Specific Language Impairment (Overview)

Prevalence and Diagnosis

SLI is a surprisingly common disorder affecting between 5 and 7 percent of the population. Boys are around three times more likely to be affected than girls. The diagnosis of SLI is currently made using exclusionary criteria. For a traditional diagnosis of SLI to be given, a functional language impairment must be present without an obvious cause such as physical impairment, nonverbal learning difficulties, hearing impairment, or autism. However in recent years, the need for these other causes to be absent has been questioned. The prevalence of SLI appears to be greater in families that already have an affected family member and in those families with children with autism. The exact diagnostic criteria for SLI differ, and there are a number of different definitions in clinical practice and research. J. B. Tomblin and colleagues conducted research showing that language scores falling below 1.25 standard deviation (SD) from the normative mean (approximately the 10th percentile) were a valid indicator of clinical language impairment, and this level was seen henceforth as a gold standard for diagnosis.

Nevertheless, academics and clinicians often use 1 SD as a threshold for investigating profiles and [Page 568]prioritizing resources, but when this less strict level is used, children's performance is usually required to fall below the threshold on a wider range of language measures. More controversially, the level of nonverbal IQ (NVIQ) needed for a diagnosis of SLI is usually defined in research as 85 points (1 SD) or higher, while United Kingdom educational practice as well as the fourth edition of the Diagnostic and Statistical Manual of Mental Health Disorders (DSM-IV) both deem a score below 70 (2 SD), rather than below 85, to indicate more global learning difficulties. There have also been questions raised regarding the use of such thresholds, for example, whether children with NVIQs of 86 and language scores of 84 really have SLI and whether children with NVIQs of 110 and language scores of 90 should be described as having a primary language impairment, even though both scores fall within the normal range.

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