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Intelligence quotient
Race and intelligence
Countries and intelligence
Intelligence: A Unifying
Construct for the Social Sciences

Intelligence has been defined in many different ways as including, but not limited to, aspects of problem solving, abstract thought, understanding, learning ability, reasoning, memory, planning, perception, and communication.

Intelligence is most widely studied in humans. Several approaches to human intelligence have been adopted. IQ tests and related tests are the most researched and by far the most widely used in practical settings.[1][1]

There have also been research on intelligence in animals and even plants. Artificial intelligence is the simulation of intelligence in machines.


From latin inter- "between" + legere "choose, pick out".[2]


The definition of intelligence is controversial. Groups of scientists have stated the following:

  1. from "Mainstream Science on Intelligence" (1994), an editorial statement by fifty-two researchers:
    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.[3]
  2. from "Intelligence: Knowns and Unknowns" (1995), a report published by the Board of Scientific Affairs of 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. Although considerable clarity has been achieved in some areas, no such conceptualization has yet answered all the important questions, and none commands universal assent. Indeed, when two dozen prominent theorists were recently asked to define intelligence, they gave two dozen, somewhat different, definitions. s (Stemberg & Detterman, 1986). Such disagreements are not cause for dismay. Scientific research rarely begins with fully agreed definitions, though it may eventually lead to them.[4][5]

Besides the foregoing definitions, these psychology and learning researchers also have defined intelligence as:

Researcher Quotation
Alfred Binet Judgment, otherwise called "good sense," "practical sense," "initiative," the faculty of adapting one's self to circumstances ... auto-critique.[6]
David Wechsler The aggregate or global capacity of the individual to act purposefully, to think rationally, and to deal effectively with his environment.[7]
Lloyd Humphreys "...the resultant of the process of acquiring, storing in memory, retrieving, combining, comparing, and using in new contexts information and conceptual skills."[8]
Cyril Burt Innate general cognitive ability[9]
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.[10]
Linda Gottfredson The ability to deal with cognitive complexity.[11]
Sternberg & Salter Goal-directed adaptive behavior.[12]
Reuven Feuerstein The theory of Structural Cognitive Modifiability describes intelligence as "the unique propensity of human beings to change or modify the structure of their cognitive functioning to adapt to the changing demands of a life situation."[13]

What is considered intelligent varies with culture. For example, when asked to sort, the Kpelle in western Africa take a functional rather than linguistic approach. A Kpelle participant stated "the knife goes with the orange because it cuts it." When asked how a fool would sort, they sorted linguistically, putting the knife with other implements and the orange with other foods, which is the style considered intelligent in other cultures.[14]

Approaches to human intelligence

IQ and related tests

Main article: Intelligence quotient

The dominant approach to understanding intelligence with by far the most published research and practical use is the psychometric approach as represented by Intelligence quotient (IQ) tests and related tests. Non-IQ psychometric tests are primarily not intended to measure intelligence itself but some closely related construct such as scholastic aptitude. One example in the US is the SAT test. Both the IQ and the non-IQ tests are highly correlated with one another which has been interpreted as evidence for the existence of an underlying general mental ability or factor (the g factor).[1]

There are critics of IQ and g who do not dispute the stability of IQ test scores or the fact that they predict certain forms of achievement rather effectively. They do argue, however, that to base a concept of intelligence on IQ test scores alone is to ignore many important aspects of mental ability.[1]

On the other hand, Linda S. Gottfredson has argued that the results of thousands of studies support the importance of IQ for school and job performance. IQ also predicts or correlates with numerous other life outcomes. In contrast, empirical support for non-g intelligences is lacking or very poor. She argued that despite this the ideas of multiple non-g intelligences are very attractive to many due to the suggestion that everyone can be smart in some way.[15]

Howard Gardner's theory of multiple intelligences

Howard Gardner's theory of multiple intelligences argues that there are multiple, only weakly correlated intelligences. Proposed intelligences include Spatial, Linguistic, Logical-mathematical, Bodily-kinesthetic, Musical, Interpersonal, Intrapersonal, Naturalistic, and possibly also Existential and Moral. In contrast, modern IQ tests measure many abilities but these are highly correlated with one another.

Howard Gardner's theory has been heavily criticized. When tested, it was found that the different "intelligences" actually correlated with the g factor, supporting the idea of a single dominant type of intelligence. According to the study, each of the domains proposed by Gardner involved a blend of g, of cognitive abilities other than g, and, in some cases, of non-cognitive abilities or of personality characteristics.[16] It has been further argued that there are no supporting empirical studies and that neuroscientific research have demonstrated that the human brain is unlikely to function as proposed by Gardner.[17][18]

Triarchic theory of intelligence

Robert Sternberg has proposed the triarchic theory of intelligence (later renamed the . It argues that there are three different intelligences. Analytic intelligence is measured by IQ tests. In addition there are also creative intelligence (see creativity below) and practical intelligence. Practical intelligence have been argued to be related to job performance but relatively uncorrelated with IQ.[1] The theory has been updated and renamed as the Theory of Successful Intelligence.[19]

Sternberg's theories and research on intelligence remain contentious within the scientific community.[20][21][22][23]

PASS Theory of Intelligence

The PASS Theory of Intelligence proposes that cognition is organized in three systems and four processes. The first is the Planning, which involves executive functions responsible for controlling and organizing behavior, selecting and constructing strategies, and monitoring performance. The second is the Attention process, which is responsible for maintaining arousal levels and alertness, and ensuring focus on relevant stimuli. The next comprise two processes, Simultaneous and Successive processing to encode, transform, and retain information. Simultaneous processing is engaged when the relationship between items and their integration into whole units of information is required. Examples of this include recognizing figures, such as a triangle within a circle vs. a circle within a triangle, or the difference between "he had a shower before breakfast" and "he had breakfast before a shower." Successive processing is required for organizing separate items in a sequence such as remembering a sequence of words or actions exactly in the order in which they had just been presented. These four processes are functions of four areas of the brain. Planning is broadly located in the front part of our brains, the frontal lobe. Attention and arousal are combined functions of the frontal lobe and the lower parts of the cortex, although the parietal lobes are also involved in attention as well. Simultaneous processing and Successive processing occur in the posterior region or the back of the brain. Simultaneous processing is broadly associated with the occipital and the parietal lobes while Successive processing is broadly associated with the frontal-temporal lobes.[24][25][26]

The PASS theory is not incompatible with IQ tests and several modern IQ tests have been influenced by it. The different processes in the PASS theory have been seen as corresponding to different abilities in the Cattell-Horn-Carroll theory which has also influenced modern IQ tests.[27]

Piaget's theory and Neo-Piagetian theories

In Piaget's theory of cognitive development, by Jean Piaget, the focus is not on mental abilities but rather on a child's mental models of the world. As a child develops, increasingly more accurate models of the world are developed which enable the child to interact with the world better. One example being object permanence where the child develops a model where objects continue to exist even when they cannot be seen, heard, or touched.

Piaget's theory described four main stages and many sub-stages in the development. Degree of progress through these is correlated with but is not identical with psychometric IQ.[28][29] Piaget conceptualizes intelligence as an activity more than a capacity.

Neo-Piagetian theories of cognitive development expand Piaget's theory in various ways such as also considering psychometric-like factors such as processing speed and working memory, "hypercognitive" factors like self-monitoring, more stages, and more consideration on how progress may vary in different domains such as spatial or social.[30][31]

Piaget's theory has been criticized for the age of appearance of a new model of the world, such as object permanence, being dependent on how the testing is done. More generally, the theory may be very difficult to test empirically due to the difficulty of proving or not proving that a certain mental model is the explanation for the results of testing.[32]

Human intelligence and human personality traits

Intelligence is not the only psychological factor determining a person's success. Personality traits such as self-discipline, good work motivation, and need for achievement are likely also important.


Several different tests with goal of measuring creativity have been constructed. The most well-known may be the Torrance Tests of Creative Thinking. Research using such tests have generally found creativity to be different from IQ.

It has also been argued that creativity is a form of personality rather than a cognitive ability. The trait openness to experience, one of the Big Five personality traits, has been argued to be particularly linked to creativity.

Evolution of human intelligence

The ancestors of modern humans evolved large and complex brains exhibiting an ever-increasing intelligence through a long evolutionary process. Different explanations have been proposed.

Recent and possible future change in human intelligence

See: Dysgenics, Eugenics, Neuroethics, Human genetic engineering, Transhumanism

Animal and plant intelligence

Although humans have been the primary focus of intelligence researchers, scientists have also attempted to investigate animal intelligence, or more broadly, animal cognition. These researchers are interested in studying both mental ability in a particular species, and comparing abilities between species. They study various measures of problem solving, as well as mathematical and language abilities. Some challenges in this area are defining intelligence so that it means the same thing across species (e.g. comparing intelligence between literate humans and illiterate animals), and then operationalizing a measure that accurately compares mental ability across different species and contexts.

It has been argued that plants should also be classified as being intelligent based on their ability to sense the environment and adjust their morphology, physiology and phenotype accordingly.[33][34]

Artificial intelligence

Artificial intelligence (or AI) is both the intelligence of machines and the branch of computer science which aims to create it, through "the study and design of intelligent agents"[35] or "rational agents", where an intelligent agent is a system that perceives its environment and takes actions which maximize its chances of success.[36] Achievements in artificial intelligence include constrained and well-defined problems such as games, crossword-solving and optical character recognition. General intelligence or strong AI has not yet been achieved and is a long-term goal of AI research.

Among the traits that researchers hope machines will exhibit are reasoning, knowledge, planning, learning, communication]], perception, and the ability to move and manipulate objects.[35][36] In the field of artificial intelligence there is no consensus on how closely the brain should be simulated.


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