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article on the topic in 1904 was barely dry before others were arguing that intellectual ability could not be adequately captured by
g
or by any other unitary quantity—and understandably so, for common sense rebels against the idea that something so important about people as their intellects can be captured even roughly by variations in a single quantity. Many of the famous names in the history of psychometrics challenged the reality of
g,
starting with Galton’s most eminent early disciple, Karl Pearson, and continuing with many other creative and influential psychometricians.
    In diverse ways, they sought the grail of a set of primary and mutually independent mental abilities. For Spearman, there was just one suchprimary ability,
g.
For Raymond Cattell, there are two kinds of
g,
crystallized
and
fluid,
with crystallized
g
being general intelligence transformed into the skills of one’s own culture, and fluid
g
being the all-purpose intellectual capacity from which the crystallized skills are formed. In Louis Thurstone’s theory of intelligence, there are a half-dozen or so
primary mental abilities,
such as verbal, quantitative, spatial, and the like. In Philip Vernon’s theory, intellectual capacities are arranged in a hierarchy with
g
at its apex; in Joy Guilford’s, the structure of intellect is refined into 120 or more intellectual components. The theoretical alternatives to unitary, general intelligence have come in many sizes, shapes, and degrees of plausibility.
    Many of these efforts proved to have lasting value. For example, Cattell’s distinction between fluid and crystallized intelligence remains a useful conceptual contrast, just as other work has done much to clarify what lies in the domain of specific abilities that
g
cannot account for. But no one has been able to devise a set of tests that do not reveal a large general factor of intellectual ability—in other words, something very like Spearman’s
g.
Furthermore, the classicists point out, the best standardized tests, such as a modern IQ test, do a reasonably good job of measuring
g.
When properly administered, the tests are not measurably biased against socioeconomic, ethnic, or racial subgroups. They predict a wide variety of socially important outcomes.
    This is not the same as saying that the classicists are satisfied with their understanding of intelligence,
g
is a statistical entity, and current research is probing the underlying neurologic basis for it. Arthur Jensen, the archetypal classicist, has been active in this effort for the last decade, returning to Galton’s intuition that performance on elementary cognitive tasks, such as reaction time in recognizing simple patterns of lights and shapes, provides an entry point into understanding the physiology of
g.
The Revisionists: Intelligence as Information Processing
     
    A theory of intelligence need not be structural. The emphasis may be on process rather than on structure. In other words, it may try to figure out what a person is
doing
when exercising his or her intelligence, rather than what elements of intelligence are put together. The great Swiss psychologist, Jean Piaget, started his career in Alfred Binet’s laboratory trying to adapt Cyril Burt’s intelligence tests for Parisian children. Piagetdiscovered quickly that he was less interested in how well the children did than in what errors they made. 35 Errors revealed what the underlying processes of thought must have been, Piaget believed. It was the processes of intelligence that fascinated him during his long and illustrious career, which led in time to his theory of the stages of cognitive development.
    Starting in the 1960s, research on human cognition became the preoccupation of experimental psychologists, displacing the animal learning experiments of the earlier period. It was inevitable that the new experimentalists would turn to the study of human intelligence in natural settings. John B. Carroll and Earl B. Hunt led the