What Causes Specific Language Impairment in Children?

Abstract

Specific language impairment (SLI) is diagnosed when a child's language development is deficient for no obvious reason. For many years, there was a tendency to assume that SLI was caused by factors such as poor parenting, subtle brain damage around the time of birth, or transient hearing loss. Subsequently it became clear that these factors were far less important than genes in determining risk for SLI. A quest to find "the gene for SLI" was undertaken, but it soon became apparent that no single cause could account for all cases. Furthermore, although fascinating cases of SLI caused by a single mutation have been discovered, in most children the disorder has a more complex basis, with several genetic and environmental risk factors interacting. The clearest evidence for genetic effects has come from studies that diagnosed SLI using theoretically motivated measures of underlying cognitive deficits rather than conventional clinical criteria.

Fig. 1

Mean z-scores on nonword repetition for individuals with specific language impairment (probands, defined as those with z-score less than 1.0) and their co-twins, in relation to whether they are monozygotic (MZ) or dizygotic (DZ) twins. The population mean score is zero. Insofar as similar environmental influences affect both twins, two members of a twin pair would be expected to resemble one another. However, if, as shown here, the similarity between MZ probands and their co-twins is greater than that between DZ probands and their co-twins, this points to a genetic influence on low scores. Data from Bishop, North, & Donlan (1996).