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Intelligence is a property of mind that encompasses many related mental abilities, such as the capacities to reason, plan, solve problems, think abstractly, comprehend ideas and language, and learn. In common parlance, the term smart, metaphorically used is frequently the synonym of situational and behavioural (i.e. observed and context dependent) intelligence.

Although many regard the concept of intelligence as having a much broader scope, for example in cognitive science and computer science, in some schools of psychology, the study of intelligence generally regards this 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 in 1995:

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 Intelligence Gottfredson, 1997, p. 13).

Individual intelligence experts have offered a number of similar definitions.

  • Alfred Binet: "...judgment, otherwise called good sense, practical sense, initiative, the faculty of adapting one's self to circumstances... auto-critique ."
  • David Wechsler: "... the aggregate or global capacity of the individual 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 behaviour."
  • John Kotter on Leadership Intelligence: A "keen mind" i.e., strong analytical ability, good judgement, and the capacity to think strategically and multi-dimensionally.

Psychometric intelligence

Despite the variety of concepts of intelligence, the most influential approach to understanding intelligence (i.e., with the most supporters and the most published research over the longest period of time) is based on psychometric testing, which regards intelligence as cognitive ability.

Intelligence, narrowly defined, can be measured by intelligence tests, also called IQ (intelligence quotient) tests. Such intelligence tests take many forms, but 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 factor". 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 principal 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). However, this is by no means universally accepted (see eg Carroll (1993) and Snow et al (1984) who put forward what might be described as an iterpenetrating position having more in common with that of Charles Spearman (eg 1924) who is credited with having developed the concept of g.

Intelligence, IQ, and g

Intelligence, Intelligence quotient (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.

Criticisms of the psychometric approach

Critics of the psychometric approach, such as Robert Sternberg (who formulated the The Triarchic Theory of Intelligence), point out that people in the general population have a somewhat different conception of intelligence than most experts. In turn, they argue that the psychometric approach measures only a part of what is commonly understood as intelligence. Other critics, such as Sir Arthur Stanley Eddington, argue that the equipment used in an experiment often determines the results and that proving that e.g. intelligence exists does not prove that current equipment measure it correctly. Sceptics often argue that so much scientific knowledge about the brain is still to be discovered that claiming the conventional IQ test methodology to be infallible is just a small step forward from claiming that Craniometry was the infallible method for measuring intelligence (which had scientific merits based on knowledge available in the nineteenth century).

One or several types of intelligence?

The phrase "intelligence is task-specific" suggests that while 'general intelligence' can indeed be assessed, all that that would really amount to is a sum total of a given individual's competencies minus any perceived incompetencies.

Most 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). The evidence for g comes from factor analysis of tests of cognitive abilities. The methods of factor analysis do not guarantee a single dominant factor will be discovered. 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). Furthermore, g theorists contend that proponents of multiple intelligences (e.g. Sternberg, Gardner) have not disproved the existence of a general factor of intelligence (Kline, 2000). 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.


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).
  • The implications of the dramatic increase in test scores over time (scores have been increasing at about 1 Standard Deviation per generation). See Flynn effect.

Stephen Jay Gould was an important popular critic of intelligence theory. In his book The Mismeasure of Man, Gould made the following claims about intelligence:

  • Intelligence is not measurable.
  • Intelligence is not innate.
  • Intelligence is only partly heritable, and what is inherited is mutable.
  • Intelligence cannot be captured in a single number.

However it is reported that he has largely ignored at least a decade of important recent research and draws from outdated information to validate his conclusions. Some of Gould's criticisms are aimed at Arthur Jensen. Jensen alleges Gould made several misrepresentations of his work.

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