Language Development in Young Twins: Biological, Genetic and Social Aspects

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Abstract
The present study has confirmed previous evidence that young twins show a marked delay in language development. This delay averaged six months at the age of 48 months, compared to a control group of singletons; it was evenly reflected in all nine subtests of the Illinois Test of Psycholinguistic Abilities, and was shown equally in MZ and DZ twins. Biological variables such as birth weight, gestational age and reproductive complications were only weakly associated with language scores at four years. The twins were not significantly retarded on non-verbal tests of general intelligence, compared with population norms or with the singleton controls. Environmental factors, particularly social class and family size were strongly correlated with language scores, though the association was stronger in singletons than in twins. Middle class twins were relatively much more retarded in language development compared with middle class singletons than working class twins compared with working class singletons. Comparison of intra-class correlations between 28 MZ and 64 DZ pairs yielded a heritability index of 44% for the test as a whole, with considerable variations in h2 over the nine subtests. Heritabilities were highest for tests sampling the visual motor channel, and lowest for tests on the auditory-vocal channel. In confirmation of this finding, an examination of the test profiles of the singleton controls suggested that social class differences were most marked on tests of the auditory-vocal channel. It is suggested that the latter are more susceptible to environmental variables.