Intelligence test

IQ tests are designed to give approximately this Gaussian distribution. Colors delineate one standard deviation. But the true frequency of low and high IQs is greater than that given by the Gaussian curve.

An intelligence quotient or IQ is a score derived from a set of standardized tests of intelligence. Intelligence tests come in many forms, and some tests use a single type of item or question. Most tests yield both an overall score and individual subtests scores. Regardless of design, all IQ tests measure the same general intelligence factor also referred to as "g." Component tests are generally designed and chosen because they are found to be predictable of later intellectual development. In some studies, IQ has been shown to correlate with job performance, socioeconomic advancement, and other "social pathologies." Recent work has demonstrated links between IQ and health, longevity, and functional literacy. However, IQ tests have engendered much controversy and it is important to note that they do not measure all meanings of "intelligence." IQ scores are relative. Meaning, that the placement of an IQ score is similar to a placement in a race as they are both dependent upon the performance of the other participants.

History

In 1905, the French psychologist Alfred Binet published the first modern test of intelligence. His principal goal was to identify students who needed special help in coping with the school curriculum. Along with his collaborator Theodore Simon, Binet published revisions of his Binet-Simon intelligence scale in 1908 and 1911, the last appearing just before his untimely death. In 1912, the abbreviation of "intelligence quotient" or IQ, a translation of the German Intelligenz-quotient, was coined by the German psychologist William Stern.

A further refinement of the Binet-Simon scale was published in 1916 by Lewis M. Terman, from Stanford University, who incorporated Stern's proposal that an individual's intelligence level be measured as an intelligence quotient (IQ). Terman's test, which he named the Stanford-Binet Intelligence Scale formed the basis for one of the modern intelligence tests still commonly used today.

Originally, IQ was calculated as a ratio with the formula $100\times {\frac {\text{mental age}}{\text{chronological age}}}.$

A 10-year-old who scored as high as the average 13-year-old, for example, would have an IQ of 130 (100*13/10).

In 1939 David Wechsler published the first intelligence test explicitly designed for an adult population, the Wechsler Adult Intelligence Scale, or WAIS. Since publication of the WAIS, Wechsler extended his scale downward to create the Wechsler Intelligence Scale for Children, or WISC. The third edition of the WAIS (WAIS-III) is the most widely used psychological test in the world, and the fourth edition of the WISC (WISC-IV) is the most widely used intelligence test for children. The Wechsler scales contained separate subscores for verbal and performance IQ, thus being less dependent on overall verbal ability than early versions of the Stanford-Binet scale, and was the first intelligence scale to base scores on a standardized normal distribution rather than an age-based quotient.

Since the publication of the WAIS, almost all intelligence scales have adopted the normal distribution method of scoring. The use of the normal distribution scoring method makes the term "intelligence quotient" an inaccurate description of the intelligence measurement, but "IQ" still enjoys colloquial usage, and is used to describe all of the intelligence scales currently in use.

IQ testing

Structure

IQ tests come in many forms, and some tests use a single type of item or question, while others use several different subtests. Most tests yield both an overall score and individual subtest scores.

A typical IQ test requires the test subject to solve a fair number of problems in a set time under supervision. Most IQ tests include items from various domains, such as short-term memory, verbal knowledge, spatial visualization, and perceptual speed. Some tests have a total time limit, others have a time limit for each group of problems, and there are a few untimed, unsupervised tests, typically geared to measuring high intelligence. The widely used standardized test for determining IQ, the WAIS (Wechsler Adult Intelligence Scale-Third Edition (WAIS-III), consists of fourteen subtests, seven verbal (Information, Comprehension, Arithmetic, Similarities, Vocabulary, Digit Span, and Letter-Number Sequencing) and seven performance (Digit Symbol-Coding, Picture Completion, Block Design, Matrix Reasoning, Picture Arrangement, Symbol Search, and Object Assembly).

Scoring

When standardizing an IQ test, a representative sample of the population is tested using each test question. IQ tests are calibrated in such a way as to yield a normal distribution, or "bell curve." Each IQ test, however, is designed and valid only for a certain IQ range. Because so few people score in the extreme ranges, IQ tests usually cannot accurately measure very low and very high IQs.

Various IQ tests measure a standard deviation with a different number of points. Thus, when an IQ score is stated, the standard deviation used should also be stated.

When an individual has scores that do not correlate with each other, there is a good reason to suspect a learning disability or other cause for this lack of correlation. Tests have been chosen for inclusion because they display the ability to use this method to predict later difficulties in learning.

An individual's IQ score may or may not be stable over the course of the individual's lifetime.

IQ and general intelligence factor

Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking), with the summary score calculated from subtest scores. The average score, according to the bell curve, is 100. Individual subtest scores tend to correlate with one another, even when seemingly disparate in content.

Mathematical analysis of individuals' scores on the subtests of a single IQ test or the scores from a variety of different IQ tests (e.g., Stanford-Binet, WISC-R, Raven's Progressive Matrices, Cattell Culture Fair III, Universal Nonverbal Intelligence Test, Primary Test of Nonverbal Intelligence, and others) find that they can be described mathematically as measuring a single common factor and various factors that are specific to each test. This kind of factor analysis has led to the theory that underlying these disparate cognitive tasks is a single factor, termed the general intelligence factor (or g), that corresponds with the common-sense concept of intelligence. In the normal population, g and IQ are roughly 90% correlated and are often used interchangeably.

Tests differ in their g-loading, which is the degree to which the test score reflects g rather than a specific skill or "group factor" (such as verbal ability, spatial visualization, or mathematical reasoning).

Mental handicaps

Individuals with an unusually low IQ score, varying from about 70 ("Educable Mentally Retarded") to as low as 20 (usually caused by a neurological condition), are considered to have developmental difficulties. However, there is no true IQ-based classification for developmental disabilities.

Positive correlations with IQ

While IQ is sometimes treated as an end unto itself, scholarly work on IQ focuses to a large extent on IQ's validity, that is, the degree to which IQ correlates with outcomes such as job performance, social pathologies, or academic achievement. Different IQ tests differ in their validity for various outcomes. Traditionally, correlation for IQ and outcomes is viewed as a means to also predict performance; however, because IQ is a known social artifact, readers should distinguish between prediction in the hard sciences and the social sciences.

Validity is the correlation between score (in this case cognitive ability, as measured, typically, by a paper-and-pencil test) and outcome (in this case job performance, as measured by a range of factors including supervisor ratings, promotions, training success, and tenure), and ranges between −1.0 (the score is perfectly wrong in predicting outcome) and 1.0 (the score perfectly predicts the outcome). See validity (psychometric).

Research shows that general intelligence plays an important role in many valued life outcomes. In addition to academic success, IQ correlates to some degree with job performance (see below), socioeconomic advancement (e.g., level of education, occupation, and income), and "social pathology" (e.g., adult criminality, poverty, unemployment, dependence on welfare, children outside of marriage). Recent work has demonstrated links between general intelligence and health, longevity, and functional literacy. Correlations between g and life outcomes are pervasive, though IQ does not correlate with subjective self-reports of happiness. IQ and g correlate highly with school performance and job performance, less so with occupational prestige, moderately with income, and to a small degree with law-abiding behaviour. IQ does not explain the inheritance of economic status and wealth.

Other tests

One study found a correlation of .82 between g and SAT scores.[2] Another correlation of .81 between g and GCSE scores.[3]

Correlations between IQ scores (general cognitive ability) and achievement test scores are reported to be .81 by Deary and colleagues, with the percentage of variance accounted for by general cognitive ability ranging "from 58.6% in Mathematics and 48% in English to 18.1% in Art and Design"

School performance

The American Psychological Association's report Intelligence: Knowns and Unknowns (1995) 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. However, this means that they explain only 25% of the variance. Successful school learning depends on many personal characteristics other than intelligence, such as memory, persistence, interest in school, and willingness to study.

Correlations between IQ scores and total years of education are about .55, implying that differences in psychometric intelligence account for about 30% of the outcome variance. Many occupations can only be entered through professional schools which base their admissions at least partly on test scores: the MCAT, the GMAT, the GRE, the DAT, the LSAT, etc. Individual scores on admission-related tests such as these are certainly correlated with scores on tests of intelligence. It is partly because intelligence test scores predict years of education that they also predict occupational status, and income to a smaller extent.

Job performance

According to Schmidt and Hunter, "for hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability." The validity depends on the type of job and varies across different studies, ranging from 0.2 to 0.6 . However IQ mostly correlates with cognitive ability only if IQ scores are below average and this rule has many (about 30 %) exceptions for people with average and higher IQ scores . Also, IQ is related to the "academic tasks" (auditory and linguistic measures, memory tasks, academic achievement levels) and much less related to tasks where even precise hand work ("motor functions") are required

A meta-analysis (Hunter and Hunter, 1984) which pooled validity results across many studies encompassing thousands of workers (32,124 for cognitive ability), reports that the validity of cognitive ability for entry-level jobs is 0.54, larger than any other measure including job try-out (0.44), experience (0.18), interview (0.14), age (−0.01), education (0.10), and biographical inventory (0.37). This implies that, across a wide range of occupations, intelligence test performance accounts for some 29% of the variance in job performance.

According to Marley Watkins and colleagues, IQ is 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 American Psychological Association's report Intelligence: Knowns and Unknowns (1995) states that other individual characteristics such as interpersonal skills, aspects of personality, etc., are probably of equal or greater importance, but at this point we do not have 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, found that IQ has a substantial effect on income independently of family background .

The American Psychological Association's report Intelligence: Knowns and Unknowns (1995) states that IQ scores account for about one-fourth 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.

One reason why some studies claim that IQ only accounts for a sixth of the variation in income is 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 offspring's inherited wealth, race, and schooling are more important as factors in determining income than IQ.

Other correlations with IQ

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

There is a correlation of -.19 between IQ scores and number of juvenile offences in a large Danish sample; with social class controlled, the correlation dropped to -. 17. Similarly, the correlations for most "negative outcome" variables are typically smaller than .20, which means that test scores are associated with less than 4% of their total variance. It is important to realize that the causal links between psychometric ability and social outcomes may be indirect. Children who are unsuccessful in - and hence alienated from - school may be more likely to engage in delinquent behaviours for that very reason, compared to other children who enjoy school and are doing well.

IQ is also associated with certain diseases.

The book IQ and the Wealth of Nations claims to show that the GDP/person of a nation can in large part be explained by the average IQ score of its citizens. This claim has been both disputed and supported in peer-reviewed papers. The data used have also been questioned.

Tambs et al. (1989) 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. (1997) report that the inequality in education and income was predominantly due to genes, with shared environmental factors playing a subordinate role.

Some argue that IQ scores are used as an excuse for not trying to reduce poverty or otherwise improve living standards for all. Claimed low intelligence has historically been used to justify the feudal system and unequal treatment of women (but note that many studies find identical average IQs among men and women; see sex and intelligence). In contrast, others claim that the refusal of "high-IQ elites" to take IQ seriously as a cause of inequality is itself immoral.

Heritability

The role of genes and environment (nature vs. nurture) in determining IQ is reviewed in Plomin et al. (2001, 2003). The degree to which genetic variation contributes to observed variation in a trait is measured by a statistic called heritability. Heritability scores range from 0 to 1, and can be interpreted as the percentage of variation (e.g. in IQ) that is due to variation in genes. Twin and adoption studies are commonly used to determine the heritability of a trait. Until recently heritability was mostly studied in children. These studies find the heritability of IQ is approximately 0.5; that is, half of the variation in IQ among the children studied was due to variation in their genes. The remaining half was thus due to environmental variation and measurement error. A heritability of 0.5 implies that IQ is "substantially" heritable. Studies with adults show that they have a higher heritability of IQ than children do and that heritability could be as high as 0.8. The American Psychological Association's 1995 task force on "Intelligence: Knowns and Unknowns" concluded that within the White population the heritability of IQ is "around .75" (p. 85).[4]

Environment

Environmental factors play a large role in determining IQ in certain situations. Proper childhood nutrition appears to be critical for cognitive development as malnutrition correlates to lower IQ. Other research indicates environmental factors such as prenatal exposure to toxins, the duration of breastfeeding, and micronutrient deficiency affecting IQ.

Nearly all personality traits show that, contrary to expectations, environmental effects actually cause adoptive siblings raised in the same family to be as different as children who were raised in different families (Harris, 1998; Plomin & Daniels, 1987). IQ is an exception among children. The IQs of adoptive siblings, who share no genetic relation but do share a common family environment, are correlated at .32. Despite attempts to isolate the factors that cause adoptive siblings to be similar, they have not been identified. It is important to note, however, that shared family effects on IQ disappear after adolescence.

Active genotype-environment correlation, also called the "nature of nurture," is observed for IQ. This phenomenon is measured similarly to heritability; but instead of measuring variation in IQ due to genes, variation in environment due to genes is determined. One study found that 40% of variation in measures of home environment are accounted for by genetic variation. This suggests that the way human beings craft their environment is due in part to genetic influences.

A study of French children adopted between the ages of 4 and 6 shows the continuing interplay of nature and nurture. The children came from poor backgrounds with I.Q.’s that averaged 77, putting them near retardation. Nine years later after adoption, they retook the I.Q. tests, and all of them did better. The amount they improved was directly related to the adopting family’s status. "Children adopted by farmers and laborers had average I.Q. scores of 85.5; those placed with middle-class families had average scores of 92. The average I.Q. scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98."[5] This study suggests that IQ is not stable over the course of ones lifetime and that, even in later childhood, a change in individual's environment can have a significant effect on IQ.

Genetics

It is reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. Surprisingly, the opposite occurs. Heritability measures in infancy are as low as 20%, around 40% in middle childhood, and as high as 80% in adulthood.

Shared family effects also seem to disappear by adulthood. Adoption studies show that, after adolescence, adopted siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: monzygotic or identical twins raised separately are highly similar in IQ (0.86), more so than fraternal or dizygotic twins raised together (0.6) and much more than adopted siblings (~0.0).

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 populations. While there is little scholarly debate about the existence of some of these differences, the reasons remain highly controversial both within academia and in the public sphere.

Health and IQ

Persons with a higher IQ have generally lower adult morbidity and mortality. Post-Traumatic Stress Disorder, severe depression, and schizophrenia are less prevalent in higher IQ bands.

A study of 11,282 individuals in Scotland who took intelligence tests at ages 7, 9 and 11 in the 1950s and 1960s, found an "inverse linear association" between childhood IQ scores and hospital admissions for injuries in adulthood. The association between childhood IQ and the risk of later injury remained even after accounting for factors such as the child's socioeconomic background. Research in Scotland has also shown that a 15-point lower IQ meant people had a fifth less chance of seeing their 76th birthday, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long.

A decrease in IQ has also been shown as an early predictor of late-onset Alzheimer's Disease and other forms of dementia. In a 2004 study, Cervilla and colleagues showed that tests of cognitive ability provide useful predictive information up to a decade before the onset of dementia. However, when diagnosing individuals with a higher level of cognitive ability, in this study those with IQ's of 120 or more, patients should not be diagnosed from the standard norm but from an adjusted high-IQ norm that measured changes against the individual's higher ability level. In 2000, Whalley and colleagues published a paper in the journal Neurology, which examined links between childhood mental ability and late-onset dementia. The study showed that mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested.

Several factors can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the blood-brain barrier is less effective. Such impairment may sometimes be permanent, or may sometimes be partially or wholly compensated for by later growth. Several harmful factors may also combine, possibly causing greater impairment.

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). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.

In terms of the effect of one's intelligence on health, high childhood IQ correlates with one's chance of becoming a vegetarian in adulthood (Gale, CR. IQ in childhood and vegetarianism in adulthood: 1970 British cohort study. British Journal of Medicine 334 (7587): 245.), and inversely correlates with the chances of smoking (Taylor, MD. Childhood IQ and social factors on smoking behaviour, lung function and smoking-related outcomes in adulthood: linking the Scottish Mental Survey 1932 and the Midspan studies. British Journal of Health Psychology 10 (3): 399-401.), becoming obese, and having serious traumatic accidents in adulthood.

Gender and IQ

Sex and intelligence research investigates differences in the distributions of cognitive skills between men and women. This research employs experimental tests of cognitive ability, which take a variety of forms, including written tests like the SAT. Research has focused on differences in individual skills as well as overall differences in general cognitive ability, which is often called g. IQ tests, specially designed to measure cognitive ability, usually test a variety of skills, and IQ scores are often used as a measure of g.

The populations of men and women differ on average in how well they perform on some of these skill tests, but do equally well on other tests. For example, women tend to score higher on certain verbal and memory tests, whereas men tend to score higher on spatial tests, particularly mental spatial rotations. While these results are relatively uncontroversial, the question of whether men and women differ on average in g is a matter of debate among experts. Most studies unambiguously find that men as a population are more varied than women in g (i.e. they have a higher variance and therefore there are more men than women at the extremes of ability).

However, determining whether men and women differ on average has been more difficult. It is easy to design an IQ test in which either males or females score higher on average, by selecting different tests or giving them different weights, so the question boils down to which weights the different tests should have for the g factor.

The primary reason for expecting that men will have higher average g than women is the male advantage in brain size. Resolving this question requires the use of sophisticated statistical techniques to extract g from the results of IQ tests. Some studies find an average male advantage in g, but most do not.

When the Stanford-Binet test was revised in the 1940ies, preliminary test yielded a higher average IQ for women; the test was consequently adjusted to give identical averages for men and women.

According to Jackson and Rushton, a scientific consensus existed during the 20th century that there are no sex differences in overall intelligence. They attribute this consensus in part to early work by Cyril Burt and Lewis Terman who found no sex differences in the first IQ tests.

A 1994 study by H. Stumpf and Douglas N. Jackson based on medical school application test scores showed that men averaged IQs about 8.4 points higher than women, while women averaged memories about 7.5 IQ points higher than men.

A 1999 study by Richard Lynn [6], found that the IQ difference between men and women is typically about 3-4 IQ points, while women usually maintain short-term memory advantages over men of about 2 IQ points. In a 2005 study published in the British Journal of Psychology which attracted media attention in the wake of the January 2005 controversy at Harvard (below), he and Paul Irwing analyzed existing studies to report that university men have an average IQ between 3.3 and 5.0 points higher than that of university women. In Nature, intelligence-test designer Steve Blinkhorn argued in reply that Lynn and Irwing's analysis was critically flawed, for example by deliberately excluding a large contrary study that made up almost 45% of the subjects in the meta-analysis; in subsequent correspondence in the same journal, Blinkhorn pointed out that their use of meta-analysis was "methodologically inappropriate, statistically bizarre and unsuited to the estimation of subpopulation parameters."

Some studies claim that men outperform women on average by 3-4 IQ points.

Evidence against differences in overall average IQ scores between men and women has come from several very large and representative studies. However, these studies did find that the scores of men show greater variance than the scores of women, and that men and women have some differences in average scores on tests of particular abilities, which tend to balance out in overall IQ scores.

Deary et al. (2003) performed an analysis of an IQ test administered to almost all children in Scotland at age 11 in 1932 (>80,000). The average IQ scores by sex were 100.64 for girls and 100.48 for boys. The difference in mean IQ was not significant. However, the standard deviation was 14.1 for girls and 14.9 for boys. This difference was statistically significant. In the sample studied, 49.6% are girls and 50.4% are boys. Because of the difference in variance between the sexes, however, girls are in excess by 2% in the middle IQ range of 90–115. At the extreme IQ ranges, 50–60 and 130–140, boys make up 58.6% and 57.7% of the population (gaps of 17.2% and 15.4%) respectively. That is, boys were overrepresented amongst the lowest and highest IQ groups. It is generally observed that males tend to hit the most positive and negative performance results of many tests.

Comparing Groups

The average scores of young men and women in mathematics, for example, will be close, but there will be more men than women in the very low scores and in the very high scores. In this sense, the red bell curve in the diagram represents women, compared to men in green. There is evidence to suggest that forms of autism may be essentially extreme expressions of certain typically male characteristics.  This is represented by the blue in the diagram.

A 2001 report by Richard J. Coley of the ETS found that females often outperformed males on various measures of verbal ability, while males tended to outperform females on measures of mathematical and spatial ability. [7]

• Spatial abilities: large differences favoring males are found in performance on visual-spatial tasks (e.g., mental rotation) and spatio-temporal tasks (e.g., tracking a moving object through space). The male advantage in visual-spatial tasks is approximately 1 standard deviation. The difference starts at twelve to sixteen years of age.
• Verbal abilities: a range of differences, some large, favoring females are found in performance on verbal tasks. Males also show higher levels of dyslexia and other reading disabilities. The incidence of stuttering is also higher among males.
• Memory: Several studies have shown women are better at certain types of memory.

• General knowledge: A study by Richard Lynn showed that men have more general knowledge than women.
• Education: In the United States, women tend to outnumber men at colleges and universities, except at technical institutions that emphasize math and science such as MIT and Caltech, where men predominate.
• Academia: Men outnumber women in tenured faculty positions in math and science. Women outnumber men in tenured faculty positions in humanities fields.
• High IQ societies: In all "high IQ societies" men outnumber women; e.g., in Mensa the male-to-female ratio is 2:1.
• Boys tend to have a higher incidence of behavioral problems in schools which may affect academic achievement.

Race and IQ

Much research has been devoted to the extent and potential causes of racial group differences in IQ.

Theories about the possibility of a relationship between race and intelligence have been the subject of speculation and debate since the 16th century. The contemporary debate focuses on the nature, causes, and importance, or lack of importance, of ethnic differences in intelligence test scores and other measures of cognitive ability, and whether "race" is a meaningful biological construct with significance other than its correlation to membership of particular ethnic groups. Thus, the question of the relative roles of nature and nurture in causing individual and group differences in cognitive ability is seen as fundamental to understanding the debate.

The modern controversy surrounding intelligence and race focuses on the results of IQ studies conducted during the second half of the 20th century in the United States, Western Europe, and other industrialized nations.

Modern theories and research on race and intelligence are often grounded in two controversial assumptions:

• that the social categories of race and ethnicity are concordant with genetic categories, such as biogeographic ancestry, and
• that intelligence is quantitatively measurable by modern tests and is dominated by a unitary "general intelligence factor"

While the general intelligence factor is an accepted and widespread view of the structure of abilities, some theorists regard it as misleading.There are a wide range of human abilities, including many that seem to have intellectual components which are outside the domain of standard psychometric tests. Most of the research is based on IQ testing of blacks and whites in the United States, and much of the current debate centers around which environmental factors may influence IQ scores the most, and whether or not there are genetic differences between races that play a significant role in creating the gap, this last question being the most controversial in the debate.

Environmental factors, such as nutrition, have been shown to influence IQ in children. Other environmental factors include: education level, richness of the early home environment, the existence of caste-like minorities, socio-economic factors, culture, pidgin language barriers, quality of education, health, racism, lack of positive role-models, exposure to violence, the Flynn effect, sociobiological differences and stereotype threat. There is also significant debate about exactly how environmental factors play their role in creating the gap and the interrelationships between these factors.

The more controversial part of the debate is whether group IQ differences are caused in part by genetic differences. Hereditarianism hypothesizes that a genetic contribution to intelligence includes genes linked to brain anatomy or physiology that vary by race. These kinds of hypotheses have drawn a great deal of media attention and criticism. Robert Sternberg writes that race intelligence research that focuses on a genetic cause for the gap is attempting to show that one group is inferior to another group. The conclusion of some researchers: that racial groups in the US vary in average IQ scores in part because of genetic differences between races has led to heated academic debates that have spilled over into the public sphere.

Observations about race and intelligence also have important applications for critics of the media portrayal of race. Stereotypes in media such as books, music, film, and television can reinforce racial stereotypes and may influence the perceived opportunities for success in academics for minority students.

Criticism and views

Binet

Alfred Binet 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.

The Mismeasure of Man

Some scientists dispute psychometrics entirely. In The Mismeasure of Man professor Stephen Jay Gould argued that intelligence tests were based on faulty assumptions and showed their history of being used as the basis for scientific racism. He wrote:

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

He spent much of the book criticizing the concept of IQ, including a historical discussion of how the IQ tests were created and a technical discussion of why g is simply a mathematical artifact. Later editions of the book included criticism of The Bell Curve.

Gould does not dispute the stability of test scores, nor the fact that they predict certain forms of achievement. He does argue, however, that to base a concept of intelligence on these test scores alone is to ignore many important aspects of mental ability.

Relation between IQ and intelligence

See also: Intelligence

Several other ways of measuring intelligence have been proposed. Daniel Schacter, Daniel Gilbert, and others have moved beyond general intelligence and IQ as the sole means to describe intelligence.

Test bias

The American Psychological Association's report Intelligence: Knowns and Unknowns (1995) states that that IQ tests as predictors of social achievement are not biased against people of African descent since they predict future performance, such as school achievement, similarly to the way they predict future performance for European descent.

However, IQ tests may well be biased when used in other situations. A 2005 study finds some evidence that the WAIS-R is not culture-fair for Mexican Americans. 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 have resulted in incorrect claims that a majority of children with autism are mentally retarded.

Outdated methodology

A 2006 paper argues that mainstream contemporary test analysis does not reflect substantial recent developments in the field and "bears an uncanny resemblance to the psychometric state of the art as it existed in the 1950s."It also claims that some of the most influential recent studies on group differences in intelligence, in order to show that the tests are unbiased, use outdated methodology.

The view of the American Psychological Association

In response to the controversy surrounding The Bell Curve, the American Psychological Association's Board of Scientific Affairs established a task force in 1995 to write a consensus statement on the state of intelligence research which could be used by all sides as a basis for discussion. 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 agree that individual (but specifically not population) differences in intelligence are substantially influenced by genetics.

They state there is little evidence to show that childhood diet influences intelligence except in cases of severe malnutrition. They agree that there are no significant differences between the average IQ scores of males and females. The task force agrees that large differences do exist between the average IQ scores of blacks and whites, and that these differences cannot be attributed to biases in test construction. The task force suggests that explanations based on social status and cultural differences are possible, and that environmental factors have raised mean test scores in many populations. Regarding genetic causes, they noted that there is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis.

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.

References

ISBN links support NWE through referral fees

• Carroll, J.B. (1993). Human cognitive abilities: A survey of factor-analytical studies. New York: Cambridge University Press.
• Coward, W.M. and Sackett, P.R. (1990). Linearity of ability-performance relationships: A reconfirmation. Journal of Applied Psychology, 75:297–300.
• Duncan, J., P. Burgess, and H. Emslie (1995) Fluid intelligence after frontal lobe lesions. Neuropsychologia, 33(3): p. 261-8.
• Duncan, J., et al., A neural basis for general intelligence. Science, 2000. 289(5478): p. 457-60.
• Frey, M.C. and Detterman, D.K. (2003) Scholastic Assessment or g? The Relationship Between the Scholastic Assessment Test and General Cognitive Ability. Psychological Science, 15(6):373–378. PDF
• Gottfredson, L. S. (1997). "Why g matters: The complexity of everyday life." Intelligence, 24(1), 79–132. PDF
• Gottfredson, L.S. (1998). The general intelligence factor. Scientific American Presents, 9(4):24–29. PDF
• Gottfredson, L. S. (2005). Suppressing intelligence research: Hurting those we intend to help. In R. H. Wright & N. A. Cummings (Eds.), Destructive trends in mental health: The well-intentioned path to harm (pp. 155–186). New York: Taylor and Francis. Pre-print PDF PDF
• Gottfredson, L. S. (in press). "Social consequences of group differences in cognitive ability (Consequencias sociais das diferencas de grupo em habilidade cognitiva)." In C. E. Flores-Mendoza & R. Colom (Eds.), Introdução à psicologia das diferenças individuals. Porto Alegre, Brazil: ArtMed Publishers. PDF
• Gray, J.R., C.F. Chabris, and T.S. Braver, Neural mechanisms of general fluid intelligence. Nat Neurosci, 2003. 6(3): p. 316-22.
• Gray, J.R. and P.M. Thompson, Neurobiology of intelligence: science and ethics. Nat Rev Neurosci, 2004. 5(6): p. 471-82.
• Haier RJ, Jung RE, Yeo RA, et al. (2005). The neuroanatomy of general intelligence: sex matters. NeuroImage 25: 320–327.
• Harris, J. R. (1998). The nurture assumption : why children turn out the way they do. New York, Free Press.
• Hunt, E. (2001). Multiple views of multiple intelligence. [Review of Intelligence Reframed: Multiple Intelligences for the 21st Century.]
• Jensen, A.R. (2006). "Clocking the Mind: Mental Chronometry and Individual Differences." Elsevier Science. --->New release scheduled for early June, 2006.
• McClearn, G. E., Johansson, B., Berg, S., Pedersen, N. L., Ahern, F., Petrill, S. A., & Plomin, R. (1997). Substantial genetic influence on cognitive abilities in twins 80 or more years old. Science, 276, 1560–1563.
• Murray, Charles (1998). Income Inequality and IQ, AEI Press PDF
• Noguera, P.A. (2001). Racial politics and the elusive quest for excellence and equity in education. In Motion Magazine article
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• Schoenemann, P.T., M.J. Sheehan, and L.D. Glotzer, Prefrontal white matter volume is disproportionately larger in humans than in other primates. Nat Neurosci, 2005.
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• Tambs K, Sundet JM, Magnus P, Berg K. "Genetic and environmental contributions to the covariance between occupational status, educational attainment, and IQ: a study of twins." Behav Genet. 1989 Mar;19(2):209–22. PMID 2719624.
• Thompson, P.M., Cannon, T.D., Narr, K.L., Van Erp, T., Poutanen, V.-P., Huttunen, M., Lönnqvist, J., Standertskjöld-Nordenstam, C.-G., Kaprio, J., Khaledy, M., Dail, R., Zoumalan, C.I., Toga, A.W. (2001). "Genetic influences on brain structure." Nature Neuroscience 4, 1253-1258.

External links

Collective statements

• The Wall Street Journal: Mainstream Science on IntelligencePDF Reprint - Mainstream science on intelligence: An editorial with 52 signatories, history, and bibliography.
• APA—Intelligence: Knowns and Unknowns
• APA Committee on Online Psychological Tests and Assessment report

Review papers

• Scientific American: The General Intelligence Factor
• Scientific American: Intelligence Considered
• Neurobiology of Intelligence: Science and Ethics PDF

IQ testing

• "Scans Show Different Growth for Intelligent Brains," New York Times, March 30, 2006
• IQ comparison site
• IQ comparison chart
• Estimated IQs of the greatest geniuses (note: these sorts of estimates are considered highly suspect; the psychological community generally regards it impossible to infer an "IQ" from writing samples or accomplishments)

Online IQ tests

• Kids IQ Test Center Free information regarding childrens intelligence.
• Online IQ Test An online IQ test (40 questions, ~ 40 minutes)
• Online Intelligence Testing IQ Test (clever method of disabusing notions about online IQ tests)

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