Intelligence quotient

Lore & Background

An intelligence quotient (IQ) is a total score derived from a set of standardized tests or subtests designed to assess human intelligence. Originally, IQ was calculated by dividing a person's estimated mental age—obtained from an intelligence test—by their chronological age, then multiplying by 100. Modern IQ tests instead transform raw scores to a normal distribution with a mean of 100 and a standard deviation of 15, meaning about two-thirds of the population scores between 85 and 115, and roughly 2 percent score above 130 or below 70. Scores are estimates of intelligence, not concrete measures like distance or mass, due to the abstract nature of the concept. IQ scores correlate with factors such as nutrition, parental socioeconomic status, morbidity, mortality, and perinatal environment. Although heritability of IQ has been studied for nearly a century, debate continues over the significance of heritability estimates and inheritance mechanisms. IQ tests are used for educational placement, intellectual assessment, and job applicant evaluation, and in research as predictors of job performance and income. Raw scores have risen an average of three points per decade since the early 1900s, known as the Flynn effect. Historically, IQ testing was misused by eugenicists to promote racial hierarchy and justify segregation, but such ideas are rejected by mainstream science. The first standardized test was attempted by Francis Galton, who hypothesized correlations with physical traits like reflexes and head size but found none. Alfred Binet and Théodore Simon succeeded in 1905 with a verbal-focused test to identify intellectual disability in children, revealing mental age. The abbreviation "IQ" was coined by William Stern. Charles Spearman’s factor analysis of test correlations led to the concept of a general mental ability, or g, underlying all mental performance.

Reader's Guide

The concept of IQ has had significant impact on education, military selection, and employment. During World War I, the U.S. Army used mental tests developed by Robert Yerkes, Terman, and Goddard to evaluate and assign recruits, leading to the first mass-produced written tests of intelligence. These tests generated controversy and public debate, and their results are considered dubious and controversial, having rested on contested assumptions about heritability and innate intelligence. Historically, many proponents of IQ testing have been eugenicists who used pseudoscience to push ideas of racial hierarchy, but such ideas have been rejected by a strong consensus of mainstream science. The heritability of IQ has been studied for nearly a century, with ongoing debate over the significance of heritability estimates and mechanisms of inheritance. The Flynn effect shows that raw scores on IQ tests have been rising at an average rate of three IQ points per decade since the early 20th century.

Did You Know?

• The Binet–Simon Intelligence test was intended to identify 'mental retardation' in school children, in contradistinction to claims that these children were 'sick' and should be removed to asylums.
• Raw scores on IQ tests have been rising at an average rate of three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect.