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.
In this preliminary work 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 the Simple Arithmetic Test (after B. N. Mukherjee) understood to be outstanding with respect to the size of loadings and purity of the numerical facility factor (Factor N). 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 in mathematical test performance was determined. All the MZ correlations, except one, were greater than the corresponding rs for the DZ twins. Four of the seven differences were significant ( p ≤ .10). The difference between the totals was significant ( p ≤ .10). The results suggest that numerical facility is a unitary ability independently inherited with as much as 72% of the within-family variance determined by hereditary factors.
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