Galton and his disciples, we might recall, were obsessed with the measurement of intelligence. Between 1890 and 1910, dozens of tests were devised in Europe and America that purported to measure intelligence in some unbiased and quantitative manner. In 1904, Charles Spearman, a British statistician, noted an important feature of these tests: people who did well in one test generally tended to do well in another test. Spearman hypothesized that this positive correlation existed because all the tests were obliquely measuring some mysterious common factor. This factor, Spearman proposed, was not
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