33 pairs of MZ twins and 12 pairs of like-sexed DZ twins 32 boys and 58 girls, 40 of whom were Negro and 50 white whose ages ranged from 13 to 18, were given a battery of psychological tests which included (1) the surface development tests. (2) Porteus Mazes, (3) the newcastle spatial-test. (4) the paper Folding test, (5) the identical pictures test, Perceptual Speed, (6) the Object-Aperture Test, Form B, and (7) Cube comparisons. Using three different habitability ratio proposed by Nichols, and (3) the F ratio (Block), the relative intra pair similarity of MZ and like sexed DZ twins on the selected perceptual tasks was determined. All the MZ correlations were greater than the corresponding for the DZ twins. Four of the nine differences were significant (p-05). The range of habitability coefficient ratios (Holzinger) was from 15 to 89, with MZ interclass is ranging from .46 to 91 and DZ rs from .08 to 72. The agreement among tests suggests that the mental abilities represented are independently inherited with as much as 89% of the within-family variance accounted for by hereditary factors.
Abstract