Intelligence quotient

"IQ" redirects here. For the Inuit traditional knowledge commonly abbreviated as "IQ", see Inuit Qaujimajatuqangit. For the British progressive rock band, see IQ (band). For the film, see I.Q. (film).

Intelligence quotient
An example of one kind of IQ test item, modeled after items in the Raven's Progressive Matrices test.
ICD-9-CM	94.01
MedlinePlus	001912

An intelligence quotient, or IQ, is a score derived from one of several standardized tests designed to assess intelligence. The abbreviation "IQ" comes from the German term Intelligenz-Quotient, originally coined by psychologist William Stern. When modern IQ tests are devised, the mean (average) score within an age group is set to 100 and the standard deviation (SD) almost always to 15, although this was not always so historically. Thus, the intention is that approximately 95% of the population scores within two SDs of the mean, i.e. has an IQ between 70 and 130.

IQ scores have been shown to be associated with such factors as morbidity and mortality, parental social status, and, to a substantial degree, biological parental IQ. While the heritability of IQ has been investigated for nearly a century, there is still debate about the significance of heritability estimates and the mechanisms of inheritance.

IQ scores are used as predictors of educational achievement, special needs, job performance and income. They are also used to study IQ distributions in populations and the correlations between IQ and other variables. The average IQ scores for many populations have been rising at an average rate of three points per decade since the early 20th century, a phenomenon called the Flynn effect. It is disputed whether these changes in scores reflect real changes in intellectual abilities.

History

See also: History of the race and intelligence controversy

Early history

The first large-scale mental test may have been the imperial examination system in China. Modern mental testing began in France in the 19th century. It contributed to separating mental retardation from mental illness and reducing the neglect, torture, and ridicule heaped on both groups.

Englishman Francis Galton coined the terms psychometrics and eugenics, and developed a method for measuring intelligence based on nonverbal sensory-motor tests. It was initially popular, but was abandoned after the discovery that it had no relationship to outcomes such as college grades.

French psychologist Alfred Binet, together with psychologists Victor Henri and Théodore Simon, after about 15 years of development, published the Binet-Simon test in 1905, which focused on verbal abilities. It was intended to identify mental retardation in school children. 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 exactly matched his chronological age, 6.0. (Fancher, 1985). 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. The eugenics movement in the USA seized on it as a means to give them credibility in diagnosing mental retardation, and thousands of American women, most of them poor African Americans, were forcibly sterilized based on their scores on IQ tests, often without their consent or knowledge. 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.

General factor (g)

Main article: g factor

The many different kinds of IQ tests use a wide variety of methods. Some tests are visual, some are verbal, some tests only use abstract-reasoning problems, and some tests concentrate on arithmetic, spatial imagery, reading, vocabulary, memory or general knowledge. The psychologist Charles Spearman in 1904 made the first formal factor analysis of correlations between the tests. He found a single common factor explained the positive correlations among tests. This is an argument still accepted in principle by many psychometricians. Spearman named it g for "general factor" and labelled the smaller, specific factors or abilities for specific areas s. In any collection of IQ tests, by definition the test that best measures g is the one that has the highest correlations with all the others. Most of these g-loaded tests typically involve some form of abstract reasoning. Therefore, Spearman and others have regarded g as the (perhaps genetically determined) real essence of intelligence. This is still a common but not universally accepted view. Other factor analyses of the data, with different results, are possible. Some psychometricians regard g as a statistical artifact. One of the best measures of g is Raven's Progressive Matrices which is a test of visual reasoning.

The War Years

During World War I, a way was needed to evaluate and assign recruits. This led to the rapid development of several mental tests. The testing generated controversy and much public debate. Nonverbal or "performance" tests were developed for those who could not speak English or were suspected of malingering. After the war, positive publicity on army psychological testing helped to make psychology a respected field. Subsequently, there was an increase in jobs and funding in psychology. Group intelligence tests were developed and became widely used in schools and industry.

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, it influenced later theories.

David Wechsler produced the first version of his test in 1939. It gradually became more popular and overtook the 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 10+ subtests provided this. Another is Binet focused on verbal abilities, while the Wechsler also included nonverbal abilities. The 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.

Cattell–Horn–Carroll theory

Main article: Cattell–Horn–Carroll theory

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. 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 he eventually identified 9 or 10 broad abilities. The theory continued to be called Gf-Gc theory.

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.

More recently (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. It has greatly influenced many of the current broad IQ tests.

It is argued that this reflects much of what is known about intelligence from research. A hierarchy of factors is used; g is at the top. Under it are 10 broad abilities that in turn are subdivided into 70 narrow abilities. The broad abilities are:

• Fluid intelligence (Gf) includes the broad ability to reason, form concepts, and solve problems using unfamiliar information or novel procedures.
• Crystallized intelligence (Gc) includes the breadth and depth of a person's acquired knowledge, the ability to communicate one's knowledge, and the ability to reason using previously learned experiences or procedures.
• Quantitative reasoning (Gq) is the ability to comprehend quantitative concepts and relationships and to manipulate numerical symbols.
• Reading and writing ability (Grw) includes basic reading and writing skills.
• Short-term memory (Gsm) is the ability to apprehend and hold information in immediate awareness, and then use it within a few seconds.
• Long-term storage and retrieval (Glr) is the ability to store information and fluently retrieve it later in the process of thinking.
• Visual processing (Gv) is the ability to perceive, analyze, synthesize, and think with visual patterns, including the ability to store and recall visual representations.
• Auditory processing (Ga) is the ability to analyze, synthesize, and discriminate auditory stimuli, including the ability to process and discriminate speech sounds that may be presented under distorted conditions.
• Processing speed (Gs) is the ability to perform automatic cognitive tasks, particularly when measured under pressure to maintain focused attention.
• Decision/reaction time/speed (Gt)reflects the immediacy with which an individual can react to stimuli or a task (typically measured in seconds or fractions of seconds; it is not to be confused with Gs, which typically is measured in intervals of 2–3 minutes). See Mental chronometry.

Modern tests do not necessarily measure of all of these broad abilities. For example, Gq and Grw may be seen as measures of school achievement and not IQ. Gt 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.

Modern comprehensive IQ tests no longer give a single 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

J.P. Guilford's Structure of Intellect (1967) model 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 due to both practical problems and theoretical criticisms.

Alexander Luria's earlier work on neuropsychological processes lead 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, mental retardation, 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 requires 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.

Modern tests

Well-known modern IQ tests include Raven's Progressive Matrices, Wechsler Adult Intelligence Scale, Wechsler Intelligence Scale for Children, Stanford-Binet, Woodcock-Johnson Tests of Cognitive Abilities, and Kaufman Assessment Battery for Children.

Approximately 95% of the population have scores within two standard deviations (SD) of the mean. If one SD is 15 points, as is common in almost all modern tests, then 95% of the population are within a range of 70 to 130, and 98% are below 131. Alternatively, two-thirds of the population have IQ scores within one SD of the mean, i.e. within the range 85-115.

IQ scales are ordinally scaled. 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 means half the cognitive ability of IQ 100. In particular, IQ points are not percentage points.

The correlation between IQ test results and achievement test results is about 0.7.

Mental age vs. modern method

The IQs of a large enough population are calculated so they conform to a normal distribution with a mean of 100 and a standard deviation of 15.

German psychologist William Stern proposed a method of scoring children's intelligence tests in 1912. He calculated what he called a Intelligenz-Quotient score, or IQ, as the quotient of the 'mental age' (the age group which scored such a result on average) of the test-taker and the 'chronological age' of the test-taker, multiplied by 100. Terman used this system for the first version of the Stanford-Binet Intelligence Scales. This method has several problems such as the fact that it cannot be used to score adults.

Wechsler introduced a different procedure for his test that is now used by almost all IQ tests. When an IQ test is constructed, a standardization sample representative of the general population takes the test. The median result is defined to be equivalent to 100 IQ points. In almost all modern tests, a standard deviation of the results is defined to be equivalent to 15 IQ points. When a subject takes an IQ test, the result is ranked compared to the results of the standardization sample and the subject is given an IQ score equal to those with the same test result in the standardization sample.

The values of 100 and 15 were chosen to get somewhat similar scores as in the older type of test. Likely as a part of the rivalry between the Binet and the Wechsler, the Binet until 2003 chose to have 16 for one SD, causing considerable confusion. Today, almost all tests use 15 for one SD. Modern scores are sometimes referred to as "deviation IQs," while older method age-specific scores are referred to as "ratio IQs."

Reliability and validity

Psychometricians generally regard IQ tests as having high statistical reliability. A high reliability implies that—although test-takers may have varying scores when taking the same test on differing occasions, and they may have varying scores when taking different IQ tests at the same age—the scores generally agree with one another and across time. A test-taker's score on any one IQ test is surrounded by an error band that shows, to a specified degree of confidence, what the test-taker's true score is likely to be. For modern tests, the standard error of measurement is about three points, or in other words, the odds are about two out of three that a person's true IQ is in range from three points above to three points below the test IQ. Another description is there is a 95% chance the true IQ is in range from four to five points above to four to five points below the test IQ, depending on the test in question. Clinical psychologists generally regard them as having sufficient statistical validity for many clinical purposes.

IQ scores can differ to some degree for the same individual on different IQ tests (age 12–13 years). (IQ score table data and pupil pseudonyms adapted from description of KABC-II norming study cited in Kaufman 2009.)

Pupil	KABC-II	WISC-III	WJ-III
Asher	90	95	111
Brianna	125	110	105
Colin	100	93	101
Danica	116	127	118
Elpha	93	105	93
Fritz	106	105	105
Georgi	95	100	90
Hector	112	113	103
Imelda	104	96	97
Jose	101	99	86
Keoku	81	78	75
Leo	116	124	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. 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.

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 or not there are social subgroup differences in the effect, and what possible causes of the effect might be. Flynn's observations have prompted much new research in psychology and "demolish some long-cherished beliefs, and raise a number of other interesting issues along the way."

IQ and age

IQ can change to some degree over the course of childhood. However, in one longitudinal study, the mean IQ scores of tests at ages 17 and 18 were correlated at r=.86 with the mean scores of tests at ages five, six and seven and at r=.96 with the mean scores of tests at ages 11, 12 and 13.

IQ scores for children are relative to children of a similar age. That is, a child of a certain age does not do as well on the tests as an older child or an adult with the same IQ. But, relative to persons of a similar age, or other adults in the case of adults, they do equally well if the IQ scores are the same. To convert a child's IQ score into an adult score the following calculation should be made: $\mathrm {IQ_{child}} *\mathrm {age} /16=\mathrm {IQ_{adult}}$. The number 16 is used to indicate the age at which supposedly the IQ reaches its peak.

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. 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. It is unclear whether any lifestyle intervention can preserve fluid intelligence into older ages.

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 the mid adulthood or later. Subsequently, intelligence seems to decline slowly.

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.

Heritability

See also: Heritability of IQ and Environment and intelligence

Heritability is defined as the proportion of variance in a trait which is attributable to genotype within a defined population in a specific environment. A number of points must be considered when interpreting heritability. Heritability measures the proportion of 'variation' in a trait can be attributed to genes, and not the proportion of a trait caused by genes. The value of heritability can change if the impact of environment (or of genes) in the population is substantially altered. A high heritability of a trait does not mean environmental effects, such as learning, are not involved. Since heritability increases during childhood and adolescence, one should be cautious drawing conclusions regarding the role of genetics and environment from studies where the participants are not followed until they are adults.

Studies have found the heritability of IQ in adult twins to be 0.7 to 0.8 and in children twins 0.45 in the Western world. It may seem reasonable to expect genetic influences on traits like IQ should become less important as one gains experiences with age. However, the opposite occurs. Heritability measures in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 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. Debate is ongoing about whether these heritability estimates are too high due to not adequately considering various factors, such as that the environment may be relatively more important in families with low socioeconomic status or the effect of the maternal (fetal) environment.

Recent research suggests that molecular genetics of psychology and social science requires approaches that go beyond the examination of candidate genes.

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 due to 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 impact on the development and functionality of the brain.

While a number of individual genes have been reported to be associated with IQ. Examples include CHRM2, microcephalin, and ASPM. However, Deary and colleagues (2009) argued no evidence has been replicated., a finding supported by Chabris et al (2012)

Recently, FNBP1L polymorphisms, specifically the SNP rs236330 have been associated with normally varying intelligence differences in adults and in children.

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. This has been replicated in infants, children and adolescents in the US, though not outside the US, for instance a reverse result was reported in the UK

Dickens and Flynn (2001) have argued that genes for high IQ initiate environment shaping feedback, as genetic effects cause bright children to seek out more stimulating environments that further increase IQ. In their model, an environment effects decay over time (the model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they caused children to persist in seeking 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 the Head Start Program have not produced lasting gains in IQ scores. More intensive, but much smaller, projects Abecedarian Project have reported lasting effects, often on Socioeconomic status variables, rather than IQ.

A placebo controlled double-blind experiment found that vegetarians who took 5 grams of creatine per day for six weeks showed a significant improvement on two separate tests of fluid intelligence, Raven's Progressive Matrices, and the backward digit span test from the WAIS. The treatment group was able to repeat longer sequences of numbers from memory and had higher overall IQ scores than the control group. The researchers concluded that "supplementation with creatine significantly increased intelligence compared with placebo." A subsequent study found that creatine supplements improved cognitive ability in the elderly. However, a study on young adults (0.03 g/kg/day for six weeks, e.g., 2 g/day for 150-pound individual) failed to find any improvements.

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. 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 of extended periods of time.

Music and IQ

Musical training in childhood has been found to correlate with higher than average IQ. In a 2004 study indicated that 6 year old children who received musical training (voice or piano lessons) had an average increase in IQ of 7.0 points while children who received alternative training (i.e. drama) or no training had an average increase in IQ of only 4.3 points (which may be consequence of the children entering grade school) as indicated by full scale IQ. Children were tested using Wechsler Intelligence Scale for Children–Third Edition, Kaufman Test of Educational Achievement and Parent Rating Scale of the Behavioral Assessment System for Children.

Listening to classical music was reported to increase IQ; specifically spatial ability. In 1994 Frances Rauscher and Gorden Shaw reported that college students who listened to 10 minutes of Mozart's Sonata for Two Pianos, showed an increase in IQ of 8 to 9 points on the spatial subtest on the Standford-Binet Intelligence Scale. The phenomenon was coined the Mozart effect.

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.

Music lessons

In 2004, Schellenberg devised an experiment to test his hypothesis that music lessons can enhance the IQ of children. He had 144 samples of 6 year old children which were put into 4 groups; keyboard lessons, vocal lessons, drama lessons or no lessons at all, for 36 weeks. The samples' IQ was measured both before and after the lessons had taken place using the Wechsler Intelligence Scale for Children–Third Edition, Kaufman Test of Educational Achievement and Parent Rating Scale of the Behavioral Assessment System for Children. All four groups had increases in IQ, most likely resulted by the entrance of grade school. The notable difference with the two music groups compared to the two controlled groups was a slightly higher increase in IQ. The children in the control groups on average had an increase in IQ of 4.3 points, while the increase in IQ of the music groups was 7.0 points. Though the increases in IQ were not dramatic, one can still conclude that musical lessons does have a positive effect for children, if taken at a young age. It is hypothesized that improvements in IQ occur after musical lessons because the music lessons encourage multiple experiences which generates progression in a wide range of abilities for the children. Testing this hypothesis however, has proven difficult.

Another test also performed by Schellenberg tested the effects of musical training in adulthood. He had two groups of adults, one group whom were musically trained and another group who were not. He administered tests of intelligence quotient and emotional intelligence to the trained and non-trained groups and found that the trained participants had an advantage in IQ over the untrained subjects even with gender, age, environmental issues (e.g. income, parent's education) held constant. The two groups, however, score similarly in the emotional intelligence test. The test results (like the previous results) show that there is a positive correlation between musical training and IQ, but it is not evident that musical training has a positive effect on emotional intelligence.

IQ and 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 and IQ

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, sometimes be partially or wholly compensated for by later growth.

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.

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 outcomes

Intelligence is a better predictor of educational and work success than any other single score.

Some measures of educational SAT aptitude are essentially IQ tests; For instance Frey and Detterman (2004) reported a correlation of 0.82 between g (general intelligence factor) and SAT scores another has found correlation of 0.81 between g and GCSE scores.

Correlations between IQ scores (general cognitive ability) and achievement test scores are reported to be 0.81 by Deary and colleagues, with the explained variance ranging "from 58.6% in Mathematics and 48% in English to 18.1% in Art and Design".

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).

It has been found IQ correlation 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 GPA of the last 60 hours. In contrast, Performance IQ correlation with the same GPA was only 0.22 in the same study.

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 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. 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) are more likely to influence performance. It is largely through the quicker acquisition of job-relevant knowledge that higher IQ mediates job performance.

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.

The US military has minimum enlistment standards at about the IQ 85 level. There have been two experiments with lowering this to 80 but in both cases these men could not master soldiering well enough to justify their costs

Some US police departments have set a maximum IQ score for new officers (for example: 125, in New London, CT), under the argument that those with overly-high IQs will become bored and exhibit high turnover in the job. This policy has been challenged as discriminatory, but upheld by at least one US District court.

The American Psychological Association's report "Intelligence: Knowns and Unknowns" states that since the explained variance is 29%, other individual characteristics such as interpersonal skills, aspects of personality etc. are probably of equal or greater importance, but at this point there are no equally reliable instruments to measure them.

Income

While it has been suggested that "in economic terms it appears that the IQ score measures something with decreasing marginal value. It is important to have enough of it, but having lots and lots does not buy you that much.", large scale longitudinal studies indicate an increase in IQ translates into an increase in performance at all levels of IQ: i.e., that ability and job performance are monotonically linked at all IQ levels. Charles Murray, coauthor of The Bell Curve, found that IQ has a substantial effect on income independently of family background.

The link from IQ to wealth is much less strong that than from IQ to job performance. Some studies indicate that IQ is unrelated to net worth.

The American Psychological Association's 1995 report Intelligence: Knowns and Unknowns stated that IQ scores accounted for (explained variance) about 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.

Some studies claim 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 claims 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 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.

IQ and crime

The American Psychological Association's 1995 report Intelligence: Knowns and Unknowns stated that the correlation between IQ and crime was -0.2. It was -0.19 between IQ scores and number of juvenile offenses in a large Danish sample; with social class controlled, the correlation dropped to -0.17. A correlation of 0.20 means that the explained variance is less than 4%. It is important to realize that 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 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.

A study of the relationship between US county-level IQ and US county-level crime rates found that higher average IQs were 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."

Other correlations with IQ

In addition, IQ and its correlation to health, violent crime, gross state product, and government effectiveness are the subject of a 2006 paper in the publication Intelligence. The paper breaks down IQ averages by U.S. states using the federal government's National Assessment of Educational Progress math and reading test scores as a source.

The American Psychological Association's 1995 report Intelligence: Knowns and Unknowns stated that the correlations for most "negative outcome" variables are typically smaller than 0.20, which means that the explained variance is less than 4%.

Tambs et al. found that occupational status, educational attainment, and IQ are individually heritable; and further found that "genetic variance influencing educational attainment ... contributed approximately one-fourth of the genetic variance for occupational status and nearly half the genetic variance for IQ." In a sample of U.S. siblings, Rowe et al. report that the inequality in education and income was predominantly due to genes, with shared environmental factors playing a subordinate role.

A recent USA study connecting political views and intelligence has shown that the mean adolescent intelligence of young adults who identify themselves as "very liberal" is 106.4, while that of those who identify themselves as "very conservative" is 94.8. Two other studies conducted in the UK reached similar conclusions.

There are also other correlations such as those between religiosity and intelligence and fertility and intelligence.

Real-life accomplishments

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

Accomplishment	IQ	Test/study	Year
MDs, JDs, or 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 between 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 intelligence measures such as IQ scores vary between ethnic and racial groups and sexes. While there is little scholarly debate about the existence of some of these differences, their causes remain highly controversial both within academia and in the public sphere.

Sex

Main article: Sex and psychology

Most IQ tests are constructed so that there are no overall score differences between females and males. Because environmental factors affect brain activity and behavior, where differences are found, it can be difficult for researchers to assess whether or not the differences are innate. Areas where differences have been found include verbal and mathematical ability.

Race

Main article: Race and intelligence

The 1996 Task Force investigation on Intelligence sponsored by the American Psychological Association concluded that there are significant variations in IQ across races. The problem of determining the causes underlying this variation relates to the question of the contributions of "nature and nurture" to IQ. Psychologists such as Alan S. Kaufman and Nathan Brody and statisticians such as Bernie Devlin argue that there are insufficient data to conclude that this is because of genetic influences. One of the most notable researchers arguing for a strong genetic influence on these average score differences is Arthur Jensen. In contrast, other researchers such as Richard Nisbett argue that environmental factors can explain all of the average group differences.

Public policy

Main article: Intelligence and public policy

In the United States, certain public policies and laws regarding military service, education, public benefits, capital punishment, 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 mental retardation is in part based on the results of IQ testing. Borderline intellectual functioning is a categorization where a person has below average cognitive ability (an IQ of 71–85), but the deficit is not as severe as mental retardation (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 old, which type of school a child should go to. They have been much less used since the widespread introduction of comprehensive schools.

Criticism and views

Relation between IQ and intelligence

See also: Intelligence

IQ is the most researched approach to intelligence and by far the most widely used in practical setting. However, although IQ attempts to measure some notion of intelligence, it may fail to act as an accurate measure of "intelligence" in its broadest sense. IQ tests only examine particular areas embodied by the broadest notion of "intelligence", failing to account for certain areas which are also associated with "intelligence" such as creativity or emotional intelligence.

There are critics such as Keith Stanovich who do not dispute the stability of IQ test scores or the fact that they predict certain forms of achievement rather effectively. They do argue, however, that to base a concept of intelligence on IQ test scores alone is to ignore many important aspects of mental ability.

Criticism of g

Some scientists dispute IQ entirely. In The Mismeasure of Man (1996), paleontologist Stephen Jay Gould criticized IQ tests and argued that that they were used for scientific racism. He argued that g was a mathematical artifact and criticized:

...the abstraction of intelligence as a single entity, its location within the brain, its quantification as one number for each individual, and the use of these numbers to rank people in a single series of worthiness, invariably to find that oppressed and disadvantaged groups—races, classes, or sexes—are innately inferior and deserve their status.(pp. 24–25)

Psychologist Peter Schönemann was also a persistent critic of IQ, calling it "the IQ myth". He argued that g is a flawed theory and that the high heritability estimates of IQ are based on false assumptions.

Psychologist Arthur Jensen has rejected the criticism by Gould and also argued that even if g was replaced by a model with several intelligences this would change the situation less than expected. All tests of cognitive ability would continue to be highly correlated with one another and there would still be a black-white gap on cognitive tests.

Test bias

See also: Stereotype threat

The American Psychological Association's report Intelligence: Knowns and Unknowns stated that in the United States IQ tests as predictors of social achievement are not biased against African Americans since they predict future performance, such as school achievement, similarly to the way they predict future performance for Caucasians.

However, IQ tests may well be biased when used in other situations. A 2005 study stated 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," 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 for children with autism; 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 children with autism are mentally retarded.

Outdated methodology

See also: Psychometrics

A 2006 article stated that contemporary psychologic research often did not reflect substantial recent developments in psychometrics and "bears an uncanny resemblance to the psychometric state of the art as it existed in the 1950s."

"Intelligence: Knowns and Unknowns"

In response to the controversy surrounding The Bell Curve, the American Psychological Association's Board of Scientific Affairs established a task force in 1995 to write a report on the state of intelligence research which could be used by all sides as a basis for discussion, "Intelligence: Knowns and Unknowns". The full text of the report is available through several websites.

In this paper the representatives of the association regret that IQ-related works are frequently written with a view to their political consequences: "research findings were often assessed not so much on their merits or their scientific standing as on their supposed political implications".

The task force concluded that IQ scores do have high predictive validity for individual differences in school achievement. They confirm the predictive validity of IQ for adult occupational status, even when variables such as education and family background have been statistically controlled. They stated that individual differences in intelligence are substantially influenced by both genetics and environment.

The report stated that a number of biological factors, including malnutrition, exposure to toxic substances, and various prenatal and perinatal stressors, result in lowered psychometric intelligence under at least some conditions. The task force agrees that large differences do exist between the average IQ scores of blacks and whites, saying:

The cause of that differential is not known; it is apparently not due to any simple form of bias in the content or administration of the tests themselves. The Flynn effect shows that environmental factors can produce differences of at least this magnitude, but that effect is mysterious in its own right. Several culturally based explanations of the Black/ White IQ differential have been proposed; some are plausible, but so far none has been conclusively supported. There is even less empirical support for a genetic interpretation. In short, no adequate explanation of the differential between the IQ means of Blacks and Whites is presently available.

The APA journal that published the statement, American Psychologist, subsequently published eleven critical responses in January 1997, several of them arguing that the report failed to examine adequately the evidence for partly genetic explanations.

Dynamic assessment

Notable and increasingly influential alternative to the wide range of standard IQ tests originated in the writings of psychologist Lev Vygotsky (1896-1934) of his most mature and highly productive period of 1932-1934. The notion of the zone of proximal development that he introduced in 1933, roughly a year before his death, served as the banner for his proposal to diagnose development as the level of actual development that can be measured by the child's independent problem solving and, at the same time, the level of proximal, or potential development that is measured in the situation of moderately assisted problem solving by the child. The maximum level of complexity and difficulty of the problem that the child is capable to solve under some guidance indicates the level of potential development. Then, the difference between the higher level of potential and the lower level of actual development indicates the 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.

The ideas on the zone of development were later developed in a number of psychological and educational theories and practices. Most notably, they were developed under the banner of dynamic assessment that focuses on the testing of learning and developmental potential (for instance, in the work of Reuven Feuerstein and his associates, who has criticized standard IQ testing for its putative assumption or acceptance of "fixed and immutable" characteristics of intelligence or cognitive functioning). Grounded in developmental theories of Vygotsky and Feuerstein, who recognized that human beings are not static entities but are always in states of transition and transactional relationships with the world, dynamic assessment received also considerable support in the recent revisions of cognitive developmental theory by Joseph Campione, Ann Brown, and John D. Bransford and in theories of multiple intelligences by Howard Gardner and Robert Sternberg.

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 on some IQ test or equivalent. Mensa International is perhaps the best known of these. There are other groups requiring a score above the 98th percentile.

Reference charts

Main article: IQ reference chart

IQ reference charts are tables suggested by test publishers to divide intelligence ranges in various categories.

See also

• Child prodigy
• Cultural intelligence
• Curiosity quotient
• Developmental disability
• Educational quotient
• Emotional quotient
• Genius
• Idiot
• IQ and the Wealth of Nations Template:Nb10
• Intellectual giftedness

• Late bloomer
• Learning disability
• Malleable intelligence
• Nature versus nurture
• Savant syndrome
• Sentience quotient
• Social IQ
• Spiritual intelligence
• Personality test

References

Notes

Further reading

• Carroll, J.B. (1993). Human cognitive abilities: A survey of factor-analytical studies. New York: Cambridge University Press. ISBN 0-521-38275-0.
• Lahn, Bruce T.; Ebenstein, Lanny (2009). "Let's celebrate human genetic diversity". Nature. 461 (7265): 726–8. doi:10.1038/461726a. PMID 19812654.
• 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.
• Duncan, J.; Seitz, RJ; Kolodny, J; Bor, D; Herzog, H; Ahmed, A; Newell, FN; Emslie, H (2000). "A Neural Basis for General Intelligence". Science. 289 (5478): 457–60. doi:10.1126/science.289.5478.457. PMID 10903207.
• Duncan, John; Burgess, Paul; Emslie, Hazel (1995). "Fluid intelligence after frontal lobe lesions". Neuropsychologia. 33 (3): 261–8. doi:10.1016/0028-3932(94)00124-8. PMID 7791994.
• Flynn, James R. (1999). "Searching for justice: The discovery of IQ gains over time" (PDF). American Psychologist. 54 (1): 5–20. doi:10.1037/0003-066X.54.1.5.
• 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. PMID 15147489.
• Gale, C. R; Deary, I. J; Schoon, I.; Batty, G D.; Batty, G D. (2006). "IQ in childhood and vegetarianism in adulthood: 1970 British cohort study". BMJ. 334 (7587): 245. doi:10.1136/bmj.39030.675069.55. PMC 1790759. PMID 17175567.
• Gottfredson, L (1997). "Why g matters: The complexity of everyday life" (PDF). Intelligence. 24 (1): 79–132. doi:10.1016/S0160-2896(97)90014-3.
• Gottfredson, Linda S. (1998). "The general intelligence factor" (PDF). Scientific American Presents. 9 (4): 24–29.
• Gottfredson, L.S. (2005). "Suppressing intelligence research: Hurting those we intend to help.". In Wright, R.H. and Cummings, N.A (Eds.) (ed.). Destructive trends in mental health: The well-intentioned path to harm (PDF). New York: Taylor and Francis. pp. 155–186. ISBN 0-415-95086-4.{{cite book}}: CS1 maint: multiple names: editors list (link)
• Gottfredson, L.S. (2006). "Social consequences of group differences in cognitive ability (Consequencias sociais das diferencas de grupo em habilidade cognitiva)". In Flores-Mendoza, C.E. and Colom, R. (Eds.) (ed.). Introdução à psicologia das diferenças individuais (PDF). Porto Alegre, Brazil: ArtMed Publishers. pp. 155–186. ISBN 85-363-0621-1.{{cite book}}: CS1 maint: multiple names: editors list (link)
• Gould, S.J. (1996). W. W. Norton & Co. (ed.). The Mismeasure of Man: Revised and Expanded Edition. New-York: Penguin. ISBN 0-14-025824-8.
• Gray, Jeremy R.; Chabris, Christopher F.; Braver, Todd S. (2003). "Neural mechanisms of general fluid intelligence". Nature Neuroscience. 6 (3): 316–22. doi:10.1038/nn1014. PMID 12592404.
• Gray, Jeremy R.; Thompson, Paul M. (2004). "Neurobiology of intelligence: science and ethics". Nature Reviews Neuroscience. 5 (6): 471–82. doi:10.1038/nrn1405. PMID 15152197.
• Haier, R; Jung, R; Yeo, R; Head, K; Alkire, M (2005). "The neuroanatomy of general intelligence: sex matters". NeuroImage. 25 (1): 320–7. doi:10.1016/j.neuroimage.2004.11.019. PMID 15734366.
• Harris, J.R. (1998). The Nurture Assumption: why children turn out the way they do. New York (NY): Free Press. ISBN 0-684-84409-5. {{cite book}}: Invalid |ref=harv (help)
• Hunt, Earl (2001). "Multiple Views of Multiple Intelligence". PsycCRITIQUES. 46 (1): 5–7. doi:10.1037/002513.
• Jensen, A.R. (1979). Bias in mental testing. New York (NY): Free Press. ISBN 0-02-916430-3.
• Jensen, A.R. (1979). The g Factor: The Science of Mental Ability. Wesport (CT): Praeger Publishers. ISBN 0-275-96103-6.
• Jensen, A.R. (2006). Clocking the Mind: Mental Chronometry and Individual Differences. Elsevier. ISBN 0-08-044939-5.
• Kaufman, Alan S. (2009). IQ Testing 101. New York (NY): Springer Publishing. ISBN 978-0-8261-0629-2. {{cite book}}: Invalid |ref=harv (help)
• Klingberg, Torkel; Forssberg, Hans; Westerberg, Helena (2002). "Training of Working Memory in Children With ADHD". Journal of Clinical and Experimental Neuropsychology (Neuropsychology, Development and Cognition: Section A). 24 (6): 781–91. doi:10.1076/jcen.24.6.781.8395. PMID 12424652. {{cite journal}}: Invalid |ref=harv (help)
• McClearn, G. E.; Johansson, B; Berg, S; Pedersen, NL; Ahern, F; Petrill, SA; Plomin, R (1997). "Substantial Genetic Influence on Cognitive Abilities in Twins 80 or More Years Old". Science. 276 (5318): 1560–3. doi:10.1126/science.276.5318.1560. PMID 9171059.
• Mingroni, M (2004). "The secular rise in IQ: Giving heterosis a closer look". Intelligence. 32 (1): 65–83. doi:10.1016/S0160-2896(03)00058-8.
• Murray, C. (1998). Income Inequality and IQ (PDF). Washington (DC): AEI Press. ISBN 0-8447-7094-9.
• Noguera, P.A (2001). "Racial politics and the elusive quest for excellence and equity in education". Motion Magazine. Article # ER010930002.
• Plomin, R.; DeFries, J.C.; Craig, I.W.; McGuffin, P (2003). Behavioral genetics in the postgenomic era. Washington (DC): American Psychological Association. ISBN 1-55798-926-5.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Plomin, R.; DeFries, J.C.; McClearn, G.E.; McGuffin, P (2000). Behavioral genetics (4th ed.). New York (NY): Worth Publishers. ISBN 0-7167-5159-3.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Rowe, D.C.; Vesterdal, W.J.; Rodgers, J.L. (1997). "The Bell Curve Revisited: How Genes and Shared Environment Mediate IQ-SES Associations". {{cite journal}}: Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)
• Schoenemann, P Thomas; Sheehan, Michael J; Glotzer, L Daniel (2005). "Prefrontal white matter volume is disproportionately larger in humans than in other primates". Nature Neuroscience. 8 (2): 242–52. doi:10.1038/nn1394. PMID 15665874.
• Shaw, P.; Greenstein, D.; Lerch, J.; Clasen, L.; Lenroot, R.; Gogtay, N.; Evans, A.; Rapoport, J.; Giedd, J. (2006). "Intellectual ability and cortical development in children and adolescents". Nature. 440 (7084): 676–9. doi:10.1038/nature04513. PMID 16572172.
• Tambs, Kristian; Sundet, Jon Martin; Magnus, Per; Berg, Kåre (1989). "Genetic and environmental contributions to the covariance between occupational status, educational attainment, and IQ: A study of twins". Behavior Genetics. 19 (2): 209–22. doi:10.1007/BF01065905. PMID 2719624.
• Thompson, Paul M.; Cannon, Tyrone D.; Narr, Katherine L.; Van Erp, Theo; Poutanen, Veli-Pekka; Huttunen, Matti; Lönnqvist, Jouko; Standertskjöld-Nordenstam, Carl-Gustaf; Kaprio, Jaakko (2001). "Genetic influences on brain structure". Nature Neuroscience. 4 (12): 1253–8. doi:10.1038/nn758. PMID 11694885.
• Wechsler, D. (1997). Wechsler Adult Intelligence Scale (3rd ed.). San Antonia (TX): The Psychological Corporation.
• Wechsler, D. (2003). Wechsler Intelligence Scale for Children (4th ed.). San Antonia (TX): The Psychological Corporation.
• Weiss, Volkmar (2009). "National IQ means transformed from Programme for International Student Assessment (PISA) Scores". The Journal of Social, Political and Economic Studies. 31 (1): 71–94.

External links

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