Intelligence is a most complex practical property of mind, integrating numerous mental abilities, such as the capacities to reason, solve problems, think abstractly, comprehend ideas and language, and learn. The study of intelligence within psychology generally regards this trait as distinct from creativity or personality. However, the definition of intelligence has been, and continues to be, subject to debate. Some claim a unitary attribute, often called "general intelligence" or g, which can be measured using standard IQ tests, and which correlates with a person's abilities on a wide range of tasks and contexts. Others have argued that there are multiple "intelligences," with different people displaying differing levels of each type. Additionally, great controversies have arisen regarding the question of whether this "intelligence" is inherited, and if so whether some groups are more intelligent than others. Of particular concern has been the claim that some races are superior, leading justification to racist expectations and behavior.
Despite research and theories from numerous scholars our understanding of intelligence is still limited. Perhaps, since researchers use only their own human intellect to discover the secrets of human intellectual abilities such limitations are to be expected. Viewing ourselves as members of one large human family, each with our own abilities and talents the use of which provide joy to ourselves and to others, allows us to have a deeper appreciation of what "intelligence" means.
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. 
Individual intelligence experts have offered a number of similar definitions.
Despite the variety of concepts of intelligence, the most influential approach to understanding intelligence (namely that which has the most supporters and the most published research over the longest period of time) is based on psychometric testing.
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, 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 statistical 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, 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."
However, not all researchers agree that g can be treated as a single factor. For example, Raymond Cattell identified fluid and crystallized intelligence (abbreviated Gf and Gc, respectively) as factors of "general intelligence." He defined fluid intelligence as the ability to find meaning in confusion and solve new problems, whereas crystallized intelligence is defined as the ability to utilize previously acquired knowledge and experience. Cattell conceived of Gf and Gc as separate though correlated mental abilities which together comprise g, or "general intelligence." Charles Spearman, who originally developed the theory of g, made a similar distinction between "eductive" and "reproductive" mental abilities.
The terms "fluid" and "crystallized" are somewhat misleading because one is not a "crystallized" form of the other. Rather, they are believed to be separate neural and mental systems. Fluid intelligence is the ability to draw inferences and understand the relationships of various concepts, independent of acquired knowledge. Crystallized intelligence is the ability to use skills, knowledge, and experience. It is not equated with memory or knowledge although it relies on accessing information from long-term memory.
Most IQ tests attempt to measure both varieties. For example, the Wechsler Adult Intelligence Scale (WAIS) measures fluid intelligence on the performance scale and crystallized intelligence on the verbal scale. The overall IQ score is based on a combination of these two scales.
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 review of the empirical research, David Geary found that g is highly correlated with many important social outcomes. He found that individuals with low IQ scores 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 IQ scores are associated with more years of education, higher status jobs, and higher income.
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 have argued that the equipment used in an experiment often determines the results and that proving that intelligence exists does not prove that current equipment measures 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).
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. 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. 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.
Yale psychologist Robert 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.
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, 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 . Furthermore, g theorists contend that proponents of multiple intelligences (such as Sternberg and Gardner) have not disproved the existence of a general factor of intelligence  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.
The Triarchic Theory of Intelligence was formulated by Robert Sternberg. The theory by itself was groundbreaking in that it was among the first to go against the psychometric approach to intelligence and take a more cognitive approach. Sternberg’s definition of intelligence is “(a) mental activity directed toward purposive adaptation to, selection and shaping of, real-world environments relevant to one’s life” (Sternberg, 1985, p. 45), which means that intelligence is how well an individual deals with environmental changes throughout their lifespan. Sternberg’s theory is comprised of three parts: componential, experiential, and practical.
The first subtheory discusses componential analysis and its application to intelligence. Sternberg divided the components of intelligence into the metacomponents, performance components, and knowledge-acquisition components (Sternberg, 1985).
The metacomponents are executive processes used in problem solving and decision making that involve the majority of managing our mind. They tell the mind how to act. Metacomponents are also sometimes referred to as a homunculus. A homunculus is a fictitious or metaphorical "person" inside our head that controls our actions, and which is often seen to invite an infinite regress of homunculi controlling each other (Sternberg, 1985).
Sternberg’s next set of components, performance components, are the processes that actually carry out the actions the metacomponents dictate. These are the basic processes that allow us to do tasks, such as perceiving problems in our long-term memory, perceiving relations between objects, and applying relations to another set of terms (Sternberg, 1997).
The last set of components, knowledge-acquisition components, are used in obtaining new information. These components complete tasks that involve selectively choosing information from irrelevant information. These components can also be used to selectively combine the various pieces of information they have gathered. Gifted individuals are proficient in using these components because they are able to learn new information at a greater rate (Sternberg, 1997).
Sternberg’s second stage of his theory is his experiential subtheory. This stage deals mainly with how well a task is performed with regard to how familiar it is. Sternberg splits the role of experience into two parts: novelty and automatization.
A novel situation is one that you have never experienced before. People that are adept at managing a novel situation can take the task and find new ways of solving it that the majority of people would not notice (Sternberg, 1997).
A process that has been automatized has been performed multiple times and can now be done with little or no extra thought. Once a process is automatized, it can be run in parallel with the same or other processes. The problem with novelty and automatization is that being skilled in one component does not ensure that you are skilled in the other (Sternberg, 1997).
Sternberg’s third subtheory of intelligence, called practical or contextual, “deals with the mental activity involved in attaining fit to context” (Sternberg, 1985, 45). Through the three processes of adaptation, shaping, and selection, individuals create an ideal fit between themselves and their environment. This type of intelligence is often referred to as "street smarts."
Adaptation occurs when one makes a change within oneself in order to better adjust to one’s surroundings (Sternberg, 1985). For example, when the weather changes and temperatures drop, people adapt by wearing extra layers of clothing to remain warm.
Shaping occurs when one changes their environment to better suit one’s needs (Sternberg, 1985). A teacher may invoke the new rule of raising hands to speak to ensure that the lesson is taught with least possible disruption.
The process of selection is undertaken when a completely new alternate environment is found to replace the previous, unsatisfying environment to meet the individual’s goals (Sternberg, 1985). For instance, immigrants leave their lives in their homeland countries where they endure economical and social hardships and come to America in search of a better and less strained life.
The theory of multiple intelligences is a psychological and educational theory put forth by psychologist Howard Gardner, which suggests that an array of different kinds of "intelligence" exists in human beings. Gardner suggests that each individual manifests varying levels of these different intelligences, and thus each person has a unique "cognitive profile." The theory was first laid out in Gardner's 1983 book, Frames of Mind: The Theory of Multiple Intelligences, and has been further refined in subsequent years.
The theory was proposed in the context of debates about the concept of intelligence, and whether methods which claim to measure intelligence (or aspects thereof) are truly scientific. Gardner's theory argues that intelligence, as it is traditionally defined, does not adequately encompass the wide variety of abilities humans display. In his conception, a child who masters the multiplication table easily is not necessarily more intelligent overall than a child who struggles to do so. The second child may be stronger in another kind of intelligence, and therefore may best learn the given material through a different approach, or may excel in a field outside of mathematics. The theory suggests that, rather than relying on a uniform curriculum, schools should offer "individual-centered education," with curricula tailored to the needs of each child. This approach includes working to help students develop the intelligences they are weaker in.
Gardner identified intelligences based upon a range of factors and criteria, including: case studies of individuals exhibiting unusual talents in a given field (child prodigies, autistic savants); neurological evidence for areas of the brain that are specialized for particular capacities (often including studies of people who have suffered brain damage affecting a specific capacity); the evolutionary relevance of the various capacities; psychometric studies; and a symbolic formulation of the area treated by each proposed intelligence. He originally identified seven core intelligences: linguistic, logical-mathematical, spatial, bodily-kinesthetic, musical, interpersonal, and intrapersonal. In 1999 he added an eighth, the naturalistic intelligence, and work continues on whether there is an existential intelligence 
The theory has been widely criticized in the psychological and educational theory communities. The most common criticisms have argued that Gardner's theory is based on his own intuition rather than empirical data and that the intelligences are just other names for talents or personality types. Despite these criticisms, the theory has enjoyed a great deal of success amongst educators over the past twenty years. There are several schools which espouse Multiple Intelligences as a pedagogy, and many individual teachers who incorporate some or all of the theory into their methodology. Many books and educational materials exist which explain the theory and how it may be applied to the classroom.
To do with words, spoken or written. People with verbal-linguistic intelligence display a facility with words and languages. They are typically good at reading, writing, telling stories, and memorizing words and dates. They tend to learn best by reading, taking notes, and listening to lectures, and via discussion and debate. They are also frequently skilled at explaining, teaching, and oration or persuasive speaking. Those with verbal-linguistic intelligence learn foreign languages very easily as they have high verbal memory and recall and an ability to understand and manipulate syntax and structure.
To do with logic, abstractions, inductive and deductive reasoning, and numbers. While it is often assumed that those with this intelligence naturally excel in mathematics, chess, computer programming, and other logical or numerical activities, a more accurate definition places emphasis less on traditional mathematical ability and more reasoning capabilities, abstract pattern recognition, scientific thinking and investigation, and the ability to perform complex calculations.
Those who automatically correlate this intelligence with skill in mathematics criticize this intelligence by arguing that logical ability is often more strongly correlated with verbal rather than mathematical ability: for example, the old Analytic section of the Graduate Record Examination correlated more strongly with the Verbal section than the Mathematical. One possibility is that formal, symbolic logic, and strict logic games are under the command of mathematical intelligence, while skills as fallacy hunting, argument construction, and so forth are under the command of verbal intelligence.
To do with vision and spatial judgment. People with strong visual-spatial intelligence are typically very good at visualizing and mentally manipulating objects. They have a strong visual memory and are often artistically inclined. Those with visual-spatial intelligence also generally have a very good sense of direction and may also have very good hand-eye coordination, although this is normally seen as a characteristic of the bodily-kinesthetic intelligence.
Some critics point out the high correlation between the spatial and mathematical abilities, which seems to disprove the clear separation of the intelligences as Gardner theorizes. A thorough understanding of the two intelligences precludes this criticism, however, as the two intelligences do not precisely conform to the definitions of visual and mathematical abilities. Although they may share certain characteristics, they are easily distinguished by several factors, and there are many with strong logical-mathematical intelligence and weak visual-spatial, and vice versa.
To do with movement and doing. In this category, people are generally adept at physical activities such as sports or dance and often prefer activities which utilize movement. They may enjoy acting or performing, and in general they are good at building and making things. They often learn best by physically doing something, rather than reading or hearing about it. Those with strong bodily-kinesthetic intelligence seem to use what might be termed "muscle memory"—they remember things through their body, rather than through words (verbal memory) or images (visual memory).
To do with rhythm, music, and hearing. Those who have a high level of musical-rhythmic intelligence display greater sensitivity to sounds, rhythms, tones, and music. They normally have good pitch and may even have absolute pitch, and are able to sing, play musical instruments, and compose music. Since there is a strong aural component to this intelligence, those who are strongest in it may learn best via lecture. In addition, they will often use songs or rhythms to learn and memorize information, and may work best with music playing.
To do with nature, nurturing, and classification. This is the newest of the intelligences and is not as widely accepted as the original seven. Those with it are said to have greater sensitivity to nature and their place within it, the ability to nurture and grow things, and greater ease in caring for, taming, and interacting with animals. They are also good at recognizing and classifying different species.
The theory behind this intelligence is often criticized, much like the spiritual or existential intelligence (see below), as it is seen by many is not indicative of an intelligence but rather an interest.
To do with interaction with others. People in this category are usually extroverts and are characterized by their sensitivity to others' moods, feelings, temperaments, and motivations and their ability to cooperate in order to work as part of a group. They communicate effectively and empathize easily with others, and may be either leaders or followers. They typically learn best by working with others and often enjoy discussion and debate.
To do with oneself. Those who are strongest in this intelligence are typically introverts and prefer to work alone. They are usually highly self-aware and capable of understanding their own emotions, goals, and motivations. They often have an affinity for thought-based pursuits such as philosophy. They learn best when allowed to concentrate on the subject by themselves. There is often a high level of perfectionism associated with this intelligence.
Other intelligences have been suggested or explored by Gardner and his colleagues, including spiritual, existential, and moral intelligence. Gardner excluded spiritual intelligence due to its failure to meet a number of his criteria. Existential intelligence (the capacity to raise and reflect on philosophical questions about life, death, and ultimate realities) meets most of the criteria with the exception of identifiable areas of the brain that specialize for this faculty. Moral capacities were excluded because they are normative rather than descriptive.
Traditionally schools have almost exclusively emphasized the development of logical intelligence and linguistic intelligence (mainly reading and writing). While many students function well in this environment, there are those who do not. Gardner's theory argues that students will be better served by a broader vision of education, wherein teachers use different methodologies, exercises, and activities to reach all students, not just those who excel at linguistic and logical intelligence.
The practical application of the theory of multiple intelligences varies widely. It runs the gamut from a teacher who, when confronted with a student having difficulties, uses a different approach to teach the material, to an entire school using MI as a framework. In general, those who subscribe to the theory strive to provide opportunities for their students to use and develop all the different intelligences, not just the few at which they naturally excel.
A Harvard-led study of 41 schools using the theory came to the conclusion that in these schools there was "a culture of hard work, respect, and caring; a faculty that collaborated and learned from each other; classrooms that engaged students through constrained but meaningful choices, and a sharp focus on enabling students to produce high-quality work." 
Of the schools implementing Gardner's theory, the most well-known is New City School, in St. Louis, Missouri, which has been using the theory since 1988. The school's teachers have produced two books for teachers, Celebrating Multiple Intelligences and Succeeding With Multiple Intelligences and the principal, Thomas Hoerr, has written Becoming a Multiple Intelligences School as well as many articles on the practical applications of the theory. The school has also hosted four conferences, each attracting over 200 educators from around the world and remains a valuable resource for teachers interested in implementing the theory in their own classrooms.
Criticisms of the theory's application in schools come in two major forms. First, opponents argue that the theory may lead to a sort of intellectual relativism, wherein students' failures are explained away as being an example of a different kind of intelligence, not a lesser one. Gardner himself has said that he never intended his theory to affirm that all people are equally gifted, but rather that the definition of intelligence was too narrow to encompass all types of intelligence.
The second major criticism is that it is fallacious to say that someone may be good in one intelligence but not in another. This criticism is largely based on a misunderstanding of the theory: people do not have one intelligence or another, but varying levels of ability in all the intelligences, and therefore someone who excels in one is perfectly capable of excelling in another, or in all. In addition, Gardner believes that most activities require the use of several intelligences at once—the conductor of a symphony clearly uses musical intelligence, but also uses interpersonal to communicate and direct the group, and bodily-kinesthetic to use his hands and body in order to conduct.
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. Some of the controversial topics include:
Stephen Jay Gould has been the preeminent popular critic of claims about intelligence. In his book The Mismeasure of Man, Gould made the following claims about intelligence:
Some of Gould's criticisms were aimed specifically at Arthur Jensen, who alleged that Gould made several misrepresentations of his work.
The study of intelligence is important because findings can give a better understanding of human creativity, psychological development, and emotion. The existence of intelligence has been attributed to both nature and nurture, but the truth behind it may be something of a mix of both. There are indeed children born with a certain genius about them that allows them to complete college courses at the age of fifteen. There are also those born who fail academia for the early part of their lives, but end up turning their lives around to come out on top of their respective schools. Was it a lack of intelligence that led to the early academic failure, a growth in intelligence that led to later success, or was that intelligence always there but just not recognized? Do intelligence tests measure a so called g-factor, or is intelligence merely relevant to the society the person resides in?
Intelligence is one of the attributes that separate human beings from animals. By saying "a human is more intelligent than an ape," most people would take that as to mean humans are able to rationalize, reason, create, conceptualize, and discuss in a way that an ape is unable to do. Intelligence also is presented as a marker of competition and privilege.
The concept of intelligence has been one of the most contentious in psychology, with controversy spanning the lifetime of the research area. The most controversial claims have come from certain scientists that intelligence varies between races, giving some races the right to govern over others. However, not only have their data been seriously questioned, the premise that inheritance is hereditary challenged, but the whole notion of a singular, quantifiable attribute called "intelligence" has become the subject of debate.
Regardless of the way in which intelligence is defined or measured, human beings clearly have great intellectual abilities, to think, reason, and understand the world in which we live, the way other people think and act, and to understand ourselves. As we advance in our development as individuals and collectively as a species, recognizing and appreciating the diversity that is contained within this concept may be more valuable than trying to ascertain how to quantify it.
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