An analysis of Koch's (1966) Primary Mental Abilities Test data for 5- to 7-year-old twins.

The purpose of this note is to present twin correlations based on analyses of Koch's (1966) Primary Mental Abilities Test data on 83 pairs of 5- to 7-year-old twins. The results yielded significant heritability for the Spatial subtest and suggested genetic influence on the Verbal and Quantitative subtests as well. An analysis of the phenotypic correlation between the Verbal and Spatial subtests suggested that verbal and spatial abilities are genetically independent at this age and that the relationship between the two abilities is mediated primarily by within-family environmental factors.