Intelligence quotient

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

Anintelligence quotient(IQ) is a total score derived from a set ofstandardized testsor subtests designed to assesshuman intelligence.Originally, IQ was a score obtained by dividing a person'smental agescore, 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.For modernIQ tests,theraw scoreistransformedto anormal distributionwith mean 100 andstandard deviation15.This results in approximately two-thirds of the population scoring between IQ 85 and IQ 115 and about 2 percent eachabove 130andbelow 70.

Intelligence quotient
One kind of IQ test item, modelled after items in theRaven's Progressive Matricestest
ICD-10-PCS	Z01.8
ICD-9-CM	94.01

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".IQ scores have been shown to be associated with such factors asnutrition,parentalsocioeconomic status,morbidityandmortality,parentalsocial status,andperinatal environment.While theheritability of IQhas been investigated for nearly a century, there is still debate about the significance of heritability estimatesand the mechanisms of inheritance.

IQ scores are used for educational placement,assessment of intellectual ability,and evaluating job applicants. In research contexts, they have been studied as predictors ofjob performanceandincome.They are also used to study distributions of psychometric intelligence in populations and thecorrelationsbetween 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 theFlynn effect.Investigation of different patterns of increases in subtest scores can also inform current research on human intelligence.

Historically, many proponents of IQ testing have beeneugenicistswho usedpseudoscienceto push now-debunked views ofracial hierarchyin order to justifysegregationand opposeimmigration.Such views are now rejected by a strong consensus of mainstream science, though fringe figures continue to promote them inpseudo-scholarshipand popular culture.

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 intointelligencecategories by observing their behavior in daily life.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 thereliabilityand error of estimation in the classification procedure.

The English statisticianFrancis Galton(1822–1911) made the first attempt at creating a standardized test for rating a person's intelligence. A pioneer ofpsychometricsand 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 meangenes,although he did develop severalpre-Mendeliantheories of particulate inheritance).He hypothesized that there should exist a correlation between intelligence and other observable traits such asreflexes,muscle grip, andhead size.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.

PsychologistAlfred Binet,co-developer of theStanford–Binet test

French psychologistAlfred Binet,together with Victor Henri andThéodore Simon,had more success in 1905, when they published theBinet–Simon Intelligence test,which focused on verbal abilities. It was intended to identify "mental retardation" in school children,but in specific contradistinction to claims made by psychiatrists that these children were "sick" (not "slow" ) and should therefore be removed from school and cared for in asylums.The score on the Binet–Simon scale would reveal the child'smental age.For example, a six-year-old child who passed all the tasks usually passed by six-year-olds—but nothing beyond—would have a mental age that matched his chronological age, 6.0. (Fancher, 1985). Binet and Simon thought that intelligence was multifaceted, but came under the control of practical judgment.

In Binet and Simon's view, there were limitations with the scale and they stressed what they saw as the remarkable diversity of intelligence and the subsequent need to study it using qualitative, as opposed to quantitative, measures (White, 2000). American psychologistHenry H. Goddardpublished a translation of it in 1910. American psychologistLewis TermanatStanford Universityrevised the Binet–Simon scale, which resulted in theStanford revision of the Binet-Simon Intelligence Scale(1916). It became the most popular test in the United States for decades.

The abbreviation "IQ" was coined by thepsychologistWilliam Sternfor theGermantermIntelligenzquotient,his term for a scoring method forintelligencetests atUniversity of Breslauhe advocated in a 1912 book.

General factor (g)

Main article:gfactor

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 psychologistCharles Spearmanin 1904 made the first formalfactor analysisofcorrelationsbetween 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 itgfor "general factor" and labeled the specific factors or abilities for specific taskss.In any collection of test items that make up an IQ test, the score that best measuresgis the composite score that has the highest correlations with all the item scores. Typically, the "g-loaded "composite score of an IQ test battery appears to involve a common strength in abstract reasoning across the test's item content.

United States military selection in World War I

During World War I, the Army needed a way to evaluate and assign recruits to appropriate tasks. This led to the development of several mental tests byRobert Yerkes,who worked with major hereditarians of American psychometrics—including Terman, Goddard—to write the test.The testing generated controversy and much public debate in the United States. Nonverbal or "performance" tests were developed for those who could not speak English or were suspected of malingering.Based on Goddard's translation of the Binet–Simon test, the tests had an impact in screening men for officer training:

...the tests did have a strong impact in some areas, particularly in screening men for officer training. At the start of the war, the army and national guard maintained nine thousand officers. By the end, two hundred thousand officers presided, and two- thirds of them had started their careers in training camps where the tests were applied. In some camps, no man scoring below C could be considered for officer training.

In total 1.75 million men were tested, making the results the first mass-produced written tests of intelligence, though considered dubious and non-usable, for reasons including high variability of test implementation throughout different camps and questions testing for familiarity with American culture rather than intelligence.After the war, positive publicity promoted by army psychologists helped to make psychology a respected field.Subsequently, there was an increase in jobs and funding in psychology in the United States.Group intelligence tests were developed and became widely used in schools and industry.

The results of these tests, which at the time reaffirmed contemporary racism and nationalism, are considered controversial and dubious, having rested on certain contested assumptions: that intelligence was heritable, innate, and could be relegated to a single number, the tests were enacted systematically, and test questions actually tested for innate intelligence rather than subsuming environmental factors.The tests also allowed for the bolstering ofjingoist narrativesin the context of increased immigration, which may have influenced the passing of theImmigration Restriction Act of 1924.

L.L. Thurstoneargued 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.

David Wechslerproduced 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.

IQ testing and the eugenics movement in the United States

Eugenics,a set of beliefs and practices aimed at improving thegeneticquality of thehuman populationby excluding people and groups judged to be inferior and promoting those judged to be superior,played a significant role in the history and culture of theUnited Statesduring theProgressive Era,from the late 19th century until US involvement inWorld War II.

TheAmerican eugenics movementwas rooted in thebiological deterministideas of the British ScientistSir Francis Galton.In 1883, Galton first used the word eugenics to describe the biological improvement of human genes and the concept of being "well-born".He believed that differences in a person's ability were acquired primarily through genetics and that eugenics could be implemented throughselective breedingin order for the human race to improve in its overall quality, therefore allowing for humans to direct their own evolution.

Henry H. Goddardwas a eugenicist. In 1908, he published his own version,The Binet and Simon Test of Intellectual Capacity,and cordially promoted the test. He quickly extended the use of the scale to the public schools (1913), to immigration (Ellis Island,1914) and to a court of law (1914).

Unlike Galton, who promoted eugenics through selective breeding for positive traits, Goddard went with the US eugenics movement to eliminate "undesirable" traits.Goddard used the term "feeble-minded"to refer to people who did not perform well on the test. He argued that" feeble-mindedness "was caused by heredity, and thus feeble-minded people should be prevented from giving birth, either by institutional isolation or sterilization surgeries.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 rulingBuck v. Bell,forced over 60,000 people to go through sterilization in the United States.

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".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 geneticistOtmar Freiherr von Verschuer) continued to work and promote their ideas in the United States.In later decades, some eugenic principles have made a resurgence as a voluntary means of selective reproduction, with some calling them "new eugenics".As it becomes possible to test for and correlate genes with IQ (and its proxies),ethicists and embryonicgenetic testingcompanies are attempting to understand the ways in which the technology can be ethically deployed.

Cattell–Horn–Carroll theory

Main article:Cattell–Horn–Carroll theory

PsychologistRaymond Cattelldefinedfluid and crystallized intelligenceand authored theCattell Culture Fair IIIIQ 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, andcrystallized 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 studentJohn 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.

John B. Carroll(1993), after a comprehensive reanalysis of earlier data, proposed thethree 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.

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), withgas 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.

Modern tests do not necessarily measure all of these broad abilities. For example,quantitative knowledgeandreading and writing abilitymay be seen as measures of school achievement and not IQ.Decision speedmay be difficult to measure without special equipment.gwas 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.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.

Other theories

An alternative to standard IQ tests, meant to test theproximal developmentof children, originated in the writings of psychologistLev Vygotsky(1896–1934) during his last two years of his life.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.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.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 ofdynamic assessment,which seeks to measure developmental potential(for instance, in the work ofReuven Feuersteinand his associates,who hascriticized standard IQ testingfor its putative assumption or acceptance of "fixed and immutable" characteristics of intelligence or cognitive functioning). Dynamic assessment has been further elaborated in the work ofAnn Brown,andJohn D. Bransfordand in theories ofmultiple intelligencesauthored byHoward GardnerandRobert Sternberg.

J.P. Guilford'sStructure 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 andtheoreticalcriticisms.

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.It has influenced some recent IQ tests, and been seen as a complement to the Cattell–Horn–Carroll theory described above.

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.The most commonly used individual IQ test series is theWechsler Adult Intelligence Scale(WAIS) for adults and theWechsler 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 theStanford–Binet Intelligence Scales,Woodcock–Johnson Tests of Cognitive Abilities,theKaufman Assessment Battery for Children,theCognitive Assessment System,and theDifferential Ability Scales.

There are various other IQ tests, including:

1. Raven's Progressive Matrices(RPM)
2. Cattell Culture Fair III(CFIT)
3. Reynolds Intellectual Assessment Scales(RIAS)
4. Thurstone's Primary Mental Abilities
5. Kaufman Brief Intelligence Test(KBIT)
6. Multidimensional Aptitude Battery II
7. Das–Naglieri cognitive assessment system(CAS)
8. Naglieri Nonverbal Ability Test(NNAT)
9. Wide Range Intelligence Test(WRIT)

IQ scales areordinally scaled.Theraw scoreof the normingsampleis usually (rank order)transformedto anormal distributionwith mean 100 andstandard deviation15.While onestandard deviationis 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

Reliability

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 inKaufman (2009).)

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

Psychometricians generally regard IQ tests as having highstatistical reliability.Reliability represents the measurement consistency of a test.A reliable test produces similar scores upon repetition.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 reportedstandard error of measurementcan be as low as about three points.Reported standard error may be an underestimate, as it does not account for all sources of error.

Outside influences such as low motivation or high anxiety can occasionally lower a person's IQ test score.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.By the same token, high IQ scores are also significantly less reliable than those near to the population median.Reports of IQ scores much higher than 160 are considered dubious.

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.While IQ tests are generally considered to measure some forms of intelligence, they may fail to serve as an accurate measure of broader definitions ofhuman intelligenceinclusive of, for example,creativityandsocial intelligence.For this reason, psychologist Wayne Weiten argues that theirconstruct validitymust be carefully qualified, and not be overstated.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."

Some scientists have disputed the value of IQ as a measure of intelligence altogether. InThe Mismeasure of Man(1981, expanded edition 1996),evolutionary biologistStephen Jay Gouldcompared IQ testing with the now-discredited practice of determining intelligence viacraniometry,arguing that both are based on the fallacy ofreification,"our tendency to convert abstract concepts into entities".Gould's argument sparked a great deal of debate,and the book is listed as one ofDiscover Magazine's "25 Greatest Science Books of All Time".

Along these same lines, critics such asKeith Stanovichdo 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.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 ofgdoes not fully account for the different skills and knowledge types that produce success in human society.

Despite these objections, clinical psychologists generally regard IQ scores as having sufficientstatistical validityfor many clinical purposes.

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 samelatent abilitiesgive different answers to specific questions on the same IQ test.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 groupswith the same underlyinglatentability levelhave a different chance of giving specific responses.Such questions are usually removed in order to make the test equally fair for both groups. Common techniques for analyzing DIF areitem response theory(IRT) based methods, Mantel-Haenszel, andlogistic regression.

A 2005 study found that "differential validity in prediction suggests that theWAIS-Rtest may contain cultural influences that reduce the validity of the WAIS-R as a measure of cognitive ability for Mexican American students, "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.Standard intelligence tests, such as the Stanford–Binet, are often inappropriate forautisticchildren; 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.

Flynn effect

Main article:Flynn effect

Since the early 20th century, raw scores on IQ tests have increased in most parts of the world.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 bookThe Bell CurveafterJames R. Flynn,the author who did the most to bring this phenomenon to the attention of psychologists.

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.A 2011 textbook,IQ and Human Intelligence,byN. 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.A 2011 psychology textbook, lead authored by Harvard Psychologist ProfessorDaniel Schacter,noted that humans' inherited intelligence could begoing downwhile acquired intelligence goes up.

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 thenegative Flynn effect.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.

Age

Ronald S. Wilsonis largely credited with the idea that IQ heritability rises with age.Researchers building on this phenomenon dubbed it "The Wilson Effect," named after the behavioral geneticist.A paper byThomas J. Bouchard Jr.,examining twin and adoption studies, including twins "reared apart," finds that IQ "reaches an asymptote at about 0.80 at 18–20 years of age and continuing at that level well into adulthood. In the aggregate, the studies also confirm that shared environmental influence decreases across age, approximating about 0.10 at 18–20 years of age and continuing at that level into adulthood."IQ can change to some degree over the course of childhood.In onelongitudinal study,the mean IQ scores of tests at ages 17 and 18 were correlated atr= 0.86with the mean scores of tests at ages five, six, and seven and atr= 0.96with the mean scores of tests at ages 11, 12, and 13.

The current consensus is thatfluid intelligencegenerally declines with age after early adulthood, whilecrystallized intelligenceremains intact.However, 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.

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 theFlynn effectand is in part acohorteffect 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. 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.It is unclear whether any lifestyle intervention can preserve fluid intelligence into older ages.

Genetics and environment

Environmentalandgeneticfactors play a role in determining IQ. Their relative importance has been the subject of much research and debate.

Heritability

See also:Heritability of IQandEnvironment and intelligence

The general figure for theheritabilityof IQ, according to anAmerican Psychological Associationreport, is 0.45 for children, and rises to around 0.75 for late adolescents and adults.Heritability measures forgfactorin infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.9 in adulthood.One proposed explanation is that people with different genes tend to reinforce the effects of those genes, for example by seeking out different environments.

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.

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.

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.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.Recent findings of gene associations with normally varying intellectual differences in adults and children continue to show weak effects for any one gene.

A 2017meta-analysisconducted on approximately 78,000 subjects identified 52 genes associated with intelligence.FNBP1Lis reported to be the single gene most associated with both adult and child intelligence.

Gene-environment interaction

David Rowereported an interaction of genetic effects withsocioeconomic status,such that the heritability was high in high-SES families, but much lower in low-SES families.In the US, this has been replicated in infants,children,adolescents,and adults.Outside the US, studies show no link between heritability and SES.Some effects may even reverse sign outside the US.

Dickens and Flynn (2001) have argued that genes for high IQ initiate an environment-shapingfeedback 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.

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 theHead Start Programhave 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.More intensive, but much smaller projects, such as theAbecedarian Project,have reported lasting effects, often on socioeconomic status variables, rather than IQ.

Recent studies have shown that training in using one'sworking memorymay 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 designedworking memory training.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.

Music

Further information:Mozart effect

Musical training in childhood correlates with higher than average IQ.However, a study of 10,500 twins found no effects on IQ, suggesting that the correlation was caused by genetic confounders.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."

It is popularly thought that listening to classical music raises IQ. However, multiple attempted replications (e.g.) 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.

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.

Health

Main articles:Impact of health on intelligenceandCognitive 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.

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.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 productsand 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 IQ increases.

Cognitive epidemiologyis 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

TheAmerican Psychological Association's reportIntelligence: Knowns and Unknownsstates 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).

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.

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

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."The validity of IQ as a predictor ofjob performanceis 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.The correlations were higher when the unreliability of measurement methods was controlled for.While IQ is more strongly correlated with reasoning and less so with motor function,IQ-test scores predict performance ratings in all occupations.

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.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."

Newer studies find that the effects of IQ on job performance have been greatly overestimated. The current estimates of the correlation between job performance and IQ are about 0.23 correcting for unreliability and range restriction.

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.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.

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 aremonotonicallylinked at all IQ levels.

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".

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.The American Psychological Association's 1995 reportIntelligence: Knowns and Unknownsstated 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.Charles Murray(1998) showed a more substantial effect of IQ on income independent of family background.In a meta-analysis, Strenze (2006) reviewed much of the literature and estimated the correlation between IQ and income to be about 0.23.

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 ofThe g Factor,Arthur Jensensays 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 Seligmancites an IQ income correlation of 0.5 (25% of the variance).

A 2002 study 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 reportIntelligence: Knowns and Unknownsstated that thecorrelation between IQ and crimewas −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.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 theexplained varianceaccounts 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.

In his bookThe g Factor(1998),Arthur Jensencited 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 2009Handbook of Crime Correlatesstated 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 withconduct disorderhave lower IQ than their peers "strongly argues" for the theory.

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."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.

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.Indeed, there is evidence that the small relationship is mediated by well-being, substance abuse, and other confounding factors that prohibit simple causal interpretation.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.A nationally representative longitudinal study has shown that this relationship is entirely mediated by school performance.

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.

Other accomplishments

Average adult combined IQs associated with real-life accomplishments by various tests:

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:

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:

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.

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.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.The existence of differences in IQ between the sexes has been debated, and largely depends on which tests are performed.

Race

Main article:Race and intelligence

While the concept of "race"is asocial construct,discussions of a purported relationship between race and intelligence, as well as claims of genetic differences in intelligence along racial lines, have appeared in bothpopular scienceandacademic researchsince the modern concept of race was first introduced.

Genetics do not explain differences in IQ test performance between racial or ethnic groups.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.In recent decades, as understanding ofhuman geneticshas advanced, claims of inherent differences in intelligence between races have been broadly rejected by scientists on boththeoreticalandempiricalgrounds.

Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap.A 1996 task force investigation on intelligence sponsored by theAmerican Psychological Associationconcluded that "because ethnic differences in intelligence reflect complex patterns, no overall generalization about them is appropriate," with environmental factors the most plausible reason for the shrinking gap.A systematic analysis byWilliam DickensandJames 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".The effects ofstereotype threathave been proposed as an explanation for differences in IQ test performance between racial groups,as have issues related to cultural difference and access to education.

Despite the strong scientific consensus to the contrary, fringe figures continue to promotescientific racismabout group-level IQ averages inpseudo-scholarshipand popular culture.

Sex

Main article:Sex differences in intelligence

With the advent of the concept ofgorgeneral intelligence,many researchers have found that there are no significant sex differences in average IQ,though ability in particular types of intelligence does vary.Thus, while some test batteries show slightly greater intelligence in males, others show greater intelligence in females.In particular, studies have shown female subjects performing better on tasks related toverbal ability,and males performing better on tasks related to rotation of objects in space, often categorized asspatial ability.These differences remain, asHunt (2011)observes, "even though men and women are essentially equal in general intelligence".

Some research indicates that male advantages on somecognitive testsare minimized when controlling for socioeconomic factors.Other research has concluded that there is slightlylarger variabilityin male scores in certain areas compared to female scores, which results in slightly more males than females in thetopandbottomof the IQ distribution.

The existence of differences between male and female performance on math-related tests is contested,and a meta-analysis focusing on average gender differences in math performance found nearly identical performance for boys and girls.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.

Public policy

Main article:Intelligence and public policy

In the United States, certainpublic policiesand laws regarding military service,education, public benefits,capital punishment,and employment incorporate an individual's IQ into their decisions. However, in the case ofGriggs v. Duke Power Co.in 1971, for the purpose of minimizing employment practices that disparately impacted racial minorities, theU.S. Supreme Courtbanned the use of IQ tests in employment, except when linked to job performance via ajob analysis.Internationally, certain public policies, such as improving nutrition and prohibitingneurotoxins,have as one of their goals raising, or preventing a decline in, intelligence.

A diagnosis ofintellectual disabilityis in part based on the results of IQ testing.Borderline intellectual functioningis 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, theeleven plus examwhich 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 ofcomprehensive schools.

Classification

Main article:IQ classification

PhysicistStephen Hawking.When asked his IQ, he replied: "I have no idea. People who boast about their IQ are losers."

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".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 Internationalis perhaps the best known of these. The largest 99.9th percentile (three standard deviations above the mean) society is theTriple Nine Society.

See also

• Emotional competence
• Emotional 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 Abilities

General and cited references

• Aiken, Lewis (1979).Psychological Testing and Assessment(3rd ed.). Boston: Allyn and Bacon.ISBN978-0-205-06613-1.
• Aiken, Lewis R. (1996).Assessment of Intellectual Functioning.Perspectives on Individual Differences (2nd ed.). New York: Plenum Press.ISBN978-0-306-48431-5.LCCN95026038.
• American Psychiatric Association (2013).Diagnostic and Statistical Manual of Mental Disorders(5th ed.). Arlington, VA: American Psychiatric Publishing.ISBN978-0-89042-555-8.
• Anastasi, Anne;Urbina, Susana(1997).Psychological Testing(7th ed.). Upper Saddle River, NJ:Prentice Hall.ISBN978-0023030857.
• Binet, Alfred;Simon, Th. (1916).The development of intelligence in children (The Binet–Simon Scale).Publications of the Training School at Vineland New Jersey Department of Research. Vol. 11. Translated by E. S. Kite. Baltimore, MD: Williams & Wilkins.Retrieved18 July2010.
• Borsboom, Denny (September 2006)."The attack of the psychometricians".Psychometrika.71(3): 425–440.doi:10.1007/s11336-006-1447-6.PMC2779444.PMID19946599.
• Brody, Nathan (2005)."Chapter 26: To g or Not to g—That Is the Question".In Wilhelm, Oliver; Engle, Randall W. (eds.).Handbook of Understanding and Measuring Intelligence.Thousand Oaks, CA:SAGE Publications.pp.489–502.ISBN978-0-7619-2887-4.
• Campbell, Jonathan M. (2006). "Chapter 3: Mental Retardation/Intellectual Disability". In Campbell, Jonathan M.; Kamphaus, Randy W. (eds.).Psychodiagnostic Assessment of Children: Dimensional and Categorical Approaches.Hoboken, NJ: Wiley.ISBN978-0-471-21219-5.
• Carroll, John B. (1993).Human cognitive abilities: A survey of factor-analytic studies(PDF).New York:Cambridge University Press.ISBN978-0-521-38275-5.Archived fromthe original(PDF)on 14 July 2014.Retrieved15 May2014.
• Carroll, John B. (1998). "Human Cognitive Abilities: A Critique". In McArdle, John J.; Woodcock, Richard W. (eds.).Human Cognitive Abilities in Theory and Practice.Mahwah, NJ: Lawrence Erlbaum Associates. pp. 5–23.ISBN978-0-8058-2717-0.
• Ceci, Stephen; Williams, Wendy M. (1 February 2009)."Should scientists study race and IQ? YES: The scientific truth must be pursued".Nature.457(7231): 788–789.doi:10.1038/457788a.PMID19212385.S2CID205044224.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.
• Cox, Catherine M. (1926).The Early Mental Traits of 300 Geniuses.Genetic Studies of Genius Volume 2. Stanford, CA: Stanford University Press.
• Deary, Ian(2001).Intelligence: A Very Short Introduction.Oxford:Oxford University Press.ISBN978-0-19-289321-5.
• Deary, Ian J.; Batty, G. David (2007)."Cognitive epidemiology".Journal of Epidemiology and Community Health.61(5): 378–384.doi:10.1136/jech.2005.039206.PMC2465694.PMID17435201.
• Deary, I. J.; Johnson, W.; Houlihan, L. M. (2009)."Genetic foundations of human intelligence"(PDF).Human Genetics.126(1): 215–232.doi:10.1007/s00439-009-0655-4.hdl:20.500.11820/c3e0a75b-dad6-4860-91c6-b242221af681.PMID19294424.S2CID4975607.
• Deary, I. J.; Strand, S.; Smith, P.; Fernandes, C. (2007). "Intelligence and educational achievement".Intelligence.35(1): 13–21.doi:10.1016/j.intell.2006.02.001.
• Detterman, D.K.; Daniel, M.H. (1989). "Correlations of mental tests with each other and with cognitive variables are highest for low IQ groups".Intelligence.13(4): 349–359.doi:10.1016/s0160-2896(89)80007-8.
• Dumont, Ron; Willis, John O.; Elliot, Colin D. (2009).Essentials of DAS-II® Assessment.Hoboken, NJ: Wiley. p.126.ISBN978-0470-22520-2.
• Dumont, Ron; Willis, John O. (2013)."Range of DAS Subtest Scaled Scores".Dumont Willis.Archived fromthe originalon 7 April 2014.
• Eysenck, Hans (1995).Genius: The Natural History of Creativity.Problems in the Behavioural Sciences No. 12. Cambridge: Cambridge University Press.ISBN978-0-5-2148508-1.
• Eysenck, Hans (1998).Intelligence: A New Look.New Brunswick, NJ:Transaction Publishers.ISBN978-0-7658-0707-6.
• Flanagan, Dawn P.; Harrison, Patti L., eds. (2012).Contemporary Intellectual Assessment: Theories, tests, and issues(3rd ed.). New York:Guilford Press.ISBN978-1-60918-995-2.
• Flanagan, Dawn P.;Kaufman, Alan S.(2009).Essentials of WISC-IV Assessment.Essentials of Psychological Assessment (2nd ed.). Hoboken, NJ:Wiley.ISBN978-0470189153.
• Fletcher, Richard B.; Hattie, John (11 March 2011).Intelligence and Intelligence Testing.Taylor & Francis.ISBN978-1-136-82321-3.Retrieved31 August2013.
• Flint, Jonathan; Greenspan, Ralph J.; Kendler, Kenneth S. (28 January 2010).How Genes Influence Behavior.Oxford University Press.ISBN978-0-19-955990-9.
  • Debby Tsuang; Andrew David (June 2011)."How Genes Influence Behavior".American Journal of Psychiatry.168(6): 656–657.doi:10.1176/appi.ajp.2011.11010097.
• Flynn, James R.(2009).What Is Intelligence?: Beyond the Flynn Effect.Cambridge: Cambridge University Press.ISBN978-0-521-74147-7.
  • Cosma Shalizi (27 April 2009)."The Domestication of the Savage Mind".bactra.org.
• Flynn, James R. (2012).Are We Getting Smarter? Rising IQ in the Twenty-First Century.Cambridge: Cambridge University Press.ISBN978-1-107-60917-4.
  • Winerman, Lea (March 2013)."Smarter than ever?".Monitor on Psychology.44(3): 30.
• Frey, Meredith C.; Detterman, Douglas K. (2004). "Scholastic Assessment org?".Psychological Science.15(6): 373–8.doi:10.1111/j.0956-7976.2004.00687.x.PMID15147489.S2CID12724085.
• Freides, David (1972). "Review of Stanford–Binet Intelligence Scale, Third Revision". In Oscar Buros (ed.).Seventh Mental Measurements Yearbook.Highland Park, NJ: Gryphon Press. pp.772–773.
• Georgas, James; Weiss, Lawrence; van de Vijver, Fons; Saklofske, Donald (2003). "Preface". In Georgas, James; Weiss, Lawrence;van de Vijver, Fons;Saklofske, Donald (eds.).Culture and Children's Intelligence: Cross-Cultural Analysis of the WISC-III.San Diego, CA: Academic Press. pp. xvx–xxxii.ISBN978-0-12-280055-9.
• Gottfredson, Linda S.(1997)."Mainstream Science on Intelligence (editorial)"(PDF).Intelligence.24:13–23.doi:10.1016/s0160-2896(97)90011-8.ISSN0160-2896.
• Gottfredson, Linda S. (1997)."Why g matters: The complexity of everyday life"(PDF).Intelligence.24(1): 79–132.CiteSeerX10.1.1.535.4596.doi:10.1016/S0160-2896(97)90014-3.ISSN0160-2896.Retrieved7 July2014.
• Gottfredson, Linda S. (1998)."The general intelligence factor"(PDF).Scientific American Presents.9(4): 24–29.
• Gottfredson, Linda S. (11 March 2005)."Chapter 9: Suppressing Intelligence Research: Hurting Those We Intend to Help"(PDF).In Wright, Rogers H.; Cummings, Nicholas A. (eds.).Destructive Trends In Mental Health: The Well-Intentioned Path to Harm.Taylor & Francis. pp. 155–186.ISBN978-0-203-95622-9.
• Gottfredson, Linda S. (2006)."Conseqüências sociais das diferenças de grupo na capacidade cognitiva"[Social consequences of group differences in cognitive ability](PDF).In Flores-Mendoza, Carmen E.; Colom, Roberto (eds.).Introdução à Psicologia das Diferenças Individuais[Introduction to the psychology of individual differences]. Porto Alegre, Brazil: ArtMed Publishers. pp. 433–456.ISBN978-85-363-1418-1.
• 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.ISBN978-1-4338-0392-5.
• Gould, Stephen Jay(1981).The Mismeasure of Man.New York: W. W. Norton.ISBN978-0-393-30056-7.
  • 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.ISBN978-0-393-31425-0.
• Gregory, Robert J. (1995)."Classification of Intelligence".InSternberg, Robert J.(ed.).Encyclopedia of human intelligence.Vol. 1. Macmillan. pp. 260–266.ISBN978-0-02-897407-1.
• Groth-Marnat, Gary (2009).Handbook of Psychological Assessment(5th ed.). Hoboken, NJ: Wiley.ISBN978-0-470-08358-1.
• Harris, Judith Rich(2009).The Nurture Assumption: Why Children Turn Out the Way They Do(2nd ed.). Free Press.ISBN978-1-4391-0165-0.
  • 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.ISBN978-0-13-236273-3.
• 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.ISBN978-0-521-70781-7.
• 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.ISBN978-0916690021.LCCN71087869.
  • "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.ISBN978-0-02-916430-3.
  •Scarr, Sandra (1981). "Implicit Messages: A Review ofBias in Mental Testing".American Journal of Education.89(3): 330–338.doi:10.1086/443584.JSTOR1084961.S2CID147214993.
• Jensen, Arthur R. (1998).The g Factor: The Science of Mental Ability.Human Evolution, Behavior, and Intelligence. Westport, CT: Praeger.ISBN978-0-275-96103-9.ISSN1063-2158.
  •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.ISBN978-0-08-044939-5.Retrieved7 July2014.
  •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.S2CID143666885.
• Jensen, Arthur R. (2011). "The Theory of Intelligence and Its Measurement".Intelligence.39(4): 171–177.doi:10.1016/j.intell.2011.03.004.ISSN0160-2896.
• 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.Thousand Oaks, CA: SAGE.ISBN978-0-85702-757-3.
  •Gamboa, Camille (May 2013)."(Review) Developmental Psychology: Revisiting the Classic Studies, [ed.] by Alan M. Slater and Paul C. Quinn".Choice.50(9). Archived fromthe originalon 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.PMC2899491.PMID20625474.Retrieved29 June2010.
• Kaufman, Alan S.(2009).IQ Testing 101.New York: Springer Publishing.ISBN978-0-8261-0629-2.
• Kaufman, Alan S.;Lichtenberger, Elizabeth O. (2006).Assessing Adolescent and Adult Intelligence(3rd ed.). Hoboken, NJ:Wiley.ISBN978-0-471-73553-3.
• Kaufman, Scott Barry (1 June 2013).Ungifted: Intelligence Redefined.Basic Books.ISBN978-0-465-02554-1.Retrieved1 October2013.
  • "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.ISBN978-1-4625-1121-1.Archived fromthe originalon 16 October 2014.Retrieved9 June2014.
• 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.ISSN0028-0836.PMID19812654.S2CID205050141.
• Lohman, David F.; Foley Nicpon, Megan (2012)."Chapter 12: Ability Testing & 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.ISBN978-1-931280-17-4.Archived fromthe original(PDF)on 15 March 2016.Retrieved7 July2014.
• Mackintosh, N. J.(1998).IQ and Human Intelligence.Oxford: Oxford University Press.ISBN978-0-19-852367-3.
• Mackintosh, N. J.(2011).IQ and Human Intelligence(2nd ed.). Oxford: Oxford University Press.ISBN978-0-19-958559-5.
• Matarazzo, Joseph D. (1972).Wechsler's Measurement and Appraisal of Adult Intelligence(5th ed.). Baltimore, MD: Williams & 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.PMC1009574.
• McIntosh, David E.; Dixon, Felicia A.; Pierson, Eric E. "Chapter 25: Use of Intelligence Tests in the Identification of Giftedness". InFlanagan & Harrison (2012),pp. 623–642.
• Murray, Charles (1998).Income Inequality and IQ(PDF).Washington, DC:AEI Press.ISBN978-0-8447-7094-9.Retrieved7 July2014.
  •Loury, Glenn C. (18 May 1998)."Charles II".Hard Questions Column.The New Republic.Archived fromthe originalon 24 September 2015.Retrieved7 July2014.
• Naglieri, Jack A. (1999).Essentials of CAS Assessment.Essentials of Psychological Assessment. Hoboken, NJ: Wiley.ISBN978-0-471-29015-5.
• 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.ISSN0003-066X.S2CID20957095.Retrieved9 October2014.
• Noguera, Pedro A. (30 September 2001)."Racial politics and the elusive quest for excellence and equity in education".In Motion Magazine.Article # ER010930002.Retrieved7 July2014.
• Perleth, Christoph; Schatz, Tanja; Mönks, Franz J. (2000). "Early Identification of High Ability". In Heller, Kurt A.; Mönks, Franz J.;Sternberg, Robert J.;Subotnik, Rena F. (eds.).International Handbook of Giftedness and Talent(2nd ed.). Amsterdam: Pergamon. pp.297–316.ISBN978-0-08-043796-5.a gifted sample gathered using IQ > 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.ISBN978-1-4292-4215-8.
• 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.ISBN978-0-316-78890-8.
  • 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.Archivedfrom 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 & York.ISBN9781981604999.LCCN14010447.OCLC4521857.Retrieved15 June2014.
  Stern, William (1912).Die psychologischen Methoden der Intelligenzprüfung: 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.Retrieved26 June2010.
• 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.Retrieved29 October2010.
• Urbina, Susana (2011). "Chapter 2: Tests of Intelligence". InSternberg, Robert J.;Kaufman, Scott Barry (eds.).The Cambridge Handbook of Intelligence.Cambridge: Cambridge University Press. pp.20–38.ISBN9780521739115.
• Wasserman, John D. "Chapter 1: A History of Intelligence Assessment: The Unfinished Tapestry". InFlanagan & Harrison (2012),pp. 3–55.
• Wechsler, David(1939).The Measurement of Adult Intelligence(1st ed.). Baltimore, MD: Williams & Witkins.LCCN39014016.
• 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 & Sons.ISBN978-0-470-89127-8.Retrieved25 November2013.
• 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.ISBN978-0-12-088763-7.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

• Classics in the History of Psychology