PSYCHOMETRIC G-RELATED TO DIFFERENCES IN HEAD SIZE

Scores on 17 diverse tests of cognitive abilities obtained from 82 pairs of monozygotic (MZ) and 61 pairs of dizygotic (DZ) twins were correlated with head size. A general factor, or psychometric g, was extracted from the tests, and g factor scores were found to be correlated with head size variables not only within individuals, but within twin pairs and between twin pairs. The size of the various tests' g loadings predicts the degree to which the tests are correlated with head size. This finding adds one more biological variable-head size and, by inference, brain size-to the list of other biological variables reported in the literature as showing a significant relation to psychometric g, the general factor common to all cognitive tests. Also, the varying magnitude of the mean difference between groups of white and black children on the 17 tests is related to the tests' loadings on g, on spatial ability, and the tests' correlations with head size.