intelligence

intelligence What is intelligence? Ability to learn? Success in adapting to new situations? The number and originality of mental associations? Skill in reasoning or producing abstract ideas or problem-solving? All of these definitions have been proposed but none has yet become definitive, either for professional psychologists or for the lay public. In general, intelligence most often refers to practical problem-solving ability, verbal ability, social competence, and effective adaptation to one's environment and to new situations and changes within it. There is often a quantitative dimension as well: some individual or group or species has more or less of it than some other. Cross-cultural studies have revealed significant differences in the ways in which various groups define the sets of characteristics associated with something like overall mental ability. And historically, even in the West, intelligence has meant a number of things. It was used most often until the twentieth century to refer simply to all the intellective functions of the mind, as distinguished from the will and the emotions, universal human properties little associated with measurable individual ability.

Francis Galton in England and Alfred Binet in France were among the most significant within psychology in developing the modern conception of intelligence. Beginning in the 1860s, Galton pursued a programme of investigating individual differences in mental ability by measuring reactions to various physical stimuli and then showing that those measurements were distributed, like height or weight, according to the normal or bell-shaped curve. Although Galton's anthropometric approach was soon abandoned, his insistence that intelligence was a biological entity that was inheritable, and normally distributed in populations, persisted, and became linked to a very different method of assessing intelligence devised by Binet. In response to a governmental education commission, Binet and his colleague Théodore Simon created a set of tests, individually administered, which were designed to track normal intellectual progress. Oriented toward the higher mental processes, the Binet–Simon Intelligence Scale (1905, 1908, 1911) was able to produce a number, the mental age (MA), that characterized the intellectual level of each child administered the examination. Not only did it allow test-takers to be ranked according to the level of their intelligence, but it suggested that intelligence itself was a discrete and measurable entity.

The Binet–Simon Intelligence Scale set the standard for all further developments in the field. Lews M. Terman's 1916 revision of the Binet–Simon scale, the Stanford–Binet, quickly became the benchmark instrument for the assessment of intelligence, and helped to introduce the concept of the intelligence quotient (IQ), a ratio of mental age to chronological age which was adopted from German psychologist Wilhelm Stern and designed to produce a measure of intelligence which was constant over time. Revised in 1937 and again in 1960, the Stanford–Binet has remained one of the pre-eminent individual measures of intelligence. Its main rivals have been the tests of child (WISC) and adult (WAIS) intelligence developed by David Wechsler, starting in the 1940s, which provide, in addition to an overall measure of IQ, individual assessments of verbal and non-verbal ability.

Wechsler's provision of two additional scores highlights one of the persistent theoretical issues pursued in studies of intelligence: whether it is one thing or many. Using factor analysis, British psychologist Charles Spearman (1904) argued that performance on intelligence tests could be explained on the basis of two factors, general intelligence (g) and task-specific abilities (s). Spearman's theory was challenged during the 1920s and 1930s, by L. L. Thurstone in the US and Godfrey Thomson in the UK, both of whom also employed factor analysis, but who used it to argue against g and in favour of the existence of a small number of relatively independent abilities. During the post-war period, Philip E. Vernon, among others, attempted to arbitrate between these competing theories using a hierarchical conception of intelligence, which depicted intelligence as extending from a single overall ability down to a large number of specific skills. This approach was rejected by Joy P. Guilford, however, who proposed instead a three-dimensional model that initially posited 120 independent mental factors and subsequently posited 150. Commencing in the 1970s, various cognitively-based models have been put forward, including most prominently those by Howard Gardner, with his seven discrete types of intelligence, and Robert J. Sternberg, with his triarchic theory of intelligence. These cognitive approaches owe a great deal to the influence of the psychometric tradition and also to developmental studies of intelligence, particularly those associated with Jean Piaget (stages of intellectual development) and Lev Vygotsky (social influences on intellectual development).

The second major theoretical issue in intelligence studies has been over the relative weights of nature and nurture. Galton's work on individual intelligence began with the assumption that intelligence was both biological and inheritable, a belief that ran strong during the heyday of eugenics (1900s–20s), and was used to support such programs as immigration restriction and sterilization of the mentally deficient. Research during the 1930s and 1940s, however, especially at the Iowa Child Welfare Research Station, emphasized the importance of nurture: IQ, for example, was found to change when children were placed in different environments. After the war, studies continued to show the powerful effects of both nature and nurture on IQ. Research on identical twins has led some psychologists to conclude that at least 60% of IQ results from heredity. At the same time, a great deal of data has been collected indicating the influence of nutrition, kind of education received, and degree of sensory stimulation on IQ score.

The enormous professional interest in intelligence has been sustained by its many practical applications. As part of mobilization for World War I, American psychologists created new instruments that could be group administered, and tested approximately 1.75 million army recruits. This programme served to introduce the nation to standardized intelligence testing, and during the 1920s intelligence testing boomed, adopted by schools and industry as a means of efficient placement and assessment of students and personnel. Although some of the infatuation with testing receded by the end of the decade, intelligence and its measurement had by then become permanent features of the social and intellectual landscape. Debates over the provision of educational opportunities, the capabilities of various ethnic or racial groups, and the value of affirmative action have all been conducted at least in part through the language of native intelligence. However ill-defined, intelligence has become a concept of much consequence within the contemporary world.

John Carson

Bibliography

Ceci, S. J. (1996). On intelligence: a bioecological treatise on intellectual development. Harvard University Press, Cambridge, MA.
Sokal, M. M. (ed.) (1987). Psychological testing and American society, 1890–1930. Rutgers University Press, New Brunswick.