What is intelligence?

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Intelligence is a hypothetical concept, rather than a tangible entity, that is used by psychologists and other scientists to explain differences in the quality and adaptive value of the behavior of humans and, to some extent, animals. Its meaning and the theoretical models used to explore it are as varied as the field of psychology itself.
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The idea that human beings differ in their capacity to adapt to their environments, to learn from experience, to exercise various skills, and in general to succeed at various endeavors has existed since ancient times. Intelligence is the attribute most often singled out as responsible for successful adaptations. Up to the end of the nineteenth century, notions about what constitutes intelligence and how differences in intelligence arise were mostly speculative. In the late nineteenth century, several trends converged to bring about an event that would change the way in which intelligence was seen and dramatically influence the way it would be studied. That event, which occurred in 1905, was the publication of the first useful instrument for measuring intelligence, the Binet-Simon scale, which was developed in France by Alfred Binet and Théodore Simon.

Although the development of intelligence tests was a great technological accomplishment, it occurred, in a sense, somewhat prematurely before much scientific attention had been paid to the concept of intelligence. This circumstance tied the issue of defining intelligence and a large part of the research into its nature and origins to the limitations of the tests that had been devised. In fact, the working definition of intelligence that many psychologists have used either explicitly or implicitly in their scientific and applied pursuits is the one expressed by Edwin Boring in 1923, which holds that intelligence is whatever intelligence tests measure. Most psychologists realize that this definition is redundant and inadequate in that it erroneously implies that the tests are perfectly accurate and able to capture all that is meant by the concept. Nevertheless, psychologists and others have proceeded to use the tests as if the definition were true, mainly because of a scarcity of viable alternatives. The general public has also been led astray by the existence of “intelligence” tests and the frequent misuse of their results. Many people have come to think of the intelligence quotient, or IQ, not as a simple score achieved on a particular test, which it is, but as a complete and stable measure of intellectual capacity, which it most definitely is not. Such misconceptions have led to an understandable resistance toward and resentment of intelligence tests.

Changing Definitions

Boring’s semifacetious definition of intelligence may be the best known and most criticized one, but it is only one among many that have been offered. Most experts in the field have defined the concept at least once in their careers. Two of the most frequently cited and influential definitions are the ones provided by Binet himself and by David Wechsler, author of a series of “second-generation” individual intelligence tests that overtook the Binet scales in terms of the frequency with which they are used. Binet believed that the essential activities of intelligence are to judge well, to comprehend well, and to reason well. He stated that intelligent thought is characterized by direction, knowing what to do and how to do it; by adaptation, the capacity to monitor one’s strategies for attaining a desired end; and by criticism, the power to evaluate and control one’s behavior. In 1975, almost sixty-five years after Binet’s death, Wechsler defined intelligence, not dissimilarly, as the global capacity of the individual to act purposefully, to think rationally, and to deal effectively with the environment.

In addition to the testing experts (psychometricians), developmental, learning, and cognitive psychologists, among others, are also vitally interested in the concept of intelligence. Specialists in each of these subfields emphasize different aspects of it in their definitions and research.

Representative definitions were sampled in 1921, when the Journal of Educational Psychology published the views of fourteen leading investigators, and again in 1986, when Robert Sternberg and Douglas Detterman collected the opinions of twenty-four experts in a book entitled What Is Intelligence? Contemporary Viewpoints on Its Nature and Definition. Most of the experts sampled in 1921 offered definitions that equated intelligence with one or more specific abilities. For example, Lewis Terman equated it with abstract thinking, which is the ability to elaborate concepts and to use language and other symbols. Others proposed definitions that emphasized the ability to adapt or learn. Some definitions centered on knowledge and cognitive components only, whereas others included nonintellectual qualities, such as perseverance.

In comparison, Sternberg and Detterman’s 1986 survey of definitions, which is even more wide ranging, is accompanied by an organizational framework consisting of fifty-five categories or combinations of categories under which the twenty-four definitions can be classified. Some theorists view intelligence from a biological perspective and emphasize differences across species or the role of the central nervous system. Some stress cognitive aspects of mental functioning, while others focus on the role of motivation and goals. Still others, such as Anne Anastasi, choose to look on intelligence as a quality that is inherent in behavior rather than in the individual. Another major perspective highlights the role of the environment, in terms of demands and values, in defining what constitutes intelligent behavior. Throughout the 1986 survey, one can find definitions that straddle two or more categories.

A review of the 1921 and 1986 surveys shows that the definitions proposed have become considerably more sophisticated and suggests that, as the field of psychology has expanded, the views of experts on intelligence may have grown farther apart. The reader of the 1986 work is left with the clear impression that intelligence is such a multifaceted concept that no single quality can define it and no single task or series of tasks can capture it completely. Moreover, it is clear that to unravel the qualities that produce intelligent behavior, one must look not only at individuals and their skills but also at the requirements of the systems in which people find themselves. In other words, intelligence cannot be defined in a vacuum.

New intelligence research focuses on different ways to measure intelligence and on paradigms for improving or training intellectual abilities and skills. Measurement paradigms allow researchers to understand ongoing processing abilities. Some intelligence researchers include measures of intellectual style and motivation in their models.

Factor Analysis

The lack of a universally accepted definition has not deterred continuous theorizing and research on the concept of intelligence. The central issue that has dominated theoretical models of intelligence is the question of whether it is a single, global ability or a collection of specialized abilities. This debate, started in England by Charles Spearman, is based on research that uses the correlations among various measures of abilities and, in particular, the method of factor analysis, which was also pioneered by Spearman. As early as 1904, Spearman, having examined the patterns of correlation coefficients among tests of sensory discrimination and estimates of intelligence, proposed that all mental functions are the result of a single general factor, which he later designated g. Spearman equated g with the ability to grasp and apply relations. He also allowed for the fact that most tasks require unique abilities, and he named those s, or specific, factors. According to Spearman, to the extent that performance on tasks was positively correlated, the correlation was attributable to the presence of g, whereas the presence of specific factors tended to lower the correlation between measures of performance on different tasks.

By 1927, Spearman had modified his theory to allow for the existence of an intermediate class of factors, known as group factors, which were neither as universal as g nor as narrow as the s factors. Group factors were seen as accounting for the fact that certain types of activities, such as tasks involving the use of numbers or the element of speed, correlate more highly with one another than they do with tasks that do not have such elements in common.

Factor-analytic research has undergone explosive growth and extensive variations and refinements in both England and the United States since the 1920s. In the United States, work in this field was influenced greatly by Truman Kelley, whose 1928 book Crossroads in the Mind of Man presented a method for isolating group factors, and L. L. Thurstone, who by further elaboration of factor-analytic procedures identified a set of about twelve factors that he designated as the “primary mental abilities.” Seven of these were repeatedly found in a number of investigations, using samples of people at different age levels, that were carried out by both Thurstone and others. These group factors or primary mental abilities are verbal comprehension, word fluency, speed and accuracy of arithmetic computation, spatial visualization, associative memory, perceptual speed, and general reasoning.

Organizational Models

As the search for distinct intellectual factors progressed, their number multiplied, and so did the number of models devised to organize them. One type of scheme, used by Cyril Burt, Philip E. Vernon, and others, is a hierarchical arrangement of factors. In these models, Spearman’s g factor is placed at the top of a pyramid and the specific factors are placed at the bottom; in between, there are one or more levels of group factors selected in terms of their breadth and arranged according to their interrelationships with the more general factors above them and the more specific factors below them.

In Vernon’s scheme, for example, the ability to change a tire might be classified as a specific factor at the base of the pyramid, located underneath an intermediate group factor labeled mechanical information, which in turn would be under one of the two major group factors identified by Vernon as the main subdivisions under g—namely, the practical-mechanical factor. The hierarchical scheme for organizing mental abilities is a useful device that is endorsed by many psychologists on both sides of the Atlantic. It recognizes that very few tasks are so simple as to require a single skill for successful performance, that many intellectual functions share some common elements, and that some abilities play a more pivotal role than others in the performance of culturally valued activities.

Another well-known scheme for organizing intellectual traits is the structure-of-intellect (SOI) model developed by J. P. Guilford. Although the SOI is grounded in extensive factor-analytic research conducted by Guilford throughout the 1940s and 1950s, the model goes beyond factor analysis and is perhaps the most ambitious attempt to classify systematically all the possible functions of the human intellect. The SOI classifies intellectual traits along three dimensions—namely, five types of operations, four types of contents, and six types of productions, for a total of 120 categories (5 × 4 × 6). Intellectual operations consist of what a person actually does (for example, evaluating or remembering something), the contents are the types of materials or information on which the operations are performed (for example, symbols, such as letters or numbers), and the products are the form in which the contents are processed (for example, units or relations). Not all the 120 categories in Guilford’s complex model have been used, but enough factors have been identified to account for about one hundred of them, and some have proved very useful in labeling and understanding the skills that tests measure. Furthermore, Guilford’s model has served to call attention to some dimensions of intellectual activity, such as creativity and interpersonal skills, that had been neglected previously.

Competence and Self-Management

Contemporary theorists in the area of intelligence have tried to avoid the reliance on factor analysis and existing tests that have limited traditional research and have tried different approaches to the subject. For example, Howard Gardner, in his 1983 book Frames of Mind: The Theory of Multiple Intelligences, starts with the premises that the essence of intelligence is competence and that there are several distinct areas in which human beings can demonstrate competence. Based on a wide-ranging review of evidence from many scientific fields and sources, Gardner designated seven areas of competence as separate and relatively independent “intelligences.” In his 1993 work Multiple Intelligences, Gardner revised his theory to include an eighth type of intelligence. This set of attributes comprises verbal, mathematical, spatial, bodily/ kinesthetic, musical, interpersonal, intrapersonal, and naturalist skills.

Another theory is the one proposed by Robert Sternberg in his 1985 book Beyond IQ: A Triarchic Theory of Human Intelligence. Sternberg defines intelligence, broadly, as mental self-management and stresses the “real-world,” in addition to the academic, aspects of the concept. He believes that intelligent behavior consists of purposively adapting to, selecting, and shaping one’s environment and that both culture and personality play significant roles in such behavior. Sternberg posits that differences in IQ scores reflect differences in individuals’ stages of developing the expertise measured by the particular IQ test, rather than attributing these scores to differences in intelligence, ability, or aptitude. Sternberg’s model has five key elements: metacognitive skills, learning skills, thinking skills, knowledge, and motivation. The elements all influence one another. In this work, Sternberg claims that measurements derived from ability and achievement tests are not different in kind; only in the point at which the measurements are being made.

Intelligence and Environment

Theories of intelligence are still grappling with the issues of defining its nature and composition. Generally, newer theories do not represent radical departures from the past. They do, however, emphasize examining intelligence in relation to the variety of environments in which people actually live rather than to only academic or laboratory environments. Moreover, many investigators, especially those in cognitive psychology, are more interested in breaking down and replicating the steps involved in information processing and problem solving than they are in enumerating factors or settling on a single definition of intelligence. These trends hold the promise of moving the work in the field in the direction of devising new ways to teach people to understand, evaluate, and deal with their environments more intelligently instead of simply measuring how well they do on intelligence tests. In their 1998 article “Teaching Triarchically Improves School Achievement,” Sternberg and his colleagues note that teaching or training interventions can be linked directly to components of intelligence. Motivation also plays a role. In their 2000 article “Intrinsic and Extrinsic Motivation,” Richard Ryan and Edward Deci provide a review of contemporary thinking about intrinsic and extrinsic motivation. The authors suggest that the use of motivational strategies should promote student self-determination.

The most heated of all the debates about intelligence is the one regarding its determinants, often described as the nature-nurture controversy. The nature side of the debate was spearheaded by ;Francis Galton, a nineteenth century English scientist who had become convinced that intelligence was a hereditary trait. Galton’s followers tried to show, through studies comparing identical and nonidentical twins reared together and reared apart and by comparisons of people related to each other in varying degrees, that genetic endowment plays a far larger role than the environment in determining intelligence. Attempts to quantify an index of heritability for intelligence through such studies abound, and the estimates derived from them vary widely. On the nurture side of the debate, massive quantities of data have been gathered in an effort to show that the environment, including factors such as prenatal care, social-class membership, exposure to certain facilitative experiences, and educational opportunities of all sorts, has the more crucial role in determining a person’s level of intellectual functioning.

Many critics, such as Anastasi (in a widely cited 1958 article entitled “Heredity, Environment, and the Question ’How?’”) have pointed out the futility of debating how much each factor contributes to intelligence. Anastasi and others argue that behavior is a function of the interaction between heredity and the total experiential history of individuals and that, from the moment of conception, the two are inextricably tied. Moreover, they point out that, even if intelligence were shown to be primarily determined by heredity, environmental influences could still modify its expression at any point. Most psychologists now accept this “interactionist” position and have moved on to explore how intelligence develops and how specific genetic and environmental factors affect it.


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