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. Most tests yield both an overall score and individual subtest scores. IQ has been shown to correlate with a number of variables including job performance and socioeconomic advancement, 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."

The definition of intelligence has been, and continues to be, subject to debate. Some claim a unitary attribute, often called "general intelligence" or g, which can be measured using standard tests, and which correlates with a person's abilities on a wide range of tasks and contexts. Others have argued that there are multiple "intelligences," with different people displaying differing levels of each type.

Additionally, great controversies have arisen regarding the question of whether this "intelligence," whatever it is that IQ tests measure, is inherited, and if so whether some groups are more intelligent than others. Of particular concern has been the claim that some races are superior, leading justification to racist expectations and behavior. Despite research and theories from numerous scholars our understanding of intelligence is still limited. Perhaps, since researchers use only their own human intellect to discover the secrets of human intellectual abilities such limitations are to be expected. Viewing ourselves as members of one large human family, each with our own abilities and talents the use of which provide joy to ourselves and to others, allows us to have a deeper appreciation of what "intelligence" means.

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, since it was designed for use with children, IQ was calculated as a ratio with the formula $100\times {\frac {\text{mental age}}{\text{chronological age}}}.$

For example, a 10-year-old who scored as high as the average 13-year-old 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 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" is colloquially still used describe all of the intelligence scales currently in use.

IQ testing

Structure

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

A typical IQ test requires the test subject to solve a 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.

Scoring

IQ tests are standardized using a representative sample of the population. The tests are calibrated in such a way in order 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.

IQ and general intelligence factor

Modern IQ tests produce scores for different areas (such as language fluency, three-dimensional thinking, and so forth), with the summary score calculated from subtest scores. The average score, according to the bell curve, is 100.

Mathematical analysis of individuals' scores on the subtests of a single IQ test or the scores from a variety of different IQ tests (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 percent correlated and are often used interchangeably.

However, not all researchers agree that g can be treated as a single factor. For example, Charles Spearman, who originally developed the theory of g, made a distinction between "eductive" and "reproductive" mental abilities. Raymond Cattell identified "fluid" and "crystallized" intelligence (abbreviated Gf and Gc, respectively) as factors of "general intelligence." Cattell conceived of Gf and Gc as separate though correlated mental abilities which together comprise g, or "general intelligence." He defined fluid intelligence as the ability to find meaning in confusion and solve new problems, whereas crystallized intelligence is defined as the ability to utilize previously acquired knowledge and experience:

It is apparent that one of these powers ... has the 'fluid' quality of being directable to almost any problem. By contrast, the other is invested in particular areas of crystallized skills which can be upset individually without affecting the others.

Many IQ tests attempt to measure both varieties, with the overall IQ score based on a combination of these two scales. The Cattell Culture Fair IQ test, the Raven Progressive Matrices, and the performance subscale of the WAIS are measures of Gf. Vocabulary tests and the verbal subscale of the WAIS are considered good measures of Gc.

Positive correlations with IQ

While the study of IQ is sometimes treated as an end unto itself, the original purpose of IQ tests was validity, that is, the degree to which IQ correlates with outcomes such as job performance, social pathologies, or academic achievement. 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). Different IQ tests differ in their validity for various outcomes.

General intelligence has been shown to play an important role in many valued life outcomes. In addition to academic success, IQ correlates to some degree with job performance, socioeconomic advancement (level of education, occupation, and income), and "social pathology" (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 behavior. IQ does not explain the inheritance of economic status and wealth. An additional complication is that an individual's IQ score may or may not be stable over the course of the individual's lifetime.

School performance

Children with high scores on tests of intelligence tend to learn more of what is taught in school than their lower-scoring peers. However, successful school learning depends on many personal characteristics other than intelligence, such as memory, persistence, interest in school, and willingness to study.

Individual scores on college admission-related tests such as the MCAT, GRE, and LSAT are correlated with scores on tests of intelligence. This is partly due to intelligence test scores predicting years of education. To a smaller extent, they also predict occupational status, and income. This may be connected to the fact that many higher status and higher paid occupations can only be entered through professional schools which base part of their admissions on test scores.

Job performance

IQ tests or IQ scores may be used as part of the hiring process for new personnel, based on the understanding that:

For hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability.

However, the validity of this measure depends on the type of job and varies across different studies. For example, 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.

The American Psychological Association's 1995 report Intelligence: Knowns and Unknowns states that other individual characteristics such as interpersonal skills, aspects of personality, and so forth, are probably of equal or greater importance in predicting job performance. However, at this point, we do not have equally reliable instruments for measuring them.

Income

Studies have shown a correlation between IQ and income. Charles Murray, coauthor of The Bell Curve, found that IQ has a substantial effect on income independently of family background.. However, the exact nature of the relationship is disputed.

The American Psychological Association's report stated 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. They concluded that psychometric intelligence appears as only one of a great many factors that influence social outcomes. Some have suggested that non-IQ factors such as inherited wealth, race, and schooling are more important in determining income than IQ.

Another view is that the relationship between IQ and income is not linear. 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).

Other correlations with IQ

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

The 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 scrutinized.

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.

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".

Shared family effects appear 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 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.

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. Other 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, and inversely correlates with the chances of smoking , becoming obese, and having serious traumatic accidents in adulthood.

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.

Gender and IQ

Gender and intelligence research investigates differences in the distributions of cognitive skills between men and women. The populations of men and women differ on average in how well they perform on some of these skill tests, but they 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).

When the Stanford-Binet test was revised in the 1940s, preliminary tests yielded a higher average IQ for women; the test was consequently adjusted to give identical averages for men and women. Early work by Cyril Burt and Lewis Terman had found no sex differences in IQ using earlier versions of the tests.

However, determining whether men and women differ on average has been difficult. It is easy to design an IQ test in which either males or females score higher on average, the question boils down to what should be emphasized for the g factor. The primary reason behind the prediction that men will have higher average g than women is the advantage of the male brain size. Some studies find an average male advantage in g, but most do not.

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.

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.

Race and IQ

Theories about the possibility of a relationship between race and intelligence have been the subject of speculation and debate since the sixteenth 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 twentieth 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 ability, 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 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 were qualified to measure intelligence. He neither invented the term "intelligence quotient" nor supported its numerical expression:

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 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.

In his books, he criticized 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 that assert that the tests are unbiased use outdated methodology.

The view of the American Psychological Association

In response to the controversy surrounding Murray and Herrnstein's 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 task force concluded that IQ scores do have high predictive validity for individual differences in school achievement. They confirmed the predictive validity of IQ for adult occupational status, even when variables such as education and family background have been statistically controlled. They agreed that individual (but specifically not population) differences in intelligence are substantially influenced by genetics. They agreed that there are no significant differences between the average IQ scores of males and females. They reported that there is little evidence to show that childhood diet influences intelligence, except in cases of severe malnutrition.

The task force agreed that large differences do exist between the average IQ scores of populations of different races, and that these differences cannot be attributed to biases in test construction. They suggested that explanations based on social status and cultural differences are possible, and that environmental factors have raised mean test scores in many populations. They noted that there is not much direct evidence on racial disparities, but what little there is fails to support the genetic hypothesis. Responses to the report included several criticisms that the evidence for partly-genetic explanations had not been adequately examined.

In their report, the representatives of the association regretted 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."

References

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External links

All links retrieved July 28, 2009.

• Mainstream Science on Intelligence Wall Street Journal December 13, 1994: A18.
• Mainstream science on intelligence: An editorial with 52 signatories, history, and bibliography.
• IQ comparison chart

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