Intelligence quotient

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Intelligence quotient
Race and intelligence
Countries and intelligence
Intelligence: A Unifying
Construct for the Social Sciences

An intelligence quotient, or IQ, is a score derived from one of several different tests designed to mesasure intelligence. The term "IQ" comes from the German term "Intelligenz-Quotient".


Modern mental testing began in France in the nineteenth century. It contributed to separating mental retardation from mental illness and reducing the neglect, torture, and ridicule heaped on both groups.[1]

Francis Galton, half-cousin to Charles Darwin, created the terms psychometrics and eugenics, and 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.[1][2]

Alfred Binet, together with co-workers, after about 15 years of development, published the first IQ test in 1905, which focused on verbal abilities. It was intended to identify mental retardation in school children. [1]

During World War I, tests were needed for evaluating and assigning draftees. This caused a rapid development of several mental tests. Nonverbal or "Performance" tests were developed for those who could not speak English or were suspected of malingering.[1]

IQ tests soon after their creation become widely used for both research and practical applications, such as diagnosing mental retardation and for evaluation of job applicants. Many achievement or aptitude tests, such as those used for gaining admission to higher education, correlate highly with IQ tests. IQ tests have also been highly controversial due to observed group differences, such as between races, as well as the use of IQ tests results in debates regarding issues such as eugenics and immigration.

Mental age vs. modern method

The term "IQ" comes from German "Intelligenz-Quotient", coined by the German psychologist William Stern in 1912, who proposed a method of scoring children's intelligence tests. He calculated the IQ score 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. This method has several problems such as not working for adults.

Modern tests therefore use a different procedure. 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 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. Although the term "IQ" is still in common use, it is now an inaccurate description, mathematically speaking, since a quotient is no longer involved.

The values of 100 and 15 were chosen in order to get scores somewhat similar to those in the older type of test. Likely as a part of the rivalry between the Binet and the Wechsler IQ tests, the Binet test 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 the older method age-specific scores are referred to as "ratio IQs".[1]

Modern tests

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Graph showing the distribution of IQ in a population. The different colors represent 15 IQ points or 1 standard deviation.

Approximately 95% of the population have scores within two standard deviations (SD) of the average result of 100. 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. 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 cognitive 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.

Reliability and validity

IQ tests are generally regarded as having high statistical reliability. A high reliability implies that while test-takers can have varying scores on differing occasions when taking the same test and can vary in scores on different IQ tests taken at the same age, the scores generally agree. 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 3 points, or in other words, the odds are about 2 out of 3 that a person's true IQ is in range from 3 points above to 3 points below the test IQ. Another description is that there is a 95% chance that the true IQ is in range from 4-5 points above to 4-5 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.[1][3][4]

The general intelligence factor (g)

Test item similar to the items in the IQ test "Raven's Progressive Matrices". The person tested should indicate what figure is missing in the lower right corner. The test is not dependent on language, which is usually considered an advantage.

Non-IQ psychometric tests are primarily not intended to measure intelligence itself, but some closely related construct, such as scholastic aptitude. In the United States, examples include the SSAT, the SAT, the ACT, the GRE, the MCAT, the LSAT, and the GMAT.[5]

There are many different kinds of IQ and non-IQ psychometric tests, using a wide variety of methods. Some tests are visual, some are verbal, some tests only use of abstract-reasoning problems, and some tests concentrate on arithmetic, spatial imagery, reading, vocabulary, memory or general knowledge. A person doing well on one test tends to do well on the other tests. Thus, the test results are correlated with one another. The psychologist Charles Spearman made the first formal factor analysis of correlations between the tests in the early 20th century. He found that a single common factor explained for the positive correlations among tests. He called this factor g, for "general intelligence", "general mental ability", or "general intelligence factor". In addition, there are also smaller, specific factors or abilities for specific areas, labeled s. This is a theory still accepted in principle by many psychometricians. In any collections 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 definitely proven, theory. Other factor analyses of the data with different results are possible. Some psychometricians regard g as a statistical artifact. The accepted best measure of g is Raven's Progressive Matrices, which is a test of visual reasoning.[6][6]

An alternative interpretation of the high correlations between IQ and non-IQ psychometric tests is the "mutualism model". It argues that intelligence depends on several independent mechanisms, none of which influences performance on all cognitive tests. These mechanisms support each other so that efficient operation of one of them makes efficient operation of the others more likely, thereby creating the positive correlations.[7]

Cattell-Horn-Carroll theory

Many of the broad, recent IQ tests have been greatly influenced by the Cattell-Horn-Carroll theory. It is argued to reflect much of what is known about intelligence from research. A hierarchy of factors is used. g is at the top. Under it, there are 10 broad abilities, that in turn are subdivided into 70 narrow abilities. The broad abilities are: [1]

  • 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): the ability to comprehend quantitative concepts and relationships and to manipulate numerical symbols.
  • Reading & 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): reflect the immediacy with which an individual can react to stimuli or a task (typically measured in seconds or fractions of seconds; not to be confused with Gs, which typically is measured in intervals of 2–3 minutes).

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.[1] 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. [1]

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.[1]

Flynn effect

Main article: Flynn effect

IQ and age

IQ can change to some degree over the course of childhood.[1] 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 5, 6 and 7 and at r=.96 with the mean scores of tests at ages 11, 12 and 13.[5]

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.[5]

IQ is highly stable during life and has been largely resistant to interventions aimed to change it long-term and substantially.[8][9][10]

There have been a variety of studies of IQ and aging 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 for to gain accurate data. It is unclear whether any lifestyle intervention can preserve fluid intelligence into older ages.[1]

The peak of capacity for both fluid intelligence and crystallized intelligence occurs at age 26. This is followed by a slow decline.[11]

Heritability of IQ

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


"Heritability" is defined as the proportion of variance of a trait that is attributable to genetic factors within a defined population in a specific environment. A heritability of 1 indicates that all variation is genetic in origin and a heritability of 0 indicates that none of the variation is genetic. The heritability figure may change if the balance between genetic and environmental factors change. For example, if the environment becomes more similar for everyone in the group, then genetic factors will determine more of the variation and the heritability figure will increase.[8]

Heritability can be estimated using twin studies.[12][13][14] The report "Intelligence: Knowns and Unknowns" stated that in the United States, heritability has been estimated to be 0.75 in adults and 0.45 in children. Newer estimates indicate that heritability might be as high as 0.80 in adulthood.[15][16] That heritability increases with age may be due persons with increasing age being increasingly able to choose their own environment. People with a genetically higher IQ may choose more intellectually stimulating environments, which reinforce their already high IQ, while the opposite occurs for people with low IQ.[5] Other researchers have argued for an adult heritability of 0.5.[1]

Brain size have in studies had a heritability of 0.5-0.8.[17]

A high heritability of a trait does not mean that environmental effects such as learning are not involved. Vocabulary size, for example, is very substantially heritable (and highly correlated with general intelligence), although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions and thus heritability is high.[5]

If the environment relevant to a given trait changes in a way that affects all members of the population equally, the mean value of the trait will change without any change in its heritability (because the variation or differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase.[5]

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. Furthermore, there may be differences regarding the effects on g and on non-g factors, with g possibly being harder to affect and environmental interventions disproportionately affecting non-g factors.[10]

Shared family environment

There are aspects of environments that family members have 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). There is a similar effect for several other psychological traits. These studies have not looked at the effects of extreme environments, such as in abusive families.[5][18][19][20]

By age 10, genetic variance is larger than shared environmental variance and heritability of IQ reaches an asymptote at about 0.80 at 18-20 years of age and continuing at that level well into adulthood.[16]

Non-shared family environment and environment outside the family

Although parents treat their children differently, such differential treatment explains only a small amount of non-shared environmental influence. One suggestion is that children react differently to the same environment due to different genes. Another influence is the impact of peers and other experiences outside the family.[5][19] Accidents and diseases not affecting the family equally are other examples.


Below are presented correlations between different groups of people demonstrating that IQ scores are more similar for people who are more similar genetically. Note that even the same person tested twice do not get a perfect correlation, but a correlation of 0.95.[1]

Reared/living together

  • Identical twins—Reared together 0.86
  • Fraternal twins—Reared together 0.55
  • Biological siblings—Reared together 0.47
  • Parent-child—Living together 0.42
  • Unrelated children—Reared together 0.30
  • Adoptive parent–child—Living together 0.19

Not reared/living together

  • Identical twins—Reared apart 0.76
  • Fraternal twins—Reared apart 0.35
  • Biological siblings—Reared apart 0.24
  • Parent-child—Living apart 0.22

Regression toward the mean

Regression towards the mean is a statistical phenomenon that occurs when an outcome is determined by many independent factors. If an outcome is extreme, then this occurred because most of the independent factors agreed by chance. This is unlikely to occur again, so the next outcome is likely to be less extreme. If IQ is determined by many factors, genetic and/or environmental, then they must mostly agree in the same direction in order to produce an extreme IQ. The child of a person with an extreme IQ is unlikely to have all the factors agree so similarly, so the child is on average likely to have a less extreme IQ.[1]

Average IQ for parents and children from different occupations

Average IQ for different occupations groups and the average IQ of children with two parents from the same occupational group.

  • Professional and technical: 112
    • Their children: 108
  • Managers and administrators: 104
    • Their children: 103
  • Clerical workers; sales workers; skilled workers, craftsmen, and foreme: 101
    • Their children: 100
  • Semi-skilled workers: 92
    • Their children: 96
  • Unskilled workers: 87
    • Their children: 87

Brain shape

People who are more similar genetically also have more similar shaped brains, according a study using modern imaging technology. This was particularly true for the frontal lobes and areas involved in speech. These areas were also associated with intelligence in the study.[17]


In the middle of the twentieth century, a large number of early childhood intervention programs, such as the Head Start program, were tried with one expectation being that these would eliminate or substantially reduce various IQ gaps, including the racial IQ gaps. Large initial IQ gains were also found, but the initial enthusiasm declined, as it become apparent that the IQ or achievement tests gains soon faded away as the children grew older. For example, a 1995 review of 36 such early intervention programs found no consistent pattern of lasting effects on IQ or achievement tests. There are a few exceptions, but these have been criticized on various grounds.[21] It has been speculated that such programs would be more likely to produce long-term IQ gains, if they taught children how to replicate outside the program the kinds of cognitively demanding experiences that may have produced the IQ gains, while they were in the programs.[22][23]

Many other interventions have also produced minor gains in IQ, but lasting gains from long-term follow-up of an experimental study is lacking. For example, listening to classical music was found to increase spatial ability in one study. However, this effect is a short term effect and usually lasts no longer than 10 to 15 minutes. This phenomenon was coined the Mozart effect.[24] Another study found that having received musical training in childhood correlated with higher than average IQ in adults. However, this was not a follow-up of an experimental study, which means that there may be other explanations, such as those who already had a higher IQ being more likely to take and continue with music training.[25] A newer study strongly suggests that associations between music practice and IQ in the general population are non-causal in nature.[26]

IQ and brain anatomy

Meta-analyses and reviews show a correlation between brain size and IQ. A 2009 literature review stated that in 28 samples using modern brain imaging techniques the mean brain size/g correlation was 0.40 (N = 1,389). In 59 samples using external head size measures it was 0.20 (N = 63,405). In 6 studies that corrected for that different IQ subtests measure g unequally well, the mean correlation was 0.63. Some studies have found the whole brain to be important for g while others have found the frontal lobes to be particularly important. Two studies founds correlations of 0.48 and 0.56 between brain size and the number of neurons in the cerebral cortex (based on counting in representative areas).[27][17]

In 2014, a large meta-analysis showed robust and significant positive associations of brain volume and IQ (r = .24) but also states that older studies overestimated the correlation.[28]

A 2009 review stated that the majority of data shows that both gray matter and white matter volume correlate with IQ, but the correlation is stronger for gray matter. Increased number of neurons in the gray matter may explain the higher correlation, but not necessarily so since glucose consumption and intelligence measures correlate negatively, which may mean intelligent individuals use their neurons more efficiently, such as being more efficient in their formation of synapses between neurons, which help to create more efficient neural circuitry. The white matter correlation may be due to more myelination or better control of pH and thus enhanced neural transmission. For more specific regions, the most frequently replicated positive correlations appear localized in the lateral and medial frontal lobe cortex. Positive correlations are also found with volume in many other areas. Cortical thickness may be a better measure than gray matter volume, although this may vary with age, with an initially negative correlation in early childhood becoming positive later. The explanation may again be that more intelligent individuals manage their synapses better. During evolution, not only brain size, but also brain folding has increased, which has increased the surface area. Convolution data may support "The Parieto-Frontal Integration Theory", which see medial cortex structures as particularly important. Volume of the corpus callosum or subareas were found to be important in several studies, which may be due to more efficient inter-hemispheric information transfer.[29]

Health and IQ

Social outcomes

IQ and associations
with social outcomes
Countries and intelligence:
Associations with other variables
Countries and intelligence:
Within-country regions
Dysgenics: Pessimism regarding
the future of Western civilization
Effects of race mixing:
Latin America
Intelligence quotient:
Social outcomes
Race and intelligence:
Historical societies
Race and intelligence:
Modern societies
Smart fraction
The Bell Curve: Tables

Outside of academic research and health care, IQ testing is often done due to its ability to predict academic achievement, future job performance, and other variables of interest. Academic research has also examined these associations, as well as the associations of IQ with many other social outcomes, such as income and wealth.

The following sections discuss associations between an individual's IQ and social outcomes. The associations between the average IQ of a group and social outcomes may be even more important.[30]

Real-life accomplishments

Average adult IQ associated with real-life accomplishments:[1]

  • MDs or PhDs 125
  • College graduates 115
  • 1–3 years of college 105-110
  • Clerical and sales workers 100-105
  • High school graduates, skilled workers (e.g., electricians, cabinetmakers) 100
  • 1–3 years of high school (completed 9–11 years of school) 95
  • 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:[1]

  • 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; farmers and farm managers) 92
  • Unskilled workers 87

Type of work that can be accomplished:[1]

  • Adults can harvest vegetables, repair furniture 60
  • Adults can do domestic work, simple carpentry 50
  • Adults can mow lawns, do simple laundry 40

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.[1]

"Explained variance"

Many of the arguments and criticisms regarding the associations between IQ and social outcomes assume that how much of the variance of an outcome that can be explained by IQ (explained variance) can be calculated as the square of the correlation coefficient between IQ and the outcome. This way of calculating explained variance has been criticized as inappropriate for most social scientific work.[31]

Other tests

A review found that certain IQ tests had an average correlation of about 0.7 with achievement tests.[1][32] Another study found a correlation of 0.82 between g and SAT scores.[33]

A study looking at English students found a correlation of 0.81 between g and GCSE scores and that the explained variance ranged "from 58.6% in Mathematics and 48% in English to 18.1% in Art and Design" (see criticism of explained variance calculation above).[34]

School performance

The 1995 report "Intelligence: Knowns and Unknowns" stated 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 0.5. 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" (see criticism of explained variance calculation above).[5]

Job performance

One review stated that "for hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability."[35] 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.[36] The correlations were higher when the unreliability of measurement methods were controlled for.[5] While IQ is more strongly correlated with reasoning and less so with motor function,[37] IQ-test scores predict performance ratings in all occupations.[35] 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.[35] It is largely mediated through the quicker acquisition of job-relevant knowledge that IQ predicts job performance.

In establishing a causal direction to the link between IQ and work performance, longitudinal studies suggest that IQ exerts a causal influence on future academic achievement, whereas academic achievement does not substantially influence future IQ scores.[38] Other studies state 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.[39]

Other studies show that ability and performance for jobs are linearly related, such that at all IQ levels, an increase in IQ translates into a concomitant increase in performance.[40]

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.[41]

The 1995 report "Intelligence: Knowns and Unknowns" stated that since the correlation is not extremely high other factors such as interpersonal skills and aspects of personality are probably also important, but at the time of the report there were no equally reliable instruments to measure them. Sometimes IQ scores have been described as the "best available predictor" of job performance.[5]

Military performance

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.[42]

During the Vietnam War and a shortage of men to draft, "Secretary of Defense Robert McNamara arrived at a more permanent workaround. The US government would draft men whose low IQ scores had hitherto disqualified them from military service. This stratagem—codenamed ‘Project 100,000’—is detailed along with its dreadful consequences in the book McNamara’s Folly by the late Hamilton Gregory. Gregory witnessed the fate of the low-IQ draftees firsthand while he was a soldier in Vietnam. These draftees—cruelly nicknamed ‘McNamara’s Morons’—were generally capable of completing simple tasks, but even a simple task imperfectly executed can be disastrous in warfare. [...] What happened to many of the 100,000 (whose actual total exceeded 350,000) is not hard to predict. “To survive in combat you had to be smart,” Gregory writes. “You had to know how to use your rifle effectively and keep it clean and operable, how to navigate through jungles and rice paddies without alerting the enemy, and how to communicate and cooperate with other members of your team.” Fulfilling all or any one of these minimum requirements for survival in a battlefield is contingent upon a certain level of verbal and visuospatial intelligence, which many of McNamara’s draftees did not possess. This ultimately led to their fatality rate in Vietnam exceeding that of other GIs by a factor of three."[43]

Income and wealth

The report 1995 Intelligence: Knowns and Unknowns stated that IQ scores account for (explained variance) about one-fourth of the social status variance and one-sixth of the income variance (a correlation of 0.4). Statistical controls for parental SES eliminate about a quarter of this predictive power (see the criticism of the explained variance calculation above).[5] This has been criticized as based on young adults (many of whom have not yet completed their education). Arthur Jensen argued 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.[44]

A 2002 study further examined the impact of non-IQ factors on income and argued that an individual's location, inherited wealth, race, and schooling are more important as factors in determining income than IQ.[45]

Researchers have argued 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."[46]

It has also been argued that while higher IQ increases income, it has little effect on absolute wealth. Very rich people may have achieved their money through inheritance or entrepreneurship. Thus, their wealth has not been achieved by accumulating high salaries.[47]

IQ and crime

The 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 would mean that the explained variance is less than 4%. Similarly, the correlations for most "negative outcome" variables was typically smaller than .20 (see criticism of explained variance calculation above). The report stated that 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.[5]

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. This is a non-linear relationship which would mean that the overall correlation would be misleadingly low.

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 argue" against the theory.[48]

A large (n=21513) Finnish study showed a mostly linear association between IQ and criminal offending.[49]

Religiosity and IQ

Several large studies in the United States have found significant but relatively weak associations between a lower IQ and religiosity. The relationship has remained after controlling for education and was strongest for fundamentalism.[50][51][52]

IQ and dysgenics

Main article: Dysgenics

Group differences

Among the most controversial issues related to the IQ is the observation that average IQ scores and/or more narrow ability average test scores vary between ethnic/racial groups and between the 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.


Main article: Race and intelligence


Map showing the average IQs of different countries according the IQ and the Wealth of Nations.


The report 1995 Intelligence: Knowns and Unknowns stated that most tests have been constructed to give equal overall scores to men and women. Men have a large advantage in visual-spatial tasks like mental rotation and spatiotemporal tasks like tracking a moving object through space. This explains their better performance in tasks involving aiming and throwing. Males also score higher on quantitative, mechanical,[53] and proportional reasoning. Females score higher on verbal tasks. Many more males than females are diagnosed with dyslexia and reading disabilities as well as stuttering. Sex hormones have been implicated as a cause of these differences.[5]

Some more recent studies have found somewhat higher average IQ for men than for women, corresponding to the on average somewhat larger brains of men.[54][55] For normal distributions, such as IQ and height, if there is a small average difference, then it will be amplified at the extremes. There is a 30:1 ratio of men to women who have a height of 5 feet ten inches; there is a 2000:1 ratio for a height of 6 feet.[56][57]

Studies have also found greater variance in the scores of men compared to that of women.[58][59] This would also cause greater differences between men and women at extreme IQ scores.

Public policies directly using IQ

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).

Internationally, certain public policies, such as improving nutrition and prohibiting neurotoxins, have as one of their goals raising, or preventing a decline of, IQ.

In the United States, certain public policies and laws regarding military service,[60][61] education, public benefits,[62] capital punishment,[63] 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 in very rare cases.[64]

Criticism and views

Relationship between IQ and intelligence

Main article: Intelligence

IQ is by far the most researched approach to intelligence and by far the most widely used in practical settings, due to its documented predictive ability. However, there may be more to intelligence in a broad sense than IQ, with creativity being one example of a trait that may be different from IQ.

Criticism of g

In The Mismeasure of Man (1996), paleontologist Stephen Jay Gould criticized IQ tests and argued that they were used for scientific racism. One argument was directed against g which was argued to be a mathematical artifact. Psychologist Peter Schönemann was another critic.[65][66]

Psychologist Arthur Jensen has rejected this criticism by Gould and also argued that even if g was replaced by a model with several intelligences, then this would change the situation less than expected. All tests of cognitive ability would continue to be highly correlated with one another, as they are currently, and there would still be a Black-White gap on cognitive tests.[67] James R. Flynn, an intelligence researcher known for his criticisms of racial theories of intelligence, similarly argued that "Gould's book evades all of [Arthur] Jensen's best arguments for a genetic component in the black-white IQ gap by positing that they are dependent on the concept of g as a general intelligence factor. Therefore, Gould believes that if he can discredit g no more need be said. This is manifestly false. Jensen's arguments would bite no matter whether blacks suffered from a score deficit on one or 10 or 100 factors."[68]

Early IQ research

Various aspects of early IQ research have sometimes been cited as criticisms. A reply has been that drawing conclusions from early intelligence research is like condemning the auto industry by criticizing the performance of the first automobiles.[69]

Test bias

Regarding various argued issues in connection with race and intelligence research, such as test bias against certain groups, see the article about the research.

Outdated methodology

A 2006 article stated that contemporary psychological 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." However, it also states that an "increasing number of psychometrically informed research papers that have been appearing in the past decade."[70]

High IQ societies

There are social organizations, some international, which limit membership to people who have high test scores. Mensa International is perhaps the most well-known of these.

Incorrect popular usage

The term IQ may be used in various ways which are not identical with psychometric IQ. There also various tests that claim to be IQ tests, but are not properly designed and validated.

External links


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