Charles Spearman originally suggested in 1927 that the varying magnitudes of the mean differences between whites and blacks in standardized scores on a variety of mental tests are directly related to the size of the tests' loadings on g, the general factor common to all complex tests of mental ability. Several independent large-scale studies involving factor analysis and the extraction of a g factor from a number of diverse tests given to white and black samples show significant correlations between tests' g loadings and the mean white-black difference (expressed in standard score units) on the tests, thus substantiating Spearman's hypothesis. The average white-black difference on diverse mental tests is interpreted as essentially a difference in Spearman's g, rather than as a difference in the more specific factors peculiar to any particular content, knowledge, acquired skills, or type of test. (Author/CTM)