A generalization of the F test is proposed to see how many independent components are statistically significant when the co-variances between DZ twin differences on several psychological tests are compared with similar co-variances for MZ twins. When applied to the 6 scores of Thurstone's Primary Mental Abilities test battery, at least 4 separate components were found to be significant independently. Then 4 components are tentatively identified as similar to, although not identical with, the Number, Verbal, Space and Word fluency scores. While the procedure bears some resemblance to factor analysis, it may lead to quite different results since it is based on the comparison of two co-variance matrices of twin differences.
In conclusion, it should be kept in mind that these results are based on small samples of MZ and DZ twins, so that a repetition of the study seems indicated. This replication is now in progress.