33 pairs of M Z twins and 12 pairs of like-sexed DZ wins, 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 verbal comprehension tests which included the Heim Self-judging Vocabulary Test, the Wide-Range Vocabulary Tesr, the Heim Vocabulary Tesr, and the Spelling Achievement Test. Using three different heritability ratios, ( 1 ) the Holzlnger heritability coefficient, ( 2 ) the heritability ratio proposed by Nichols, and ( 3 ) the F ratio (Block), the relative incrapair similarity of MZ and I~ke-sexed DZ twins on the verbal comprehension tests was determined. All the MZ correlations were greater than the corresponding 7s for the DZ twin-pairs. Two of the six differences were significant ( p 4 .05). The range of heritability coefficient ratios (Holzinger) was from .35 to .78, with MZ intraclass rs ranging from .45 to ,815, with DZ rs from -3 4 to .40. The agreemenr among rests suggests that verbal comprehension as tested by the various performance tests is heritable with as much as .78 of the within-family variance attributable to hereditary factors.
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