ is the older formula of mental age divided by chronological age times 100; it breaks down for adults, so most tests replaced it with the deviation IQ. Learn more →
- Standard deviation (SD)
- Standard deviation measures how widely scores spread around the mean; on the IQ scale one SD is 15 points, so about 68% of people score 85–115 and about 95% score 70–130. Learn more →
- Percentile rank
- A percentile rank tells you what percentage of same-age peers score below you; IQ 115 is about the 84th percentile and IQ 130 about the 98th. Learn more →
- Hensachi (T-score)
- Hensachi is the Japanese name for a score scaled to a mean of 50 and SD of 10; it maps to IQ as 50 + (IQ − 100) ÷ 15 × 10, so IQ 130 equals a hensachi of 70. Learn more →
- g factor (general intelligence)
- The g factor is the common ability inferred from the fact that performance on very different mental tasks is positively correlated; Charles Spearman proposed it in 1904, and IQ is considered a good index of g. Learn more →
- Fluid intelligence (Gf)
- Fluid intelligence is the ability to reason through novel problems and find rules without relying on prior knowledge; it is measured by tasks such as figural matrix reasoning and tends to decline with age in adulthood. Learn more →
- Crystallized intelligence (Gc)
- Crystallized intelligence is the store of vocabulary, knowledge and skills acquired through education and experience; it is measured with vocabulary and general-knowledge items and tends to hold up or grow into old age. Learn more →
- CHC theory
- CHC (Cattell–Horn–Carroll) theory is a hierarchical model of intelligence with general ability g at the top and broad abilities such as fluid reasoning, crystallized knowledge and processing speed beneath it; it underpins the design of modern intelligence tests. Learn more →
- Working memory
- Working memory is the capacity to hold information in mind for a short time while manipulating it; it supports mental arithmetic and following conversations and is a separate index on the WISC and WAIS. Learn more →
- Processing speed
- Processing speed is the ability to carry out simple visual judgments and tasks quickly and accurately; the WISC and WAIS measure it with symbol-coding and search tasks. Learn more →
- Matrix reasoning
- Matrix reasoning is a task in which you infer the rules governing a grid of figures and choose the one that fills the blank; it is the standard language-free format for measuring fluid intelligence. Learn more →
- Raven's Progressive Matrices
- Raven's Progressive Matrices is a nonverbal intelligence test published by John C. Raven in 1938, made of matrix-reasoning items of increasing difficulty and widely used because it depends little on language or culture. Learn more →
- WAIS (Wechsler Adult Intelligence Scale)
- The WAIS is the leading clinician-administered intelligence test for ages 16 and up, reporting a full-scale IQ plus index scores for verbal comprehension, perceptual reasoning, working memory and processing speed. Learn more →
- WISC (Wechsler Intelligence Scale for Children)
- The WISC is the Wechsler individual intelligence test for children; the current WISC-V covers ages 6:0–16:11 in the US edition (5:0–16:11 in Japan) and reports a full-scale IQ and five primary index scores. Learn more →
- Item response theory (IRT)
- Item response theory is a statistical model that estimates a test taker's ability θ while accounting for each item's difficulty (and discrimination), so the score depends on which items you answered correctly rather than just how many. Learn more →
- Flynn effect
- The Flynn effect is the rise in average intelligence-test scores of roughly 3 points per decade seen in many countries during the 20th century, named after researcher James Flynn. Learn more →
- Heritability
- Heritability is the share of individual differences within a population that is explained by genetic differences; for adult intelligence it is estimated at roughly 50–80%, but it does not say how much of one person's intelligence is genetic. Learn more →
- Borderline intellectual functioning
- Borderline intellectual functioning refers to an IQ of roughly 70–84, below average but above the threshold for intellectual disability; on the normal curve it covers about 14% of the population. Learn more →
- Intellectual disability (intellectual developmental disorder)
- Intellectual disability is a condition in which limitations in intellectual functioning (an IQ of roughly 70 or below) appear during the developmental period together with difficulties in adapting to everyday life that require support; it is judged from adaptive behavior as well as the IQ score. Learn more →
- Gifted
- Gifted describes people with markedly higher intellectual ability or talent than same-age peers; IQ 130 or above (about the top 2%) is a common benchmark, but there is no single agreed definition. Learn more →
- Twice-exceptional (2e)
- Twice-exceptional (2e) describes people who combine high intellectual ability with a developmental condition such as ADHD, a learning disability or autism; because strengths and difficulties can mask each other, it is easily overlooked. Learn more →
- Normal distribution
- The normal distribution is a symmetric, bell-shaped probability distribution centered on the mean; IQ is expressed on a normal distribution with a mean of 100 and a standard deviation of 15, so about 68% of people fall between 85 and 115 and about 95% between 70 and 130. Learn more →
- Standardization (norming)
- Standardization is the procedure of giving an intelligence test to a sample chosen to match the population, examining the score distribution, and building conversion norms so that each age group has a mean of 100 and a standard deviation of 15. Learn more →
- Measurement error (confidence interval)
- Measurement error is the amount a score varies when the same person takes the same test, because of condition, concentration and the particular items; an IQ should therefore be read as a range (a confidence interval) rather than a single point. Learn more →
- Full-scale IQ (FSIQ)
- Full-scale IQ (FSIQ) is the overall IQ that Wechsler tests compute by combining several subtests; when the index scores differ widely, the full-scale IQ alone describes the person poorly. Learn more →
- Mental age
- Mental age is the Binet-style measure of which age group's average performance a person's test result matches; it is used to calculate ratio IQ (mental age ÷ chronological age × 100). Learn more →
- Tanaka-Binet Intelligence Scale
- The Tanaka-Binet Intelligence Scale is the Japanese version of the Binet test, first published by psychologist Kan'ichi Tanaka in 1947; it covers age 2 through adulthood and is the individual test most often used in Japan for ryoiku techo (intellectual disability certificate) assessments. Learn more →
- Mensa
- Mensa is an international high-IQ society founded in England in 1946; membership is open to people who score in the top 2% of the population on a standardized intelligence test. Learn more →
- Spatial ability
- Spatial ability is the capacity to grasp the shape, position and orientation of objects in the mind and to rotate or combine them; it is measured with mental-rotation and paper-folding style items. Learn more →
- Cognitive and non-cognitive abilities
- Cognitive ability is the umbrella term for the capacities used to perceive, understand, remember, reason and decide; IQ is a standardized index of part of it and is distinguished from non-cognitive abilities such as motivation, self-control and cooperativeness. Learn more →
- EQ (emotional intelligence)
- EQ, or emotional intelligence, is the ability to recognize, understand and regulate your own and other people's emotions and to use them in action; it measures something different from IQ and has no single agreed scale. Learn more →
- Theory of multiple intelligences
- The theory of multiple intelligences, proposed by psychologist Howard Gardner in 1983, treats intelligence as several independent domains such as linguistic, logical-mathematical, spatial and musical; it differs from mainstream psychometrics, which emphasizes the g factor. Learn more →
- Metacognition
- Metacognition is the process of treating your own thinking and understanding as an object, monitoring it and adjusting it as needed; the concept was proposed by psychologist John H. Flavell in 1979. Learn more →
- Dunning–Kruger effect
- The Dunning–Kruger effect is the tendency for low performers to overestimate their own performance while high performers slightly underestimate theirs; it was reported by psychologists Kruger and Dunning in 1999. Learn more →
- Savant syndrome
- Savant syndrome is a condition in which a person with a developmental disability or brain injury shows outstanding ability in one specific area such as memory, calculation, music or art. Learn more →
- Neurodiversity
- Neurodiversity is the view that differences in brain and nervous-system traits, such as those seen in ADHD and autism, are part of human diversity rather than defects; the term was popularized by sociologist Judy Singer in the late 1990s. Learn more →
- Cognitive reserve
- Cognitive reserve is the concept that explains why people with the same amount of brain pathology differ in symptoms; it is thought to be built through education and intellectual and social activity and to delay the point at which symptoms appear. Learn more →
All IQ figures assume a normal distribution with a mean of 100 and a standard deviation of 15. Definitions follow standard psychometric usage and were compiled by the BrainRank editorial team.