IQ tests take their name because many IQ tests have been scaled based on the testers' assessment of the subject's relative cognitive abilities ("intelligence") as compared to different age groups. IQ tests do not purport to 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. Likewise, IQ tests are generally designed to meaningfully assess learning discrepancies and deficiencies in comparison to relatively homogenous cultural groups.
The principal practical purpose of IQ tests remains the diagnosis and treatment of learning disabilities, and the assessment of individual strengths in educational and employment contexts. Variances between different component tests often indicate a persons performance on some component tests is limited by a neurological or other learning disability, who may be assisted by intensive educational intervention. Substantial test design and research focuses on the ability of a test component to predict later difficulties with reading and learning.
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. That is, significant variation in IQ between adults can be attributed to genetic variation, with the remaining variation attributable to environmental sources that are not shared within families. In the United States, marked variation in IQ occurs within families, with siblings differing on average by 12 points.
IQ scores are designed to correlate with academic success, and have been found, in differing degrees, to correlate to job performance, socioeconomic advancement, and "social pathologies". IQ scores have been taken by many psychologists to be a proxy for intelligence, and a measurable definition of certain types of intellectual ability, but generally not taken to measure all meanings of "intelligence" (e.g., creativity). Recent work has demonstrated links between IQ and health, longevity, and functional literacy.
The average IQ scores for many populations were rising during the 20th century: a phenomenon called the Flynn effect. It is not known whether these changes in scores reflect real changes in intellectual abilities. On average, IQ scores are stable over a person's lifetime, but some individuals undergo large changes. For example, scores can be affected by the presence of learning disabilities (Shaywitz, 1995).