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Intelligence quotient

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The IQs of a population fits a Normal Distribution.

An Intelligence quotient, or an IQ is a number. This number is the score (result) of one of several standard tests. The tests are designed to measure the intelligence of an individual. A person's IQ score tells how much his intelligence is above or below the average. The idea of the test was developed at the start of the 20th century.[1][2] The tests try to avoid specific knowledge, and try to ask questions which, in principle, anyone might be able to answer.

One modern IQ test is the Wechsler Adult Intelligence Scale. It says where the subject's score is on the Gaussian bell. The bell curve used has a center value of 100 and a standard deviation of 15; other tests may have different standard deviations.

IQ scores can tell some things about a person, as well as intelligence. This is because intelligence is linked to other aspects of life. "All the cognitive tests completed in 1983 predicted onset of dementia and Alzheimer’s disease up to 11 years later".[3] They can predict social status (how much money they earn and how important their job is) of the parents,[4] and the parents' IQ. There is still disagreement about how much is heritable (passed from parents to children). People still disagree about how much of a person's IQ comes his parents and how much depends on his environment (what his home is like).[5]

IQ scores are used in various ways:

  1. to predict what kind of education can benefit from
  2. to tell what kind of jobs a person could probably do
  3. to study what the IQ scores of a population are like
  4. to study what other things about a person are related to his IQ

The average IQ scores for many populations have been rising about three points per decade since the early 20th century. Most of the increase is in the lower half of the IQ range. This is called the Flynn effect. People who study it disagree whether these changes in scores are really happening, or it they mean that there were mistakes with how people were tested in the past.

General factor (g)

There are many different kinds of intelligence tests that use many different methods. Some kinds of tests are

  • visual (they only use pictures)
  • verbal (they only use words)
  • abstract-reasoning (thinking about puzzles)
  • arithmetic (simple maths)
  • spatial imagery (thinking about shapes)
  • reading
  • vocabulary (how many words a person knows)
  • memory
  • general knowledge

The psychologist Charles Spearman in 1904 first studied how the scores from different kinds of intelligence tests are related to each other. He found that if a person got a high (or low) score on one kind of test, he probably (but not always) would get a similar score on the other kinds of tests. Because of this, he said that a person's intelligence could be described with one number. He called this number g (for general factor). Tests that use abstract reasoning are usually the best to tell what the scores on the other kinds of tests probably will be. Because of that, Spearman thought that a person's abstract reasoning ability (how good he was at solving puzzles or problems) was what other kinds of intelligence are based on.

Therefore, the number g is what an IQ test is supposed to measure. One of the most commonly used measures of g is Raven's Progressive Matrices, which is a test of visual reasoning.[6]

The war years in the United States

During World War I, a way was needed to test if people could be in the military and then decide what kind of job they could do best. Because of this people wanted to learn the best way to test the minds of recruits (people serving in the military). Because of this, people in America (and other countries) worked to develop good intelligence tests.

The testing generated controversy and much public debate. Nonverbal or "performance" tests were developed for those who could not speak English or were suspected of malingering.[7] After the war, positive publicity on army psychological testing helped to make psychology a respected field.[8] Afterwards, there was an increase in jobs and funding in psychology in the United States.[9] Group intelligence tests were developed and became widely used in schools and industry.[10][11]

Criticisms of IQ tests

There are a number of problems with intelligence quotients. They relate to different fields of the subject. The problems can be grouped:

  • There is no general agreement on what intelligence really is. Different branches of science use different definitions. So, it is problematic to claim that the intelligence quotient is a measure of intelligence.
  • Some think it is problematic that different aspects of intelligence can be combined into one "measurement".
  • The first tests were done on children in school, to determine which children would likely need more attention. Some think this is different from measuring "intelligence". A child that needs more help in school is not necessarily less intelligent; they might simply come from a different background.
  • Some tests favour those tested from a certain cultural background. People outside the culture will test less well, but they are not necessarily less intelligent.

Test does not measure intelligence

Alfred Binet, a French psychologist who had designed one of the first such tests in 1905 had this opinion. He used the test to see which pupils would need special help with the school curriculum. He believed that the test scales were not able to measure intelligence:

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

He argued that with good education programs, most students could catch up and perform quite well in school. This was independent of the background of the pupil. He did not believe that intelligence was a measurable fixed entity.

Some dispute psychometrics entirely. Paleontologist 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 opinion, the General intelligence factor g (which these tests measure), is simply a mathematical artifact.

…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)

However, as explained above, IQ tests were highly successful in assessing recruits during wartime. Therefore, it must be true that they are measuring a relevant mental capability. Therefore IQ are not simply a mathematical fiction: they relate to the ability of individuals to perform certain functions. Even if experts do not agree on a definition of intelligence, that does not disprove the usefulness (or otherwise) of the tests. In every day life people do notice the relative intelligence of others. The issue is central to human nature and evolutionary psychology, because humans evolved the characteristics which helped them survive and reproduce.[12]

Tests are biased

The American Psychological Association's report Intelligence: knowns and unknowns states that IQ tests as predictors of social achievement are not biased against people of African descent. They predict future performance, such as school achievement, similarly to the way they predict future performance for European descent.[13]

However, IQ tests may well be biased when used in other situations. A 2005 study stated that "differential validity in prediction suggests that the WAIS-R test may contain cultural influences that reduce the validity of the WAIS-R as a measure of cognitive ability for Mexican American students,"[14] indicating a weaker positive correlation relative to sampled white students. Other recent studies have questioned the culture-fairness of IQ tests when used in South Africa.[15] Standard intelligence tests, such as the Stanford-Binet, are often inappropriate for children with autism and dyslexia; 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.[16]

Claimed low intelligence has historically been used to justify the feudal system and unequal treatment of women. In contrast, others claim that the refusal of "high-IQ elites" to take IQ seriously as a cause of inequality is itself immoral.[17]

The view of the American Psychological Association

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.[13][18]

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 found that individual differences in intelligence are substantially influenced by genetics and that both genes and environment, in complex interplay, are essential to the development of intellectual competence.

They state there is little evidence to show that childhood diet influences intelligence except in cases of severe malnutrition. 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.

The APA journal that published the statement, American Psychologist, later published responses in January 1997. Several of these argued that the report failed to examine adequately the evidence for partly-genetic explanations.


  1. Indiana University (2007). "William Stern". Indiana University. Retrieved July 15 2009.
  2. i.e. as a quotient of "mental age" and "chronological age."
  3. Cervilla et al. (2004). "Premorbid cognitive testing predicts the onset of dementia and Alzheimer's disease better than and independently of APOE genotype". Psychiatry 2004;75:1100-1106.. Retrieved August 6 2006.
  4. Intelligence: Knowns and Unknowns Report of a Task Force established by the Board of Scientific Affairs of the American Psychological Association - Released August 7, 1995 - A slightly edited version was published in the American Psychologist, Feb 1996. Official Journal of the APA
  5. Devlin B. Daniels M. & Roeder K. (1997). "The heritability of IQ". Nature 388 (6641): 468–71. doi:10.1038/41319 . PMID 9242404 .
    The same study suggests that the heritable component of IQ becomes more significant with age.
  6. Neisser U (1997). "Rising scores on intelligence tests". American Scientist 85: 440–7.
  7. Kaufman, Alan S. 2009. IQ Testing 101. New York: Springer. ISBN 978-0-8261-0629-2
  8. Kennedy, Carrie H. & McNeil, Jeffrey A. 2006. A history of military psychology. In Kennedy, Carrie H. & Zillmer, Eric. Military psychology: clinical and operational applications. New York: Guilford Press. pp. 1–17. ISBN 1-57230-724-2
  9. Katzell, Raymond A.; Austin, James T. (1992). "From then to now: The development of industrial-organizational psychology in the United States". Journal of Applied Psychology 77 (6): 803–35. doi:10.1037/0021-9010.77.6.803 .
  10. Terman, Lewis M. 1919 (and many later editions). The measurement of intelligence: an explanation of and a complete guide for the use of the Stanford revision and extension of the Binet–Simon intelligence scale. London:Harrap.
  11. Kevles, D. J. (1968). "Testing the Army's intelligence: psychologists and the military in World War I". The Journal of American History 55 (3): 565–81. doi:10.2307/1891014 .
  12. Pinker, Steven 2002. The blank slate: the modern denial of human nature. New York: Viking. ISBN 0-670-03151-8
  13. 13.0 13.1 Neisser et al. (1995). "Intelligence: knowns and unknowns". Board of Scientific Affairs of the American Psychological Association. Retrieved August 6 2006.
  14. Culture-Fair Cognitive Ability Assessment Steven P. Verney Assessment, Vol. 12, No. 3, 303-319 (2005)
  15. Shuttleworth-Edwards AB et al (2004). "Cross-cultural effects on IQ test performance: a review and preliminary normative indications on WAIS-III test performance". J Clin Exp Neuropsychol 26 (7): 903–920. doi:10.1080/13803390490510824 . PMID 15742541 .
  16. Edelson, MG (2006). "Are the majority of children with autism mentally retarded? a systematic evaluation of the data". Focus Autism Other Dev Disabl 21 (2): 66–83. doi:10.1177/10883576060210020301 . Retrieved 2007-04-15.
  17. Steve Sailer (2000). "How to help the left half of the bell curve". Retrieved 2006-08-06.
  18. [1]