Is specific language impairment a valid diagnostic category? Genetic and psycholinguistic evidence

Specific language impairment (SLI) is diagnosed when a child fails to develop language normally for no apparent reason: hearing and intelligence are adequate and the social environment is unexceptional. Definitions of SLI typically specify that the child must have a substantial discrepancy between language ability and non-verbal IQ. However, data from a twin study question the validity of this requirement, and indicate that SLI is not genetically distinct from less specific disorders where language impairment occurs in the context of low average or borderline non-verbal ability. A second question concerns the heterogeneous language symptoms seen in SLI: do these correspond to distinct conditions, or to different phenotypic manifestations of a common underlying disorder, or are they merely random variations resulting from unreliable assessments? The last of these possibilities is ruled out by the finding that twins who are concordant for language disorder show good agreement in terms of the pattern of language impairment. However, systematic variation in the age and ability of children in different SLI subgroups suggest that these may correspond to variable manifestations of a core inherited language disorder, rather than distinct diagnostic entities.