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Intelligence quotient

April 11, 2023

"IQ" redirects here. For other uses, see IQ (disambiguation).

An intelligence quotient (IQ) is a total score derived from a set of standardised tests or subtests designed to assess human intelligence.[1] The abbreviation "IQ" was coined by the psychologist William Stern for the German term Intelligenzquotient, his term for a scoring method for intelligence tests at University of Breslau he advocated in a 1912 book.[2]

Intelligence quotient
One kind of IQ test item, modelled after items in the Raven's Progressive Matrices test
ICD-10-PCSZ01.8
ICD-9-CM94.01

Historically, IQ was a score obtained by dividing a person's mental age score, obtained by administering an intelligence test, by the person's chronological age, both expressed in terms of years and months. The resulting fraction (quotient) was multiplied by 100 to obtain the IQ score.[3] For modern IQ tests, the raw score is transformed to a normal distribution with mean 100 and standard deviation 15.[4] This results in approximately two-thirds of the population scoring between IQ 85 and IQ 115 and about 2.5 percent each above 130 and below 70.[5][6]

Scores from intelligence tests are estimates of intelligence. Unlike, for example, distance and mass, a concrete measure of intelligence cannot be achieved given the abstract nature of the concept of "intelligence".[7] IQ scores have been shown to be associated with such factors as nutrition,[8][9][10] parental socioeconomic status,[11][12] morbidity and mortality,[13][14] parental social status,[15] and perinatal environment.[16] While the heritability of IQ has been investigated for nearly a century, there is still debate about the significance of heritability estimates[17][18] and the mechanisms of inheritance.[19]

IQ scores are used for educational placement, assessment of intellectual disability, and evaluating job applicants. In research contexts, they have been studied as predictors of job performance[20] and income.[21] 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 that scales to three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect. Investigation of different patterns of increases in subtest scores can also inform current research on human intelligence.

History

See also: History of the race and intelligence controversy

Precursors to IQ testing

Historically, even before IQ tests were devised, there were attempts to classify people into intelligence categories by observing their behavior in daily life.[22][23] Those other forms of behavioral observation are still important for validating classifications based primarily on IQ test scores. Both intelligence classification by observation of behavior 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 standardized 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).[24][25][26] He hypothesized that there should exist a correlation between intelligence and other observable traits such as reflexes, muscle grip, and head size.[27] He set up the first mental testing center 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.[28][29]

 
Psychologist Alfred Binet, co-developer of the Stanford–Binet test

French psychologist Alfred Binet, together with Victor Henri and Théodore Simon, had more success in 1905, when they published the Binet–Simon test, which focused on verbal abilities. It was intended to identify "mental retardation" in school children,[30] 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.[31] 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 thought that intelligence was multifaceted, but came under the control of practical judgment.

In Binet's view, there were limitations with the scale and he stressed what he saw as the remarkable diversity of intelligence and the subsequent 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–Binet Intelligence Scales (1916). It became the most popular test in the United States for decades.[30][32][33][34]

General factor (g)

Main article: g factor

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 conceptualized 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 labeled the specific factors or abilities for specific tasks s.[35] 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.[citation needed]

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.[36] 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.[30] 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.[36]

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.[36] After the war, positive publicity promoted by army psychologists helped to make psychology a respected field.[37] Subsequently, there was an increase in jobs and funding in psychology in the United States.[38] Group intelligence tests were developed and became widely used in schools and industry.[39]

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.[36] The tests also allowed for the bolstering of jingoist narratives in the context of increased immigration, which may have influenced the passing of the Immigration Restriction Act of 1924.[36]

L.L. Thurstone argued for a model of intelligence that included seven unrelated factors (verbal comprehension, word fluency, number facility, spatial visualization, associative memory, perceptual speed, reasoning, and induction). While not widely used, Thurstone's model influenced later theories.[30]

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 now similar to the Wechsler in several aspects, but the Wechsler continues to be the most popular test in the United States.[30]

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,[40][41][42] 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.[43][44]

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".[45][46] 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.[47]

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).[48]

Unlike Galton, who promoted eugenics through selective breeding for positive traits, Goddard went with the US eugenics movement to eliminate "undesirable" traits.[49] 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 sterilization surgeries.[48] At first, sterilization 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 sterilization. Different states adopted the sterilization 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 sterilization in the United States.[50]

California's sterilization program was so effective that the Nazis turned to the government for advice on how to prevent the birth of the "unfit".[51] While the US eugenics movement lost much of its momentum in the 1940s in view of the horrors of Nazi Germany, advocates of eugenics (including Nazi geneticist Otmar Freiherr von Verschuer) continued to work and promote their ideas in the United States.[51] In later decades, some eugenic principles have made a resurgence as a voluntary means of selective reproduction, with some calling them "new eugenics".[52] As it becomes possible to test for and correlate genes with IQ (and its proxies),[53] ethicists and embryonic genetic testing companies are attempting to understand the ways in which the technology can be ethically deployed.[54]

Cattell–Horn–Carroll theory

Main article: Cattell–Horn–Carroll theory
 
Psychologist Raymond Cattell defined fluid and crystallized intelligence and authored the Cattell Culture Fair III IQ test.

Raymond Cattell (1941) proposed two types of cognitive abilities in a revision of Spearman's concept of general intelligence. Fluid intelligence (Gf) was hypothesized as the ability to solve novel problems by using reasoning, and crystallized intelligence (Gc) was hypothesized as a knowledge-based ability that was very dependent on education and experience. In addition, fluid intelligence was hypothesized to decline with age, while crystallized intelligence was largely resistant to the effects of aging. The theory was almost forgotten, but was revived by his student John L. Horn (1966) who later argued Gf and Gc were only two among several factors, and who eventually identified nine or ten broad abilities. The theory continued to be called Gf-Gc theory.[30]

John B. Carroll (1993), after a comprehensive reanalysis of earlier data, proposed the three stratum theory, which is a hierarchical model with three levels. The bottom stratum consists of narrow abilities that are highly specialized (e.g., induction, spelling ability). The second stratum consists of broad abilities. Carroll identified eight second-stratum abilities. Carroll accepted Spearman's concept of general intelligence, for the most part, as a representation of the uppermost, third stratum.[55][56]

In 1999, a merging of the Gf-Gc theory of Cattell and Horn with Carroll's Three-Stratum theory has led to the Cattell–Horn–Carroll theory (CHC Theory), with g as the top of the hierarchy, ten broad abilities below, and further subdivided into seventy narrow abilities on the third stratum. CHC Theory has greatly influenced many of the current broad IQ tests.[30]

Modern tests do not necessarily measure all of these broad abilities. For example, quantitative knowledge and reading & writing ability may be seen as measures of school achievement and not IQ.[30] Decision speed may be difficult to measure without special equipment. g was earlier often subdivided into only Gf and Gc, which were thought to correspond to the nonverbal or performance subtests and verbal subtests in earlier versions of the popular Wechsler IQ test. More recent research has shown the situation to be more complex.[30] Modern comprehensive IQ tests do not stop at reporting a single IQ score. Although they still give an overall score, they now also give scores for many of these more restricted abilities, identifying particular strengths and weaknesses of an individual.[30]

Other theories

An alternative to standard IQ tests, meant to test the proximal development of children, originated in the writings of psychologist Lev Vygotsky (1896–1934) during his last two years of his life.[57][58] According to Vygotsky, the maximum level of complexity and difficulty of problems that a child is capable to solve under some guidance indicates their level of potential development. The difference between this level of potential and the lower level of unassisted performance indicates the child's zone of proximal development.[59] Combination of the two indexes—the level of actual and the zone of the proximal development—according to Vygotsky, provides a significantly more informative indicator of psychological development than the assessment of the level of actual development alone.[60][61] His ideas on the zone of development were later developed in a number of psychological and educational theories and practices, most notably under the banner of dynamic assessment, which seeks to measure developmental potential[62][63][64] (for instance, in the work of Reuven Feuerstein and his associates,[65] who has criticized standard IQ testing for its putative assumption or acceptance of "fixed and immutable" characteristics of intelligence or cognitive functioning). Dynamic assessment has been further elaborated in the work of Ann Brown, and John D. Bransford and in theories of multiple intelligences authored by Howard Gardner and Robert Sternberg.[66][67]

J.P. Guilford's Structure of Intellect (1967) model of intelligence used three dimensions, which, when combined, yielded a total of 120 types of intelligence. It was popular in the 1970s and early 1980s, but faded owing to both practical problems and theoretical criticisms.[30]

Alexander Luria's earlier work on neuropsychological processes led to the PASS theory (1997). It argued that only looking at one general factor was inadequate for researchers and clinicians who worked with learning disabilities, attention disorders, intellectual disability, and interventions for such disabilities. The PASS model covers four kinds of processes (planning process, attention/arousal process, simultaneous processing, and successive processing). The planning processes involve decision making, problem solving, and performing activities and require goal setting and self-monitoring.

The attention/arousal process involves selectively attending to a particular stimulus, ignoring distractions, and maintaining vigilance. Simultaneous processing involves the integration of stimuli into a group and requires the observation of relationships. Successive processing involves the integration of stimuli into serial order. The planning and attention/arousal components comes from structures located in the frontal lobe, and the simultaneous and successive processes come from structures located in the posterior region of the cortex.[68][69][70] It has influenced some recent IQ tests, and been seen as a complement to the Cattell–Horn–Carroll theory described above.[30]

Current tests

 
Normalized IQ distribution with mean 100 and standard deviation 15

There are a variety of individually administered IQ tests in use in the English-speaking world.[71][72] The most commonly used individual IQ test series is the Wechsler Adult Intelligence Scale (WAIS) for adults and the Wechsler Intelligence Scale for Children (WISC) for school-age test-takers. Other commonly used individual IQ tests (some of which do not label their standard scores as "IQ" scores) include the current versions of the Stanford–Binet Intelligence Scales, Woodcock–Johnson Tests of Cognitive Abilities, the Kaufman Assessment Battery for Children, the Cognitive Assessment System, and the Differential Ability Scales.

IQ tests also include:

  1. Raven's Progressive Matrices
  2. Cattell Culture Fair III
  3. Reynolds Intellectual Assessment Scales
  4. Thurstone's Primary Mental Abilities[73][74]
  5. Kaufman Brief Intelligence Test[75]
  6. Multidimensional Aptitude Battery II
  7. Das–Naglieri cognitive assessment system
  8. Naglieri Nonverbal Ability Test
  9. Wide Range Intelligence Test
  10. AJT Cognitive Test - Indonesia

IQ scales are ordinally scaled.[76][77][78][79][80] The raw score of the norming sample is usually (rank order) transformed to a normal distribution with mean 100 and standard deviation 15.[4] While one standard deviation is 15 points, and two SDs are 30 points, and so on, this does not imply that mental ability is linearly related to IQ, such that IQ 50 would mean half the cognitive ability of IQ 100. In particular, IQ points are not percentage points.

Reliability and validity

IQ scores can differ to some degree for the same person on different IQ tests, so a person does not always belong to the same IQ score range each time the person is tested. (IQ score table data and pupil pseudonyms adapted from description of KABC-II norming study cited in Kaufman (2009).[81][82])
Pupil KABC-II WISC-III WJ-III
A 90 95 111
B 125 110 105
C 100 93 101
D 116 127 118
E 93 105 93
F 106 105 105
G 95 100 90
H 112 113 103
I 104 96 97
J 101 99 86
K 81 78 75
L 116 124 102

Reliability

Psychometricians generally regard IQ tests as having high statistical reliability.[15][83] Reliability represents the measurement consistency of a test.[84] A reliable test produces similar scores upon repetition.[84] On aggregate, IQ tests exhibit high reliability, although test-takers may have varying scores when taking the same test on differing occasions, and may have varying scores when taking different IQ tests at the same age. Like all statistical quantities, any particular estimate of IQ has an associated standard error that measures uncertainty about the estimate. For modern tests, the confidence interval can be approximately 10 points and reported standard error of measurement can be as low as about three points.[85] Reported standard error may be an underestimate, as it does not account for all sources of error.[86]

Outside influences such as low motivation or high anxiety can occasionally lower a person's IQ test score.[84] For individuals with very low scores, the 95% confidence interval may be greater than 40 points, potentially complicating the accuracy of diagnoses of intellectual disability.[87] By the same token, high IQ scores are also significantly less reliable than those near to the population median.[88] Reports of IQ scores much higher than 160 are considered dubious.[89]

Validity as a measure of intelligence

Reliability and validity are very different concepts. While reliability reflects reproducibility, validity refers to whether the test measures what it purports to measure.[84] While IQ tests are generally considered to measure some forms of intelligence, they may fail to serve as an accurate measure of broader definitions of human intelligence inclusive of, for example, creativity and social intelligence. For this reason, psychologist Wayne Weiten argues that their construct validity must be carefully qualified, and not be overstated.[84] According to Weiten, "IQ tests are valid measures of the kind of intelligence necessary to do well in academic work. But if the purpose is to assess intelligence in a broader sense, the validity of IQ tests is questionable."[84]

Some scientists have disputed the value of IQ as a measure of intelligence altogether. In The Mismeasure of Man (1981, expanded edition 1996), evolutionary biologist Stephen Jay Gould compared IQ testing with the now-discredited practice of determining intelligence via craniometry, arguing that both are based on the fallacy of reification, "our tendency to convert abstract concepts into entities".[90] Gould's argument sparked a great deal of debate,[91][92] and the book is listed as one of Discover Magazine's "25 Greatest Science Books of All Time".[93]

Along these same lines, critics such as Keith Stanovich do not dispute the capacity of IQ test scores to predict some kinds of achievement, but argue that basing a concept of intelligence on IQ test scores alone neglects other important aspects of mental ability.[15][94] Robert Sternberg, another significant critic of IQ as the main measure of human cognitive abilities, argued that reducing the concept of intelligence to the measure of g does not fully account for the different skills and knowledge types that produce success in human society.[95]

Despite these objections, clinical psychologists generally regard IQ scores as having sufficient statistical validity for many clinical purposes.[specify][30][96]

Test bias or differential item functioning

Differential item functioning (DIF), sometimes referred to as measurement bias, is a phenomenon when participants from different groups (e.g. gender, race, disability) with the same latent abilities give different answers to specific questions on the same IQ test.[97] DIF analysis measures such specific items on a test alongside measuring participants' latent abilities on other similar questions. A consistent different group response to a specific question among similar types of questions can indicate an effect of DIF. It does not count as differential item functioning if both groups have an equally valid chance of giving different responses to the same questions. Such bias can be a result of culture, educational level and other factors that are independent of group traits. DIF is only considered if test-takers from different groups with the same underlying latent ability level have a different chance of giving specific responses.[98] Such questions are usually removed in order to make the test equally fair for both groups. Common techniques for analyzing DIF are item response theory (IRT) based methods, Mantel-Haenszel, and logistic regression.[98]

A 2005 study found that "differential validity in prediction suggests that the WAIS-R test may contain cultural influences that reduce the validity of the WAIS-R as a measure of cognitive ability for Mexican American students,"[99] indicating a weaker positive correlation relative to sampled white students. Other recent studies have questioned the culture-fairness of IQ tests when used in South Africa.[100][101] Standard intelligence tests, such as the Stanford–Binet, are often inappropriate for autistic children; the alternative of using developmental or adaptive skills measures are relatively poor measures of intelligence in autistic children, and may have resulted in incorrect claims that a majority of autistic children are of low intelligence.[102]

Flynn effect

Main article: Flynn effect

Since the early 20th century, raw scores on IQ tests have increased in most parts of the world.[103][104][105] When a new version of an IQ test is normed, the standard scoring is set so performance at the population median results in a score of IQ 100. The phenomenon of rising raw score performance means if test-takers are scored by a constant standard scoring rule, IQ test scores have been rising at an average rate of around three IQ points per decade. This phenomenon was named the Flynn effect in the book The Bell Curve after James R. Flynn, the author who did the most to bring this phenomenon to the attention of psychologists.[106][107]

Researchers have been exploring the issue of whether the Flynn effect is equally strong on performance of all kinds of IQ test items, whether the effect may have ended in some developed nations, whether there are social subgroup differences in the effect, and what possible causes of the effect might be.[108] A 2011 textbook, IQ and Human Intelligence, by N. J. Mackintosh, noted the Flynn effect demolishes the fears that IQ would be decreased. He also asks whether it represents a real increase in intelligence beyond IQ scores.[109] A 2011 psychology textbook, lead authored by Harvard Psychologist Professor Daniel Schacter, noted that humans' inherited intelligence could be going down while acquired intelligence goes up.[110]

Research has suggested that the Flynn effect has slowed or reversed course in some Western countries beginning in the late 20th century. The phenomenon has been termed the negative Flynn effect.[111] A study of Norwegian military conscripts' test records found that IQ scores have been falling for generations born after the year 1975, and that the underlying cause of both initial increasing and subsequent falling trends appears to be environmental rather than genetic.[111]

Age

IQ can change to some degree over the course of childhood.[112] In one longitudinal study, the mean IQ scores of tests at ages 17 and 18 were correlated at r=0.86 with the mean scores of tests at ages five, six, and seven and at r=0.96[further explanation needed] with the mean scores of tests at ages 11, 12, and 13.[15]

For decades, practitioners' handbooks and textbooks on IQ testing have reported IQ declines with age after the beginning of adulthood. However, later researchers pointed out this phenomenon is related to the Flynn effect and is in part a cohort effect rather than a true aging effect. A variety of studies of IQ and aging have been conducted since the norming of the first Wechsler Intelligence Scale drew attention to IQ differences in different age groups of adults. The current consensus is that fluid intelligence generally declines with age after early adulthood, while crystallized intelligence remains intact. Both cohort effects (the birth year of the test-takers) and practice effects (test-takers taking the same form of IQ test more than once) must be controlled to gain accurate data.[inconsistent] It is unclear whether any lifestyle intervention can preserve fluid intelligence into older ages.[113]

The exact peak age of fluid intelligence or crystallized intelligence remains elusive. Cross-sectional studies usually show that especially fluid intelligence peaks at a relatively young age (often in the early adulthood) while longitudinal data mostly show that intelligence is stable until mid-adulthood or later. Subsequently, intelligence seems to decline slowly.[114]

Genetics and environment

Environmental and genetic factors play a role in determining IQ. Their relative importance has been the subject of much research and debate.[115]

Heritability

See also: Heritability of IQ and Environment and intelligence

The general figure for the heritability of IQ, according to an American Psychological Association report, is 0.45 for children, and rises to around 0.75 for late adolescents and adults.[15] Heritability measures for g factor in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.9 in adulthood.[116][117] One proposed explanation is that people with different genes tend to reinforce the effects of those genes, for example by seeking out different environments.[15][118]

Shared family environment

Family members have aspects of environments in common (for example, characteristics of the home). This shared family environment accounts for 0.25–0.35 of the variation in IQ in childhood. By late adolescence, it is quite low (zero in some studies). The effect for several other psychological traits is similar. These studies have not looked at the effects of extreme environments, such as in abusive families.[15][119][120][121]

Non-shared family environment and environment outside the family

Although parents treat their children differently, such differential treatment explains only a small amount of nonshared environmental influence. One suggestion is that children react differently to the same environment because of different genes. More likely influences may be the impact of peers and other experiences outside the family.[15][120]

Individual genes

A very large proportion of the over 17,000 human genes are thought to have an effect on the development and functionality of the brain.[122] While a number of individual genes have been reported to be associated with IQ, none have a strong effect. Deary and colleagues (2009) reported that no finding of a strong single gene effect on IQ has been replicated.[123] Recent findings of gene associations with normally varying intellectual differences in adults and children continue to show weak effects for any one gene.[124][125]

Gene-environment interaction

David Rowe reported an interaction of genetic effects with socioeconomic status, such that the heritability was high in high-SES families, but much lower in low-SES families.[126] In the US, this has been replicated in infants,[127] children,[128] adolescents,[129] and adults.[130] Outside the US, studies show no link between heritability and SES.[131] Some effects may even reverse sign outside the US.[131][132]

Dickens and Flynn (2001) have argued that genes for high IQ initiate an environment-shaping feedback cycle, with genetic effects causing bright children to seek out more stimulating environments that then further increase their IQ. In Dickens' model, environment effects are modeled as decaying over time. In this model, the Flynn effect can be explained by an increase in environmental stimulation independent of it being sought out by individuals. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they enduringly raised children's drive to seek out cognitively demanding experiences.[133][134]

Interventions

In general, educational interventions, as those described below, have shown short-term effects on IQ, but long-term follow-up is often missing. For example, in the US, very large intervention programs such as the Head Start Program have not produced lasting gains in IQ scores. Even when students improve their scores on standardized tests, they do not always improve their cognitive abilities, such as memory, attention and speed.[135] More intensive, but much smaller projects, such as the Abecedarian Project, have reported lasting effects, often on socioeconomic status variables, rather than IQ.[15]

Recent studies have shown that training in using one's working memory may increase IQ. A study on young adults published in April 2008 by a team from the Universities of Michigan and Bern supports the possibility of the transfer of fluid intelligence from specifically designed working memory training.[136] Further research will be needed to determine nature, extent and duration of the proposed transfer. Among other questions, it remains to be seen whether the results extend to other kinds of fluid intelligence tests than the matrix test used in the study, and if so, whether, after training, fluid intelligence measures retain their correlation with educational and occupational achievement or if the value of fluid intelligence for predicting performance on other tasks changes. It is also unclear whether the training is durable for extended periods of time.[137]

Music

Musical training in childhood correlates with higher than average IQ.[138][139] However, a study of 10,500 twins found no effects on IQ, suggesting that the correlation was caused by genetic confounders.[140] A meta-analysis concluded that "Music training does not reliably enhance children and young adolescents' cognitive or academic skills, and that previous positive findings were probably due to confounding variables."[141]

It is popularly thought that listening to classical music raises IQ. However, multiple attempted replications (e.g.[142]) have shown that this is at best a short-term effect (lasting no longer than 10 to 15 minutes), and is not related to IQ-increase.[143]

Brain anatomy

Main article: Neuroscience and intelligence

Several neurophysiological factors have been correlated with intelligence in humans, including the ratio of brain weight to body weight and the size, shape, and activity level of different parts of the brain. Specific features that may affect IQ include the size and shape of the frontal lobes, the amount of blood and chemical activity in the frontal lobes, the total amount of gray matter in the brain, the overall thickness of the cortex, and the glucose metabolic rate.[144]

Health

Main articles: Impact of health on intelligence and Cognitive epidemiology

Health is important in understanding differences in IQ test scores and other measures of cognitive ability. Several factors can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the blood–brain barrier is less effective. Such impairment may sometimes be permanent, or sometimes be partially or wholly compensated for by later growth.[citation needed]

Since about 2010, researchers such as Eppig, Hassel, and MacKenzie have found a very close and consistent link between IQ scores and infectious diseases, especially in the infant and preschool populations and the mothers of these children.[145] They have postulated that fighting infectious diseases strains the child's metabolism and prevents full brain development. Hassel postulated that it is by far the most important factor in determining population IQ. However, they also found that subsequent factors such as good nutrition and regular quality schooling can offset early negative effects to some extent.

Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring fortification of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury, and organochlorides). Improvements in nutrition, and in public policy in general, have been implicated in worldwide IQ increases.[citation needed]

Cognitive epidemiology is a field of research that examines the associations between intelligence test scores and health. Researchers in the field argue that intelligence measured at an early age is an important predictor of later health and mortality differences.

Social correlations

School performance

The American Psychological Association's report Intelligence: Knowns and Unknowns states that wherever it has been studied, children with high scores on tests of intelligence tend to learn more of what is taught in school than their lower-scoring peers. The correlation between IQ scores and grades is about .50. This means that the explained variance is 25%. Achieving good grades depends on many factors other than IQ, such as "persistence, interest in school, and willingness to study" (p. 81).[15]

It has been found that the correlation of IQ scores with school performance depends on the IQ measurement used. For undergraduate students, the Verbal IQ as measured by WAIS-R has been found to correlate significantly (0.53) with the grade point average (GPA) of the last 60 hours (credits). In contrast, Performance IQ correlation with the same GPA was only 0.22 in the same study.[146]

Some measures of educational aptitude correlate highly with IQ tests – for instance, Frey & Detterman (2004) reported a correlation of 0.82 between g (general intelligence factor) and SAT scores;[147] another research found a correlation of 0.81 between g and GCSE scores, with the explained variance ranging "from 58.6% in Mathematics and 48% in English to 18.1% in Art and Design".[148]

Job performance

According to Schmidt and Hunter, "for hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability."[20] The validity of IQ as a predictor of job performance is above zero for all work studied to date, but varies with the type of job and across different studies, ranging from 0.2 to 0.6.[149] The correlations were higher when the unreliability of measurement methods was controlled for.[15] While IQ is more strongly correlated with reasoning and less so with motor function,[150] IQ-test scores predict performance ratings in all occupations.[20] That said, for highly qualified activities (research, management) low IQ scores are more likely to be a barrier to adequate performance, whereas for minimally-skilled activities, athletic strength (manual strength, speed, stamina, and coordination) is more likely to influence performance.[20] The prevailing view among academics is that it is largely through the quicker acquisition of job-relevant knowledge that higher IQ mediates job performance. This view has been challenged by Byington & Felps (2010), who argued that "the current applications of IQ-reflective tests allow individuals with high IQ scores to receive greater access to developmental resources, enabling them to acquire additional capabilities over time, and ultimately perform their jobs better."[151]

In establishing a causal direction to the link between IQ and work performance, longitudinal studies by Watkins and others suggest that IQ exerts a causal influence on future academic achievement, whereas academic achievement does not substantially influence future IQ scores.[152] Treena Eileen Rohde and Lee Anne Thompson write that general cognitive ability, but not specific ability scores, predict academic achievement, with the exception that processing speed and spatial ability predict performance on the SAT math beyond the effect of general cognitive ability.[153]

Income

It has been suggested that "in economic terms it appears that the IQ score measures something with decreasing marginal value" and it "is important to have enough of it, but having lots and lots does not buy you that much".[154][155] However, large-scale longitudinal studies indicate an increase in IQ translates into an increase in performance at all levels of IQ: i.e. ability and job performance are monotonically linked at all IQ levels.[156][157]

The link from IQ to wealth is much less strong than that from IQ to job performance. Some studies indicate that IQ is unrelated to net worth.[158][159] The American Psychological Association's 1995 report Intelligence: Knowns and Unknowns stated that IQ scores accounted for about a quarter of the social status variance and one-sixth of the income variance. Statistical controls for parental SES eliminate about a quarter of this predictive power. Psychometric intelligence appears as only one of a great many factors that influence social outcomes.[15] Charles Murray (1998) showed a more substantial effect of IQ on income independent of family background.[160] In a meta-analysis, Strenze (2006) reviewed much of the literature and estimated the correlation between IQ and income to be about 0.23.[21]

Some studies assert that IQ only accounts for (explains) a sixth of the variation in income because many studies are based on young adults, many of whom have not yet reached their peak earning capacity, or even their education. On pg 568 of The g Factor, Arthur Jensen says that although the correlation between IQ and income averages a moderate 0.4 (one-sixth or 16% of the variance), the relationship increases with age, and peaks at middle age when people have reached their maximum career potential. In the book, A Question of Intelligence, Daniel Seligman cites an IQ income correlation of 0.5 (25% of the variance).

A 2002 study[161] further examined the impact of non-IQ factors on income and concluded that an individual's location, inherited wealth, race, and schooling are more important as factors in determining income than IQ.

Crime

The American Psychological Association's 1995 report Intelligence: Knowns and Unknowns stated that the correlation between IQ and crime was −0.2. This association is generally regarded as small and prone to disappearance or a substantial reduction after controlling for the proper covariates, being much smaller than typical sociological correlates.[162] It was −0.19 between IQ scores and the number of juvenile offenses in a large Danish sample; with social class controlled for, the correlation dropped to −0.17. A correlation of 0.20 means that the explained variance accounts for 4% of the total variance. The causal links between psychometric ability and social outcomes may be indirect. Children with poor scholastic performance may feel alienated. Consequently, they may be more likely to engage in delinquent behavior, compared to other children who do well.[15]

In his book The g Factor (1998), Arthur Jensen cited data which showed that, regardless of race, people with IQs between 70 and 90 have higher crime rates than people with IQs below or above this range, with the peak range being between 80 and 90.

The 2009 Handbook of Crime Correlates stated that reviews have found that around eight IQ points, or 0.5 SD, separate criminals from the general population, especially for persistent serious offenders. It has been suggested that this simply reflects that "only dumb ones get caught" but there is similarly a negative relation between IQ and self-reported offending. That children with conduct disorder have lower IQ than their peers "strongly argues" for the theory.[163]

A study of the relationship between US county-level IQ and US county-level crime rates found that higher average IQs were very weakly associated with lower levels of property crime, burglary, larceny rate, motor vehicle theft, violent crime, robbery, and aggravated assault. These results were "not confounded by a measure of concentrated disadvantage that captures the effects of race, poverty, and other social disadvantages of the county."[164] However, this study is limited in that it extrapolated Add Health estimates to the respondent's counties, and as the dataset was not designed to be representative on the state or county level, it may not be generalizable.[165]

It has also been shown that the effect of IQ is heavily dependent on socioeconomic status and that it cannot be easily controlled away, with many methodological considerations being at play.[166] Indeed, there is evidence that the small relationship is mediated by well-being, substance abuse, and other confounding factors that prohibit simple causal interpretation.[167] A recent meta-analysis has shown that the relationship is only observed in higher risk populations such as those in poverty without direct effect, but without any causal interpretation.[168] A nationally representative longitudinal study has shown that this relationship is entirely mediated by school performance.[169]

Health and mortality

Multiple studies conducted in Scotland have found that higher IQs in early life are associated with lower mortality and morbidity rates later in life.[170][171]

Other accomplishments

Average adult combined IQs associated with real-life accomplishments by various tests:[172][173]
Accomplishment IQ Test/study Year
MDs, JDs, and PhDs 125 WAIS-R 1987
College graduates 112 KAIT 2000
K-BIT 1992
115 WAIS-R
1–3 years of college 104 KAIT
K-BIT
105–110 WAIS-R
Clerical and sales workers 100–105
High school graduates, skilled workers (e.g., electricians, cabinetmakers) 100 KAIT
WAIS-R
97 K-BIT
1–3 years of high school (completed 9–11 years of school) 94 KAIT
90 K-BIT
95 WAIS-R
Semi-skilled workers (e.g. truck drivers, factory workers) 90–95
Elementary school graduates (completed eighth grade) 90
Elementary school dropouts (completed 0–7 years of school) 80–85
Have 50/50 chance of reaching high school 75
Average IQ of various occupational groups:[174]
Accomplishment IQ Test/study Year
Professional and technical 112
Managers and administrators 104
Clerical workers, sales workers, skilled workers, craftsmen, and foremen 101
Semi-skilled workers (operatives, service workers, including private household) 92
Unskilled workers 87
Type of work that can be accomplished:[172]
Accomplishment IQ Test/study Year
Adults can harvest vegetables, repair furniture 60
Adults can do domestic work 50

There is considerable variation within and overlap among these categories. People with high IQs are found at all levels of education and occupational categories. The biggest difference occurs for low IQs with only an occasional college graduate or professional scoring below 90.[30]

Group differences

Among the most controversial issues related to the study of intelligence is the observation that IQ scores vary on average between ethnic and racial groups, though these differences have fluctuated and in many cases steadily decreased over time.[175] While there is little scholarly debate about the continued existence of some of these differences, the current scientific consensus is that they stem from environmental rather than genetic causes.[176][177][178] The existence of differences in IQ between the sexes remains controversial, and largely depends on which tests are performed.[179][180]

Race

Main article: Race and intelligence

While the concept of "race" is a social construct,[181] discussions of a purported relationship between race and intelligence, as well as claims of genetic differences in intelligence along racial lines, have appeared in both popular science and academic research since the modern concept of race was first introduced. Despite the tremendous amount of research done on the topic, no scientific evidence has emerged that the average IQ scores of different population groups can be attributed to genetic differences between those groups.[182][183][184] Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap.[185][186][178]

A 1996 task force investigation on intelligence sponsored by the American Psychological Association concluded that there were significant variations in IQ across races.[15] However, a systematic analysis by William Dickens and James Flynn (2006) showed the gap between black and white Americans to have closed dramatically during the period between 1972 and 2002, suggesting that, in their words, the "constancy of the Black-White IQ gap is a myth".[187]

The problem of determining the causes underlying racial variation has been discussed at length as a classic question of "nature versus nurture", for instance by Alan S. Kaufman[188] and Nathan Brody.[189] Researchers such as statistician Bernie Devlin have argued that there are insufficient data to conclude that the black-white gap is due to genetic influences.[190] Dickens and Flynn argued more positively that their results refute the possibility of a genetic origin, concluding that "the environment has been responsible" for observed differences.[187] A review article published in 2012 by leading scholars on human intelligence reached a similar conclusion, after reviewing the prior research literature, that group differences in IQ are best understood as environmental in origin.[191] More recently, geneticist and neuroscientist Kevin Mitchell has argued, on the basis of basic principles of population genetics, that "systematic genetic differences in intelligence between large, ancient populations" are "inherently and deeply implausible".[192]

The effects of stereotype threat have been proposed as an explanation for differences in IQ test performance between racial groups,[193][194] as have issues related to cultural difference and access to education.[195][196]

Sex

Main article: Sex differences in intelligence

With the advent of the concept of g or general intelligence, many researchers have argued that there are no significant sex differences in general intelligence,[180][197][198] though ability in particular types of intelligence does appear to vary.[179][198] Thus, while some test batteries show slightly greater intelligence in males, others show greater intelligence in females.[179][198] In particular, studies have shown female subjects performing better on tasks related to verbal ability,[180] and males performing better on tasks related to rotation of objects in space, often categorized as spatial ability.[199] These differences remain, as Hunt (2011) observes, "even though men and women are essentially equal in general intelligence".

Some research indicates that male advantages on some cognitive tests are minimized when controlling for socioeconomic factors.[179][197] Other research has concluded that there is slightly larger variability in male scores in certain areas compared to female scores, which results in slightly more males than females in the top and bottom of the IQ distribution.[200]

The existence of differences between male and female performance on math-related tests is contested,[201] and a meta-analysis focusing on average gender differences in math performance found nearly identical performance for boys and girls.[202] Currently, most IQ tests, including popular batteries such as the WAIS and the WISC-R, are constructed so that there are no overall score differences between females and males.[15][203][204]

Public policy

Main article: Intelligence and public policy

In the United States, certain public policies and laws regarding military service,[205][206] education, public benefits,[207] capital punishment,[105] and employment incorporate an individual's IQ into their decisions. However, in the case of Griggs v. Duke Power Co. in 1971, for the purpose of minimizing employment practices that disparately impacted racial minorities, the U.S. Supreme Court banned the use of IQ tests in employment, except when linked to job performance via a job analysis. Internationally, certain public policies, such as improving nutrition and prohibiting neurotoxins, have as one of their goals raising, or preventing a decline in, intelligence.

A diagnosis of intellectual disability is in part based on the results of IQ testing. Borderline intellectual functioning is the categorization of individuals of below-average cognitive ability (an IQ of 71–85), although not as low as those with an intellectual disability (70 or below).

In the United Kingdom, the eleven plus exam which incorporated an intelligence test has been used from 1945 to decide, at eleven years of age, which type of school a child should go to. They have been much less used since the widespread introduction of comprehensive schools.

Classification

Main article: IQ classification
 
Physicist Stephen Hawking. When asked his IQ, he replied: "I have no idea. People who boast about their IQ are losers."[208]

IQ classification is the practice used by IQ test publishers for designating IQ score ranges into various categories with labels such as "superior" or "average".[173] IQ classification was preceded historically by attempts to classify human beings by general ability based on other forms of behavioral observation. Those other forms of behavioral observation are still important for validating classifications based on IQ tests.

High IQ societies

Main article: High IQ society

There are social organizations, some international, which limit membership to people who have scores as high as or higher than the 98th percentile (two standard deviations above the mean) on some IQ test or equivalent. Mensa International is perhaps the best known of these. The largest 99.9th percentile (three standard deviations above the mean) society is the Triple Nine Society.

See also

Citations

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External links

 
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  • Classics in the History of Psychology
  • Human Intelligence: biographical profiles, current controversies, resources for teachers

April 11, 2023
intelligence, quotient, redirects, here, other, uses, disambiguation, intelligence, quotient, total, score, derived, from, standardised, tests, subtests, designed, assess, human, intelligence, abbreviation, coined, psychologist, william, stern, german, term, i. IQ redirects here For other uses see IQ disambiguation An intelligence quotient IQ is a total score derived from a set of standardised tests or subtests designed to assess human intelligence 1 The abbreviation IQ was coined by the psychologist William Stern for the German term Intelligenzquotient his term for a scoring method for intelligence tests at University of Breslau he advocated in a 1912 book 2 Intelligence quotientOne kind of IQ test item modelled after items in the Raven s Progressive Matrices testICD 10 PCSZ01 8ICD 9 CM94 01Historically IQ was a score obtained by dividing a person s mental age score obtained by administering an intelligence test by the person s chronological age both expressed in terms of years and months The resulting fraction quotient was multiplied by 100 to obtain the IQ score 3 For modern IQ tests the raw score is transformed to a normal distribution with mean 100 and standard deviation 15 4 This results in approximately two thirds of the population scoring between IQ 85 and IQ 115 and about 2 5 percent each above 130 and below 70 5 6 Scores from intelligence tests are estimates of intelligence Unlike for example distance and mass a concrete measure of intelligence cannot be achieved given the abstract nature of the concept of intelligence 7 IQ scores have been shown to be associated with such factors as nutrition 8 9 10 parental socioeconomic status 11 12 morbidity and mortality 13 14 parental social status 15 and perinatal environment 16 While the heritability of IQ has been investigated for nearly a century there is still debate about the significance of heritability estimates 17 18 and the mechanisms of inheritance 19 IQ scores are used for educational placement assessment of intellectual disability and evaluating job applicants In research contexts they have been studied as predictors of job performance 20 and income 21 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 that scales to three IQ points per decade since the early 20th century a phenomenon called the Flynn effect Investigation of different patterns of increases in subtest scores can also inform current research on human intelligence Contents 1 History 1 1 Precursors to IQ testing 1 2 General factor g 1 3 United States military selection in World War I 1 4 IQ testing and the eugenics movement in the United States 1 5 Cattell Horn Carroll theory 1 6 Other theories 2 Current tests 3 Reliability and validity 3 1 Reliability 3 2 Validity as a measure of intelligence 3 3 Test bias or differential item functioning 3 4 Flynn effect 3 5 Age 4 Genetics and environment 4 1 Heritability 4 2 Shared family environment 4 3 Non shared family environment and environment outside the family 4 4 Individual genes 4 5 Gene environment interaction 5 Interventions 6 Music 7 Brain anatomy 8 Health 9 Social correlations 9 1 School performance 9 2 Job performance 9 3 Income 9 4 Crime 9 5 Health and mortality 9 6 Other accomplishments 10 Group differences 10 1 Race 10 2 Sex 11 Public policy 12 Classification 13 High IQ societies 14 See also 15 Citations 16 General and cited references 17 External linksHistory EditSee also History of the race and intelligence controversy Precursors to IQ testing Edit Historically even before IQ tests were devised there were attempts to classify people into intelligence categories by observing their behavior in daily life 22 23 Those other forms of behavioral observation are still important for validating classifications based primarily on IQ test scores Both intelligence classification by observation of behavior 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 standardized 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 24 25 26 He hypothesized that there should exist a correlation between intelligence and other observable traits such as reflexes muscle grip and head size 27 He set up the first mental testing center 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 28 29 Psychologist Alfred Binet co developer of the Stanford Binet test French psychologist Alfred Binet together with Victor Henri and Theodore Simon had more success in 1905 when they published the Binet Simon test which focused on verbal abilities It was intended to identify mental retardation in school children 30 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 31 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 thought that intelligence was multifaceted but came under the control of practical judgment In Binet s view there were limitations with the scale and he stressed what he saw as the remarkable diversity of intelligence and the subsequent 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 Binet Intelligence Scales 1916 It became the most popular test in the United States for decades 30 32 33 34 General factor g Edit Main article g factor 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 conceptualized 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 labeled the specific factors or abilities for specific tasks s 35 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 citation needed United States military selection in World War I Edit 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 36 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 30 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 36 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 36 After the war positive publicity promoted by army psychologists helped to make psychology a respected field 37 Subsequently there was an increase in jobs and funding in psychology in the United States 38 Group intelligence tests were developed and became widely used in schools and industry 39 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 36 The tests also allowed for the bolstering of jingoist narratives in the context of increased immigration which may have influenced the passing of the Immigration Restriction Act of 1924 36 L L Thurstone argued for a model of intelligence that included seven unrelated factors verbal comprehension word fluency number facility spatial visualization associative memory perceptual speed reasoning and induction While not widely used Thurstone s model influenced later theories 30 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 now similar to the Wechsler in several aspects but the Wechsler continues to be the most popular test in the United States 30 IQ testing and the eugenics movement in the United States Edit 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 40 41 42 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 43 44 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 45 46 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 47 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 48 Unlike Galton who promoted eugenics through selective breeding for positive traits Goddard went with the US eugenics movement to eliminate undesirable traits 49 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 sterilization surgeries 48 At first sterilization 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 sterilization Different states adopted the sterilization 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 sterilization in the United States 50 California s sterilization program was so effective that the Nazis turned to the government for advice on how to prevent the birth of the unfit 51 While the US eugenics movement lost much of its momentum in the 1940s in view of the horrors of Nazi Germany advocates of eugenics including Nazi geneticist Otmar Freiherr von Verschuer continued to work and promote their ideas in the United States 51 In later decades some eugenic principles have made a resurgence as a voluntary means of selective reproduction with some calling them new eugenics 52 As it becomes possible to test for and correlate genes with IQ and its proxies 53 ethicists and embryonic genetic testing companies are attempting to understand the ways in which the technology can be ethically deployed 54 Cattell Horn Carroll theory Edit Main article Cattell Horn Carroll theory Psychologist Raymond Cattell defined fluid and crystallized intelligence and authored the Cattell Culture Fair III IQ test Raymond Cattell 1941 proposed two types of cognitive abilities in a revision of Spearman s concept of general intelligence Fluid intelligence Gf was hypothesized as the ability to solve novel problems by using reasoning and crystallized intelligence Gc was hypothesized as a knowledge based ability that was very dependent on education and experience In addition fluid intelligence was hypothesized to decline with age while crystallized intelligence was largely resistant to the effects of aging The theory was almost forgotten but was revived by his student John L Horn 1966 who later argued Gf and Gc were only two among several factors and who eventually identified nine or ten broad abilities The theory continued to be called Gf Gc theory 30 John B Carroll 1993 after a comprehensive reanalysis of earlier data proposed the three stratum theory which is a hierarchical model with three levels The bottom stratum consists of narrow abilities that are highly specialized e g induction spelling ability The second stratum consists of broad abilities Carroll identified eight second stratum abilities Carroll accepted Spearman s concept of general intelligence for the most part as a representation of the uppermost third stratum 55 56 In 1999 a merging of the Gf Gc theory of Cattell and Horn with Carroll s Three Stratum theory has led to the Cattell Horn Carroll theory CHC Theory with g as the top of the hierarchy ten broad abilities below and further subdivided into seventy narrow abilities on the third stratum CHC Theory has greatly influenced many of the current broad IQ tests 30 Modern tests do not necessarily measure all of these broad abilities For example quantitative knowledge and reading amp writing ability may be seen as measures of school achievement and not IQ 30 Decision speed may be difficult to measure without special equipment g was earlier often subdivided into only Gf and Gc which were thought to correspond to the nonverbal or performance subtests and verbal subtests in earlier versions of the popular Wechsler IQ test More recent research has shown the situation to be more complex 30 Modern comprehensive IQ tests do not stop at reporting a single IQ score Although they still give an overall score they now also give scores for many of these more restricted abilities identifying particular strengths and weaknesses of an individual 30 Other theories Edit An alternative to standard IQ tests meant to test the proximal development of children originated in the writings of psychologist Lev Vygotsky 1896 1934 during his last two years of his life 57 58 According to Vygotsky the maximum level of complexity and difficulty of problems that a child is capable to solve under some guidance indicates their level of potential development The difference between this level of potential and the lower level of unassisted performance indicates the child s zone of proximal development 59 Combination of the two indexes the level of actual and the zone of the proximal development according to Vygotsky provides a significantly more informative indicator of psychological development than the assessment of the level of actual development alone 60 61 His ideas on the zone of development were later developed in a number of psychological and educational theories and practices most notably under the banner of dynamic assessment which seeks to measure developmental potential 62 63 64 for instance in the work of Reuven Feuerstein and his associates 65 who has criticized standard IQ testing for its putative assumption or acceptance of fixed and immutable characteristics of intelligence or cognitive functioning Dynamic assessment has been further elaborated in the work of Ann Brown and John D Bransford and in theories of multiple intelligences authored by Howard Gardner and Robert Sternberg 66 67 J P Guilford s Structure of Intellect 1967 model of intelligence used three dimensions which when combined yielded a total of 120 types of intelligence It was popular in the 1970s and early 1980s but faded owing to both practical problems and theoretical criticisms 30 Alexander Luria s earlier work on neuropsychological processes led to the PASS theory 1997 It argued that only looking at one general factor was inadequate for researchers and clinicians who worked with learning disabilities attention disorders intellectual disability and interventions for such disabilities The PASS model covers four kinds of processes planning process attention arousal process simultaneous processing and successive processing The planning processes involve decision making problem solving and performing activities and require goal setting and self monitoring The attention arousal process involves selectively attending to a particular stimulus ignoring distractions and maintaining vigilance Simultaneous processing involves the integration of stimuli into a group and requires the observation of relationships Successive processing involves the integration of stimuli into serial order The planning and attention arousal components comes from structures located in the frontal lobe and the simultaneous and successive processes come from structures located in the posterior region of the cortex 68 69 70 It has influenced some recent IQ tests and been seen as a complement to the Cattell Horn Carroll theory described above 30 Current tests Edit Normalized IQ distribution with mean 100 and standard deviation 15 There are a variety of individually administered IQ tests in use in the English speaking world 71 72 The most commonly used individual IQ test series is the Wechsler Adult Intelligence Scale WAIS for adults and the Wechsler Intelligence Scale for Children WISC for school age test takers Other commonly used individual IQ tests some of which do not label their standard scores as IQ scores include the current versions of the Stanford Binet Intelligence Scales Woodcock Johnson Tests of Cognitive Abilities the Kaufman Assessment Battery for Children the Cognitive Assessment System and the Differential Ability Scales IQ tests also include Raven s Progressive Matrices Cattell Culture Fair III Reynolds Intellectual Assessment Scales Thurstone s Primary Mental Abilities 73 74 Kaufman Brief Intelligence Test 75 Multidimensional Aptitude Battery II Das Naglieri cognitive assessment system Naglieri Nonverbal Ability Test Wide Range Intelligence Test AJT Cognitive Test IndonesiaIQ scales are ordinally scaled 76 77 78 79 80 The raw score of the norming sample is usually rank order transformed to a normal distribution with mean 100 and standard deviation 15 4 While one standard deviation is 15 points and two SDs are 30 points and so on this does not imply that mental ability is linearly related to IQ such that IQ 50 would mean half the cognitive ability of IQ 100 In particular IQ points are not percentage points Reliability and validity EditIQ scores can differ to some degree for the same person on different IQ tests so a person does not always belong to the same IQ score range each time the person is tested IQ score table data and pupil pseudonyms adapted from description of KABC II norming study cited in Kaufman 2009 81 82 Pupil KABC II WISC III WJ IIIA 90 95 111B 125 110 105C 100 93 101D 116 127 118E 93 105 93F 106 105 105G 95 100 90H 112 113 103I 104 96 97J 101 99 86K 81 78 75L 116 124 102Reliability Edit Psychometricians generally regard IQ tests as having high statistical reliability 15 83 Reliability represents the measurement consistency of a test 84 A reliable test produces similar scores upon repetition 84 On aggregate IQ tests exhibit high reliability although test takers may have varying scores when taking the same test on differing occasions and may have varying scores when taking different IQ tests at the same age Like all statistical quantities any particular estimate of IQ has an associated standard error that measures uncertainty about the estimate For modern tests the confidence interval can be approximately 10 points and reported standard error of measurement can be as low as about three points 85 Reported standard error may be an underestimate as it does not account for all sources of error 86 Outside influences such as low motivation or high anxiety can occasionally lower a person s IQ test score 84 For individuals with very low scores the 95 confidence interval may be greater than 40 points potentially complicating the accuracy of diagnoses of intellectual disability 87 By the same token high IQ scores are also significantly less reliable than those near to the population median 88 Reports of IQ scores much higher than 160 are considered dubious 89 Validity as a measure of intelligence Edit Reliability and validity are very different concepts While reliability reflects reproducibility validity refers to whether the test measures what it purports to measure 84 While IQ tests are generally considered to measure some forms of intelligence they may fail to serve as an accurate measure of broader definitions of human intelligence inclusive of for example creativity and social intelligence For this reason psychologist Wayne Weiten argues that their construct validity must be carefully qualified and not be overstated 84 According to Weiten IQ tests are valid measures of the kind of intelligence necessary to do well in academic work But if the purpose is to assess intelligence in a broader sense the validity of IQ tests is questionable 84 Some scientists have disputed the value of IQ as a measure of intelligence altogether In The Mismeasure of Man 1981 expanded edition 1996 evolutionary biologist Stephen Jay Gould compared IQ testing with the now discredited practice of determining intelligence via craniometry arguing that both are based on the fallacy of reification our tendency to convert abstract concepts into entities 90 Gould s argument sparked a great deal of debate 91 92 and the book is listed as one of Discover Magazine s 25 Greatest Science Books of All Time 93 Along these same lines critics such as Keith Stanovich do not dispute the capacity of IQ test scores to predict some kinds of achievement but argue that basing a concept of intelligence on IQ test scores alone neglects other important aspects of mental ability 15 94 Robert Sternberg another significant critic of IQ as the main measure of human cognitive abilities argued that reducing the concept of intelligence to the measure of g does not fully account for the different skills and knowledge types that produce success in human society 95 Despite these objections clinical psychologists generally regard IQ scores as having sufficient statistical validity for many clinical purposes specify 30 96 Test bias or differential item functioning Edit Differential item functioning DIF sometimes referred to as measurement bias is a phenomenon when participants from different groups e g gender race disability with the same latent abilities give different answers to specific questions on the same IQ test 97 DIF analysis measures such specific items on a test alongside measuring participants latent abilities on other similar questions A consistent different group response to a specific question among similar types of questions can indicate an effect of DIF It does not count as differential item functioning if both groups have an equally valid chance of giving different responses to the same questions Such bias can be a result of culture educational level and other factors that are independent of group traits DIF is only considered if test takers from different groups with the same underlying latent ability level have a different chance of giving specific responses 98 Such questions are usually removed in order to make the test equally fair for both groups Common techniques for analyzing DIF are item response theory IRT based methods Mantel Haenszel and logistic regression 98 A 2005 study found that differential validity in prediction suggests that the WAIS R test may contain cultural influences that reduce the validity of the WAIS R as a measure of cognitive ability for Mexican American students 99 indicating a weaker positive correlation relative to sampled white students Other recent studies have questioned the culture fairness of IQ tests when used in South Africa 100 101 Standard intelligence tests such as the Stanford Binet are often inappropriate for autistic children the alternative of using developmental or adaptive skills measures are relatively poor measures of intelligence in autistic children and may have resulted in incorrect claims that a majority of autistic children are of low intelligence 102 Flynn effect Edit Main article Flynn effect Since the early 20th century raw scores on IQ tests have increased in most parts of the world 103 104 105 When a new version of an IQ test is normed the standard scoring is set so performance at the population median results in a score of IQ 100 The phenomenon of rising raw score performance means if test takers are scored by a constant standard scoring rule IQ test scores have been rising at an average rate of around three IQ points per decade This phenomenon was named the Flynn effect in the book The Bell Curve after James R Flynn the author who did the most to bring this phenomenon to the attention of psychologists 106 107 Researchers have been exploring the issue of whether the Flynn effect is equally strong on performance of all kinds of IQ test items whether the effect may have ended in some developed nations whether there are social subgroup differences in the effect and what possible causes of the effect might be 108 A 2011 textbook IQ and Human Intelligence by N J Mackintosh noted the Flynn effect demolishes the fears that IQ would be decreased He also asks whether it represents a real increase in intelligence beyond IQ scores 109 A 2011 psychology textbook lead authored by Harvard Psychologist Professor Daniel Schacter noted that humans inherited intelligence could be going down while acquired intelligence goes up 110 Research has suggested that the Flynn effect has slowed or reversed course in some Western countries beginning in the late 20th century The phenomenon has been termed the negative Flynn effect 111 A study of Norwegian military conscripts test records found that IQ scores have been falling for generations born after the year 1975 and that the underlying cause of both initial increasing and subsequent falling trends appears to be environmental rather than genetic 111 Age Edit IQ can change to some degree over the course of childhood 112 In one longitudinal study the mean IQ scores of tests at ages 17 and 18 were correlated at r 0 86 with the mean scores of tests at ages five six and seven and at r 0 96 further explanation needed with the mean scores of tests at ages 11 12 and 13 15 For decades practitioners handbooks and textbooks on IQ testing have reported IQ declines with age after the beginning of adulthood However later researchers pointed out this phenomenon is related to the Flynn effect and is in part a cohort effect rather than a true aging effect A variety of studies of IQ and aging have been conducted since the norming of the first Wechsler Intelligence Scale drew attention to IQ differences in different age groups of adults The current consensus is that fluid intelligence generally declines with age after early adulthood while crystallized intelligence remains intact Both cohort effects the birth year of the test takers and practice effects test takers taking the same form of IQ test more than once must be controlled to gain accurate data inconsistent It is unclear whether any lifestyle intervention can preserve fluid intelligence into older ages 113 The exact peak age of fluid intelligence or crystallized intelligence remains elusive Cross sectional studies usually show that especially fluid intelligence peaks at a relatively young age often in the early adulthood while longitudinal data mostly show that intelligence is stable until mid adulthood or later Subsequently intelligence seems to decline slowly 114 Genetics and environment EditEnvironmental and genetic factors play a role in determining IQ Their relative importance has been the subject of much research and debate 115 Heritability Edit See also Heritability of IQ and Environment and intelligence The general figure for the heritability of IQ according to an American Psychological Association report is 0 45 for children and rises to around 0 75 for late adolescents and adults 15 Heritability measures for g factor in infancy are as low as 0 2 around 0 4 in middle childhood and as high as 0 9 in adulthood 116 117 One proposed explanation is that people with different genes tend to reinforce the effects of those genes for example by seeking out different environments 15 118 Shared family environment Edit Family members have aspects of environments in common for example characteristics of the home This shared family environment accounts for 0 25 0 35 of the variation in IQ in childhood By late adolescence it is quite low zero in some studies The effect for several other psychological traits is similar These studies have not looked at the effects of extreme environments such as in abusive families 15 119 120 121 Non shared family environment and environment outside the family Edit Although parents treat their children differently such differential treatment explains only a small amount of nonshared environmental influence One suggestion is that children react differently to the same environment because of different genes More likely influences may be the impact of peers and other experiences outside the family 15 120 Individual genes Edit A very large proportion of the over 17 000 human genes are thought to have an effect on the development and functionality of the brain 122 While a number of individual genes have been reported to be associated with IQ none have a strong effect Deary and colleagues 2009 reported that no finding of a strong single gene effect on IQ has been replicated 123 Recent findings of gene associations with normally varying intellectual differences in adults and children continue to show weak effects for any one gene 124 125 Gene environment interaction Edit David Rowe reported an interaction of genetic effects with socioeconomic status such that the heritability was high in high SES families but much lower in low SES families 126 In the US this has been replicated in infants 127 children 128 adolescents 129 and adults 130 Outside the US studies show no link between heritability and SES 131 Some effects may even reverse sign outside the US 131 132 Dickens and Flynn 2001 have argued that genes for high IQ initiate an environment shaping feedback cycle with genetic effects causing bright children to seek out more stimulating environments that then further increase their IQ In Dickens model environment effects are modeled as decaying over time In this model the Flynn effect can be explained by an increase in environmental stimulation independent of it being sought out by individuals The authors suggest that programs aiming to increase IQ would be most likely to produce long term IQ gains if they enduringly raised children s drive to seek out cognitively demanding experiences 133 134 Interventions EditIn general educational interventions as those described below have shown short term effects on IQ but long term follow up is often missing For example in the US very large intervention programs such as the Head Start Program have not produced lasting gains in IQ scores Even when students improve their scores on standardized tests they do not always improve their cognitive abilities such as memory attention and speed 135 More intensive but much smaller projects such as the Abecedarian Project have reported lasting effects often on socioeconomic status variables rather than IQ 15 Recent studies have shown that training in using one s working memory may increase IQ A study on young adults published in April 2008 by a team from the Universities of Michigan and Bern supports the possibility of the transfer of fluid intelligence from specifically designed working memory training 136 Further research will be needed to determine nature extent and duration of the proposed transfer Among other questions it remains to be seen whether the results extend to other kinds of fluid intelligence tests than the matrix test used in the study and if so whether after training fluid intelligence measures retain their correlation with educational and occupational achievement or if the value of fluid intelligence for predicting performance on other tasks changes It is also unclear whether the training is durable for extended periods of time 137 Music EditMusical training in childhood correlates with higher than average IQ 138 139 However a study of 10 500 twins found no effects on IQ suggesting that the correlation was caused by genetic confounders 140 A meta analysis concluded that Music training does not reliably enhance children and young adolescents cognitive or academic skills and that previous positive findings were probably due to confounding variables 141 It is popularly thought that listening to classical music raises IQ However multiple attempted replications e g 142 have shown that this is at best a short term effect lasting no longer than 10 to 15 minutes and is not related to IQ increase 143 Brain anatomy EditMain article Neuroscience and intelligence Several neurophysiological factors have been correlated with intelligence in humans including the ratio of brain weight to body weight and the size shape and activity level of different parts of the brain Specific features that may affect IQ include the size and shape of the frontal lobes the amount of blood and chemical activity in the frontal lobes the total amount of gray matter in the brain the overall thickness of the cortex and the glucose metabolic rate 144 Health EditMain articles Impact of health on intelligence and Cognitive epidemiology This section needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Intelligence quotient news newspapers books scholar JSTOR October 2020 Learn how and when to remove this template message Health is important in understanding differences in IQ test scores and other measures of cognitive ability Several factors can lead to significant cognitive impairment particularly if they occur during pregnancy and childhood when the brain is growing and the blood brain barrier is less effective Such impairment may sometimes be permanent or sometimes be partially or wholly compensated for by later growth citation needed Since about 2010 researchers such as Eppig Hassel and MacKenzie have found a very close and consistent link between IQ scores and infectious diseases especially in the infant and preschool populations and the mothers of these children 145 They have postulated that fighting infectious diseases strains the child s metabolism and prevents full brain development Hassel postulated that it is by far the most important factor in determining population IQ However they also found that subsequent factors such as good nutrition and regular quality schooling can offset early negative effects to some extent Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function These include laws requiring fortification of certain food products and laws establishing safe levels of pollutants e g lead mercury and organochlorides Improvements in nutrition and in public policy in general have been implicated in worldwide IQ increases citation needed Cognitive epidemiology is a field of research that examines the associations between intelligence test scores and health Researchers in the field argue that intelligence measured at an early age is an important predictor of later health and mortality differences Social correlations EditSchool performance Edit The American Psychological Association s report Intelligence Knowns and Unknowns states that wherever it has been studied children with high scores on tests of intelligence tend to learn more of what is taught in school than their lower scoring peers The correlation between IQ scores and grades is about 50 This means that the explained variance is 25 Achieving good grades depends on many factors other than IQ such as persistence interest in school and willingness to study p 81 15 It has been found that the correlation of IQ scores with school performance depends on the IQ measurement used For undergraduate students the Verbal IQ as measured by WAIS R has been found to correlate significantly 0 53 with the grade point average GPA of the last 60 hours credits In contrast Performance IQ correlation with the same GPA was only 0 22 in the same study 146 Some measures of educational aptitude correlate highly with IQ tests for instance Frey amp Detterman 2004 reported a correlation of 0 82 between g general intelligence factor and SAT scores 147 another research found a correlation of 0 81 between g and GCSE scores with the explained variance ranging from 58 6 in Mathematics and 48 in English to 18 1 in Art and Design 148 Job performance Edit According to Schmidt and Hunter for hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability 20 The validity of IQ as a predictor of job performance is above zero for all work studied to date but varies with the type of job and across different studies ranging from 0 2 to 0 6 149 The correlations were higher when the unreliability of measurement methods was controlled for 15 While IQ is more strongly correlated with reasoning and less so with motor function 150 IQ test scores predict performance ratings in all occupations 20 That said for highly qualified activities research management low IQ scores are more likely to be a barrier to adequate performance whereas for minimally skilled activities athletic strength manual strength speed stamina and coordination is more likely to influence performance 20 The prevailing view among academics is that it is largely through the quicker acquisition of job relevant knowledge that higher IQ mediates job performance This view has been challenged by Byington amp Felps 2010 who argued that the current applications of IQ reflective tests allow individuals with high IQ scores to receive greater access to developmental resources enabling them to acquire additional capabilities over time and ultimately perform their jobs better 151 In establishing a causal direction to the link between IQ and work performance longitudinal studies by Watkins and others suggest that IQ exerts a causal influence on future academic achievement whereas academic achievement does not substantially influence future IQ scores 152 Treena Eileen Rohde and Lee Anne Thompson write that general cognitive ability but not specific ability scores predict academic achievement with the exception that processing speed and spatial ability predict performance on the SAT math beyond the effect of general cognitive ability 153 Income Edit It has been suggested that in economic terms it appears that the IQ score measures something with decreasing marginal value and it is important to have enough of it but having lots and lots does not buy you that much 154 155 However large scale longitudinal studies indicate an increase in IQ translates into an increase in performance at all levels of IQ i e ability and job performance are monotonically linked at all IQ levels 156 157 The link from IQ to wealth is much less strong than that from IQ to job performance Some studies indicate that IQ is unrelated to net worth 158 159 The American Psychological Association s 1995 report Intelligence Knowns and Unknowns stated that IQ scores accounted for about a quarter of the social status variance and one sixth of the income variance Statistical controls for parental SES eliminate about a quarter of this predictive power Psychometric intelligence appears as only one of a great many factors that influence social outcomes 15 Charles Murray 1998 showed a more substantial effect of IQ on income independent of family background 160 In a meta analysis Strenze 2006 reviewed much of the literature and estimated the correlation between IQ and income to be about 0 23 21 Some studies assert that IQ only accounts for explains a sixth of the variation in income because many studies are based on young adults many of whom have not yet reached their peak earning capacity or even their education On pg 568 of The g Factor Arthur Jensen says that although the correlation between IQ and income averages a moderate 0 4 one sixth or 16 of the variance the relationship increases with age and peaks at middle age when people have reached their maximum career potential In the book A Question of Intelligence Daniel Seligman cites an IQ income correlation of 0 5 25 of the variance A 2002 study 161 further examined the impact of non IQ factors on income and concluded that an individual s location inherited wealth race and schooling are more important as factors in determining income than IQ Crime Edit The American Psychological Association s 1995 report Intelligence Knowns and Unknowns stated that the correlation between IQ and crime was 0 2 This association is generally regarded as small and prone to disappearance or a substantial reduction after controlling for the proper covariates being much smaller than typical sociological correlates 162 It was 0 19 between IQ scores and the number of juvenile offenses in a large Danish sample with social class controlled for the correlation dropped to 0 17 A correlation of 0 20 means that the explained variance accounts for 4 of the total variance The causal links between psychometric ability and social outcomes may be indirect Children with poor scholastic performance may feel alienated Consequently they may be more likely to engage in delinquent behavior compared to other children who do well 15 In his book The g Factor 1998 Arthur Jensen cited data which showed that regardless of race people with IQs between 70 and 90 have higher crime rates than people with IQs below or above this range with the peak range being between 80 and 90 The 2009 Handbook of Crime Correlates stated that reviews have found that around eight IQ points or 0 5 SD separate criminals from the general population especially for persistent serious offenders It has been suggested that this simply reflects that only dumb ones get caught but there is similarly a negative relation between IQ and self reported offending That children with conduct disorder have lower IQ than their peers strongly argues for the theory 163 A study of the relationship between US county level IQ and US county level crime rates found that higher average IQs were very weakly associated with lower levels of property crime burglary larceny rate motor vehicle theft violent crime robbery and aggravated assault These results were not confounded by a measure of concentrated disadvantage that captures the effects of race poverty and other social disadvantages of the county 164 However this study is limited in that it extrapolated Add Health estimates to the respondent s counties and as the dataset was not designed to be representative on the state or county level it may not be generalizable 165 It has also been shown that the effect of IQ is heavily dependent on socioeconomic status and that it cannot be easily controlled away with many methodological considerations being at play 166 Indeed there is evidence that the small relationship is mediated by well being substance abuse and other confounding factors that prohibit simple causal interpretation 167 A recent meta analysis has shown that the relationship is only observed in higher risk populations such as those in poverty without direct effect but without any causal interpretation 168 A nationally representative longitudinal study has shown that this relationship is entirely mediated by school performance 169 Health and mortality Edit Multiple studies conducted in Scotland have found that higher IQs in early life are associated with lower mortality and morbidity rates later in life 170 171 Other accomplishments Edit Average adult combined IQs associated with real life accomplishments by various tests 172 173 Accomplishment IQ Test study YearMDs JDs and PhDs 125 WAIS R 1987College graduates 112 KAIT 2000K BIT 1992115 WAIS R1 3 years of college 104 KAITK BIT105 110 WAIS RClerical and sales workers 100 105High school graduates skilled workers e g electricians cabinetmakers 100 KAITWAIS R97 K BIT1 3 years of high school completed 9 11 years of school 94 KAIT90 K BIT95 WAIS RSemi skilled workers e g truck drivers factory workers 90 95Elementary school graduates completed eighth grade 90Elementary school dropouts completed 0 7 years of school 80 85Have 50 50 chance of reaching high school 75Average IQ of various occupational groups 174 Accomplishment IQ Test study YearProfessional and technical 112Managers and administrators 104Clerical workers sales workers skilled workers craftsmen and foremen 101Semi skilled workers operatives service workers including private household 92Unskilled workers 87Type of work that can be accomplished 172 Accomplishment IQ Test study YearAdults can harvest vegetables repair furniture 60Adults can do domestic work 50There is considerable variation within and overlap among these categories People with high IQs are found at all levels of education and occupational categories The biggest difference occurs for low IQs with only an occasional college graduate or professional scoring below 90 30 Group differences EditAmong the most controversial issues related to the study of intelligence is the observation that IQ scores vary on average between ethnic and racial groups though these differences have fluctuated and in many cases steadily decreased over time 175 While there is little scholarly debate about the continued existence of some of these differences the current scientific consensus is that they stem from environmental rather than genetic causes 176 177 178 The existence of differences in IQ between the sexes remains controversial and largely depends on which tests are performed 179 180 Race Edit Main article Race and intelligenceWhile the concept of race is a social construct 181 discussions of a purported relationship between race and intelligence as well as claims of genetic differences in intelligence along racial lines have appeared in both popular science and academic research since the modern concept of race was first introduced Despite the tremendous amount of research done on the topic no scientific evidence has emerged that the average IQ scores of different population groups can be attributed to genetic differences between those groups 182 183 184 Growing evidence indicates that environmental factors not genetic ones explain the racial IQ gap 185 186 178 A 1996 task force investigation on intelligence sponsored by the American Psychological Association concluded that there were significant variations in IQ across races 15 However a systematic analysis by William Dickens and James Flynn 2006 showed the gap between black and white Americans to have closed dramatically during the period between 1972 and 2002 suggesting that in their words the constancy of the Black White IQ gap is a myth 187 The problem of determining the causes underlying racial variation has been discussed at length as a classic question of nature versus nurture for instance by Alan S Kaufman 188 and Nathan Brody 189 Researchers such as statistician Bernie Devlin have argued that there are insufficient data to conclude that the black white gap is due to genetic influences 190 Dickens and Flynn argued more positively that their results refute the possibility of a genetic origin concluding that the environment has been responsible for observed differences 187 A review article published in 2012 by leading scholars on human intelligence reached a similar conclusion after reviewing the prior research literature that group differences in IQ are best understood as environmental in origin 191 More recently geneticist and neuroscientist Kevin Mitchell has argued on the basis of basic principles of population genetics that systematic genetic differences in intelligence between large ancient populations are inherently and deeply implausible 192 The effects of stereotype threat have been proposed as an explanation for differences in IQ test performance between racial groups 193 194 as have issues related to cultural difference and access to education 195 196 Sex Edit Main article Sex differences in intelligence With the advent of the concept of g or general intelligence many researchers have argued that there are no significant sex differences in general intelligence 180 197 198 though ability in particular types of intelligence does appear to vary 179 198 Thus while some test batteries show slightly greater intelligence in males others show greater intelligence in females 179 198 In particular studies have shown female subjects performing better on tasks related to verbal ability 180 and males performing better on tasks related to rotation of objects in space often categorized as spatial ability 199 These differences remain as Hunt 2011 observes even though men and women are essentially equal in general intelligence Some research indicates that male advantages on some cognitive tests are minimized when controlling for socioeconomic factors 179 197 Other research has concluded that there is slightly larger variability in male scores in certain areas compared to female scores which results in slightly more males than females in the top and bottom of the IQ distribution 200 The existence of differences between male and female performance on math related tests is contested 201 and a meta analysis focusing on average gender differences in math performance found nearly identical performance for boys and girls 202 Currently most IQ tests including popular batteries such as the WAIS and the WISC R are constructed so that there are no overall score differences between females and males 15 203 204 Public policy EditMain article Intelligence and public policy In the United States certain public policies and laws regarding military service 205 206 education public benefits 207 capital punishment 105 and employment incorporate an individual s IQ into their decisions However in the case of Griggs v Duke Power Co in 1971 for the purpose of minimizing employment practices that disparately impacted racial minorities the U S Supreme Court banned the use of IQ tests in employment except when linked to job performance via a job analysis Internationally certain public policies such as improving nutrition and prohibiting neurotoxins have as one of their goals raising or preventing a decline in intelligence A diagnosis of intellectual disability is in part based on the results of IQ testing Borderline intellectual functioning is the categorization of individuals of below average cognitive ability an IQ of 71 85 although not as low as those with an intellectual disability 70 or below In the United Kingdom the eleven plus exam which incorporated an intelligence test has been used from 1945 to decide at eleven years of age which type of school a child should go to They have been much less used since the widespread introduction of comprehensive schools Classification EditMain article IQ classification Physicist Stephen Hawking When asked his IQ he replied I have no idea People who boast about their IQ are losers 208 IQ classification is the practice used by IQ test publishers for designating IQ score ranges into various categories with labels such as superior or average 173 IQ classification was preceded historically by attempts to classify human beings by general ability based on other forms of behavioral observation Those other forms of behavioral observation are still important for validating classifications based on IQ tests High IQ societies EditMain article High IQ society There are social organizations some international which limit membership to people who have scores as high as or higher than the 98th percentile two standard deviations above the mean on some IQ test or equivalent Mensa International is perhaps the best known of these The largest 99 9th percentile three standard deviations above the mean society is the Triple Nine Society See also EditEmotional intelligence EI also known as emotional quotient EQ and emotional intelligence quotient EIQ Raven s Progressive Matrices Stanford Binet Intelligence Scales Wechsler Adult Intelligence Scale Woodcock Johnson Tests of Cognitive AbilitiesCitations Edit Braaten Ellen B Norman Dennis 1 November 2006 Intelligence IQ Testing Pediatrics in Review 27 11 403 408 doi 10 1542 pir 27 11 403 ISSN 0191 9601 PMID 17079505 Retrieved 22 January 2020 Stern 1914 pp 70 84 1914 English translation pp 48 58 1912 original German edition intelligence quotient IQ Glossary of Important Assessment and Measurement Terms Philadelphia PA National Council on Measurement in Education 2016 Archived from the original on 22 July 2017 Retrieved 1 July 2017 a b Gottfredson 2009 pp 31 32 Neisser Ulrich 1997 Rising Scores on Intelligence Tests American Scientist 85 5 440 447 Bibcode 1997AmSci 85 440N Archived from the original on 4 November 2016 Retrieved 1 December 2017 Hunt 2011 p 5 As mental testing expanded to the evaluation of adolescents and adults however there was a need for a measure of intelligence that did not depend upon mental age Accordingly the intelligence quotient IQ was developed The narrow definition of IQ is a score on an intelligence test where average intelligence that is the median level of performance on an intelligence test receives a score of 100 and other scores are assigned so that the scores are distributed normally about 100 with a standard deviation of 15 Some of the implications are that 1 Approximately two thirds of all scores lie between 85 and 115 2 Five percent 1 20 of all scores are above 125 and one percent 1 100 are above 135 Similarly five percent are below 75 and one percent below 65 Haier Richard 28 December 2016 The Neuroscience of Intelligence Cambridge University Press pp 18 19 ISBN 9781107461437 Cusick Sarah E Georgieff Michael K 1 August 2017 The Role of Nutrition in Brain Development The Golden Opportunity of the First 1000 Days The Journal of Pediatrics 175 16 21 doi 10 1016 j jpeds 2016 05 013 PMC 4981537 PMID 27266965 Saloojee Haroon Pettifor John M 15 December 2001 Iron deficiency and impaired child development British Medical Journal 323 7326 1377 1378 doi 10 1136 bmj 323 7326 1377 ISSN 0959 8138 PMC 1121846 PMID 11744547 Qian Ming Wang Dong Watkins William E Gebski Val Yan Yu Qin Li Mu Chen Zu Pei 2005 The effects of iodine on intelligence in children a meta analysis of studies conducted in China Asia Pacific Journal of Clinical Nutrition 14 1 32 42 ISSN 0964 7058 PMID 15734706 Poh Bee Koon Lee Shoo Thien Yeo Giin Shang Tang Kean Choon Noor Afifah Ab Rahim Siti Hanisa Awal Parikh Panam Wong Jyh Eiin Ng Alvin Lai Oon SEANUTS Study Group 13 June 2019 Low socioeconomic status and severe obesity are linked to poor cognitive performance in Malaysian children BMC Public Health 19 Suppl 4 541 doi 10 1186 s12889 019 6856 4 ISSN 1471 2458 PMC 6565598 PMID 31196019 Galvan Marcos Uauy Ricardo Corvalan Camila Lopez Rodriguez Guadalupe Kain Juliana September 2013 Determinants of cognitive development of low SES children in Chile a post transitional country with rising childhood obesity rates Maternal and Child Health Journal 17 7 1243 1251 doi 10 1007 s10995 012 1121 9 ISSN 1573 6628 PMID 22915146 S2CID 19767926 Markus Jokela G David Batty Ian J Deary Catharine R Gale Mika Kivimaki 2009 Low Childhood IQ and Early Adult Mortality The Role of Explanatory Factors in the 1958 British Birth Cohort Pediatrics 124 3 e380 e388 doi 10 1542 peds 2009 0334 PMID 19706576 S2CID 25256969 Deary amp Batty 2007 a b c d e f g h i j k l m n o Neisser et al 1995 Ronfani Luca Vecchi Brumatti Liza Mariuz Marika Tognin Veronica 2015 The Complex Interaction between Home Environment Socioeconomic Status Maternal IQ and Early Child Neurocognitive Development A Multivariate Analysis of Data Collected in a Newborn Cohort Study PLOS ONE 10 5 e0127052 Bibcode 2015PLoSO 1027052R doi 10 1371 journal pone 0127052 PMC 4440732 PMID 25996934 Johnson Wendy Turkheimer Eric Gottesman Irving I Bouchard Thomas J August 2009 Beyond Heritability Current Directions in Psychological Science 18 4 217 220 doi 10 1111 j 1467 8721 2009 01639 x PMC 2899491 PMID 20625474 Turkheimer 2008 Devlin B Daniels Michael Roeder Kathryn 1997 The heritability of IQ Nature 388 6641 468 71 Bibcode 1997Natur 388 468D doi 10 1038 41319 PMID 9242404 S2CID 4313884 a b c d Schmidt Frank L Hunter John E 1998 The validity and utility of selection methods in personnel psychology Practical and theoretical implications of 85 years of research findings PDF Psychological Bulletin 124 2 262 74 CiteSeerX 10 1 1 172 1733 doi 10 1037 0033 2909 124 2 262 S2CID 16429503 Archived from the original PDF on 2 June 2014 Retrieved 25 October 2017 a b Strenze Tarmo September 2007 Intelligence and socioeconomic success A meta analytic review of longitudinal research Intelligence 35 5 401 426 doi 10 1016 j intell 2006 09 004 The correlation with income is considerably lower perhaps even disappointingly low being about the average of the previous meta analytic estimates 15 by Bowles et al 2001 and 27 by Ng et al 2005 But other predictors studied in this paper are not doing any better in predicting income which demonstrates that financial success is difficult to predict by any variable This assertion is further corroborated by the meta analysis of Ng et al 2005 where the best predictor of salary was educational level with a correlation of only 29 It should also be noted that the correlation of 23 is about the size of the average meta analytic result in psychology Hemphill 2003 and cannot therefore be treated as insignificant Terman 1916 p 79 What do the above IQ s imply in such terms as feeble mindedness border line intelligence dullness normality superior intelligence genius etc When we use these terms two facts must be born in mind 1 That the boundary lines between such groups are absolutely arbitrary a matter of definition only and 2 that the individuals comprising one of the groups do not make up a homogeneous type Wechsler 1939 p 37 The earliest classifications of intelligence were very rough ones To a large extent they were practical attempts to define various patterns of behavior 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Eugenics is the science which deals with all influences that improve the inborn qualities of a race also with those that develop them to the utmost advantage Susan Currell Christina Cogdell 2006 Popular Eugenics National Efficiency and American Mass Culture in the 1930s Ohio University Press pp 2 3 ISBN 978 0 8214 1691 4 Eugenics and Economics in the Progressive Era PDF Origins of Eugenics From Sir Francis Galton to Virginia s Racial Integrity Act of 1924 University of Virginia Historical Collections at the Claude Moore Health Sciences Library Retrieved 25 October 2019 Norrgard K 2008 Human testing the eugenics movement and IRBs Nature Education 1 170 Galton Francis 1869 Hereditary Genius PDF p 64 Retrieved 1 October 2019 a b The birth of American intelligence testing Retrieved 11 November 2017 America s Hidden History The Eugenics Movement Learn Science at Scitable www nature com Retrieved 11 November 2017 Social Origins of Eugenics www eugenicsarchive org Retrieved 11 November 2017 a 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error Hanna Bradley amp Holen 1981 which influence test scores in clinical assessments Another factor that must be considered is the extent to which subtest scores reflect portions of true score variance due to a hierarchical general intelligence factor and variance due to specific group factors because these sources of true score variance are conflated Whitaker Simon April 2010 Error in the estimation of intellectual ability in the low range using the WISC IV and WAIS III Personality and Individual Differences 48 5 517 521 doi 10 1016 j paid 2009 11 017 Retrieved 22 January 2020 Lohman amp Foley Nicpon 2012 p page needed The concerns associated with SEMs standard errors of measurement are actually substantially worse for scores at the extremes of the distribution especially when scores approach the maximum possible on a test when students answer most of the items correctly In these cases errors of measurement for scale scores will increase substantially at the extremes of the 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Bulletin 96 1 72 98 doi 10 1037 0033 2909 96 1 72 S2CID 26858912 Warner Molly Ernst John Townes Brenda Peel John Preston Michael 1987 Relationships Between IQ and Neuropsychological Measures in Neuropsychiatric Populations Within Laboratory and Cross Cultural Replications Using WAIS and WAIS R Journal of Clinical and Experimental Neuropsychology 9 5 545 62 doi 10 1080 01688638708410768 PMID 3667899 Byington Eliza Felps Will 2010 Why do IQ scores predict job performance Research in Organizational Behavior 30 175 202 doi 10 1016 j riob 2010 08 003 Watkins M Lei P Canivez G 2007 Psychometric intelligence and achievement A cross lagged panel analysis Intelligence 35 1 59 68 CiteSeerX 10 1 1 397 3155 doi 10 1016 j intell 2006 04 005 Rohde T Thompson L 2007 Predicting academic achievement with cognitive ability Intelligence 35 1 83 92 doi 10 1016 j intell 2006 05 004 Detterman amp Daniel 1989 Hunt Earl B July 1995 The Role of Intelligence in Modern Society July Aug 1995 American Scientist pp 4 Nonlinearities in Intelligence Archived from the original on 21 May 2006 Coward W Mark Sackett Paul R 1990 Linearity of ability performance relationships A reconfirmation Journal of Applied Psychology 75 3 297 300 doi 10 1037 0021 9010 75 3 297 Robertson Kimberley Ferriman Smeets Stijn Lubinski David Benbow Camilla P 14 December 2010 Beyond the Threshold Hypothesis PDF Current Directions in Psychological Science 19 6 346 351 doi 10 1177 0963721410391442 S2CID 46218795 Henderson Mark 25 April 2007 Brains don t make you rich IQ study finds The Times London Retrieved 5 May 2010 You Don t Have To Be Smart To Be Rich Study Finds ScienceDaily Retrieved 26 August 2014 Murray 1998 p page needed Bowles Samuel Gintis Herbert 2002 The Inheritance of Inequality Journal of Economic Perspectives 16 3 3 30 doi 10 1257 089533002760278686 Cullen Francis T Gendreau Paul Jarjoura G Roger Wright John Paul October 1997 Crime and the Bell Curve Lessons from Intelligent Criminology Crime amp Delinquency 43 4 387 411 doi 10 1177 0011128797043004001 S2CID 145418972 Handbook of Crime Correlates Lee Ellis Kevin M Beaver John Wright 2009 Academic Press Beaver Kevin M Schwartz Joseph A Nedelec Joseph L Connolly Eric J Boutwell Brian B Barnes J C September 2013 Intelligence is associated with criminal justice processing Arrest through incarceration Intelligence 41 5 277 288 doi 10 1016 j intell 2013 05 001 Beaver Kevin M Wright John Paul January 2011 The association between county level IQ and county level crime rates Intelligence 39 1 22 26 doi 10 1016 j intell 2010 12 002 Mears Daniel P Cochran Joshua C November 2013 What is the effect of IQ on offending Criminal Justice and Behavior 40 11 1280 1300 doi 10 1177 0093854813485736 S2CID 147219554 Freeman James January 2012 The relationship between lower intelligence crime and custodial outcomes a brief literary review of a vulnerable group Vulnerable Groups amp Inclusion 3 1 14834 doi 10 3402 vgi v3i0 14834 S2CID 145305072 Ttofi Maria M Farrington David P Piquero Alex R Losel Friedrich DeLisi Matthew Murray Joseph 1 June 2016 Intelligence as a protective factor against offending A meta analytic review of prospective longitudinal studies Journal of Criminal Justice 45 4 18 doi 10 1016 j jcrimjus 2016 02 003 McGloin Jean Marie Pratt Travis C Maahs Jeff 1 September 2004 Rethinking the IQ delinquency relationship A longitudinal analysis of multiple theoretical models Justice Quarterly 21 3 603 635 doi 10 1080 07418820400095921 S2CID 143305924 Gottfredson Linda S Deary Ian J 22 June 2016 Intelligence Predicts Health and Longevity but Why Current Directions in Psychological Science 13 1 1 4 doi 10 1111 j 0963 7214 2004 01301001 x S2CID 15176389 Batty G David Deary Ian J Gottfredson Linda S 2007 Premorbid early life IQ and Later Mortality Risk Systematic Review Annals of Epidemiology 17 4 278 288 CiteSeerX 10 1 1 693 9671 doi 10 1016 j annepidem 2006 07 010 PMID 17174570 a b Kaufman 2009 p 126 a b Kaufman amp Lichtenberger 2006 Kaufman 2009 p 132 Nisbett Richard E Aronson Joshua Blair Clancy Dickens William Flynn James Halpern Diane F Turkheimer Eric 2012 Intelligence New findings and theoretical developments American Psychologist 67 2 130 159 doi 10 1037 a0026699 ISSN 1935 990X PMID 22233090 Ceci amp Williams 2009 pp 788 789 There is an emerging consensus about racial and gender equality in genetic determinants of intelligence most researchers including ourselves agree that genes do not explain between group differences Intelligence research should not be held back by its past Nature 545 7655 385 386 25 May 2017 Bibcode 2017Natur 545R 385 doi 10 1038 nature 2017 22021 PMID 28541341 S2CID 4449918 Intelligence science has undoubtedly been dogged by ugly prejudice Historical measurements of skull volume and brain weight were done to advance claims of the racial superiority of white people More recently the genuine but closing gap between the average IQ scores of groups of black and white people in the United States has been falsely attributed to genetic differences between the races a b Nisbett Richard E Aronson Joshua Blair Clancy Dickens William Flynn James Halpern Diane F Turkheimer Eric 2012 Group differences in IQ are best understood as environmental in origin PDF American Psychologist 67 6 503 504 doi 10 1037 a0029772 ISSN 0003 066X PMID 22963427 Retrieved 22 July 2013 a b c d Mackintosh 2011 pp 362 363 a b c Hunt 2011 p 389 Templeton A 2016 Evolution and Notions of Human Race In Losos J Lenski R eds How Evolution Shapes Our Lives Essays on Biology and Society Princeton Oxford Princeton University Press pp 346 361 doi 10 2307 j ctv7h0s6j 26 That this view reflects the consensus among American anthropologists is stated in Wagner Jennifer K Yu Joon Ho Ifekwunigwe Jayne O Harrell Tanya M Bamshad Michael J Royal Charmaine D February 2017 Anthropologists views on race ancestry and genetics American Journal of Physical Anthropology 162 2 318 327 doi 10 1002 ajpa 23120 PMC 5299519 PMID 27874171 See also American Association of Physical Anthropologists 27 March 2019 AAPA Statement on Race and Racism American Association of Physical Anthropologists Retrieved 19 June 2020 Jencks Christopher Phillips Meredith 2011 1998 The Black White Test Score Gap Brookings Institution Press p 503 ISBN 9780815746119 The available evidence reviewed by several authors in this volume provides as Richard E Nisbett puts it no evidence for genetic superiority of either race while providing strong evidence for a substantial environmental contribution to the black white IQ gap Birney Ewan Raff Jennifer Rutherford Adam Scally Aylwyn 24 October 2019 Race genetics and pseudoscience an explainer Ewan s Blog Bioinformatician at large Human biodiversity proponents sometimes assert that alleged differences in the mean value of IQ when measured in different populations such as the claim that IQ in some sub Saharan African countries is measurably lower than in European countries are caused by genetic variation and thus are inherent Such tales and the claims about the genetic basis for population differences are not scientifically supported In reality for most traits including IQ it is not only unclear that genetic variation explains differences between populations it is also unlikely Aaron Panofsky Dasgupta Kushan 28 September 2020 How White nationalists mobilize genetics From genetic ancestry and human biodiversity to counterscience and metapolitics American Journal of Biological Anthropology 175 2 387 398 doi 10 1002 ajpa 24150 PMC 9909835 PMID 32986847 S2CID 222163480 T he claims that genetics defines racial groups and makes them different that IQ and cultural differences among racial groups are caused by genes and that racial inequalities within and between nations are the inevitable outcome of long evolutionary processes are neither new nor supported by science either old or new Kaplan Jonathan Michael January 2015 Race IQ and the search for statistical signals associated with so called X factors environments racism and the hereditarian hypothesis Biology amp Philosophy 30 1 1 17 doi 10 1007 s10539 014 9428 0 ISSN 0169 3867 S2CID 85351431 Dickens William T Flynn James R 2006 Black Americans Reduce the Racial IQ Gap Evidence from Standardization Samples PDF Psychological Science 17 10 913 920 doi 10 1111 j 1467 9280 2006 01802 x PMID 17100793 S2CID 6593169 a b Dickens William T Flynn James R 2006 Black Americans Reduce the Racial IQ Gap Evidence from Standardization Samples PDF Psychological Science 17 10 913 920 doi 10 1111 j 1467 9280 2006 01802 x PMID 17100793 S2CID 6593169 Kaufman 2009 p 173 Brody 2005 p page needed Bernie Devlin Stephen E Fienberg Daniel P Resnick Kathryn Roeder eds 1997 Intelligence Genes and Success Scientists Respond to the Bell Curve New York Springer Verlag ISBN 978 0 387 98234 2 page needed Nisbett Richard E Aronson Joshua Blair Clancy Dickens William Flynn James Halpern Diane F Turkheimer Eric 2012 Group differences in IQ are best understood as environmental in origin PDF American Psychologist 67 6 503 504 doi 10 1037 a0029772 ISSN 0003 066X PMID 22963427 Retrieved 22 July 2013 Mitchell Kevin 2 May 2018 Why genetic IQ differences between races are unlikely The idea that intelligence can differ between populations has made headlines again but the rules of evolution make it implausible The Guardian Retrieved 13 June 2020 Mackintosh 2011 p 348 Inzlicht Michael 2011 Stereotype Threat Theory Process and Application Oxford University Press pp 5 141 143 ISBN 978 0199732449 Shuttleworth Edwards Ann B Van der Merwe Adele S 2002 WAIS III and WISC IV South African Cross Cultural Normative Data Stratified for Quality of Education In Ferraro F Richard ed Minority and cross cultural aspects of neuropsychological assessment Exton PA Swets amp Zeitlinger pp 72 75 ISBN 9026518307 Barbara P Uzzell Marcel Ponton Alfredo Ardila International Handbook of Cross Cultural Neuropsychology book ISBN 978 0805835854 2013 a b Plotnik R Kouyoumdjian H 2013 Introduction to Psychology Cengage Learning pp 282 283 ISBN 978 1133939535 a b c Hunt 2011 pp 378 379 Terry WS 2015 Learning and Memory Basic Principles Processes and Procedures Fourth Edition Psychology Press p 356 ISBN 978 1317350873 Chrisler JC McCreary DR 2010 Handbook of Gender Research in Psychology Volume 1 Gender Research in General and Experimental Psychology Springer Science amp Business Media p 302 ISBN 978 1441914651 Hyde J S Linn M C 27 October 2006 DIVERSITY Enhanced Gender Similarities in Mathematics and Science Science 314 5799 599 600 doi 10 1126 science 1132154 PMID 17068246 S2CID 34045261 Hyde Janet S Fennema Elizabeth Lamon Susan J 1990 Gender differences in mathematics performance A meta analysis Psychological Bulletin 107 2 139 155 doi 10 1037 0033 2909 107 2 139 PMID 2138794 Nisbett Richard E Aronson Joshua Blair Clancy Dickens William Flynn James Halpern Diane F Turkheimer Eric 2012 Intelligence New findings and theoretical developments American Psychologist 67 2 130 159 doi 10 1037 a0026699 PMID 22233090 S2CID 7001642 Jensen 1998 p 531 Kavanagh Jennifer 2005 Determinants of Productivity for Military Personnel PDF Report Santa Monica CA RAND Corporation ISBN 0 8330 3754 4 Retrieved 1 July 2017 Kilburn M Rebecca Hanser Lawrence M Klerman Jacob A 2009 Estimating AFQT Scores for National Educational Longitudinal Study NELS Respondents PDF Report Santa Monica CA RAND Corporation Retrieved 1 July 2017 12 00 Mental Disorders Adult www ssa gov U S Social Security Administration Retrieved 1 July 2017 Solomon Deborah 12 December 2004 The Science of Second Guessing The New York Times General and cited references EditAiken Lewis 1979 Psychological Testing and Assessment 3rd ed Boston Allyn and Bacon ISBN 978 0 205 06613 1 Aiken Lewis R 1996 Assessment of Intellectual Functioning Perspectives on Individual Differences 2nd ed New York Plenum Press ISBN 978 0 306 48431 5 LCCN 95026038 American Psychiatric 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Consequencias sociais das diferencas de grupo na capacidade cognitiva Social consequences of group differences in cognitive ability PDF In Flores Mendoza Carmen E Colom Roberto eds Introducao a Psicologia das Diferencas Individuais Introduction to the psychology of individual differences Porto Alegre Brazil ArtMed Publishers pp 433 456 ISBN 978 85 363 1418 1 Gottfredson Linda S 2009 Chapter 1 Logical Fallacies Used to Dismiss the Evidence on Intelligence Testing In Phelps Richard F ed Correcting Fallacies about Educational and Psychological Testing Washington DC American Psychological Association ISBN 978 1 4338 0392 5 Gould Stephen Jay 1981 The Mismeasure of Man New York W W Norton ISBN 978 0 393 30056 7 Christopher Lehmann Haupt 21 October 1981 Books Of The Times The Mismeasure of Man The New York Times Review Gould Stephen Jay 1996 The Mismeasure of Man Rev and expanded ed New York W W Norton ISBN 978 0 393 31425 0 Gregory Robert J 1995 Classification of Intelligence In Sternberg Robert J ed Encyclopedia of human intelligence Vol 1 Macmillan pp 260 266 ISBN 978 0 02 897407 1 Groth Marnat Gary 2009 Handbook of Psychological Assessment 5th ed Hoboken NJ Wiley ISBN 978 0 470 08358 1 Harris Judith Rich 2009 The Nurture Assumption Why Children Turn Out the Way They Do 2nd ed Free Press ISBN 978 1 4391 0165 0 Judith Rich Harris 9 April 2009 Do Parents Matter Scientific American Interview Interviewed by Jonah Lehrer Hopkins Kenneth D Stanley Julian C 1981 Educational and Psychological Measurement and Evaluation 6th ed Engelwood Cliffs NJ Prentice Hall ISBN 978 0 13 236273 3 Hunt Earl 2001 Multiple Views of Multiple Intelligence PsycCRITIQUES 46 1 5 7 doi 10 1037 002513 Hunt Earl B 2011 Human Intelligence Cambridge Cambridge University Press ISBN 978 0 521 70781 7 Jensen Arthur 1969 How Much Can We Boost IQ and Scholastic Achievement Environment Heredity and Intelligence Harvard Educational Review Reprint Series Vol 2 Cambridge MA Harvard Educational Review pp 1 123 ISBN 978 0916690021 LCCN 71087869 Jensen A R How much can we boost IQ and scholastic achievement PDF Citation Classics Review No 41 9 October 1978 Jensen Arthur R 1980 Bias in mental testing New York Free Press ISBN 978 0 02 916430 3 Scarr Sandra 1981 Implicit Messages A Review of Bias in Mental Testing American Journal of Education 89 3 330 338 doi 10 1086 443584 JSTOR 1084961 S2CID 147214993 Jensen Arthur R 1998 The g Factor The Science of Mental Ability Human Evolution Behavior and Intelligence Westport CT Praeger ISBN 978 0 275 96103 9 ISSN 1063 2158 Locurto Charles 1999 A Balance Sheet on Persistence Book Review of Jensen on Intelligence g Factor Psycoloquy 10 59 9 Jensen Arthur R 10 July 2006 Clocking the Mind Mental Chronometry and Individual Differences Elsevier ISBN 978 0 08 044939 5 Retrieved 7 July 2014 Wai Jonathan 2008 Book Review Jensen A R 2006 Clocking the mind Mental chronometry and individual differences Amsterdam Elsevier ISBN 978 0 08 044939 5 PDF Gifted Child Quarterly 52 99 doi 10 1177 0016986207310434 S2CID 143666885 Jensen Arthur R 2011 The Theory of Intelligence and Its Measurement Intelligence 39 4 171 177 doi 10 1016 j intell 2011 03 004 ISSN 0160 2896 Johnson Wendy 2012 How Much Can We Boost IQ An Updated Look at Jensen s 1969 Question and Answer In Slater Alan M Quinn Paul C eds Developmental Psychology Revisiting the Classic Studies Psychology Revisiting the Classic Studies Thousand Oaks CA SAGE ISBN 978 0 85702 757 3 Gamboa Camille May 2013 Review Developmental Psychology Revisiting the Classic Studies ed by Alan M Slater and Paul C Quinn Choice 50 9 Archived from the original on 10 October 2014 Johnson Wendy Turkheimer E Gottesman Irving Bouchard Thomas 2009 Beyond Heritability Twin Studies in Behavioral Research PDF Current Directions in Psychological Science 18 4 217 220 doi 10 1111 j 1467 8721 2009 01639 x PMC 2899491 PMID 20625474 Retrieved 29 June 2010 Kaufman Alan S 2009 IQ Testing 101 New York Springer Publishing ISBN 978 0 8261 0629 2 Kaufman Alan S Lichtenberger Elizabeth O 2006 Assessing Adolescent and Adult Intelligence 3rd ed Hoboken NJ Wiley ISBN 978 0 471 73553 3 Kaufman Scott Barry 1 June 2013 Ungifted Intelligence Redefined Basic Books ISBN 978 0 465 02554 1 Retrieved 1 October 2013 Ungifted Intelligence Redefined The Truth about Talent Practice Creativity and the Many Paths to Greatness Publishers Weekly Review Kranzler John H Floyd Randy G 1 August 2013 Assessing Intelligence in Children and Adolescents A Practical Guide Guilford Press ISBN 978 1 4625 1121 1 Archived from the original on 16 October 2014 Retrieved 9 June 2014 Lahn Bruce T Ebenstein Lanny 2009 Let s celebrate human genetic diversity Nature 461 7265 726 728 Bibcode 2009Natur 461 726L doi 10 1038 461726a ISSN 0028 0836 PMID 19812654 S2CID 205050141 Lohman David F Foley Nicpon Megan 2012 Chapter 12 Ability Testing amp Talent Identification PDF In Hunsaker Scott ed Identification The Theory and Practice of Identifying Students for Gifted and Talented Education Services Waco TX Prufrock pp 287 386 ISBN 978 1 931280 17 4 Archived from the original PDF on 15 March 2016 Retrieved 7 July 2014 Mackintosh N J 1998 IQ and Human Intelligence Oxford Oxford University Press ISBN 978 0 19 852367 3 Mackintosh N J 2011 IQ and Human Intelligence 2nd ed Oxford Oxford University Press ISBN 978 0 19 958559 5 Matarazzo Joseph D 1972 Wechsler s Measurement and Appraisal of Adult Intelligence 5th ed Baltimore MD Williams amp Witkins R D Savage April 1974 Wechsler s Measurement and Appraisal of Adult Intelligence 5th ed British Journal of Industrial Medicine Review 31 2 169 PMC 1009574 McIntosh David E Dixon Felicia A Pierson Eric E Chapter 25 Use of Intelligence Tests in the Identification of Giftedness In Flanagan amp Harrison 2012 pp 623 642 Murray Charles 1998 Income Inequality and IQ PDF Washington DC AEI Press ISBN 978 0 8447 7094 9 Retrieved 7 July 2014 Loury Glenn C 18 May 1998 Charles II Hard Questions Column The New Republic Archived from the original on 24 September 2015 Retrieved 7 July 2014 Naglieri Jack A 1999 Essentials of CAS Assessment Essentials of Psychological Assessment Hoboken NJ Wiley ISBN 978 0 471 29015 5 Neisser Ulrich Boodoo Gwyneth Bouchard Thomas J Boykin A Wade Brody Nathan Ceci Stephen J Halpern Diane F Loehlin John C et al 1996 Intelligence Knowns and unknowns PDF American Psychologist 51 2 77 101 doi 10 1037 0003 066x 51 2 77 ISSN 0003 066X S2CID 20957095 Retrieved 9 October 2014 Noguera Pedro A 30 September 2001 Racial politics and the elusive quest for excellence and equity in education In Motion Magazine Article ER010930002 Retrieved 7 July 2014 Perleth Christoph Schatz Tanja Monks Franz J 2000 Early Identification of High Ability In Heller Kurt A Monks Franz J Sternberg Robert J Subotnik Rena F eds International Handbook of Giftedness and Talent 2nd ed Amsterdam Pergamon pp 297 316 ISBN 978 0 08 043796 5 a gifted sample gathered using IQ gt 132 using the old SB L M in 1985 does not contain the top 2 of the population but the best 10 Plomin Robert DeFries John C Knopik Valerie S Neiderhiser Jenae M 2013 Behavioral Genetics 6th ed Worth Publishers ISBN 978 1 4292 4215 8 Reddy Ajitha 2008 The Eugenic Origins of IQ Testing Implications for Post Atkins Litigation DePaul Law Review 57 667 677 Shurkin Joel 1992 Terman s Kids The Groundbreaking Study of How the Gifted Grow Up Boston MA Little Brown ISBN 978 0 316 78890 8 Frederic Golden 31 May 1992 Tracking the IQ Elite TERMAN S KIDS The Groundbreaking Study of How the Gifted Grow Up By Joel N Shurkin Los Angeles Times Archived from the original on 8 November 2012 Stern William 1914 The Psychological Methods of Testing Intelligence Educational psychology monographs Vol 13 Translated by Guy Montrose Whipple Baltimore MD Warwick amp York ISBN 9781981604999 LCCN 14010447 OCLC 4521857 Retrieved 15 June 2014 Stern William 1912 Die psychologischen Methoden der Intelligenzprufung und deren Anwendung an Schulkindern The Psychological Methods of Testing Intelligence in German Leipzig J A Barth Terman Lewis M Lyman Grace Ordahl George Ordahl Louise Galbreath Neva Talbert Wilford 1915 The Stanford revision of the Binet Simon scale and some results from its application to 1000 non selected children Journal of Educational Psychology 6 9 551 62 doi 10 1037 h0075455 Terman Lewis M 1916 Ellwood P Cubberley ed The Measurement of Intelligence An Explanation of and a Complete Guide to the Use of the Stanford Revision and Extension of the Binet Simon Intelligence Scale Riverside Textbooks in Education Boston Houghton Mifflin Retrieved 26 June 2010 Terman Lewis M Merrill A Maude 1937 Measuring Intelligence A Guide to the Administration of the New Revised Stanford Binet Tests of Intelligence Boston Houghton Mifflin Terman Lewis M Merrill Maude A 1960 Stanford Binet Intelligence Scale Manual for the Third Revision Form L M with Revised IQ Tables by Samuel R Pinneau Boston MA Houghton Mifflin Turkheimer Eric April 2008 A Better Way to Use Twins for Developmental Research PDF LIFE Newsletter 2 1 2 5 Retrieved 29 October 2010 Urbina Susana 2011 Chapter 2 Tests of Intelligence In Sternberg Robert J Kaufman Scott Barry eds The Cambridge Handbook of Intelligence Cambridge Cambridge University Press pp 20 38 ISBN 9780521739115 Wasserman John D Chapter 1 A History of Intelligence Assessment The Unfinished Tapestry In Flanagan amp Harrison 2012 pp 3 55 Wechsler David 1939 The Measurement of Adult Intelligence 1st ed Baltimore MD Williams amp Witkins LCCN 39014016 Wechsler David 1997 Wechsler Adult Intelligence Scale 3rd ed San Antonio TX The Psychological Corporation Wechsler David 2003 Wechsler Intelligence Scale for Children 4th ed San Antonio TX The Psychological Corporation Weiner Irving B Graham John R Naglieri Jack A eds 2 October 2012 Handbook of Psychology Volume 10 Assessment Psychology John Wiley amp Sons ISBN 978 0 470 89127 8 Retrieved 25 November 2013 Weiss Lawrence G Saklofske Donald H Prifitera Aurelio Holdnack James A eds 2006 WISC IV Advanced Clinical Interpretation Practical Resources for the Mental Health Professional Burlington MA Academic Press ISBN 978 0 12 088763 7 This practitioner s handbook includes chapters by L G Weiss J G Harris A Prifitera T Courville E Rolfhus D H Saklofske J A Holdnack D Coalson S E Raiford D M Schwartz P Entwistle V L Schwean and T Oakland Wicherts Jelte M Dolan Conor V Carlson Jerry S van der Maas Han L J 2010 Raven s test performance of sub Saharan Africans Average performance psychometric properties and the Flynn Effect Learning and Individual Differences 20 3 135 151 doi 10 1016 j lindif 2009 12 001 Wicherts Jelte M Dolan Conor V van der Maas Han L J 2010 A systematic literature review of the average IQ of sub Saharan Africans Intelligence 38 1 1 20 doi 10 1016 j intell 2009 05 002 External links Edit Wikiquote has quotations related to Intelligence quotient Look up IQer or IQed in Wiktionary the free dictionary Classics in the History of Psychology Human Intelligence biographical profiles current controversies resources for teachers Retrieved from https en wikipedia org w index php title Intelligence quotient amp oldid 1149046976, wikipedia, wiki, book, books, library,

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