Intelligence is usually said to involve mental capabilities such as the ability to reason, plan, solve problems, think abstractly, comprehend ideas and language, and learn. Although nonscientists generally regard the concept of intelligence as having much broader scope, in psychology, the study of intelligence generally regards this behavioral trait as distinct from creativity, personality, character, or wisdom.
Definitions of intelligence
At least two major "consensus" definitions of intelligence have been proposed. First, from "Intelligence: Knowns and Unknowns" a report of a task force convened by the American Psychological Association:
- Individuals differ from one another in their ability to understand complex ideas, to adapt effectively to the environment, to learn from experience, to engage in various forms of reasoning, to overcome obstacles by taking thought. Although these individual differences can be substantial, they are never entirely consistent: a given person’s intellectual performance will vary on different occasions, in different domains, as judged by different criteria. Concepts of "intelligence" are attempts to clarify and organize this complex set of phenomena. 
A second definition of intelligence comes from "Mainstream Science on Intelligence", which was signed by 52 intelligence researchers in 1994:
- a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings—"catching on", "making sense" of things, or "figuring out" what to do. (reprinted in Gottfredson, 1997, p. 13) 
Individual intelligence experts have offered a number of similar definitions.
David Wechsler: "... the aggregate or global capacity of the indiviudal to act purposefully, to think rationally, and to deal effectively
with his environment."
Cyril Burt: "...innate general cognitive ability."
Howard Gardner: "To my mind, a human intellectual competence must entail a set of skills of problem solving—enabling the individual to resolve genuine problems or difficulties that he or she encounters and, when appropriate, to create an effective product—and must also entail the potential for finding or creating problems—and thereby laying the groundwork for the acquisition of new knowledge."
Herrnstein and Murray: "...cognitive ability."
Sternberg and Salter: "...goal-directed adaptive behavior."
Main articles: IQ, General intelligence factor
Despite the variety of concepts of intelligence, the most influential approach to understanding intelligence (i.e., the one that has generated the most systematic research) is based on psychometric testing.
Intelligence, narrowly defined, can be measured by intelligence tests, also called IQ (intelligence quotient) tests. Such tests are among the most accurate (reliable and valid) psychological tests. Such intelligence tests take many forms, but g theory proponents argue that the common tests (Stanford-Binet, Raven's Progressive Matrices, Wechsler Adult Intelligence Scale, Wechsler-Bellevue I, and others) all measure the same dominant form of intelligence, g or "general intelligence". The abstraction of g stems from the observation that scores on all forms of cognitive tests correlate positively with one another. g can be derived as the principle factor from cognitive test scores using the method of factor analysis.
In the psychometric view, the concept of intelligence is most closely identified with g, or Gf ("fluid g"). However, psychometricians can measure a wide range of abilities, which are distinct yet correlated. One common view is that these abilities are hierarchically arranged with g at the vertex (or top, overlaying all other cognitive abilities).
Intelligence, IQ, and g
Intelligence, IQ, and g are distinct. Intelligence is the term used in ordinary discourse to refer to cognitive ability. However, it is generally regarded as too imprecise to be useful for a scientific treatment of the subject. The intelligence quotient (IQ) is an index calculated from the scores on test items judged by experts to encompass the abilities covered by the term intelligence. IQ measures a multidimensional quantity: it is an amalgam of different kinds of abilities, the proportions of which may differ between IQ tests. The dimensionality of IQ scores can be studied by factor analysis, which reveals a single dominant factor underlying the scores on all IQ tests. This factor, which is a hypothetical construct, is called g. Variation in g corresponds closely to the intuitive notion of intelligence, and thus g is sometimes called general cognitive ability or general intelligence.
One or several types of intelligence?
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). A single factor is not guaranteed. Other psychological tests which do not measure cognitive ability, such as personality tests, generate multiple factors.
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.
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. These theories grew from observations of human development and of brain injury victims who demonstrate an acute loss of a particular cognitive function -- e.g. the ability to think numerically, or the ability to understand written language -- without showing any loss in other cognitive areas.
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 IQ), 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).
The fundamental argument for 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, and g thus emerges in a factor analysis. This suggests that the tests are not unrelated, but that they all tap a common factor.
This correlation could exist for other reasons, however -- for example, our educational methods might depend on some very narrow range of
cognitive functions, and thus strongly develop fundamentally unrelated abilities in people who excel within that narrow range. This remains
an open debate in cognitive psychology.
According to Jeff Hawkins, the brain's cortex implements a memory prediction system to form the basis of intelligence.
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, 2005). 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 (see IQ).
the genetic and environmental contributions to individual variation in intelligence (see Nature versus nurture).
differences in average measured intelligence between different groups and the source and meaning of these differences (see Race and intelligence and Sex and intelligence).
Stephen Jay Gould is the preeminent popular critic of claims about intelligence. In his book The Mismeasure of Man, Gould makes the following claims about intelligence:
Intelligence is not measurable.
Intelligence is not innate.
Intelligence is not heritable.
Intelligence can not be captured in a single number.
Correlates of intelligence
Certain studies have shown correlations between intelligence and some traits, such as:
|Birth order||Belmont & Marolla, 1973|
|Brain size||McDaniel, 2005||Neuroscience and intelligence|
|Family size||Belmont & Marolla, 1973||See birth order|
|Glucose metabolic rate in the brain||Haier et al., 1995||Neuroscience and intelligence|
|Grades in school||Jensen, 1998|
|Hybrid vigor||Nagoshi & Johnson, 1986|
|Race||Lynn, 1991||Race and intelligence|
|Religiousness||Clark, 2004||Religiousness and intelligence|
|Sex||Lynn, 1999||Sex and intelligence|
|Skin color in African Americans||Lynn, 2002|
- Belmont, M., & Marolla, F.A. (1973). Birth order, family size, and intelligence, Science, 182: 1096–1101.
- Coward, W.M. and Sackett, P.R. (1990). Linearity of ability-performance relationships: A reconfirmation. Journal of Applied Psychology, 75:297–300.
- Gardner, H., Kornhaber, M. and Wake, W. (1996). Intelligence: Multiple Perspectives. Fort Worth: Harcourt Brace College Publishers.
- Gottfredson, L. S. (Ed.) (1997). Intelligence and social policy. Intelligence, 24(1). (Special issue) 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
- Haier, R. J., Chueh, D., Touchette, P., Lott, I., Buchsbaum, M., Macmillan, D., et al. (1995). Brain size and cerebral glucose metabolic rate in nonspecific mental retardation and Down syndrome, Intelligence, 20: 191–210.
- Hawkings, Jeff (2005). On intelligence, Times Books, Henry Holt and Co. ISBN 0-8050-7456-2
- 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.
- Lynn, R. (1991). Race differences in intelligence: A global perspective, Mankind Quarterly, 31: 255–296.
- Lynn, R. (1999). Sex differences in intelligence and brain size: a developmental theory, Intelligence, 27: 1–12.
- Lynn, R. (2002). Skin color and intelligence in African Americans, Population and Environment, 33: 365–375.
- 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.
- Michael A. McDaniel, Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence, Intelligence, Volume 33, Issue 4, July-August 2005, Pages 337-346. 
- Murray, Charles (1998). Income Inequality and IQ, AEI Press PDF
- Nagoshi, C. T. & Johnson, R. C. (1986). The ubiquity of g, Personality and Individual Differences, 7: 201–207.
- Noguera, P.A. (2001). Racial politics and the elusive quest for excellence and equity in education. In Motion Magazine article
- R. Plomin, J. C. DeFries, G. E. McClearn, M. Rutter, Behavioral Genetics (Freeman, New York, ed. 3, 1997).
- Rushton, J.P. (1990). Creativity, intelligence, and psychoticism, Personality and Individual Differences, 11: 1291–1298.