What is IQ? A Complete Guide to the Intelligence Quotient

The intelligence quotient — IQ — is one of psychology's most studied and most misunderstood metrics. It is treated by some as a definitive measure of human worth and by others as a debunked relic of nineteenth-century anthropometry. Both extremes are wrong. The truth, as usual, sits in the middle: IQ measures something real, something stable across decades of research, and something narrow enough that it cannot tell you most of what you might want to know about a person.

A short history

The first intelligence test was developed by Alfred Binet in 1905, on commission from the French Ministry of Education. The goal was practical: identify children who needed extra help in school. Binet was clear that his test measured a snapshot of cognitive ability, not innate intellectual capacity, and he warned against treating the score as a verdict.

Lewis Terman of Stanford University adapted Binet's work for American use, creating the Stanford-Binet test in 1916. He coined the formula that gave IQ its name: mental age divided by chronological age, multiplied by 100. A 10-year-old who solved problems typical of a 12-year-old had an IQ of 120.

What modern IQ tests measure

Most current tests draw on what psychologists call the CHC model (Cattell-Horn-Carroll), which divides general intelligence into a hierarchy of abilities:

• Fluid reasoning — solving novel problems without prior knowledge
• Crystallized intelligence — applying learned knowledge
• Working memory — holding and manipulating information mentally
• Processing speed — how quickly you handle simple cognitive tasks
• Visual-spatial reasoning — manipulating shapes and patterns

A full IQ test (Wechsler, Stanford-Binet) measures all of these and produces a composite score plus subscale scores. Online tests typically focus on fluid reasoning — the type of ability least affected by education and culture, and the type that correlates most strongly with general intelligence (g).

The bell curve

IQ scores in the general population follow a normal distribution:

The mean is set to 100 by definition; the standard deviation is 15 points. About 68% of the population scores between 85 and 115; about 95% scores between 70 and 130. The fraction above 145 is roughly 0.13% — fewer than two people in a thousand.

What IQ predicts

Decades of research show IQ is a meaningful — though far from complete — predictor of:

• Academic achievement (correlation ~0.5 with grades)
• Job performance, especially in cognitively demanding roles (correlation 0.3-0.5)
• Income, with diminishing returns above ~120
• Health outcomes and longevity, partly through downstream behaviors

These correlations are statistically robust but explain only a fraction of the variance. Two people with identical IQs can have wildly different lives. Two people with IQs differing by 30 points can end up in similar places. IQ is one variable in a complex equation.

What IQ does not measure

This is the part people skip. IQ tests do not measure:

• Creativity — divergent thinking is largely independent of IQ above ~120
• Emotional intelligence — separate construct, separate tests
• Conscientiousness — the personality trait that predicts life outcomes nearly as well as IQ
• Wisdom and judgment — these depend on experience, not raw ability
• Specific talents — musical, athletic, social skills

A high IQ helps but does not guarantee success in any domain that requires more than abstract reasoning. Conversely, a moderate IQ does not preclude success; many people with IQs in the 100-115 range outperform their high-IQ peers because of better discipline, better social skills, or better luck.

Stability and change

IQ scores are remarkably stable from late childhood onward. A child tested at age 11 and again at age 80 typically scores within 10 points of their original result. This stability is part of what makes IQ a useful research tool — it is not a mood-of-the-day measurement.

But stability is not the same as immutability. The Flynn effect — the observation that average IQ scores rose by about 3 points per decade through most of the twentieth century — proves that population-level scores can shift dramatically over generations, presumably due to better nutrition, education, and abstract reasoning practice. At the individual level, scores can change with brain injury, disease, or sustained cognitive training, though the size of trainable gains is debated.

Cultural fairness

Critics rightly point out that early IQ tests were often biased — culturally specific vocabulary, references to objects and customs unfamiliar outside Western middle-class life. Modern tests have largely eliminated these obvious biases by focusing on abstract patterns (Raven's matrices), spatial reasoning, and digit-span tasks. But subtler biases remain: test-taking experience, motivation, comfort with the testing situation, and exposure to similar puzzles all affect performance.

The reasonable conclusion is that IQ tests measure cognitive ability with reasonable fairness across groups, but the score is always a reflection of ability as expressed in this particular task, on this particular day, not a measurement of some Platonic intelligence essence.

FAQ

Is IQ inherited?

Twin studies suggest 50-80% of variance in adult IQ is heritable, with environmental factors (especially in childhood) explaining the rest. "Heritable" here means "varies with genetics in this population," not "fixed at birth."

Can I improve my IQ?

You can improve your performance on specific tasks through practice, and you can improve broader cognitive function through education, sleep, exercise, and avoiding harm to the brain. Whether any of this raises your "true" g-factor is debated. Most researchers think the answer is "a little, maybe."

Are online IQ tests accurate?

A well-designed online test can give you a reasonable estimate (within 5-10 points) of your IQ in the abilities it measures. It is not equivalent to a full clinical assessment — those take 1-2 hours, are administered by a psychologist, and measure subscale scores that an online quiz cannot.

What's the difference between IQ and g?

g (general intelligence) is the latent factor that explains why people who do well on one cognitive task tend to do well on others. IQ is the score you get on a specific test designed to estimate g. The two are closely related but not identical.

References

• Deary, I. J. (2012). Intelligence: A Very Short Introduction. Oxford University Press.
• Mackintosh, N. J. (2011). IQ and Human Intelligence. Oxford University Press.
• Flynn, J. R. (2007). What Is Intelligence? Cambridge University Press.
• Plomin, R., & von Stumm, S. (2018). The new genetics of intelligence. Nature Reviews Genetics, 19(3), 148-159.