An intelligence quotient (IQ) is a total score derived from a set of standardised tests or subtests designed to assess human intelligence. Originally, IQ was a quotient obtained by dividing a person's estimated mental age, obtained by administering an intelligence test, by the person's chronological age. The resulting fraction was multiplied by 100 to obtain the IQ score. For modern IQ tests, the raw score is transformed to a normal distribution with mean 100 and standard deviation 15. This results in approximately two-thirds of the population scoring between IQ 85 and IQ 115 and about 2 percent each above 130 and below 70.
Scores from intelligence tests are estimates of intelligence. Unlike quantities such as distance or mass, a concrete measure of intelligence cannot be achieved given the abstract nature of the concept. IQ scores have been shown to be associated with factors such as nutrition, parental socioeconomic status, morbidity and mortality, parental social status, and perinatal environment. While the heritability of IQ has been studied for nearly a century, there is still debate over the significance of heritability estimates and the mechanisms of inheritance.
IQ scores have been used for educational placement, assessment of intellectual ability, and evaluating job applicants. In research contexts, they have been studied as predictors of job performance and income. They are also used to study distributions of psychometric intelligence in populations and the correlations between it and other variables. Raw scores on IQ tests for many populations have been rising at an average rate of three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect.
Historically, many proponents of IQ testing have been eugenicists who used pseudoscience to push ideas of racial hierarchy in order to justify segregation and oppose immigration. Such ideas have been rejected by a strong consensus of mainstream science, though fringe figures continue to promote them in pseudo-scholarship and popular culture.
History
Precursors to IQ testing Historically, there were attempts to classify people into intelligence categories by observing their behaviour in daily life. Those other forms of behavioural observation are still important for validating classifications based primarily on IQ test scores. Both intelligence classification by observation of behaviour outside the testing room and classification by IQ testing depend on the definition of "intelligence" used in a particular case and on the reliability and error of estimation in the classification procedure.
The English statistician Francis Galton (1822–1911) made the first attempt at creating a standardised test for rating a person's intelligence. A pioneer of psychometrics and the application of statistical methods to the study of human diversity and the study of inheritance of human traits, he believed that intelligence was largely a product of heredity (by which he did not mean genes, although he did develop several pre-Mendelian theories of particulate inheritance). He hypothesised that there should exist a correlation between intelligence and other observable traits such as reflexes, muscle grip, and head size. He set up the first mental testing centre in the world in 1882 and he published "Inquiries into Human Faculty and Its Development" in 1883, in which he set out his theories. After gathering data on a variety of physical variables, he was unable to show any such correlation, and he eventually abandoned this research.
French psychologist Alfred Binet and psychiatrist Théodore Simon, had more success in 1905, when they published the Binet–Simon Intelligence test, which focused on verbal abilities. It was intended to identify "mental retardation" in school children, but in specific contradistinction to claims made by psychiatrists that these children were "sick" (not "slow") and should therefore be removed from school and cared for in asylums. (This is an open access article, made freely available by Elsevier.) The score on the Binet–Simon scale would reveal the child's mental age. For example, a six-year-old child who passed all the tasks usually passed by six-year-olds—but nothing beyond—would have a mental age that matched his chronological age, 6.0. (Fancher, 1985). Binet and Simon thought that intelligence was multifaceted, but came under the control of practical judgment.
In Binet and Simon's view, there were limitations with the scale and they stressed what they saw as the remarkable diversity of intelligence and the consequent need to study it using qualitative, as opposed to quantitative, measures (White, 2000). American psychologist Henry H. Goddard published a translation of it in 1910. American psychologist Lewis Terman at Stanford University revised the Binet–Simon scale, which resulted in the Stanford revision of the Binet-Simon Intelligence Scale (1916). It became the most popular test in the United States for decades.
The abbreviation "IQ" was coined by the psychologist William Stern for the German term , his term for a scoring method for intelligence tests at University of Breslau he advocated in a 1912 book.
General factor (g)
The many different kinds of IQ tests include a wide variety of item content. Some test items are visual, while many are verbal. Test items vary from being based on abstract-reasoning problems to concentrating on arithmetic, vocabulary, or general knowledge.
The British psychologist Charles Spearman in 1904 made the first formal factor analysis of correlations between the tests. He observed that children's school grades across seemingly unrelated school subjects were positively correlated, and reasoned that these correlations reflected the influence of an underlying general mental ability that entered into performance on all kinds of mental tests. He suggested that all mental performance could be conceptualised in terms of a single general ability factor and a large number of narrow task-specific ability factors. Spearman named it g for "general factor" and labelled the specific factors or abilities for specific tasks s. In any collection of test items that make up an IQ test, the score that best measures g is the composite score that has the highest correlations with all the item scores. Typically, the "g-loaded" composite score of an IQ test battery appears to involve a common strength in abstract reasoning across the test's item content.
United States military selection in World War I During World War I, the Army needed a way to evaluate and assign recruits to appropriate tasks. This led to the development of several mental tests by Robert Yerkes, who worked with major hereditarians of American psychometrics—including Terman, Goddard—to write the test. The testing generated controversy and much public debate in the United States. Nonverbal or "performance" tests were developed for those who could not speak English or were suspected of malingering. Based on Goddard's translation of the Binet–Simon test, the tests had an impact in screening men for officer training:
...the tests did have a strong impact in some areas, particularly in screening men for officer training. At the start of the war, the army and national guard maintained nine thousand officers. By the end, two hundred thousand officers presided, and two- thirds of them had started their careers in training camps where the tests were applied. In some camps, no man scoring below C could be considered for officer training.
In total 1.75 million men were tested, making the results the first mass-produced written tests of intelligence, though considered dubious and non-usable, for reasons including high variability of test implementation throughout different camps and questions testing for familiarity with American culture rather than intelligence. After the war, positive publicity promoted by army psychologists helped to make psychology a respected field. Subsequently, there was an increase in jobs and funding in psychology in the United States. Group intelligence tests were developed and became widely used in schools and industry.
The results of these tests, which at the time reaffirmed contemporary racism and nationalism, are considered controversial and dubious, having rested on certain contested assumptions: that intelligence was heritable, innate, and could be relegated to a single number, the tests were enacted systematically, and test questions actually tested for innate intelligence rather than subsuming environmental factors. The tests also bolstered jingoist narratives opposing the high rates of immigration at the time, which may have influenced the passing of the Immigration Restriction Act of 1924.
L.L. Thurstone argued for a model of intelligence that included seven unrelated factors (verbal comprehension, word fluency, number facility, spatial visualisation, associative memory, perceptual speed, reasoning, and induction). While not widely used, Thurstone's model influenced later theories.
David Wechsler produced the first version of his test in 1939. It gradually became more popular and overtook the Stanford–Binet in the 1960s. It has been revised several times, as is common for IQ tests, to incorporate new research. One explanation is that psychologists and educators wanted more information than the single score from the Binet. Wechsler's ten or more subtests provided this. Another is that the Stanford–Binet test reflected mostly verbal abilities, while the Wechsler test also reflected nonverbal abilities. The Stanford–Binet has also been revised several times and is similar to the Wechsler in several aspects, but the Wechsler continues to be the most popular test in the United States.
IQ testing and the eugenics movement in the United States Eugenics, a set of beliefs and practices aimed at improving the genetic quality of the human population by excluding people and groups judged to be inferior and promoting those judged to be superior, played a significant role in the history and culture of the United States during the Progressive Era, from the late 19th century until US involvement in World War II.
The American eugenics movement was rooted in the biological determinist ideas of the British Scientist Sir Francis Galton. In 1883, Galton first used the word eugenics to describe the biological improvement of human genes and the concept of being "well-born". He believed that differences in a person's ability were acquired primarily through genetics and that eugenics could be implemented through selective breeding in order for the human race to improve in its overall quality, therefore allowing for humans to direct their own evolution.
Henry H. Goddard was a eugenicist. In 1908, he published his own version, The Binet and Simon Test of Intellectual Capacity, and cordially promoted the test. He quickly extended the use of the scale to the public schools (1913), to immigration (Ellis Island, 1914) and to a court of law (1914).
Unlike Galton, who promoted eugenics through selective breeding for positive traits, Goddard went with the US eugenics movement to eliminate "undesirable" traits. Goddard used the term "feeble-minded" to refer to people who did not perform well on the test. He argued that "feeble-mindedness" was caused by heredity, and thus feeble-minded people should be prevented from giving birth, either by institutional isolation or sterilisation surgeries. At first, sterilisation targeted the disabled, but was later extended to poor people. Goddard's intelligence test was endorsed by the eugenicists to push for laws for forced sterilisation. Different states adopted the sterilisation laws at different paces. These laws, whose constitutionality was upheld by the Supreme Court in their 1927 ruling Buck v. Bell, forced over 60,000 people to go through sterilisation in the United States.