What Is IQ? Meaning, Measurement and the Science
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Quick answer
IQ — short for 'intelligence quotient' — is a score from a standardized test designed to measure reasoning ability relative to other people. It is not a measure of knowledge, wisdom or worth. This guide explains what IQ actually captures, where the idea came from, and what a score can and cannot tell you.
What IQ actually measures
An IQ test samples your performance across reasoning tasks — pattern recognition, logical deduction, working memory, spatial manipulation, numerical and verbal reasoning — and converts it into a single standardized score. Because the scale is normed, your score is always a comparison to other people, not an absolute quantity of intelligence.
Most well-built tests load onto a 'general factor' of intelligence, often written as g. The idea, dating to Charles Spearman, is that people who do well on one kind of mental task tend to do better on others too — and g is the shared variance that all the subtests have in common.
How the score is built
Scores are standardized to a mean of 100 and a standard deviation of 15. That means 100 is exactly average, and the further from 100 you go, the rarer the score.
| IQ Range | Classification | % of People | What it means |
|---|---|---|---|
| ≤69 | Extremely Low | ~2.2% | Well below average. On clinical tests this range may warrant professional assessment. |
| 70–79 | Borderline | ~6.7% | Below average reasoning on this scale. |
| 80–89 | Low Average | ~16.1% | Slightly below the population average. |
| 90–109 | Average | ~50% | The middle of the distribution — where most people score. |
| 110–119 | High Average | ~16.1% | Above average reasoning ability. |
| 120–129 | Superior | ~6.7% | Notably above average — roughly the top 10%. |
| 130–144 | Gifted | ~2.1% | The conventional 'gifted' threshold (130) and above — top ~2%. Mensa qualifies here. |
| 145+ | Highly Gifted / Genius | ~0.1% | Exceptionally rare — the far right tail of the distribution. |
The age-norming step is crucial: your raw performance is compared to a representative sample of people your own age, then converted to the 100-centered scale.
Where IQ came from
In 1905, French psychologist Alfred Binet built the first practical intelligence scale to identify schoolchildren who needed extra help. William Stern later proposed dividing 'mental age' by chronological age to get a quotient — the original 'IQ'. Modern tests dropped the literal quotient in favor of the standardized 'deviation IQ' we use today.
Binet himself warned against treating his scale as a fixed measure of a single, permanent capacity — a caution often ignored in the century since.— On the history of intelligence testing
What IQ does not measure
- Creativity and original thinking.
- Emotional intelligence (EQ) — reading and managing emotions.
- Practical 'street smarts', skills and expertise.
- Motivation, conscientiousness and character — which often predict life outcomes better than IQ.
A number, not a verdict
IQ captures a real but narrow slice of human ability. A score is a snapshot under specific conditions — not a ceiling on what you can learn or achieve.
Frequently asked questions
What does IQ stand for?+
IQ stands for 'intelligence quotient'. The term comes from an early method of calculating the score by dividing a person's mental age by their chronological age and multiplying by 100.
What is a normal IQ?+
A normal or average IQ is between 90 and 109. By design, the average across the population is exactly 100, and about 68% of people score between 85 and 115.
Does a high IQ mean you'll be successful?+
Not necessarily. IQ correlates modestly with academic and job performance on average, but motivation, conscientiousness, opportunity and emotional skills are often stronger predictors of real-world success.
Is IQ genetic or learned?+
Both. Research suggests intelligence is influenced by genetics and by environment — education, nutrition, health and stimulation all play a role, especially in childhood.
Sources
This guide draws on standard psychometric references and peer-reviewed research:
- 1.Spearman, C. (1904). “‘General Intelligence,’ Objectively Determined and Measured.” American Journal of Psychology, 15.
- 2.Binet, A., & Simon, T. (1905). New methods for the diagnosis of the intellectual level of subnormals. L’Année Psychologique, 12.
- 3.Deary, I. J. (2020). Intelligence: A Very Short Introduction (2nd ed.). Oxford University Press.
- 4.Neisser, U., et al. (1996). “Intelligence: Knowns and Unknowns.” American Psychologist, 51(2). APA.
- 5.American Psychological Association (APA)
Sources are provided for further reading. Organization links point to official sites; academic works are cited in full. See our research standards and editorial team.
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