Why do scores on most cognitive ability tests correlate positively? The fact that people who score highly on one test tend to score highly on others ± that some people are more intelligent than others ± is so intuitively obvious that most psychologists, at least since it was ®rst discovered by Spearman (1904), take it for granted. Even the harshest critics of IQ profess no surprise at the positive correlations among tests; Stephen J. Gould (1981, p. 315) wrote, ``The fact of pervasive intercorrelation between mental tests must be among the most unsurprising major discoveries in the history of science.'' Psychologists are generally not interested in talking about, much less thinking about, let alone investigating, individual differences in cognitive ability. And there is nothing wrong with this; research topics and approaches fade in and out of popularity. But if we could step out of our normal modes of thinking, if we could free our minds of the ``debauchery of learning'' that William James famously noted (1890, as quoted by Cosmides & Tooby, 1994), we might wonder why there should be any correlation among cognitive tests, let alone the strong, consistent, positive correlations found in hundreds of studies over the past century (Carroll, 1993).