IQ – intelligence quotient – is a number derived from a set of standardized tests developed to measure a person’s cognitive abilities (“intelligence” ) in relation to their age group. An IQ test does not measure intelligence the way a ruler measures height (absolutely), but rather the way a race measures speed (relatively); IQ is described as a “quotient” because, originally, it represented the ratio between a person’s “mental age” and actual chronological age.
For people living in the prevailing conditions of the developed world, IQ is highly heritable, and by adulthood the influence of family environment on IQ is undetectable. IQ test scores are correlated with measures of brain structure and function, as well as performance on simple tasks that anyone can complete within a few seconds.
IQ is correlated with academic success, job performance, socioeconomic advancement, and “social pathologies”. It is taken by psychologists to be an excellent proxy for intelligence, and possibly the best measurable definition of intellectual ability, but generally not taken to represent intelligence perfectly. Recent work has demonstrated links between IQ and health, longevity, and functional literacy.