Since its discovery in 1904, the general factor of intelligence (g) has generated considerable controversy. A historical review reveals that g has survived the scrutiny of psychometricians, educators, and politicians who simply wish that its importance in explaining individual and group differences would disappear. However, even in an atmosphere of political correctness, g survives as a robust variable explaining educational and societal attainment. In their efforts to control educative and societal outcomes, policy makers and educators commit an injustice in their active disavowal of such an important psychological construct.
Four thousand years ago, the Chinese were the first to use written mental tests in order to predict performance. Civil servants were administered standardized tests, which included measures of mathematical ability, nonverbal reasoning, and reading comprehension. Ironically, the Chinese ended this practice in 1905, the same year Alfred Binet introduced the first standardized test designed to measure the construct most psychologists and educators long identified as intelligence.
Robert Thorndike (1997) pointed to two historical influences in the development of measures of intelligence. First, in the late nineteenth century, universal education became a reality. Prior to this time, only a select few in the upper strata of society attended school. Intelligence testing became a means of ensuring that educational resources were allocated to those students who could most benefit; in particular, it helped remove genuinely dull children from normal classes. Second, in the work of Wundt, Fechner, and Galton there appeared the growing belief that individual differences in cognitive ability could be empirically quantified. Previously, scientists had relied on qualitative techniques such as introspection to gain insight into the workings of the human mind. The quantification of the mind established psychology as a science, with intelligence tests as the primary instrument for measuring cognitive ability.
Many theories of intelligence have been offered through the years. However, most have not endured the rigors of empirical scrutiny. Because they lend themselves so readily to statistical review, factor analytic theories offer the most promise to researchers and educators hoping to explain the structure and nature of intelligence. The following presents an overview of key psychometric theories of intelligence and outlines the important contribution of the general factor of intelligence in explaining individual differences.
Charles Spearman invented factor analysis in 1904 as part of an experiment to “find out whether, as Galton has indicated, the abilities commonly taken to be intellectual had any correlation with each other or with sensory discrimination” (Spearman, 1927, p. 322). Spearman obtained teacher evaluations of 36 students from a village school. Students were rated on the usual academic subjects (Latin, English, and math) as well as music and pitch discrimination. The observed correlation between the variables prompted Spearman to hypothesize that the variables shared a common source of variance, which he termed a “general factor of intelligence” (Spearman, 1904, p. 36). From the observation that variables had different levels of intercorrelation, he concluded that the variables had different levels of saturation with the general factor, which he called g (distinguishing little g from the big G of Newtonian gravity). The different levels of saturation represented different loadings on the general factor.
It was manifest to Spearman that g did not account for all the variance in students’ scores. Therefore, he posited a two-factor theory of intelligence. That is, each variable may be described as involving two factors, g and a factor specific to the variable. (Figure 1 depicts the Spearman-Holzinger model of intelligence.)
Spearman devoted the rest of his academic career to explicating the general factor, which he later referred to as g. He conceived of g as “mental energy,” and stated that it was the “leading part of intelligence, and is displayed by the ability to handle not merely abstract ideas, but above all symbols” (p. 211). Later, working from several data sets collected over twenty years, he evolved the idea that g consisted of three processes. The first process is “the apprehension of experience.” Spearman assumed, as did Galton, that the greater the sphere of experiences an individual can draw upon, the more complex is the problem solving that he can undertake. The second process is the “eduction of relationships.” Spearman conceived of eduction as the “drawing out of a logical relationship between two stimuli” (p. 227). The third process is the “eduction of correlates,” in which an individual notes the similarities between two stimuli. A cognitive task was loaded on g to the extent that it drew on these three processes. Although Spearman recognized that g may not account for all the variance in a matrix of scores, he preferred to emphasize g as the explanatory factor in intelligence.
Various theories of psychometric intelligence offer insights into the nature of cognitive abilities. These theories also provide a means for understanding personal and societal outcomes. Central to any empirically based model of intelligence is the crucial position and function of Spearman’s g factor. Spearman’s g routinely accounts for more variance than all other cognitive factors combined, and therefore assumes a position of hierarchical prominence in any model depicting the structure of human cognitive abilities.
Although egalitarians resist the obvious, Western societies are increasingly dependent upon Spearman’s g as a causal factor in educational and societal outcomes. The relationship of g to occupational and educational attainment is so large as to effectively stratify society according to IQ. When egalitarian fallacies are set aside, the ubiquity of g is obvious. As Scarr (1985) noted ”…there are few human endeavors that could not be included in the domain of intelligence, if one considered all of the correlates of ’g’.” Because of a reluctance of some researchers to discuss a construct that accentuates individual differences rather than individual equalities, Spearman’s g has become one of the best-kept secrets in science and education. As global societies continue their movement toward technology-based economies, intelligence and Spearman’s g will become increasingly important in discussions of education and public policy. Therefore, the salient issue is not the existence or importance of intelligence and its constituent factors, but rather whether policy makers will accept its impact on reality.
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