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Intelligence (trait)

Intelligence is a general mental capability that involves the ability to reason, plan, solve problems, think abstractly, comprehend ideas and language, and learn. In psychology, the study of intelligence is related to the study of personality.



While the definition and importance of intelligence are somewhat controversial, especially in the popular press, a consensus opinion exists among intelligence researchers on many issues.

The most influential approach to the scientific study of intelligence is based on psychometric testing. Some experts accept the concept of a single dominant factor of intelligence, general mental ability or g, while others argue that intelligence consists of a set of relatively independent abilities (American Psychological Association task force report, Gottfredson 1998).

Yale psychologist Robert J. Sternberg has proposed a Triarchic Theory of Intelligence. Harvard psychologist Howard Gardner's theory of multiple intelligences breaks intelligence down into at least eight different components: logical, linguistic, spatial, musical, kinesthetic, naturalist, intra-personal and inter-personal intelligences. Daniel Goleman and several other researchers have developed the concept of emotional intelligence and claim it is at least as important as more traditional sorts of intelligence.

Proponents of multiple-intelligence theories often claim that g is, at best, a measure of academic ability. Other types of intelligence, they claim, might be just as important outside of a school setting. One theory even suggests the existence of two types of g (see Fluid and crystallized intelligence).

In response, g theorists have pointed out that g's predictive validity has been repeatedly demonstrated, for example in predicting important non-academic outcomes such as job performance (see below), while no multiple-intelligences theory has shown comparable validity. Meanwhile, they argue, the relevance, and even the existence, of multiple intelligences have not been borne out when actually tested (Hunt 2001).

When considering animal intelligence, a more general definition of intelligence might be applied: the "ability to adapt effectively to the environment, either by making a change in oneself or by changing the environment or finding a new one" (Encyclopędia Britannica).

Intelligence tests are often used to quantify human intelligence. This is not without controversy; see below for more information.

Some thinkers have explored the idea of collective intelligence, arising from the coordination of many people. Computer science has developed the field of artificial intelligence, which seeks to make computers act in increasingly intelligent ways. Many people have also speculated about the possibility of extraterrestrial intelligence.

Intelligence tests

See main article: IQ

Intelligence, narrowly defined, can be measured by intelligence tests, also called IQ tests. Such tests are among the most accurate (reliable and valid) psychological tests, but they are not intended to measure creativity, personality, character, or wisdom. Intelligence tests take many forms, but the common tests (Stanford-Binet, Raven's Standard Progressive Matrices, Wechsler Adult Intelligence Scale (WAIS), Wechsler-Bellevue I, and others) all measure the same intelligence. The general factor measured by intelligence tests is known as g (see g theory).

The fundamental indicator of a general factor is that test scores on a wide range of seemingly unrelated cognitive ability tests (such as sentence completion, arithmetic, and memorization) are positively correlated. People who score highly on one test tend to score highly on all of them. This suggests that the tests are not unrelated, but that they all tap a common factor. The common factor, g, can be extracted using mathematical techniques such as factor analysis or principal components analysis. IQ tests measure g better than any other test.

A single factor is not guaranteed. Other psychological tests which do not measure cognitive ability, such as personality tests, generate multiple factors.

IQ tests have been strongly criticized as biased, particularly against minorities. Bias in cognitive ability testing means, in part, that two groups with different average scores on the test have similar average scores on the outcome the test is supposed to predict (for example, academic achievement or job performance). Considerable scientific effort in the middle of the 20th century, led by researchers such as Arthur Jensen, centered on constructing tests which were demonstrably free of this type of bias. While these efforts generally succeeded, group differences in test scores persisted. Some people consider the different scores between racial groups (see Race and intelligence) prima facie evidence of test bias. Others, including most experts, support the alternative: that the different scores reflect real ability differences. The source of these differences are not known and are an area of active research.

Practical importance

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 only 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 great practical importance in economic terms. The utility of using a 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). Legal barriers, most prominently the 1971 United States Supreme Court decision Griggs vs. Duke Power Co., have prevented American employers from directly using cognitive ability tests to select employees, despite the tests' high validity. Using cognitive ability scores in selection adversely affects some minority groups, because different groups have different mean scores on tests of cognitive ability (see Race and intelligence).

Interpretation and explanation of intelligence differences

The correlations discussed above are well-established. However, their interpretation is not without controversy. Some maintain that intelligence 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.

Contrary to the claim that IQ is a social construct, cognitive ability is heritable. Adoption studies show that, by 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 also support a partly genetic basis for IQ: monozygotic (identical) twins raised separately are highly similar in IQ (0.75), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adopted siblings (~0.0). According to some studies the heritability of IQ increases with age, such that differences in family advantage are lost by adolescence.

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

The social-construct and real-ability interpretations for IQ differences can also 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.

Environmental factors may also play an important role in explaining IQ differences. 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.

Worldwide, IQ scores appear to be slowly rising, a trend known as the Flynn effect.


Researchers in the field of human intelligence have encountered a considerable amount of public concern and criticism - much more than many scientists would be accustomed to or comfortable with (for examples, see Gottfredson, 2003). Some of the controversial topics include:

  • the relevance of psychometric intelligence to the common-sense understanding of the topic
  • the importance of intelligence in everyday life (practical intelligence , problem solving)
  • the genetic and environmental contributions to individual variation in intelligence (see Nature versus nurture).
  • differences in average measured intelligence between racial groups, and the source and meaning of these differences (see Race and intelligence).


  • Gottfredson, L.S. (1998). The general intelligence factor. Scientific American Presents, 9(4):24-29. PDF
  • Gottfredson, L.S. (2003). Suppressing intelligence research: hurting those we intend to help. Unpublished. PDF
  • Hunt, E. (2001). Multiple views of multiple intelligence. [Review of Intelligence Reframed: Multiple Intelligences for the 21st Century.] Contemporary Psychology, 46:5-7.
  • Hunter, J.E. and Hunter, R.F. (1984). Validity and utility of alternate predictors of job performance. Psychological Bulletin, 96(1):72-98.
  • Jensen, A.R. (1998). The g Factor. Praeger, Connecticut, USA.

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Last updated: 05-09-2005 12:26:55
Last updated: 05-13-2005 07:56:04