Genetic basis of specific language impairment: evidence from a twin study.

Concordance rates were compared for 63 monozygotic (MZ) and 27 dizygotic (DZ) same-sex twin pairs, aged seven years and over, selected because at least one twin met diagnostic criteria for specific speech or language impairment. There was significant heritability for developmental speech and language disorder, defined according to DSM-II-R criteria. When the definition of the phenotype was broadened to include those with a past history of disorder and those with a less pronounced discrepancy between verbal and non-verbal ability, concordance for MZ twins was close to 100 per cent, and that for DZ twins approximately 50 per cent. There was also close similarity between concordant twins for type of disorder. There is good evidence that genetic factors play a role in the aetiology of speech and language impairment; twin data may help us arrive at a clearer conception of the phenotype as well as quantifying the extent of the genetic contribution.