Heritability of Numerical Facility

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Abstract
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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