The Syntactic Abilities of Identical Twins, Fraternal Twins, and Their Siblings.

MUNSINGER, HARRY, and DOUGLASS, ARTHUR, II. The Syntactic Abilities of Identical Twins, Fraternal Twins, and Their Siblings. CmILD DEVELOPMENT, 1976, 47, 40-50. New data are presented based on the method of twin-pair resemblances to estimate how much of the individual variation in children's language skills results from genetic factors and how much from environmental experience. The subjects were 37 pairs of monozygotic twins, 37 pairs of same-sex dizygotic twins, 11 siblings of the MZ twins, and 18 siblings of DZ twins. 2 different language measures were used: the Assessment of Children's Language Comprehension and the Northwestern Syntax Screening Test. Also, to unconfound language ability and intelligence, nonverbal IQ variance was partialed out before the language scores were analyzed. The main conclusion of this paper is that the heritability [2(rMz rDz)] of children's language abilities is .79, and that the total environmental effect on language skills cannot be much over .10. These results are most noteworthy because they are independent of nonverbal IQ covariation and are based on a substantial number of subjects (N = 206), the language measures used are reliable and valid, and the results are clear of genetic/environmental confounding.