Intelligence quotient: Encyclopedia - Intelligence quotient

Intelligence quotient

An intelligence quotient or IQ is a score derived from a set of standardized tests developed to measure a person's cognitive abilities ("intelligence") in relation to their age group. An IQ test does not measure intelligence the way a ruler measures height (absolutely), but rather the way a race measures speed (relatively).

For people living in the prevailing conditions of the developed world, IQ is highly heritable, and by adulthood the influence of family environment on IQ is undetectable. IQ test scores are correlated with measures of brain structure and function, as well as performance on simple tasks that anyone can complete within a few seconds.

IQ is strongly correlated with academic success, but can also predict important life outcomes such as job performance, socioeconomic advancement, and "social pathologies". Recent work has demonstrated links between IQ and health, longevity, and functional literacy.

Intelligence quotient - History

Alfred Binet and his colleague Theodore Simon created the Binet-Simon scale in 1905, which used testing to identify students who could benefit from extra help in school. Their assumption was that lower scores indicated the need for more teaching, not an inability to learn. This interpretation is still held by some modern experts.

Notably, Binet himself made no claim that his test properly measured intelligence. He stated in his paper New Methods for the Diagnosis of the Intellectual Level of Subnormals that

In 1910, Henry H. Goddard proposed three categories for the "feeble-minded" based on IQ scores: moron (IQ of 51-70), imbecile (IQ of 26-50), and idiot (IQ of 0-25). This taxonomy was the standard of intelligence research for decades.

In 1916, Stanford University psychologist Lewis Terman released the "Stanford Revision of the Binet-Simon Scale", generally known as the Stanford-Binet test. This became the most commonly administered test for many decades. The term "intelligence quotient," in which each student's score was the quotient of his or her tested mental age with his or her actual age, was adopted by Terman from a 1912 proposal by German psychologist William Stern. This led to refined testing developed by Robert Yerkes for United States Army recruits.

Today, the most commonly administered IQ test is the WISC-III test, originally developed by David Wechsler in 1974. The WISC-III test comprises ten types of problems, categorized by difficulty and by skill type (verbal and performance scales). A revised version, the WISC-IV, was released in 2003 and is used regularly in assessments. However, the interpretation of various combinations of subscales is still being researched. Another notable type of IQ test is the Bailey Scale of Infant Development, regarded as the 'best' means of testing cognitive development in infants.

Today, informal online IQ tests are popular, but they are at best rough approximations. The tests are not expert certified and notable limitations include a small number of questions and a lack of the time limit.

Intelligence quotient - IQ score distribution

IQ scores are expressed as a number normalized so that the average IQ in an age group is 100. In other words, an individual scoring 115 is above average when compared to people in the same age group. It is common practice to standardize so that the standard deviation (σ) of scores is 15. (The Stanford Binet IQ test uses a standard deviation of 16.) Tests are designed so that the distribution of IQ scores is Gaussian; that is, it follows a bell curve. A difference has been documented between the IQ score distributions of left-handed and right-handed test subjects; the distribution in left-handed people tends to cluster at the two extremes of the IQ scale.

(The following numbers apply to IQ scales standard deviation σ = 15.) Roughly 68% of the population has an IQ between 85 and 115. The "normal" range, or range between -2 and +2 standard deviations from the mean, is between 70 and 130, and contains about 95% of the population. An accurate score below 70 may indicate mental retardation, and a score above 130 may indicate intellectual giftedness. Retardation may result from normal variation or from a genetic or developmental malady; analogously, some otherwise normal people are very short, and others have dwarfism. Giftedness appears to be normal variation; autistic savants have often astonishing cognitive powers but below-average IQ's.

It has been observed that scores outside the range 55 to 145 must be cautiously interpreted because there are smaller numbers of respondents with which to make comparisons in those ranges. Moreover, at such extreme values, the normal distribution is a less accurate estimate of the true IQ distribution.

In actuality there is a higher percentage of the population measured at 3 or more standard deviation levels on the test than the probabilities of the normal distribution would predict. Some IQ scoring procedures may attempt to integrate such clusters of statistical outliers into the curve by adjusting the scores so that they better represent actual probabilities (according to Silverman) and in these cases, scores around 145 and above may actually have been notably higher, were they not so adjusted.

Intelligence quotient - IQ and General Intelligence Factor

Main article: General intelligence factor

Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking, etc.), with the summary score calculated from subtest scores. Individual subtest scores tend to correlate with one another, even when seemingly disparate in content. Analyses of an individual's 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 and others) will reveal that they all measure 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.

Intelligence quotient - Genetics vs environment

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. Twins studies and adoption studies are commonly used to determine the heritability of a trait. Until recently heritability was mostly studied in children. These studies yield an estimate of heritability of 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, though it is probably not this high.

Considerable research has focused on biological correlates of g; see General intelligence factor and the section on brain size below. For example, general intelligence and MRI brain volume measurements are correlated, and the effect is primarily determined by genetic factors.

Intelligence quotient - Environment

Environmental factors play a large role in determining IQ in situations where environmental conditions are variable. Proper childhood nutrition appears critical for cognitive development; malnutrition can lower IQ. Other research indicates environmental factors such as prenatal exposure to toxins, duration of breastfeeding, and micronutrient deficiency can affect IQ. However, in the developed world, none of these effects are sufficiently pronounced to be important.

In the developed world, there is some environmental effect on the IQ of children, accounting for up to a quarter of the variance. However, by adulthood, this correlation disappears, so that the cognitive ability of adults living in the prevailing conditions of the developed world is highly heritable.

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 raised in different families (Harris, 1998; Plomin & Daniels, 1987). Put another way, shared environmental variation for personality is zero, and all environmental effects are nonshared. Intelligence is actually an exception to this rule, at least 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 them, the factors that cause adoptive siblings to be similar have not been identified, though it could be related to parents choosing the type of children they will adopt. However, as explained below, 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.

Intelligence quotient - Development

It is reasonable to expect that genetic influences on traits like IQ should become less important as we gain 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: monozygotic (identical) twins raised separately are highly similar in IQ (0.86), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adopted siblings (~0.0).

Most of the IQ studies described above were conducted in developed countries, such as the United States, Japan, and Western Europe. Also, a few studies have been conducted in Moscow, East Germany, and India, and those studies produce similar results. Any such investigation is limited to describing the genetic and environmental variation found within the populations studied. This is a caveat of any heritability study.

Intelligence quotient - Mental retardation

About 75-80 percent of mental retardation is familial (runs in families), and 20-25 percent is due to organic problems, such as chromosomal abnormalities or brain damage.[2] Mild to severe mental retardation is a symptom of several hundred single-gene disorders and many chromosomal abnormalities, including small deletions. Based on twin studies, moderate to severe mental retardation does not appear to be familial, but mild mental retardation does. That is, the relatives of the moderate to severely mentally retarded have normal ranges of IQs, whereas the families of the mildly mentally retarded have IQs skewing lower.

IQ score ranges (from DSM-IV):

• mild mental retardation: IQ 50-55 to 70; children require mild support; formally called "Educable Mentally Retarded".
• moderate retardation: IQ 35-40 to 50-55; children require moderate supervision and assistance; formally called "Trainable Mentally Retarded".
• severe mental retardation: IQ 20-25 to 35-40; can be taught basic life skills and simple tasks with supervision.
• profound mental retardation: IQ below 20-25; usually caused by a neurological condition; require constant care.

The rate of mental retardation is higher among males than females, and higher among blacks than whites, according to a 1991 U.S. Centers for Disease Control and Prevention (CDC) study.[3]

By race, the overall rate was 16.6 per 1000 for blacks and 6.8 per 1000 for whites. Rates of mental retardation for black males, the group with the highest rates, were 1.7 times higher than black females, 2.4 times higher than white males, and 3.1 times higher than white females.

Individuals with IQs below 70 have been exempted from the death penalty in the U.S. since 2002.[4]

Intelligence quotient - IQ education and income

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 US 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.

Intelligence quotient - Brain size and IQ

Main article: Brain size and intelligence

Modern studies using MRI imaging have shown that brain size correlates with IQ by a factor of roughly .35 to .40. In 1991, Willerman et al. used data from 40 White American university students and reported a correlation coefficient of .35. Other studies done on samples of Caucasians show similar results, with Andreasen et al (1993) determining a correlation of .38, while Raz et al (1993) obtained a figure of .43 and Wickett et al (1994) obtained a figure of .40. The correlation between brain size and IQ seems to hold for comparisons between and within families (Gignac et al. 2003; Jensen 1994; Jensen & Johnson 1994). However, one study found correlation within family (Schoenemann et al. 2000). A study on twins (Thompson et al., 2001) showed that frontal gray matter volume was correlated with g and highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al 2002).

Intelligence quotient - The Flynn effect

Main article: Flynn effect

Worldwide, IQ scores appear to be slowly rising, a trend known as the Flynn effect. However, tests are only renormalized occasionally to obtain mean scores of 100, for example WISC-R (1974), WISC-III (1991) and WISC-IV (2003). Hence it is difficult to compare IQ scores measured years apart.

Intelligence quotient - IQ correlations

Intelligence quotient - Sex and IQ

Main article: Sex and intelligence

Most IQ tests are designed so that the average IQs of males and females are equal. However, men tend to score their highest in the parts of the test that cover spatial and quantitative abilities, and women generally score their highest in the verbal sections. Some research has indicated that the variance in men's IQ scores is greater than the variance among women's, as seen in other cognitive test scores. This would mean that men are more likely than women to have both very high and very low IQs.

Intelligence quotient - Race and IQ

Main article: Race and intelligence

Intelligence quotient - Religiousness and IQ

Main article: Religiousness and intelligence

Some studies claim correlations between IQ, SAT scores, GPA and degree of religious belief. While most of the research indicates a negative correlation between IQ and religiosity [5], this remains a controversial point.

Intelligence quotient - Health and IQ

Persons with a higher IQ have generally lower adult morbidity and mortality. This may be because they better avoid injury and take better care of their own health. It also decreases the risk of Post-Traumatic Stress Disorder, severe depression, and schizophrenia. On the other hand, it increases the risk of Obsessive Compulsive Disorder [6].

Research in Scotland has 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 [7].

Intelligence quotient - Economic development and IQ

A controversial book IQ and the Wealth of Nations, claims to show that the wealth of a nation can in large part be explained by the average IQ score. This claim has been both disputed and supported in peer-reviewed papers. The data used has also been questioned.

Intelligence quotient - Practical validity

Evidence for the practical validity of IQ comes from examining the correlation between IQ scores and life outcomes.

Research shows that intelligence plays an important role in many valued life outcomes. In addition to academic success, intelligence correlates 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 intelligence and health, longevity, and functional literacy. Correlations between g and life outcomes are pervasive, though IQ and happiness do not correlate. 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-abidingness.

General intelligence (in the literature typically called "cognitive ability") is the best predictor of job performance by the standard measure, validity. 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). The validity of cognitive ability for job performance tends to increase with job complexity and varies across different studies, ranging from 0.2 for unskilled jobs to 0.8 for the most complex jobs.

A large 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 tryout (0.44), experience (0.18), interview (0.14), age (-0.01), education (0.10), and biographical inventory (0.37).

Because higher test validity allows more accurate prediction of job performance, companies have a strong incentive to use cognitive ability tests to select and promote employees. IQ thus has high practical validity in economic terms. The utility of using one measure over another is proportional to the difference in their validities, all else equal. This is one economic reason why companies use job interviews (validity 0.14) rather than randomly selecting employees (validity 0.0).

However, legal barriers, most prominently the 1971 United States Supreme Court decision Griggs v. Duke Power Co., have prevented American employers from directly using cognitive ability tests to select employees, despite the tests' high validity. This is largely based on that cognitive ability scores in selection adversely affects some minority groups, due to that different groups have different mean scores on tests of cognitive ability. However, cognitive ability tests are still used in some organizations. The U.S. military uses the Armed Services Vocational Aptitude Battery, as higher scores correlate with significant increases in effectiveness of both individual soldiers and units,[8] [9] and Microsoft is known for using tests that correlate with IQ tests as part of the interview process.

Some researchers have echoed the popular 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." (Detterman and Daniel, 1989)[10]

However, some studies suggest IQ continues to confer large benefits even at very high levels. 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 (Coward and Sackett, 1990). In an analysis of hundreds of siblings, it was found that IQ has a substantial effect on income independently of family background (Murray, 1998).

Other studies question the real-world importance of whatever is measured with IQ tests, especially for differences in accumulated wealth and general economic inequality in a nation. IQ correlates highly with school performance but the correlations decrease the closer one gets to real-world outcomes, like with job performance, and still lower with income. It explains less than one sixth of the income variance [11]. Even for school grades, other factors explain most the variance. Regarding economic inequality, one study found that if we could magically give everyone identical IQs, we would still see 90 to 95 percent of the inequality we see today. [12]. Another recent study (2002) found that wealth, race, and schooling are important to the inheritance of economic status, but IQ is not a major contributor and the genetic transmission of IQ is even less important [13]. 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.[14]

Intelligence quotient - Validity and g-loading of specific tests

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 predicts outcomes such as job performance, social pathologies, or academic achievement. Different IQ tests differ in their validity for various outcomes.

Tests also differ in their g-loading, which is the degree to which the test score reflects general mental ability rather than a specific skill or "group factor" such as verbal ability, spatial visualization, or mathematical reasoning). g-loading and validity are related in the sense that most IQ tests derive their validity mostly or entirely from the degree to which they measure g (Jensen 1998).

Intelligence quotient - Social construct?

Some maintain that IQ is a social construct invented by the privileged classes, used to maintain their privilege. Others maintain that intelligence, measured by IQ or g, reflects a real ability, is a useful tool in performing life tasks and has a biological reality.

The social-construct and real-ability interpretations for IQ differences can be distinguished because they make opposite predictions about what would happen if people were given equal opportunities. The social explanation predicts that equal treatment will eliminate differences, while the real-ability explanation predicts that equal treatment will accentuate differences. Evidence for both outcomes exists. Achievement gaps persist in socioeconomically advantaged, integrated, liberal, suburban school districts in the United States (see Noguera, 2001). Test-score gaps tend to be larger at higher socioeconomic levels (Gottfredson, 2003). Some studies have reported a narrowing of score gaps over time.

The reduction of intelligence to a single score seems extreme and wrong to many people. Opponents argue that it is much more useful to know a person's strengths and weaknesses than to know their IQ score. Such opponents often cite the example of two people with the same overall IQ score but very different ability profiles. As measured by IQ tests, most people have highly balanced ability profiles, with differences in subscores being greater among the more intelligent.

IQ scores are not intended to gauge a person's worth, and in many situations, IQ may have little relevance.

Intelligence quotient - The Mismeasure of Man

Some scientists dispute psychometrics entirely. In The Mismeasure of Man, Professor Stephen Jay Gould argues that intelligence tests are based on faulty assumptions and shows their history of being used as the basis for scientific racism. He writes:

He spends much of the book debunking 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 include criticism of The Bell Curve.

Intelligence quotient - 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 special task force to publish an investigative report on the research presented in the book. The full text of the report is available at a third-party website. [15]

The findings of the task force state that IQ scores do have high predictive validity for individual (but not necessarily population) 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 (again, not necessarily 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 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. While they admit there is no empirical evidence supporting it, the APA task force suggests that explanations based on social status and cultural differences may be possible. Regarding genetic causes, noted that there is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis.

The report was published in 1995 and thus does not include a decade of recent research.

Intelligence quotient - Improving IQ

While a large amount of one's IQ is predetermined by genetic factors, the environment can play a role as well. IQ can be improved to a certain extent through reading and application. Improvement in diet and regular exercise can help certain cognitive functions, and getting a little extra sleep can help as well. Depression and stress reduce IQ somewhat, so removal of these factors might also help.

Drugs designed to improve cognitive function, and sometimes IQ scores are called nootropics.

Working memory training, an experimental treatment which has according to one study by Klingberg et al, improved raw scores substantially on Ravens progressive matrices and Ravens advanced progressive matrices, both IQ tests. It has also been claimed in some studies that neurofeedback can increase IQ. However, some would argue that these studies should not necessarily be interpreted as proof that neurofeedback can increase IQ as (a) they don't have a double blind component and (b) it is unknown whether their effects would apply to persons without ADHD, as most of these studies were performed on persons with ADHD. It is possible that the increase in IQ was just a result of better concentration in the subjects.

A recent scientific article on the concept of cognitive reserve included an argument that education and application of the mind can substantially increase IQ.

The "Mozart effect" is the claimed ability of certain musics to enhance intelligence, especially spatial reasoning. However, this effect is not universally accepted. In fact, there is now something of a majority (though by no means universal) consenseus that the "Mozart effect" does not exist. Musical education, as opposed to appreciation, has been shown a number of times to marginally increase IQ in children, there is sparsity of information on whether such an effect might apply to adults.

The levels of a variety of chemicals in the brain, such as chlorine, have been shown to relate to intelligence in a variety of ways. It is possible that by adjusting diet, these could be substantially changed.

Future possibilities for improving the skills IQ tests measure include stem cells treatment, genetic modification, better education based on neurological and cognitive discoveries, better nootropics, etc. [16]

Intelligence quotient - Controversy

See article on IQ test controversy.

Intelligence quotient - End material

See also

• Nature versus nurture
• Emotional intelligence
• Gifted
• SAT
• IQ test controversy
• List of countries by IQ
• Societies:
  • International High IQ Society - 95th percentile, or SD16 IQ of 126
  • Sigma Society - 97.7th percentile, or SD16 IQ of 132
  • Mensa International - 98th percentile, or SD16 IQ of 133
  • Colloquy - 99.5th percentile
  • Cerebrals Society - 99.7th percentile, or SD16 IQ of 144
  • Civiq Society - 99.87th percentile, or SD16 IQ of 148
  • Sigma Society III - 99.87th percentile, or SD16 IQ of 148
  • International Society for Philosophical Enquiry - 99.9th percentile
  • Intertel (group)
  • Triple Nine Society - 99.9th percentile, or SD16 IQ of 150
  • Prometheus Society - 99.997th percentile, or SD16 IQ of 164
  • Heliq Society - 99.997th percentile, or SD16 IQ of 164
  • Sigma Society IV - 99.997th percentile, or SD16 IQ of 164
  • Pi Society - 99.9999th percentile, or SD16 IQ of 176
  • Mega Society - 99.9999th percentile, or SD16 IQ of 176
  • Olympiq Society - 99.99997th percentile, or SD16 IQ of 180
  • Pars Society - 99.99997th percentile, or SD16 IQ of 180
  • Sigma Society V - 99.99997th percentile, or SD16 IQ of 180
  • Giga Society - 99.9999999th percentile, or SD16 IQ of 196
  • Sigma Society VI - 99.9999999th percentile, or SD16 IQ of 196

Intelligence quotient - External links

• Sigma Test (available in 14 languages)
• IQ test trainer- software for improving IQ and IQ score
• IQ Comparison Site
• APA - Intelligence: Knowns and Unknowns
• Wall Street Journal: Mainstream Science on Intelligence
• Scientific American: Intelligence Considered
• Scientific American: The General Intelligence Factor
• iqte.st: Online IQ test reviews
• APA Committee on Online Psychological Tests and Assessment report
• Estimated IQs of the greatest geniuses
• Number of highly gifted relatives of high IQ people
• The Basic Period of Individual Mental Speed, Underlying IQ
• Uncommonly difficult IQ tests
• Alternative to typical IQ Test
• IQs of celebrities
• 5 minute IQ test that uses paper folding to test your intelligence

Intelligence quotient - References

• Coward, W.M. and Sackett, P.R. (1990). Linearity of ability-performance relationships: A reconfirmation. Journal of Applied Psychology, 75:297–300.
• 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.), Introducau a psicologia das diferncas individuais. Porto Allegre, Brazil: ArtMed Publishers. PDF
• Haier RJ, Jung RE, Yeo RA, et al. (2005). The neuroanatomy of general intelligence: sex matters. NeuroImage 25: 320–327.
• Hunt, E. (2001). Multiple views of multiple intelligence. [Review of Intelligence Reframed: Multiple Intelligences for the 21st Century.]
• Jensen, A.R. (1998). The g Factor. Praeger, Connecticut, USA.
• 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
• Plomin, R., DeFries, J. C., Craig, I. W., & McGuffin, P. (2003). Behavioral genetics in the postgenomic era. Washington, DC: American Psychological Association.
• Plomin, R., DeFries, J. C., McClearn, G. E., & McGuffin, P. (2001). Behavioral genetics (4th ed.). New York: Worth Publishers.
• Rowe, D. C., W. J. Vesterdal, and J. L. Rodgers, "The Bell Curve Revisited: How Genes and Shared Environment Mediate IQ-SES Associations," University of Arizona, 1997
• 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.

Categories: Cognitive tests | Psychometrics

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