Intelligence quotient
Encyclopedia
An intelligence quotient, or IQ, is a score derived from one of several different standardized test
s designed to assess intelligence
. When modern IQ tests are constructed, the mean (average) score within an age group is set to 100 and the standard deviation
(SD) to 15. Today almost all IQ tests adhere to the assignment of 15 IQ points to each standard deviation, but this has not been the case historically. Approximately 95% of the population have scores within two SDs of the mean, i.e., 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, parental IQ. While the heritability
of IQ has been investigated for nearly a century, controversy remains regarding the significance of heritability estimates, and the mechanisms of inheritance are still a matter of some debate
.
IQ scores are used in many contexts: as predictors of education
al achievement or special needs
, by social scientists who study the distribution of IQ scores in populations and the relationships between IQ score and other variables, and as predictors of job performance and income
. 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.
Whether or not IQ tests are an accurate measure of intelligence is open to debate. It is difficult to define exactly what constitutes intelligence; it may be the case that IQ scores represent a very specific type of intelligence.
system in China. 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.
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
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 6 year-old child who passed all the tasks usually passed by 6 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.
made the first formal factor analysis
of correlation
s between the tests in the early 20th century. Spearman's model used a single common factor that accounted for the positive correlations across intelligence tests. His analysis is considered to be the first theory of intelligence. His statistical model of intelligence came to be known as the general intelligence theory
. Using his model, Spearman argued that intelligence is largely a single global ability. For three quarters of a century, Spearman's g theory was the dominant theory of intelligence.
The g-loading of an intelligence test is the degree to the test score reflects general mental ability, as opposed to specific skills related to the test. Most highly g-loaded tests 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 proven view. 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.
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 in order 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 that the Binet focused on verbal abilities while the Wechsler also included non-verbal 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.
(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 revived by his student John L. Horn
(1966) who later argued that 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 re-analysis of earlier data proposed the Three Stratum Theory
, which is a hierarchical model with three levels. At the bottom is the first stratum which 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 IQ tests.
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:
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.
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. The (P)lanning processes involve decision making, problem solving, and performing activities and requires goal setting and self-monitoring. The (A)ttention/arousal process involves selectively attending to a particular stimulus, ignoring distractions, and maintaining vigilance. (S)imultaneous processing involves the integration of stimuli into a group and requires the observation of relationships. (S)uccessive 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.
Well-known modern IQ tests include Wechsler Adult Intelligence Scale
, Wechsler Intelligence Scale for Children
, Stanford-Binet, Woodcock-Johnson Tests of Cognitive Abilities
, Kaufman Assessment Battery for Children
, and Raven's Progressive Matrices
.
Approximately 95% of the population have scores within two standard deviation
s (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. 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.
The correlation
between IQ test results and achievement test
results is about 0.7.
German psychologist William Stern proposed a method of scoring children's intelligence tests in 1902. 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 normalization sample and the subject is given an IQ score equal to those with the same test result in the normalization sample.
The values of 100 and 15 were chosen in order 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."
. 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 persons 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.
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 observation has prompted much new research in psychology and "demolish some long-cherished beliefs, and raise a number of other interesting issues along the way."
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.
For decades, it has been reported in practitioners' handbooks and textbooks on IQ testing that IQ declines with age after the beginning of adulthood. However, later researchers pointed out that this phenomenon is related to the Flynn effect
and is in part a cohort
effect rather than a true aging effect.
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.
The peak of capacity for both fluid intelligence and crystallized intelligence occurs at age 26. This is followed by a slow decline.
factors play a role in determining IQ. Their relative importance have been the subject of much research and debate.
is defined as the proportion of variance
in a trait
which is attributable to genotype
within a defined population in a specific environment. There are a number of points to consider when interpreting heritability. Heritability measures the proportion of variation in a trait that 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 that 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 twins to be between 0.7 and 0.8 in adults and 0.45 in childhood in the Western world
. It may seem reasonable to expect that 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 seek out different environments that reinforce the effects of those genes. There is an ongoing debate 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 socio-economic status or the effect of the maternal (fetal) environment.
, and ASPM. However, Deary and colleagues (2009) argued that there are still almost no replicated evidence. About 20,000 genes are thought to have an impact on the development and functionality of the brain.
on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback
loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues (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 taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program.
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. A study on young adults (0.03 g/kg/day for six weeks, e.g., 2 g/day for 150-pound individual) failed, however, to find any improvements.
Music
al training in childhood has also been found to correlate with higher than average IQ.
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.
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.
Many of the arguments and criticisms assume that explained variance can be calculated as the square of the correlation coefficient
. This way of calculating explained variance has been criticized as inappropriate for most social scientific work. Also, as for the heritability figure, the explained variance only refers to the proportion of variation in an outcome that is explained by a factor, and not the proportion of an outcome that is explained by a 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".
'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 that the 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.
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
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.
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. Charles Murray, coauthor of The Bell Curve
, found that IQ has a substantial effect on income independently of family background.
Taking the above two principles together, very high IQ produces very high job performance, but no greater income than slightly high IQ. Studies also show that high IQ is related to higher 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 (explained variance) a sixth of the variation in income because many studies are based on young adults (many of whom have not yet completed 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.
'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 argue" against 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."
, 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.
There are also other correlations such as those between political views and intelligence, religiosity and intelligence
and fertility and intelligence
.
Average IQ of various occupational groups:
Type of work that can be accomplished:
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.
" 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.
, certain public policies
and law
s 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 school
s.
Binet had designed the Binet-Simon intelligence scale in order to identify students who needed special help in coping with the school curriculum. He argued that with proper remedial education programs, most students regardless of background could catch up and perform quite well in school. He did not believe that intelligence was a measurable fixed entity.
Binet cautioned:
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 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.
(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:
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.
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.
, 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. "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.
is perhaps the most well known of these. There are other groups requiring a score above the 98th percentile.
Standardized test
A standardized test is a test that is administered and scored in a consistent, or "standard", manner. Standardized tests are designed in such a way that the questions, conditions for administering, scoring procedures, and interpretations are consistent and are administered and scored in a...
s designed to assess intelligence
Intelligence
Intelligence has been defined in different ways, including the abilities for abstract thought, understanding, communication, reasoning, learning, planning, emotional intelligence and problem solving....
. When modern IQ tests are constructed, the mean (average) score within an age group is set to 100 and the standard deviation
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
(SD) to 15. Today almost all IQ tests adhere to the assignment of 15 IQ points to each standard deviation, but this has not been the case historically. Approximately 95% of the population have scores within two SDs of the mean, i.e., an IQ between 70 and 130.
IQ scores have been shown to be associated with such factors as morbidity and mortality
Mortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
, parental social status, and, to a substantial degree, parental IQ. While the heritability
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
of IQ has been investigated for nearly a century, controversy remains regarding the significance of heritability estimates, and the mechanisms of inheritance are still a matter of some debate
Nature versus nurture
The nature versus nurture debate concerns the relative importance of an individual's innate qualities versus personal experiences The nature versus nurture debate concerns the relative importance of an individual's innate qualities ("nature," i.e. nativism, or innatism) versus personal experiences...
.
IQ scores are used in many contexts: as predictors of education
Education
Education in its broadest, general sense is the means through which the aims and habits of a group of people lives on from one generation to the next. Generally, it occurs through any experience that has a formative effect on the way one thinks, feels, or acts...
al achievement or special needs
Mental retardation
Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...
, by social scientists who study the distribution of IQ scores in populations and the relationships between IQ score and other variables, and as predictors of job performance and income
Income
Income is the consumption and savings opportunity gained by an entity within a specified time frame, which is generally expressed in monetary terms. However, for households and individuals, "income is the sum of all the wages, salaries, profits, interests payments, rents and other forms of earnings...
. 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
Flynn effect
The Flynn effect is the name given to a substantial and long-sustained increase in intelligence test scores measured in many parts of the world. When intelligence quotient tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100...
. It is disputed whether these changes in scores reflect real changes in intellectual abilities.
Whether or not IQ tests are an accurate measure of intelligence is open to debate. It is difficult to define exactly what constitutes intelligence; it may be the case that IQ scores represent a very specific type of intelligence.
Early history
The first large scale mental test may have been the imperial examinationImperial examination
The Imperial examination was an examination system in Imperial China designed to select the best administrative officials for the state's bureaucracy. This system had a huge influence on both society and culture in Imperial China and was directly responsible for the creation of a class of...
system in China. Modern mental testing began in France in the nineteenth century. It contributed to separating mental retardation
Mental retardation
Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...
from mental illness
Mental illness
A mental disorder or mental illness is a psychological or behavioral pattern generally associated with subjective distress or disability that occurs in an individual, and which is not a part of normal development or culture. Such a disorder may consist of a combination of affective, behavioural,...
and reducing the neglect, torture, and ridicule heaped on both groups.
Englishman Francis Galton
Francis Galton
Sir Francis Galton /ˈfrɑːnsɪs ˈgɔːltn̩/ FRS , cousin of Douglas Strutt Galton, half-cousin of Charles Darwin, was an English Victorian polymath: anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician...
coined the terms psychometrics
Psychometrics
Psychometrics is the field of study concerned with the theory and technique of psychological measurement, which includes the measurement of knowledge, abilities, attitudes, personality traits, and educational measurement...
and eugenics
Eugenics
Eugenics is the "applied science or the bio-social movement which advocates the use of practices aimed at improving the genetic composition of a population", usually referring to human populations. The origins of the concept of eugenics began with certain interpretations of Mendelian inheritance,...
, 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
Alfred Binet
Alfred Binet was a French psychologist who was the inventor of the first usable intelligence test, known at that time as the Binet test and today referred to as the IQ test. His principal goal was to identify students who needed special help in coping with the school curriculum...
, together with psychologists Victor Henri and Théodore Simon
Theodore Simon
Théodore Simon was a French psychologist and psychometrician. He co-created the Binet-Simon Intelligence Scale tests with Alfred Binet.- Biography :...
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 6 year-old child who passed all the tasks usually passed by 6 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
Henry H. Goddard
Henry Herbert Goddard was a prominent American psychologist and eugenicist in the early 20th century...
published a translation of it in 1910. The eugenics
Eugenics
Eugenics is the "applied science or the bio-social movement which advocates the use of practices aimed at improving the genetic composition of a population", usually referring to human populations. The origins of the concept of eugenics began with certain interpretations of Mendelian inheritance,...
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
Lewis Terman
Lewis Madison Terman was an American psychologist, noted as a pioneer in educational psychology in the early 20th century at the Stanford University School of Education. He is best known as the inventor of the Stanford-Binet IQ test...
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.
The general intelligence factor (g)
The psychologist Charles SpearmanCharles Spearman
Charles Edward Spearman, FRS was an English psychologist known for work in statistics, as a pioneer of factor analysis, and for Spearman's rank correlation coefficient...
made the first formal factor analysis
Factor analysis
Factor analysis is a statistical method used to describe variability among observed, correlated variables in terms of a potentially lower number of unobserved, uncorrelated variables called factors. In other words, it is possible, for example, that variations in three or four observed variables...
of correlation
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
s between the tests in the early 20th century. Spearman's model used a single common factor that accounted for the positive correlations across intelligence tests. His analysis is considered to be the first theory of intelligence. His statistical model of intelligence came to be known as the general intelligence theory
General intelligence factor
The g factor, where g stands for general intelligence, is a statistic used in psychometrics to model the mental ability underlying results of various tests of cognitive ability...
. Using his model, Spearman argued that intelligence is largely a single global ability. For three quarters of a century, Spearman's g theory was the dominant theory of intelligence.
The g-loading of an intelligence test is the degree to the test score reflects general mental ability, as opposed to specific skills related to the test. Most highly g-loaded tests 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 proven view. 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
Raven's Progressive Matrices
Raven's Progressive Matrices are non-verbal multiple choice measures of the reasoning component of Spearman's g , which is often referred to as general intelligence. The tests were originally developed by John C. Raven in 1936...
which is a test of visual reasoning.
The War Years
During World War IWorld War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
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
Louis Leon Thurstone
Louis Leon Thurstone was a U.S. pioneer in the fields of psychometrics and psychophysics. He conceived the approach to measurement known as the law of comparative judgment, and is well known for his contributions to factor analysis.-Background and history:Louis Leon Thurstone was born in Chicago,...
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
David Wechsler
David "Wex" Wechsler was a leading American psychologist. He developed well-known intelligence scales, such as the Wechsler Adult Intelligence Scale and the Wechsler Intelligence Scale for Children .-Biography:...
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 in order 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 that the Binet focused on verbal abilities while the Wechsler also included non-verbal 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
Raymond CattellRaymond Cattell
Raymond Bernard Cattell was a British and American psychologist, known for his exploration of many areas in psychology...
(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 revived by his student John L. Horn
John L. Horn
John L. Horn was a scholar, cognitive psychologist and a pioneer in developing theories of multiple intelligence. Dr. Horn, together with Raymond Cattell, developed the crystallized and fluid theory of intelligence. This theory was later combined with the work of John Carroll and called the CHC...
(1966) who later argued that 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
John Bissell Carroll
John Bissell Carroll was an American psychologist known for his contributions to psychology, educational linguistics and psychometrics.- Early years :...
(1993) after a comprehensive re-analysis of earlier data proposed the Three Stratum Theory
Three Stratum Theory
In 1993 John Carroll published "Human cognitive abilities: A survey of factor-analytic studies", which outlined his hierarchical, Three-Stratum Theory of cognitive abilities....
, which is a hierarchical model with three levels. At the bottom is the first stratum which 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 IQ tests.
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:
- 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). See Mental chronometryMental chronometryMental chronometry is the use of response time in perceptual-motor tasks to infer the content, duration, and temporal sequencing of cognitive operations....
.
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.
Modern 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
Alexander Luria
Alexander Romanovich Luria was a famous Soviet neuropsychologist and developmental psychologist. He was one of the founders of neuropsychology and the jointly led the Vygotsky Circle.- Biography :...
'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. The (P)lanning processes involve decision making, problem solving, and performing activities and requires goal setting and self-monitoring. The (A)ttention/arousal process involves selectively attending to a particular stimulus, ignoring distractions, and maintaining vigilance. (S)imultaneous processing involves the integration of stimuli into a group and requires the observation of relationships. (S)uccessive 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.
Well-known modern IQ tests include Wechsler Adult Intelligence Scale
Wechsler Adult Intelligence Scale
The Wechsler Adult Intelligence Scale intelligence quotient tests are the primary clinical instruments used to measure adult and adolescent intelligence. The original WAIS was published in February 1955 by David Wechsler, as a revision of the Wechsler-Bellevue Intelligence Scale...
, Wechsler Intelligence Scale for Children
Wechsler Intelligence Scale for Children
The Wechsler Intelligence Scale for Children , developed by Dr. David Wechsler, is an individually administered intelligence test for children between the ages of 6 and 16 inclusive that can be completed without reading or writing...
, Stanford-Binet, Woodcock-Johnson Tests of Cognitive Abilities
Woodcock-Johnson Tests of Cognitive Abilities
The Woodcock-Johnson Tests of Cognitive Abilities is a set of intelligence tests first developed in 1977 by Woodcock and Johnson. It was revised in 1989 and again in 2001; this last version is commonly referred to as WJ-III. They may be administered to children from age two right up to the oldest...
, Kaufman Assessment Battery for Children
Kaufman Assessment Battery for Children
The Kaufman Assessment Battery for Children is a clinical instrument for assessing cognitive development. Its construction incorporates several recent developments in both psychological theory and statistical methodology. The test was developed by Alan S. Kaufman and Nadeen L...
, and Raven's Progressive Matrices
Raven's Progressive Matrices
Raven's Progressive Matrices are non-verbal multiple choice measures of the reasoning component of Spearman's g , which is often referred to as general intelligence. The tests were originally developed by John C. Raven in 1936...
.
Approximately 95% of the population have scores within two standard deviation
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
s (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. 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
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
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.
The correlation
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
between IQ test results and achievement test
Achievement test
An achievement test is a test of developed skill or knowledge. The most common type of achievement test is a standardized test developed to measure skills and knowledge learned in a given grade level, usually through planned instruction, such as training or classroom instruction...
results is about 0.7.
Mental age vs. modern method
Quotient
In mathematics, a quotient is the result of division. For example, when dividing 6 by 3, the quotient is 2, while 6 is called the dividend, and 3 the divisor. The quotient further is expressed as the number of times the divisor divides into the dividend e.g. The quotient of 6 and 2 is also 3.A...
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
Standardization
Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
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
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
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 normalization sample and the subject is given an IQ score equal to those with the same test result in the normalization sample.
The values of 100 and 15 were chosen in order 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 reliabilityReliability (statistics)
In statistics, reliability is the consistency of a set of measurements or of a measuring instrument, often used to describe a test. Reliability is inversely related to random error.-Types:There are several general classes of reliability estimates:...
. 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 persons 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
Validity (statistics)
In science and statistics, validity has no single agreed definition but generally refers to the extent to which a concept, conclusion or measurement is well-founded and corresponds accurately to the real world. The word "valid" is derived from the Latin validus, meaning strong...
for many clinical purposes.
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 that performance at the population median results in a score of IQ 100. The phenomenon of rising raw score performance means that 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 effectFlynn effect
The Flynn effect is the name given to a substantial and long-sustained increase in intelligence test scores measured in many parts of the world. When intelligence quotient tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100...
in the book The Bell Curve
The Bell Curve
The Bell Curve is a best-selling and controversial 1994 book by the Harvard psychologist Richard J. Herrnstein and political scientist Charles Murray...
after James R. Flynn
James R. Flynn
James Robert Flynn PhD FRSNZ , aka Jim Flynn, Emeritus Professor of Political Studies at the University of Otago in Dunedin, New Zealand, researches intelligence and has become well known for his discovery of the Flynn effect, the continued year-after-year increase of IQ scores in all parts of the...
, 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 observation has 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 5, 6 and 7 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.
For decades, it has been reported in practitioners' handbooks and textbooks on IQ testing that IQ declines with age after the beginning of adulthood. However, later researchers pointed out that this phenomenon is related to the Flynn effect
Flynn effect
The Flynn effect is the name given to a substantial and long-sustained increase in intelligence test scores measured in many parts of the world. When intelligence quotient tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100...
and is in part a cohort
Cohort (statistics)
In statistics and demography, a cohort is a group of subjects who have shared a particular time together during a particular time span . Cohorts may be tracked over extended periods in a cohort study. The cohort can be modified by censoring, i.e...
effect rather than a true aging effect.
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.
The peak of capacity for both fluid intelligence and crystallized intelligence occurs at age 26. This is followed by a slow decline.
Genetics and environment
Environmental and geneticGenetic
Genetic may refer to:*Genetics, in biology, the science of genes, heredity, and the variation of organisms**Genetic, used as an adjective, refers to heredity of traits**Gene, a unit of heredity in the genome of an organism...
factors play a role in determining IQ. Their relative importance have been the subject of much research and debate.
Heritability
HeritabilityHeritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
is defined as the proportion of variance
Variance
In probability theory and statistics, the variance is a measure of how far a set of numbers is spread out. It is one of several descriptors of a probability distribution, describing how far the numbers lie from the mean . In particular, the variance is one of the moments of a distribution...
in a trait
Trait (biology)
A trait is a distinct variant of a phenotypic character of an organism that may be inherited, environmentally determined or be a combination of the two...
which is attributable to genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
within a defined population in a specific environment. There are a number of points to consider when interpreting heritability. Heritability measures the proportion of variation in a trait that 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 that 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 twins to be between 0.7 and 0.8 in adults and 0.45 in childhood in the Western world
Western world
The Western world, also known as the West and the Occident , is a term referring to the countries of Western Europe , the countries of the Americas, as well all countries of Northern and Central Europe, Australia and New Zealand...
. It may seem reasonable to expect that 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 seek out different environments that reinforce the effects of those genes. There is an ongoing debate 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 socio-economic status or the effect of the maternal (fetal) environment.
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.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. More likely influences may be the impact of peers and other experiences outside the family.Individual genes
A number of individual genes have been reported to be associated with IQ. Examples include CHRM2, microcephalinMicrocephalin
Microcephalin is one of six genes causing primary microcephaly when non-functional mutations exist in the homozygous state...
, and ASPM. However, Deary and colleagues (2009) argued that there are still almost no replicated evidence. About 20,000 genes are thought to have an impact on the development and functionality of the brain.
Gene-environment interaction
Dickens and Flynn (2001) argued that the "heritability" figure includes both a direct effect of the genotypeGenotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues (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 taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program.
Interventions
In general, educational interventions, as those described below, have shown short-term effects on IQ, but long-term follow-up is often missing. In the US for example some interventive programs such as the Head Start Program have not produced lasting gains in IQ scores, although the more intensive Abecedarian Project have.A placebo controlled double-blind experiment found that vegetarians who took 5 grams of creatine
Creatine
Creatine is a nitrogenous organic acid that occurs naturally in vertebrates and helps to supply energy to all cells in the body, primarily muscle. This is achieved by increasing the formation of Adenosine triphosphate...
per day for six weeks showed a significant improvement on two separate tests of fluid intelligence
Fluid and crystallized intelligence
In psychology, fluid and crystallized intelligence are factors of general intelligence originally identified by Raymond Cattell...
, Raven's Progressive Matrices
Raven's Progressive Matrices
Raven's Progressive Matrices are non-verbal multiple choice measures of the reasoning component of Spearman's g , which is often referred to as general intelligence. The tests were originally developed by John C. Raven in 1936...
, and the backward digit span test from the WAIS
Wechsler Adult Intelligence Scale
The Wechsler Adult Intelligence Scale intelligence quotient tests are the primary clinical instruments used to measure adult and adolescent intelligence. The original WAIS was published in February 1955 by David Wechsler, as a revision of the Wechsler-Bellevue Intelligence Scale...
. 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. A study on young adults (0.03 g/kg/day for six weeks, e.g., 2 g/day for 150-pound individual) failed, however, to find any improvements.
Music
Music
Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
al training in childhood has also been found to correlate with higher than average IQ.
Recent studies have shown that training in using one's working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...
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
Working memory training
Working memory training is any endeavor intended to increase a person's working memory, a core cognitive ability. The prospect that systematic exercises could increase working memory drives research at certain groups such as Torkel Klingberg at the Karolinska Institute. In 1999, Klingberg proposed...
. 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.
IQ and brain anatomy
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 Intelligence Quotient (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
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
Outside of academic research and medicine, IQ testing is often done due to its ability to predict future job performance, social pathologies, or academic achievement. Academic research has also examined these associations, as well as the effect of IQ on other social outcomes, such as income and wealth.Many of the arguments and criticisms assume that explained variance can be calculated as the square of the correlation coefficient
Pearson product-moment correlation coefficient
In statistics, the Pearson product-moment correlation coefficient is a measure of the correlation between two variables X and Y, giving a value between +1 and −1 inclusive...
. This way of calculating explained variance has been criticized as inappropriate for most social scientific work. Also, as for the heritability figure, the explained variance only refers to the proportion of variation in an outcome that is explained by a factor, and not the proportion of an outcome that is explained by a factor.
Other tests
One study found a correlation of 0.82 between g (general intelligence factorGeneral intelligence factor
The g factor, where g stands for general intelligence, is a statistic used in psychometrics to model the mental ability underlying results of various tests of cognitive ability...
) and SAT
SAT
The SAT Reasoning Test is a standardized test for college admissions in the United States. The SAT is owned, published, and developed by the College Board, a nonprofit organization in the United States. It was formerly developed, published, and scored by the Educational Testing Service which still...
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 AssociationAmerican Psychological Association
The American Psychological Association is the largest scientific and professional organization of psychologists in the United States. It is the world's largest association of psychologists with around 154,000 members including scientists, educators, clinicians, consultants and students. The APA...
'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 that the 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 Frank Schmidt and John 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 were 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 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 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
The American Psychological Association's report "Intelligence: Knowns and Unknowns
Intelligence: Knowns and Unknowns
Intelligence: Knowns and Unknowns was a 1995 report issued by a Task Force created by the Board of Scientific Affairs of the American Psychological Association.- Background :...
" 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
Some researchers claim 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."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. Charles Murray, coauthor of The Bell Curve
The Bell Curve
The Bell Curve is a best-selling and controversial 1994 book by the Harvard psychologist Richard J. Herrnstein and political scientist Charles Murray...
, found that IQ has a substantial effect on income independently of family background.
Taking the above two principles together, very high IQ produces very high job performance, but no greater income than slightly high IQ. Studies also show that high IQ is related to higher net worth.
The American Psychological Association
American Psychological Association
The American Psychological Association is the largest scientific and professional organization of psychologists in the United States. It is the world's largest association of psychologists with around 154,000 members including scientists, educators, clinicians, consultants and students. The APA...
'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 (explained variance) a sixth of the variation in income because many studies are based on young adults (many of whom have not yet completed their education). On pg 568 of The g Factor
The g Factor
The g Factor: The Science of Mental Ability is a book by Arthur Jensen on the general factor of human mental ability .-External links:* , Psycoloquy, Volumes 10, 1999 and 11, 2000...
, 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
Daniel Seligman
Daniel Seligman was an editor and columnist at Fortune magazine from 1950 to 1997. He also wrote for Forbes,Commentary, The American Mercury, Commonweal, and The New Leader.-Biography:...
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 AssociationAmerican Psychological Association
The American Psychological Association is the largest scientific and professional organization of psychologists in the United States. It is the world's largest association of psychologists with around 154,000 members including scientists, educators, clinicians, consultants and students. The APA...
'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
The g Factor
The g Factor: The Science of Mental Ability is a book by Arthur Jensen on the general factor of human mental ability .-External links:* , Psycoloquy, Volumes 10, 1999 and 11, 2000...
(1998), Arthur Jensen
Arthur Jensen
Arthur Robert Jensen is a Professor Emeritus of educational psychology at the University of California, Berkeley. Jensen is known for his work in psychometrics and differential psychology, which is concerned with how and why individuals differ behaviorally from one another.He is a major proponent...
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
Conduct disorder
Conduct disorder is psychological disorder diagnosed in childhood that presents itself through a repetitive and persistent pattern of behavior in which the basic rights of others or major age-appropriate norms are violated...
have lower IQ than their peers "strongly argue" against 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 crimeViolent crime
A violent crime or crime of violence is a crime in which the offender uses or threatens to use violent force upon the victim. This entails both crimes in which the violent act is the objective, such as murder, as well as crimes in which violence is the means to an end, such as robbery. Violent...
, gross state product
Gross state product
Gross state product is a measurement of the economic output of a state or province...
, 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
National Assessment of Educational Progress
The National Assessment of Educational Progress is the largest continuing and nationally representative assessment of what our nation’s students know and can do in core subjects. NAEP is a congressionally mandated project administered by the National Center for Education Statistics , within the ...
math and reading test scores as a source.
The American Psychological Association
American Psychological Association
The American Psychological Association is the largest scientific and professional organization of psychologists in the United States. It is the world's largest association of psychologists with around 154,000 members including scientists, educators, clinicians, consultants and students. The APA...
'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.
There are also other correlations such as those between political views and intelligence, religiosity and intelligence
Religiosity and intelligence
The topic of religiosity and intelligence pertains to relationships between intelligence and religiosity, the extent to which someone is religious...
and fertility and intelligence
Fertility and intelligence
Fertility and intelligence research investigates the relationship between fertility and intelligence. Demographic studies have indicated that in humans, fertility and intelligence tend to be inversely correlated, that is to say, the more intelligent, as measured by IQ tests, exhibit a lower total...
.
Real-life accomplishments
Average adult combined IQs associated with real-life accomplishments by various tests:- MDs, JDs, or PhDs 125+ (WAIS-R, 1987)
- College graduates 112 (KAITAlan S. KaufmanAlan S. Kaufman is an American psychology professor known for his work on intelligence testing.- Early life and career :Born in Brooklyn and raised on Long Island, Kaufman earned his bachelor's degree from the University of Pennsylvania in 1965; M.A. in Educational Psychology from Columbia...
, 2000; K-BITAlan S. KaufmanAlan S. Kaufman is an American psychology professor known for his work on intelligence testing.- Early life and career :Born in Brooklyn and raised on Long Island, Kaufman earned his bachelor's degree from the University of Pennsylvania in 1965; M.A. in Educational Psychology from Columbia...
, 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:
- 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:
- Adults can harvest vegetables, repair furniture 60
- Adults can do domestic work 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.
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
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
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 nurtureNature versus nurture
The nature versus nurture debate concerns the relative importance of an individual's innate qualities versus personal experiences The nature versus nurture debate concerns the relative importance of an individual's innate qualities ("nature," i.e. nativism, or innatism) versus personal experiences...
" to IQ. Psychologists such as Alan S. Kaufman
Alan S. Kaufman
Alan S. Kaufman is an American psychology professor known for his work on intelligence testing.- Early life and career :Born in Brooklyn and raised on Long Island, Kaufman earned his bachelor's degree from the University of Pennsylvania in 1965; M.A. in Educational Psychology from Columbia...
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
Arthur Jensen
Arthur Robert Jensen is a Professor Emeritus of educational psychology at the University of California, Berkeley. Jensen is known for his work in psychometrics and differential psychology, which is concerned with how and why individuals differ behaviorally from one another.He is a major proponent...
. In contrast, other researchers such as Richard Nisbett argue that environmental factors can explain all of the average group differences.
Public policy
In the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, certain public policies
Public policy (law)
In private international law, the public policy doctrine or ordre public concerns the body of principles that underpin the operation of legal systems in each state. This addresses the social, moral and economic values that tie a society together: values that vary in different cultures and change...
and law
Law
Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...
s 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.
Griggs v. Duke Power Co.
Griggs v. Duke Power Co., , was a court case argued before the United States Supreme Court on December 14, 1970. It concerned employment discrimination and the disparate impact theory and was decided on March 8, 1971...
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
Job analysis
Job analysis is the process of describing and recording aspects of jobs and specifying the skills and other requirements necessary to perform the job.-Purpose:...
. 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
Mental retardation
Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...
is in part based on the results of IQ testing. Borderline intellectual functioning
Borderline intellectual functioning
Borderline intellectual functioning is a categorization of intelligence wherein a person has below average cognitive ability , but the deficit is not as severe as mental retardation...
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 school
Comprehensive school
A comprehensive school is a state school that does not select its intake on the basis of academic achievement or aptitude. This is in contrast to the selective school system, where admission is restricted on the basis of a selection criteria. The term is commonly used in relation to the United...
s.
Binet
Alfred Binet, a French psychologist who developed the first IQ test, did not believe that IQ test scales qualified to measure intelligence. He neither invented the term "intelligence quotient" nor supported its numerical expression. He stated:The scale, properly speaking, does not permit the measure of intelligence, because intellectual qualities are not superposable, and therefore cannot be measured as linear surfaces are measured.
—Binet, 1905
Binet had designed the Binet-Simon intelligence scale in order to identify students who needed special help in coping with the school curriculum. He argued that with proper remedial education programs, most students regardless of background could catch up and perform quite well in school. He did not believe that intelligence was a measurable fixed entity.
Binet cautioned:
Some recent thinkers seem to have given their moral support to these deplorable verdicts by affirming that an individual's intelligence is a fixed quantity, a quantity that cannot be increased. We must protest and react against this brutal pessimism; we must try to demonstrate that it is founded on nothing.[142]
Relation between IQ and intelligence
IQ is the most researched approach to intelligenceIntelligence
Intelligence has been defined in different ways, including the abilities for abstract thought, understanding, communication, reasoning, learning, planning, emotional intelligence and problem solving....
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
Creativity
Creativity refers to the phenomenon whereby a person creates something new that has some kind of value. What counts as "new" may be in reference to the individual creator, or to the society or domain within which the novelty occurs...
or emotional intelligence
Emotional intelligence
Emotional intelligence is a skill or ability in the case of the trait EI model, a self-perceived ability to identify, assess, and control the emotions of oneself, of others, and of groups. Various models and definitions have been proposed of which the ability and trait EI models are the most...
.
There are critics 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 ManThe Mismeasure of Man
The Mismeasure of Man , by Stephen Jay Gould, is a history and critique of the statistical methods and cultural motivations underlying biological determinism, the belief that “the social and economic differences between human groups — primarily races, classes, and sexes — arise from inherited,...
(1996), paleontologist Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
criticized IQ tests and argued that that they were used for scientific racism
Scientific racism
Scientific racism is the use of scientific techniques and hypotheses to sanction the belief in racial superiority or racism.This is not the same as using scientific findings and the scientific method to investigate differences among the humans and argue that there are races...
. He argued that g was a mathematical artifact and criticized:
Psychologist Peter Schönemann
Peter Schonemann
Peter H. Schönemann was a German born psychometrician and statistical expert. He was Professor Emeritus in the Department of Psychological Sciences at Purdue University. His research interests included multivariate statistics, multidimensional scaling and measurement, quantitative behavior...
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
Arthur Jensen
Arthur Robert Jensen is a Professor Emeritus of educational psychology at the University of California, Berkeley. Jensen is known for his work in psychometrics and differential psychology, which is concerned with how and why individuals differ behaviorally from one another.He is a major proponent...
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
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 Whites.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
Autism
Autism is a disorder of neural development characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their...
; 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
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." However, it also states that an "increasing number of psychometrically informed research papers that have been appearing in the past decade.""Intelligence: Knowns and Unknowns"
In response to the controversy surrounding The Bell CurveThe Bell Curve
The Bell Curve is a best-selling and controversial 1994 book by the Harvard psychologist Richard J. Herrnstein and political scientist Charles Murray...
, the American Psychological Association
American Psychological Association
The American Psychological Association is the largest scientific and professional organization of psychologists in the United States. It is the world's largest association of psychologists with around 154,000 members including scientists, educators, clinicians, consultants and students. The APA...
'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
Intelligence: Knowns and Unknowns
Intelligence: Knowns and Unknowns was a 1995 report issued by a Task Force created by the Board of Scientific Affairs of the American Psychological Association.- Background :...
". 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. "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
American Psychologist
The American Psychologist is the official academic journal of the American Psychological Association. It contains archival documents and articles covering current issues in psychology, the science and practice of psychology, and psychology's contribution to public policy...
, 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.
High IQ societies
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 InternationalMensa International
Mensa is the largest and oldest high-IQ society in the world. It is a non-profit organization open to people who score at the 98th percentile or higher on a standardised, supervised IQ or other approved intelligence test...
is perhaps the most well known of these. There are other groups requiring a score above the 98th percentile.
Reference charts
IQ reference charts are tables suggested by test publishers to divide intelligence ranges in various categories.See also
- Child prodigyChild prodigyA child prodigy is someone who, at an early age, masters one or more skills far beyond his or her level of maturity. One criterion for classifying prodigies is: a prodigy is a child, typically younger than 18 years old, who is performing at the level of a highly trained adult in a very demanding...
- Cultural intelligenceCultural IntelligenceCultural intelligence, cultural quotient or CQ, is a theory within management and organisational psychology, positing that understanding the impact of an individual's cultural background on their behaviour is essential for effective business, and measuring an individual's ability to engage...
- Curiosity quotientCuriosity quotientCuriosity quotient is a term put forth by author and journalist Thomas L. Friedman as part of an illustrative formula to explain how individuals can be powerfully motivated to learn about a personally interesting subject, whether or not they possess a particularly high intelligence quotient...
- Developmental disabilityDevelopmental disabilityDevelopmental disability is a term used in the United States and Canada to describe lifelong disabilities attributable to mental or physical impairments, manifested prior to age 18. It is not synonymous with "developmental delay" which is often a consequence of a temporary illness or trauma during...
- Educational quotientEducational quotientAn educational quotient, or EQ, is a score designed to assess a subject's level of general education. Though related to intelligence quotient, there is no direct correlation between the two. A person of high IQ, may have a low EQ, and vice-versa...
- GeniusGeniusGenius is something or someone embodying exceptional intellectual ability, creativity, or originality, typically to a degree that is associated with the achievement of unprecedented insight....
- Intellectual giftednessIntellectual giftednessIntellectual giftedness is an intellectual ability significantly higher than average. It is different from a skill, in that skills are learned or acquired behaviors...
- Late bloomerLate bloomerA late bloomer is a person whose talents or capabilities are not visible to others until later than usual. The term is used metaphorically to describe a child or adolescent who develops more slowly than others in their age group, but eventually catches up and in some cases overtakes their peers, or...
- Learning disabilityLearning disabilityLearning disability is a classification including several disorders in which a person has difficulty learning in a typical manner, usually caused by an unknown factor or factors...
- Malleable intelligenceMalleable intelligenceMalleability of intelligence describes the processes by which human intelligence may be augmented through changes in neuroplasticity. These changes may come as a result of genetics, pharmacological factors, psychological factors, behavior, or environmental conditions...
- Nature versus nurtureNature versus nurtureThe nature versus nurture debate concerns the relative importance of an individual's innate qualities versus personal experiences The nature versus nurture debate concerns the relative importance of an individual's innate qualities ("nature," i.e. nativism, or innatism) versus personal experiences...
- Savant syndromeSavant syndromeSavant syndrome , sometimes referred to as savantism, is a rare condition in which people with developmental disorders have one or more areas of expertise, ability, or brilliance that are in contrast with the individual's overall limitations...
- Sentience quotientSentience QuotientThe sentience quotient concept was introduced by Robert A. Freitas Jr. in the late 1970s. It defines sentience as the relationship between the information processing rate of each individual processing unit , the weight/size of a single unit and the total number of processing units .This is a...
- Social IQ
- Spiritual intelligenceSpiritual intelligenceSpiritual intelligence is a term used to indicate a spiritual correlate to IQ and EQ . Like EQ, SQ is becoming more mainstream in scientific inquiry and philosophical/psychological discussion....