The concept of intelligence is broad in nature and refers to a person's complex mental abilities that include, among other things, the variables of amount of knowledge available at a given time; speed with which new knowledge is acquired; the ability to adapt to new situations; and the ability to deal with new and old concepts, abstract symbols, and cognitive relationships. The process of developing general mental "schemas" to classify events in the environment is called abstract intelligence/ reasoning (cf., concretistic reasoning or solving specific problems with specific materials) and formal operations (cf, Piaget, 1963), and may be measured in various ways. Many of the theories of intelligence are tied to particular tests, methods, and assessments of this complex concept, and these constitute what might be called measured intelligence. Some researchers in this area refer, also, to the "adaptive ability" (e.g., grades in school, performance and success at work) of the person as an indication of general intelligence. The modern concept of measured intelligence in psychology began with the French psychologist Alfred Binet (1857-1911), who was called upon in 1904 by the Paris minister of public instruction to develop a test that would identify subnormal children in the Paris schools for the purpose of placing and educating them in special schools. In 1905, Binet presented a scale for the measurement of intelligence, along with the French psychologist Theodore Simon (1873-1961). The scale consisted of tasks that were arranged in increasing difficulty according to the age at which an "average" child (i.e., criterion of 60-90% students passing the tasks) could master them. Using this method, Binet and Simon could identify a "mentally retarded" child, who performed below the average child for his or her particular age group. A total mental age (MA) score was calculated for each student and compared with his or her chronological age (CA). Bi-net's work on intelligence was developed further after 1911 by the German psychologist Lewis William Stern (1871-1938), who formulated the concept of intelligence quotient, or IQ, where overall IQ score equals MA divided by CA x 100 (cf., Heinis' constant/law of mental growth; Zusne, 1987). Thus, in the case of Binet, Simon, and Stern, intelligence was considered to be one general or common factor of global functioning based on the person's test score on their standardized test. The Binet-Simon Test was translated from French into English and brought to the United States by the American psychologist Henry Goddard (1866-1957) in 1908. In 1916, the American psychologist/psychometrician Lewis Terman (1877-1956) at Stanford University adapted the Binet-Simon Test to create the Stanford-Binet Test, which provided a single score of intelligence. In 1904, the English psychologist and psychometrician Charles Spearman (1863-1945) proposed a slightly different theoretical viewpoint (called the two-factor theory) concerning intelligence, where he showed the presence of one general factor (g) in classroom tests of intellectual ability and achievement (cf., law of inertia - posits that persons who are slow in starting a mental process take a long time to finish, and such people have difficulty in going back and forth between two ideas, as well as difficulty in applying an earlier learned skill to new ideas; also, J. Raven's progressive matrices theory), as well as a number of specific factors (s). The question of whether intelligence is basically a single ability (g) or a group of specific abilities (s) has been debated by psychologists for more than three generations and still remains unresolved [cf., the notion of fluid intelligence/ability, derived from factor analysis by the English-born American psychologist Raymond B. Cattell (1905-1998), referring to nonverbal reasoning abilities requiring rapid understanding of novel relationships; fluid intelligence, along with crystallized intelligence comprises Cattell's dual-intelligence theory]. The Romanian-American psychologist David Wechsler (1896-1981) developed a set of individually administered intelligence tests that measured several factors such as comprehension, vocabulary, similarities, block design, and object assembly on scales that provide an overall intelligence score as well as subscale scores on verbal and performance abilities (cf., multimodal theory of intelligence, or multiple-factor theory, which generally postulates that more than one common factor can account for a given phenomenon and, in particular, assumes that intelligence is made up of a number of different factors or abilities). The American psychologist/psychometrician Louis
Leon Thurstone (1887-1955) devised a ratio scale for the measurement of intelligence in 1928 after he pointed out the psychometric inadequacies of the Binet-Simon-Stern concept of MA as a scale for mental ability and of the IQ concept when used as the ratio of MA to CA. That is, the concept of MA is ambiguous and can mean two different things: the average age of children obtaining a given score or the average test score of children of a given age. Thurstone's theoretical approach emphasizes the use of percentile ranks or standard scores instead of MA and IQ. This viewpoint is the basis for all modern intelligence tests, and such intelligence scores today are actually standard scores with a population mean of 100 and standard deviation of 15 or 16. Thurstone's approach is based, also, on the statistical method of factor analysis, wherein he extracted a number of factors he called primary mental abilities. Thurstone developed "factor-pure" tests for the seven factors of reasoning, verbal comprehension, word fluency, number, spatial visualization, perceptual speed, and associative memory. Currently, however, it is thought that each "factor-pure" test measures not only one of Thurstone's primary abilities but, also, Spearman's g factor to some degree. The American psychologist Joy Paul Guilford (1897-1987) made numerous factor analytic studies of intellectual abilities and developed an ambitious and creative model called the structure-of-intellect model/theory, which suggests the existence of as many as 150 separate and distinct functions or abilities. In at least 120 of these "intelligences," Guilford conceives of a stack of building blocks six blocks high, four blocks wide, and five blocks deep, and where each block, theoretically, is a separate "intelligence." This novel approach (also called Guilford's cube) suggests there are five "operations" (evaluation, convergent production, divergent production, memory, cognition), four "contents" (figural, symbolic, semantic, behavioral), and six "products" (units, classes, relations, systems, transformations, implications). According to Guilford, a person uses various operations, contents, and products whenever she or he is engaged in intellectual activities (cf., Sternberg & Lubart's theory of creativity). Thus, to oversimplify the range of classical theories of intelligence, one may go from the Binet-Simon-Stern concept of one general/global factor, to the Spearman-Wechsler concept of one general plus several specific factors, to Jean Piaget's emphasis on cognitive development, to Thurstone's seven (or eight) factors/abilities and, finally, to Guilford's theoretical approach that invokes 150 distinct abilities or "intelligences" [cf., aggregation theory - refers to a conjecture by the American psychologist James McKeen Cattell (18601944) that intelligence is a function of the number of neurons in the brain and the interconnections among them]. The more modern/contemporary theories of intelligence focus on concepts such as multiple intelligences (Gardner, 1983), triarchic (componential, experiential, and contextual, or analytic, creative, and practical) intelligence (Sternberg, 1985, 1988), emotional/social intelligence (Goleman, 1995), and mental self-government (Sternberg, 1986). A general class of theories that attempts to account for individual differences ranging from reaction time, through intelligence, to study of personal values is called individuality theory. Questions concerning intelligence that remain to be answered by further theoretical and empirical developments before the "laws of intelligence" may be stated are: Is intelligence truly one general ability or several specific abilities? Is intelligence a matter of rapid information processing? In intelligence genetically, environmentally, or culturally defined? The American psychologist Arthur Robert Jensen (1923- ) developed an information-processing theory of intelligence and succeeded - where Francis Galton (1822-1911) failed earlier - in showing a relationship between general intelligence and measures derived from choice reaction-time tasks; Jensen studied, also, several physical and neurological correlates of general intelligence [cf., Flynn effect - named after the American-born New Zealand political scientist James Robert Flynn (1934- ), refers to a mysterious increase in average IQ scores, about three IQ points per decade, that has been occurring in many industrial societies since the introduction of IQ tests; and collective memory theory - attempts to explain the general rise of IQ scores over time, whereas the general level of intelligence remains relatively constant for any given population; the theory assumes that intelligence test items become more familiar longitudinally - vis-à-vis popular tests - as an increasing number of people learn the answers to questions over time; cf., dysgenic trend theory - claims that the general level of intelligence is falling/decreasing over time; the overlap hypothesis - attempts to explain the increasing consistency of IQ scores as individuals become older by stating that knowledge and skills remain constant and, thus, overlap increasingly with time; and resource depletion theory - a questionable proposition which states that with a reduction of resources in families - dependent on the number of children - the possible result, among other things, would be lower intelligence test scores for children in large families]. One popular explanation for the observed social class and racial differences on intelligence tests is called the cultural bias hypothesis, which states that the typical experiences involving the acquisition of skills and knowledge are different for different subpopulations, and the content of test items is selected much more from the experiences of certain groups of individuals such as whites, Anglo-Saxons, Protestants, and "middle" class persons than from the experiences of other groups of persons such as blacks, poor, and "lower" class individuals (cf., Herrnstein & Murray, 1994) . See also EMOTIONAL INTELLIGENCE, THEORY OF; GALTON'S LAWS; NATURE VERSUS NURTURE THEORIES; PERSONALITY THEORIES; PIAGET'S THEORY OF DEVELOPMENTAL STAGES. REFERENCES
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