Reviews literature on statistical correlations of mental abilities (1912-1913). Simpson has contributed to the study of the problem of correlation of mental abilities with each other, and with "general intelligence'. He found justification for the assumption of "general intelligence' but not for Spearman's common central factor. He has proposed to test the Spearman hypothetical explanation of correlation through the relationship to one common central factor by showing that 2 tests correlate more closely with each other than the common element in them correlates with the common element in 2 other tests. Correlational studies have been carried out for: mental deficiency and alcoholism; correctness of memory to volitional attitudes of truth telling; husband and wife correlations for factors influencing mating; and hours of sleep and performance in school. Formulae have been contributed by Pearson and Spearman.