This 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. Eleven large-scale studies (discussed in the appendix), each one comprising anywhere from 6 to 13 diverse tests, show a significant and substantial correlation between tests' g loadings and the mean black-white difference on the various tests. 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. 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 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. (Author/PN)