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Research on individual differences in mental abilities is discussed from three viewpoints: the psychometric structure of ability differences, the predictive validity of mental test scores, and some putative causes of psychometric intelligence differences in terms of psychometric and cognitive components and biological indices. A hierarchical descriptive structure for mental ability differences, as it emerged in the 1980s and 1990s, prominently displays British discoveries and suggestions, especially those of Spearman, Burt and P. E. Vernon from the first half of the 20th century. Galton and Spearman's largely unproductive search for the origins of ability differences has seen new activity since the 1970s, and there are several replicable associations that are yet to be explained. Over the 20th century the emphasis has been on measuring mental ability differences; at its beginning there was an emphasis (largely British) on understanding psychometric intelligence. The new century is likely to see a continuation of this re-emphasis on explaining human ability differences.
The construction of stories around centenaries of, or British contributions to, a scientific topic invites the respective dangers of contrivance and regional overemphasis. Fortunately, neither is risked in addressing individual differences in psychometric intelligence. An abundance of centenaries surrounds mental ability research, and British contributions were to the fore during much of the 1900s. Which centenaries to highlight? The 100-year gaps since the discovery of g (the supposedly neutral signifier for the abstract concept of general mental ability; Spearman, 1904) and the devising of the first serviceable mental ability tests (Binet, 1905) are still a few years away from closing. The centenary of the first suggested battery of mental tests passed a decade ago (J. McK. Cattell, 1890; with remarks by Galton, 1890). Two key papers in human cognitive ability research appeared on the precise centenaries of the writing of much of this article (Sharp, 1898-99) and its publication (Wissler, 1901). Inauspiciously, both were from leading American universities and both were reckoned to toll the death knells of the British (Spearman-Galton) approach to human intelligence differences.
Humans differ in their powers of mental work: that much has been recognized from antiquity (Huarte, 1575/1969). Since the beginnings of scientific psychology three principal questions have been asked about human cognitive abilities: Which domains of mental performance can be delineated? Does...