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 Test, (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 heritability ratios, (1) the Holzinger heritability coefficient, (2) the heritability 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 rs for the DZ twins. Four of the nine differences were significant (p == .05). The range of heritability coefficient ratios (Holzinger) was from .15 to .89, with MZ intraclass rs 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.
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