Intelligence

For other uses, see Intelligence (disambiguation).

Intelligence has been defined in many different ways including logic, abstract thought, understanding, self-awareness, communication, learning, having emotional knowledge, retaining, planning, and problem solving.

Intelligence is most widely studied in humans, but has also been observed in animals and in plants. Artificial intelligence is the simulation of intelligence in machines.

Within the discipline of psychology, various approaches to human intelligence have been adopted. The psychometric approach is especially familiar to the general public, as well as being the most researched and by far the most widely used in practical settings.

History of the term

Main article: Nous

Intelligence derives from the Latin verb intelligere. A form of this verb, intellectus, became the medieval technical term for understanding, and a translation for the Greek philosophical term nous. This term was however strongly linked to the metaphysical and cosmological theories of teleological scholasticism, including theories of the immortality of the soul, and the concept of the Active Intellect (also known as the Active Intelligence). This entire approach to the study of nature was strongly rejected by the early modern philosophers such as Francis Bacon, Thomas Hobbes, John Locke, and David Hume, all of whom preferred the word "understanding" in their English philosophical works. Hobbes for example, in his Latin De Corpore, used "intellectus intelligit" (translated in the English version as "the understanding understandeth") as a typical example of a logical absurdity. The term "intelligence" has therefore become less common in English language philosophy, but it has later been taken up (with the scholastic theories which it now implies) in more contemporary psychology.

Definitions

The definition of intelligence is controversial. Some groups of psychologists have suggested the following definitions:

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.

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.

Besides those definitions, psychology and learning researchers also have suggested definitions of intelligence such as:

Researcher	Quotation
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.
Lloyd Humphreys	"...the resultant of the process of acquiring, storing in memory, retrieving, combining, comparing, and using in new contexts information and conceptual skills."
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.
Sternberg & Salter	Goal-directed adaptive behavior.
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."

What is considered intelligent varies with culture. For example, when asked to sort, the Kpelle people take a functional 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.

Human intelligence

Psychometrics

Main articles: Intelligence quotient and Psychometrics

The IQs of a large enough population are calculated so that they conform to a normal distribution.

The approach to understanding intelligence with the most supporters and published research over the longest period of time is based on psychometric testing. It is also by far the most widely used in practical settings. Intelligence quotient (IQ) tests include the Stanford-Binet, Raven's Progressive Matrices, the Wechsler Adult Intelligence Scale and the Kaufman Assessment Battery for Children. There are also psychometric tests that are not intended to measure intelligence itself but some closely related construct such as scholastic aptitude. In the United States examples include the SSAT, the SAT, the ACT, the GRE, the MCAT, the LSAT, and the GMAT.

Intelligence tests are widely used in educational, business, and military settings because of their efficacy in predicting behavior. IQ and g (discussed in the next section) are correlated with many important social outcomes—individuals with low IQs are more likely to be divorced, have a child out of marriage, be incarcerated, and need long-term welfare support, while individuals with high IQs are associated with more years of education, higher status jobs and higher income. Intelligence is significantly correlated with successful training and performance outcomes, and IQ/g is the single best predictor of successful job performance.

General intelligence factor or g

Main article: g factor (psychometrics)

There are many different kinds of IQ tests using a wide variety of test tasks. Some tests consist of a single type of task, others rely on a broad collection of tasks with different contents (visual-spatial, verbal, numerical) and asking for different cognitive processes (e.g., reasoning, memory, rapid decisions, visual comparisons, spatial imagery, reading, and retrieval of general knowledge). The psychologist Charles Spearman early in the 20th century carried out the first formal factor analysis of correlations between various test tasks. He found a trend for all such tests to correlate positively with each other, which is called a positive manifold. Spearman found that a single common factor explained the positive correlations among tests. Spearman named it g for "general intelligence factor". He interpreted it as the core of human intelligence that, to a larger or smaller degree, influences success in all cognitive tasks and thereby creates the positive manifold. This interpretation of g as a common cause of test performance is still dominant in psychometrics. An alternative interpretation was recently advanced by van der Maas and colleagues. Their mutualism model assumes that intelligence depends on several independent mechanisms, none of which influences performance on all cognitive tests. These mechanisms support each other so that efficient operation of one of them makes efficient operation of the others more likely, thereby creating the positive manifold.

IQ tasks and tests can be ranked by how highly they load on the g factor. Tests with high g-loadings are those that correlate highly with most other tests. One comprehensive study investigating the correlations between a large collection of tests and tasks has found that the Raven's Progressive Matrices have a particularly high correlation with most other tests and tasks. The Raven's is a test of inductive reasoning with abstract visual material. It consists of a series of problems, sorted approximately by increasing difficulty. Each problem presents a 3 x 3 matrix of abstract designs with one empty cell; the matrix is constructed according to a rule, and the person must find out the rule to determine which of 8 alternatives fits into the empty cell. Because of its high correlation with other tests, the Raven's Progressive Matrices are generally acknowledged as a good indicator of general intelligence. This is problematic, however, because there are substantial gender differences on the Raven's, which are not found when g is measured directly by computing the general factor from a broad collection of tests.

Historical psychometric theories

Main article: Intelligence quotient § History

Several different theories of intelligence have historically been important. Often they emphasized more factors than a single one like in g factor.

Cattell-Horn-Carroll theory

Main article: Cattell-Horn-Carroll theory

Many of the broad, recent IQ tests have been greatly influenced by the Cattell-Horn-Carroll theory. It is argued to reflect much of what is known about intelligence from research. A hierarchy of factors is used. g is at the top. Under it there are 10 broad abilities that in turn are subdivided into 70 narrow abilities. The broad abilities are:

• Fluid Intelligence (Gf): includes the broad ability to reason, form concepts, and solve problems using unfamiliar information or novel procedures.
• Crystallized Intelligence (Gc): includes the breadth and depth of a person's acquired knowledge, the ability to communicate one's knowledge, and the ability to reason using previously learned experiences or procedures.
• Quantitative Reasoning (Gq): the ability to comprehend quantitative concepts and relationships and to manipulate numerical symbols.
• Reading & Writing Ability (Grw): includes basic reading and writing skills.
• Short-Term Memory (Gsm): is the ability to apprehend and hold information in immediate awareness and then use it within a few seconds.
• Long-Term Storage and Retrieval (Glr): is the ability to store information and fluently retrieve it later in the process of thinking.
• Visual Processing (Gv): is the ability to perceive, analyze, synthesize, and think with visual patterns, including the ability to store and recall visual representations.
• Auditory Processing (Ga): is the ability to analyze, synthesize, and discriminate auditory stimuli, including the ability to process and discriminate speech sounds that may be presented under distorted conditions.
• Processing Speed (Gs): is the ability to perform automatic cognitive tasks, particularly when measured under pressure to maintain focused attention.
• Decision/Reaction Time/Speed (Gt): reflect the immediacy with which an individual can react to stimuli or a task (typically measured in seconds or fractions of seconds; not to be confused with Gs, which typically is measured in intervals of 2–3 minutes). See Mental chronometry.

Modern tests do not necessarily measure of all of these broad abilities. For example, Gq and Grw may be seen as measures of school achievement and not IQ. Gt may be difficult to measure without special equipment.

g was earlier often subdivided into only Gf and Gc which were thought to correspond to the Nonverbal or Performance subtests and Verbal subtests in earlier versions of the popular Wechsler IQ test. More recent research has shown the situation to be more complex.

Controversies

While not necessarily a dispute about the psychometric approach itself, there are several controversies regarding the results from psychometric research. Examples are the role of genetics vs. environment, the causes of average group differences, or the Flynn effect.

One criticism has been against the early research such as craniometry. A reply has been that drawing conclusions from early intelligence research is like condemning the auto industry by criticizing the performance of the Model T.

Several critics, such as Stephen Jay Gould, have been critical of g, seeing it as a statistical artifact, and that IQ tests instead measure a number of unrelated abilities. The American Psychological Association's report "Intelligence: Knowns and Unknowns" stated that IQ tests do correlate and that the view that g is a statistical artifact is a minority one.

Other theories

There are critics of IQ, 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.

On the other hand, Linda S. Gottfredson (2006) has argued that the results of thousands of studies support the importance of IQ for school and job performance (see also the work of Schmidt & Hunter, 2004). 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 because they suggest that everyone can be intelligent in some way.

Multiple intelligences

Main article: Theory of multiple intelligences

Howard Gardner's theory of multiple intelligences is based on studies not only of 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 a number of different components. In the first edition of his book "Frames of Mind" (1983), he described seven distinct types of intelligence - logical-mathematical, linguistic, spatial, musical, kinesthetic, interpersonal, and intrapersonal. In a second edition of this book, he added two more types of intelligence - naturalist and existential intelligences. He argues that psychometric tests address only linguistic and logical plus some aspects of spatial intelligence. A major criticism of Gardner's theory is that it has never been tested, or subjected to peer review, by Gardner or anyone else, and indeed that it is unfalsifiable. Others (e.g. Locke, 2005) have suggested that recognizing many specific forms of intelligence (specific aptitude theory) implies a political—rather than scientific—agenda, intended to appreciate the uniqueness in all individuals, rather than recognizing potentially true and meaningful differences in individual capacities. Schmidt and Hunter (2004) suggest that the predictive validity of specific aptitudes over and above that of general mental ability, or "g", has not received empirical support.

Howard Gardner mentions in his Multiple Intelligences The Theory in Practice book, briefly about his main seven intelligences he introduced. In his book, he starts off describing Linguistic and Logical Intelligence because he believed that in society, we have put these two intelligences on a pedestal. However, Gardner believes all of the intelligences he found are equal. Note: At the time of the publication of Gardner's book Multiple Intelligences The Theory in Practice, naturalist and existential intelligences were not mentioned.

Linguistic Intelligence: People high in linguistic Intelligence have an affinity for words, both spoken and written.

Logical-Mathematics Intelligence: Is logical and mathematical ability, as well as scientific ability. Howard Gardner believed Jean Piaget may have thought he was studying all intelligence, but in truth, Piaget was really only focusing on the logical mathematical intelligence.

Spatial intelligence: The ability to form a mental model of a spatial world and to be able to maneuver and operate using that model.

Musical Intelligence: Those with musical Intelligence have excellent pitch, and may even be absolute pitch.

Bodily-kinesthetic intelligence: The ability to solve problems or to fashion products using one's whole body, or parts of the body. For example, dancers, athletes, surgeons, craftspeople, etc.

Interpersonal intelligence: The ability to see things from the perspective of others, or to understand people in the sense of empathy. Strong interpersonal intelligence would be an asset in those who are teachers, politicians, clinicians, religious leaders, etc.

Intrapersonal intelligence: A correlative ability, turned inward. It is a capacity to form an accurate, veridical model of oneself and to be able to use that model to operate effectively in life.

Triarchic theory of intelligence

Main article: Triarchic theory of intelligence

Robert Sternberg proposed the triarchic theory of intelligence to provide a more comprehensive description of intellectual competence than traditional differential or cognitive theories of human ability. The triarchic theory describes three fundamental aspects of intelligence. Analytic intelligence comprises the mental processes through which intelligence is expressed. Creative intelligence is necessary when an individual is confronted with a challenge that is nearly, but not entirely, novel or when an individual is engaged in automatizing the performance of a task. Practical intelligence is bound in a sociocultural milieu and involves adaptation to, selection of, and shaping of the environment to maximize fit in the context. The triarchic theory does not argue against the validity of a general intelligence factor; instead, the theory posits that general intelligence is part of analytic intelligence, and only by considering all three aspects of intelligence can the full range of intellectual functioning be fully understood.

More recently, the triarchic theory has been updated and renamed the Theory of Successful Intelligence by Sternberg. Intelligence is defined as an individual's assessment of success in life by the individual's own (idiographic) standards and within the individual's sociocultural context. Success is achieved by using combinations of analytical, creative, and practical intelligence. The three aspects of intelligence are referred to as processing skills. The processing skills are applied to the pursuit of success through what were the three elements of practical intelligence: adapting to, shaping of, and selecting of one's environments. The mechanisms that employ the processing skills to achieve success include utilizing one's strengths and compensating or correcting for one's weaknesses.

Sternberg's theories and research on intelligence remain contentious within the scientific community.

PASS Theory of Intelligence

Main article: PASS Theory of Intelligence

Based on A. R. Luria’s (1966) seminal work on the modularization of brain function, and supported by decades of neuroimaging research, 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 two are called Simultaneous and Successive processing and they involve encoding, transforming, and retaining 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. The PASS (Planning/Attention/Simultaneous/Successive) theory is heavily indebted to both Luria (1966, 1973), and studies in cognitive psychology involved in promoting a better look at intelligence.

Piaget's theory and Neo-Piagetian theories

Main articles: Piaget's theory of cognitive development and Neo-Piagetian theories of cognitive development

In Piaget's theory of cognitive development 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. These four main stages are:

• sensory motor stage (birth-2yrs);
• pre-operational stage (2yrs-7rs);
• concrete operational stage (7rs-11yrs); and
• formal operations stage (11yrs-16yrs)

Degree of progress through these stages are correlated, but not identical with psychometric IQ. Piaget conceptualizes intelligence as an activity more than a capacity.

One of Piaget's most famous studies focused purely on the discriminative abilities of children between the ages of two and a half years old, and four and a half years old. He began the study by taking children of different ages and placing two lines of sweets, one with the sweets in a line spread further apart, and one with the same number of sweets in a line placed more closely together. He found that, "Children between 2 years, 6 months old and 3 years, 2 months old correctly discriminate the relative number of objects in two rows; between 3 years, 2 months and 4 years, 6 months they indicate a longer row with fewer objects to have "more"; after 4 years, 6 months they again discriminate correctly". Initially younger children were not studied, because if at four years old a child could not conserve quantity, then a younger child presumably could not either. The results show however that children that are younger than three years and two months have quantity conservation, but as they get older they lose this quality, and do not recover it until four and a half years old. This attribute may be lost temporarily because of an overdependence on perceptual strategies, which correlates more candy with a longer line of candy, or because of the inability for a four-year-old to reverse situations. By the end of this experiment several results were found. First, younger children have a discriminative ability that shows the logical capacity for cognitive operations exists earlier than acknowledged. This study also reveals that young children can be equipped with certain qualities for cognitive operations, depending on how logical the structure of the task is. Research also shows that children develop explicit understanding at age 5 and as a result, the child will count the sweets to decide which has more. Finally the study found that overall quantity conservation is not a basic characteristic of humans' native inheritance.

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 (see the article on object permanence). More generally, the theory may be very difficult to test empirically because of the difficulty of proving or disproving that a mental model is the explanation for the results of the testing.

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.

Latent inhibition

Main article: Latent inhibition

Latent inhibition has been related to elements of intelligence, namely creativity and genius.

Evolution of intelligence

Main article: Evolution of human intelligence

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

Improving intelligence

Eugenics is a social philosophy which advocates the improvement of human hereditary traits through various forms of intervention. Conscious efforts to influence intelligence raise ethical issues. Eugenics has variously been regarded as meritorious or deplorable in different periods of history, falling greatly into disrepute after the defeat of Nazi Germany in World War II.

Neuroethics considers the ethical, legal and social implications of neuroscience, and deals with issues such as the difference between treating a human neurological disease and enhancing the human brain, and how wealth impacts access to neurotechnology. Neuroethical issues interact with the ethics of human genetic engineering.

Because intelligence appears to be at least partly dependent on brain structure and the genes shaping brain development, it has been proposed that genetic engineering could be used to enhance the intelligence, a process sometimes called biological uplift in science fiction. Experiments on mice have demonstrated superior ability in learning and memory in various behavioral tasks.

IQ leads to greater success in education, but independently education raises IQ scores. Attempts to raise IQ with brain training have led to increases on the training tasks – for instance working memory – but it is as yet unclear if these generalise to increased intelligence per se.

Transhumanist theorists study the possibilities and consequences of developing and using techniques to enhance human abilities and aptitudes, and individuals ameliorating what they regard as undesirable and unnecessary aspects of the human condition.

Substances which actually or purportedly improve intelligence or other mental functions are called nootropics.

A 2008 research paper claimed that practicing a dual n-back task can increase fluid intelligence (Gf), as measured in several different standard tests. This finding received some attention from popular media, including an article in Wired. However, a subsequent criticism of the paper's methodology questioned the experiment's validity and took issue with the lack of uniformity in the tests used to evaluate the control and test groups. For example, the progressive nature of Raven's Advanced Progressive Matrices (APM) test may have been compromised by modifications of time restrictions (i.e., 10 minutes were allowed to complete a normally 45-minute test).

Correlates

According to Rosemary Hopcroft, a sociologist at the University of North Carolina at Charlotte, intelligence is inversely linked with sexual frequency (people with higher levels of education often report lower numbers of sexual partners). In parallel, self-reported intelligence has been linked to unconventional sexual practices and frequent sexual activity, thoughts and fantasies.

Some studies have shown a direct link between an increased birth weight and an increased intelligence quotient.

Animal and plant intelligence

Main articles: Animal cognition and Plant intelligence

The common chimpanzee can use tools. This chimpanzee is using a stick to get food.

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.

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. (See also: Dog intelligence.) Nonhuman animals particularly noted and studied for their intelligence include chimpanzees, bonobos (notably the language-using Kanzi) and other great apes, dolphins, elephants and to some extent parrots, rats and ravens. Controversy exists over the extent to which these judgments of intelligence are accurate.

Cephalopod intelligence also provides important comparative study. Cephalopods appear to exhibit characteristics of significant intelligence, yet their nervous systems differ radically from those of most other notably intelligent life-forms (mammals and birds).

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.

Artificial intelligence

Main article: 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" or "rational agents", where an intelligent agent is a system that perceives its environment and takes actions which maximize its chances of success. 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. In the field of artificial intelligence there is no consensus on how closely the brain should be simulated.

See also

• Active intellect
• Cognitive epidemiology
• Downing effect
• Educational psychology
• Environment and intelligence
• Fertility and intelligence
• Flynn effect
• Health and intelligence
• Height and intelligence
• Heritability of IQ
• History of the race and intelligence controversy
• Individual differences psychology
• Intellectual giftedness
• Intelligence (journal)
• Knowledge
• Malleable intelligence
• Nations and intelligence
• Neuroscience and intelligence
• Passive intellect
• Race and intelligence
• Religiosity and intelligence
• Sex and psychology
• Theory of multiple intelligences
• Job Performance
• Typical Versus Maximum Performance

Further reading

Books listed in chronological order of publication

• Binet, Alfred; Simon, Th. (1916). The development of intelligence in children: The Binet-Simon Scale. Publications of the Training School at Vineland New Jersey Department of Research No. 11. E.S. Kite (Trans.). Baltimore: Williams & Wilkins. Retrieved 18 July 2010.
• Terman, Lewis M. (1916). The Measurement of Intelligence. Boston: Houghton Mifflin. ISBN 0-405-06480-2. Retrieved 26 June 2010.
• Das, J.P.; Naglieri, J.A.; Kirby, J.R. (1994). Assessment of Cognitive Processes. Needham Heights, MA, USA: Allyn & Bacon. ISBN 0-205-14164-1. {{cite book}}: Unknown parameter |laydate= ignored (help)
• Wake, Warren K.; Gardner, Howard; Kornhaber, Mindy L. (1996). Intelligence: Multiple perspectives. Fort Worth, TX: Harcourt Brace College Publishers. ISBN 0-03-072629-8. OCLC 34414874.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Khalfa, Jean, ed. (1996). What Is Intelligence?. Darwin College Lectures. Cambridge: Cambridge University Press. ISBN 978-0-521-56685-8. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |laydate= ignored (help); Unknown parameter |laysummary= ignored (help)
• Sternberg, Robert J., ed. (2000). Handbook of Intelligence. Cambridge: Cambridge University Press. ISBN 978-0-521-59648-0. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |laydate= ignored (help); Unknown parameter |laysummary= ignored (help)
• Richardson, Ken (2000). The Making of Intelligence. New York (NY): Columbia University Press. ISBN 978-0-231-12005-0. {{cite book}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysummary= ignored (help)
• Bock, Gregory; Goode, Jamie; Webb, Kate, eds. (2000). The Nature of Intelligence. Novartis Foundation Symposium 233. Chichester: Wiley. doi:10.1002/0470870850. ISBN 978-0-471-49434-8. Retrieved 16 July 2010.
• Blakeslee, Sandra; Hawkins, Jeff (2004). On intelligence. New York: Times Books. ISBN 0-8050-7456-2. OCLC 55510125.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Sternberg, Robert J., ed. (2004). International Handbook of Intelligence. Cambridge: Cambridge University Press. ISBN 978-0-521-00402-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |laydate= ignored (help); Unknown parameter |laysummary= ignored (help)
• Flynn, James R. (2009). What Is Intelligence: Beyond the Flynn Effect (expanded paperback ed.). Cambridge: Cambridge University Press. ISBN 978-0-521-74147-7. {{cite book}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysummary= ignored (help)
• Stanovich, Keith (2009). What Intelligence Tests Miss: The Psychology of Rational Thought. New Haven (CT): Yale University Press. ISBN 978-0-300-12385-2. {{cite book}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysummary= ignored (help)
• Garlick, Dennis (2010). Intelligence and the Brain: Solving the Mystery of Why People Differ in IQ and How a Child Can Be a Genius. Burbank (CA): Aesop Press. ISBN 978-0-615-31921-6. {{cite book}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysummary= ignored (help)

External links

• Intelligence on In Our Time at the BBC
• APA Task Force Examines the Knowns and Unknowns of Intelligence - American Psychologist, February 1996
• The cognitive-psychology approach vs. psychometric approach to intelligence - American Scientist magazine
• History of Influences in the Development of Intelligence Theory and Testing - Developed by Jonathan Plucker at Indiana University
• The Limits of Intelligence: The laws of physics may well prevent the human brain from evolving into an ever more powerful thinking machine by Douglas Fox in Scientific American, June 14, 2011.
• A Collection of Definitions of Intelligence

Scholarly journals and societies

• Intelligence (journal homepage)
• International Society for Intelligence Research (homepage)