The definition of intelligence has long been a matter of controversy.
Intelligence comes from the Latin verb "intellegere", which means "to understand". By this rationale, intelligence (as understanding) is arguably different from being "smart" (able to adapt to one's environment), or being "clever" (able to creatively adapt).
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. 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.
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.
 Other definitions
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Additionally, many researchers, prominent in the fields of Psychology and Learning, have offered their own definitions of human intelligence:
- Carolus Slovinec: "Intelligence is the ability to recognize connections."
- 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."
- Linda Gottfredson: "... the ability to deal with cognitive complexity."
- Herrnstein and Murray: "...cognitive ability."
- Sternberg and Salter: "...goal-directed adaptive behavior."
- John Kotter on Leadership Intelligence: A "keen mind" i.e., strong analytical ability, good judgement, and the capacity to think strategically and multi-dimensionally.
- D. Samuel Nuessle: "A mind's ability to apply knowledge to a problem-solving situation."
Other researchers prominent in the fields of Mathematics and Engineering, have offered their own definitions of intelligence:
- Alan Turing: "To respond like a human being"
- G.N. Saridis: "The entropy of control responses"
In an educational context, one's intelligence should not be equated with one's academic performance, or with the volume of knowledge one has acquired through formal education. A person's ability to think critically and analytically about his or her knowledge and experience is more important than command of a large number of facts. Intelligence is not confined to thinking either. Purposeful actions demonstrating appropriate responses to the situation and reasoned application of one's knowledge are evidence of intelligence. It is also important to note that analytic skills only constitute one part of intelligence -- mimesis, synthesis, creative and the ability to find innovative solutions to unfamiliar problems are also important.
 Psychometric approach
- Main articles: IQ, General intelligence factor
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. Such intelligence quotient (IQ) tests include the Stanford-Binet, Raven's Progressive Matrices, the Wechsler Adult Intelligence Scale and the Wechsler-Bellevue.
All forms of IQ tests correlate highly with one another. The traditional view is that these tests measure g or "general intelligence factor". g can be derived as the principal factor using the mathematical method of factor analysis. However, psychometricians can also measure a wide range of abilities, which are distinct yet correlated. For example, g itself is sometimes considered to be a two part construct, gF and gC, which stand for fluid and crystallized intelligence.
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. Carroll (1993) and Snow (1984) put forward what might be described as an interpenetrating position having more in common with that of Charles Spearman (1924) who is credited with having developed the concept of g.
Intelligence, as measured by IQ and other aptitude tests, is widely used in educational, business, and military settings because it is an effective predictor of behavior. Intelligence is significantly correlated with successful training and performance outcomes. According to research by Ree and Earles (1992), g is the single best predictor of job performance, with minimal statistical improvements gained by the addition of more specific ability measures. Using data from thousands of cases, they demonstrated that the average magnitude of correlation of g with various criterion measures ranges from r =.33 to .76. 
In a recent review of the empirical research, David Geary found that g is highly correlated with many important social outcomes. Individuals with low IQs are more likely to be divorced, more likely to have a child out of marriage, more likely to be incarcerated, and more likely to need long term welfare support. Furthermore, he found that high IQs are associated with more years of education, higher status jobs, and higher income.
IQ tests were originally devised specifically to predict educational achievement. The inventors of the IQ did not believe they were measuring fixed intelligence. Despite this, critics argue that intelligence tests have been used to support nativistic theories in which intelligence is viewed as a qualitatively unique faculty with a relatively fixed quantity.
Critics of the psychometric approach point out that people in the general population have a somewhat different and broader conception of intelligence than what is measured in IQ tests. In turn, they argue that the psychometric approach measures only a part of what is commonly understood as intelligence. Furthermore, skeptics argue that even though tests of mental abilities are correlated, people still have unique strengths and weaknesses in specific areas. Consequently they argue that psychometric theorists over-emphasize g.
Researchers in the field of human intelligence have encountered a considerable amount of public concern and criticism-- much more than scientists in other areas normally receive (see Gottfredson, 2005). For example, a number of critics have challenged the relevance of psychometric intelligence in the context of everyday life. There have also been controversies over genetic factors in intelligence, particularly the question of whether these differences relate to race and gender (see Race and intelligence and Sex and intelligence). Another controversy in the field is how to interpret the increases in test scores that have occurred over time, the so-called Flynn effect.
Stephen Jay Gould was one of the most vocal critics of intelligence testing. In his book, The Mismeasure of Man, Gould argued that intelligence is not truly measurable, and also challenged the hereditarian viewpoint on intelligence. Many of Gould's criticisms were aimed at Arthur Jensen. Jensen responded that his work had been misrepresented. He further replied that making conclusions about modern IQ tests by criticizing the flaws of early intelligence research is like condemning the auto industry by criticizing the performance of the Model T.
 Multiple intelligences
Dissatisfaction with traditional IQ tests has led to the development of a number of alternative theories, all of which suggest that intelligence is the result of a number of independent abilities that uniquely contribute to human performance. Most of these theories are relatively recent in origin, though it should be noted that Louis Thurstone proposed a theory of multiple "primary abilities" in the early 20th Century.
Howard Gardner's Theory of multiple intelligences is based on studies not only on normal children and adults but also by studies of gifted individuals (including so-called 'savants"), of persons who have suffered brain damage, of experts and virtuosos, and of individuals from diverse cultures. This led Gardner to break intelligence down into at least eight different components: logical, linguistic, spatial, musical, kinesthetic, naturalist, intrapersonal and interpersonal intelligences. He argues that psychometric tests address only linguistic and logical plus some aspects of spatial intelligence; other forms have been entirely ignored. Moreover, the paper and-pencil format of most tests rules out many kinds of intelligent performance that matter in everyday life, such as giving an extemporaneous talk (linguistic) or being able to find one's way in a new town (spatial).
Robert Sternberg's Triarchic theory of intelligence proposes three fundamental aspects of intelligence-analytic, creative, and practical--of which only the first is measured to any significant extent by mainstream tests. His investigations suggest the need for a balance between analytic intelligence, on the one hand, and creative and especially practical intelligence on the other.
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.
IQ proponents have pointed out that IQ's predictive validity has been repeatedly demonstrated, for example in predicting important non-academic outcomes such as job performance (see IQ), whereas the various multiple intelligence theories have little or no such support. Meanwhile, the relevance and even the existence of multiple intelligences have not been borne out when actually tested. Thus far, no one has been able to develop a set of ability tests that do not correlate together, and this refutes the claim that multiple intelligences are independent of each other.
 Other species
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, and then operationalizing a measure that accurately compares mental ability across different species and contexts.
Wolfgang Köhler's pioneering research on the intelligence of apes is a classic example of research in this area. Stanley Coren's book, The Intelligence of Dogs is a notable popular book on the topic