ABSTRACT
This chapter focuses on the problem of defining language behavior phenotypes. Language disorders result from a wide variety of etiologies, for example, mental retardation, hearing loss, environmental deprivation, affecting one or more levels of language performance. This broad view of language disorder is contrasted with specific language disorder (SLI), a term commonly understood to refer to deficits in language performance with no known cause, that is, not due to mental retardation, hearing loss, neurological disease, trauma, social-emotional disturbance, or environmental deprivation. The hypothesis asserted here is that SLI should be considered a behavioral classification, describing delays in the onset and rate of language development as well as asynchronies in development across language levels and deviant language performance. The notion of SLI as a unitary construct will be reviewed as well as the evidence supporting the existence of different types of disordered language performance, that is, different phenotypes of SLI. Insight into the genetics of SLI may be gained by reviewing the language outcomes of two syndromes that result from major chromosomal anomalies, Down syndrome and fragile X syndrome. Although both of these syndromes are associated with mental retardation, the language-cognition relationship for each may cast doubt on excluding mental retardation from the definition of SLI. Coming to some agreement on the defining properties of SLI will be central to documenting language phenotypes that may be genetically determined.
