Definition
IQ — intelligence quotient — is a standardised score derived from a battery of cognitive tests, scaled so the population mean is 100 and the standard deviation is 15. The score summarises how a test-taker performed on the battery relative to a representative norming sample of the same age. Modern IQ tests like the Wechsler scales and the Stanford-Binet combine subtests that probe verbal comprehension, perceptual reasoning, working memory, and processing speed, yielding both a full-scale score and a profile across these domains. The history of IQ runs from Binet's early-twentieth-century work on identifying schoolchildren who needed extra support, through its often-troubling deployment in immigration, military, and eugenic policy, to its contemporary clinical and research use.
Why it matters
How it works
A modern IQ test is administered individually by a trained psychologist using a standardised protocol. The test-taker works through subtests probing different cognitive domains; raw scores are converted to scaled scores using age-based norms, and the scaled scores combine into composite indices and a full-scale IQ. The instruments are revised periodically — partly to keep norms current as performance shifts in the population, partly to repair items that have proven biased against particular groups, and partly to incorporate updated cognitive theory. Reliability is high; validity is more nuanced, because the construct intelligence is itself partly a social-scientific decision about what to measure.
What IQ scores actually predict has been studied extensively. They correlate moderately with school achievement, occupational status, income, and some health outcomes, though never strongly enough to determine an individual's path. Twin and adoption studies find substantial heritability that nonetheless rises from childhood through adulthood — heritability estimates are not fixed properties but functions of the environment a population shares. The Flynn effect, secular score gains of roughly three points per decade for much of the twentieth century in many countries, indicates large environmental contributions that any genetic component must coexist with. Contemporary practice avoids treating IQ as a verdict on a person and instead uses it alongside profiles of fluid versus crystallized abilities, executive function, and adaptive behaviour to inform clinical and educational decisions.