GENETIC-BASIS OF SPECIFIC LANGUAGE IMPAIRMENT - EVIDENCE FROM A TWIN STUDY

Concordance rates were compared for 63 monozygotic (MZ) and 27 dizygot ic (DZ) same-sex twin pairs, aged seven years and over, selected becau se at least one twin met diagnostic criteria for specific speech or la nguage impairment. There was significant heritability for developmenta l speech and language disorder, defined according to DSM-III-R criteri a. When the definition of the phenotype was broadened to include those with a past history of disorder and those with a less pronounced disc repancy between verbal and non-verbal ability, concordance for MZ twin s was close to 100 per cent, and that for DZ twins approximately 50 pe r cent. There was also close similarity between concordant twins for t ype of disorder. There is good evidence that genetic factors play a ro le in the aetiology of speech and language impairment; twin data may h elp us arrive at a clearer conception of the phenotype as well as quan tifying the extent of the genetic contribution.