Although the black and white populations in the United States differ, on average, by about one standard deviation (equivalent to 15 IQ points) on current IQ tests, they differ by various amounts on different tests. The present study examines the nature of the highly variable black–white difference across diverse tests and indicates the major systematic source of this between-population variation, namely, Spearman's g. Charles Spearman originally suggested in 1927 that the varying magnitude of the mean difference between black and white populations on a variety of mental tests is directly related to the size of the test's loading on g, the general factor common to all complex tests of mental ability. Eleven large-scale studies, each comprising anywhere from 6 to 13 diverse tests, show a significant and substantial correlation between tests' g loadings and the mean black–white difference (expressed in standard score units) on the various tests. Hence, in accord with Spearman's hypothesis, the average black–white difference on diverse mental tests may be interpreted as chiefly a difference in g, rather than as a difference in the more specific sources of test score variance associated with any particular informational content, scholastic knowledge, specific acquired skill, or type of test. The results of recent chronometric studies of relatively simple cognitive tasks suggest that the g factor is related, at least in part, to the speed and efficiency of certain basic information-processing capacities. The consistent relationship of these processing variables to g and to Spearman's hypothesis suggests the hypothesis that the differences between black and white populations in the rate of information processing may account for a part of the average black–white difference on standard IQ tests and their educational and occupational correlates.