Familial aggregation in specific language impairment.

A case-control family study design, in which the current language-related abilities of all biological, primary relatives (mother, father, siblings) of probands with specific language impairment (SLI) and matched controls were assessed, was used to investigate familial aggregation for language disorders. Current test data from each family member showed the rate of language impairment for mothers, fathers, sisters, and brothers of the SLI probands to be significantly higher than for members of control families. Impairment rates for fathers and mothers were approximately equal, whereas rates for brothers were significantly higher than for sisters. In SLI proband families, Language Impairment (LI) occurred in 13.0% of offspring (excluding proband) with neither parent affected, 40% of offspring with one parent affected, and 71.4% of offspring in families in which both parents were language impaired. Rates of impairment as determined in current testing were compared directly to impairment rates estimated from family-history questionnaires collected from the same families. Group data showed impairment rates estimated from the family-history questionnaires to be similar to the rates based on actual testing. Furthermore, both appeared in line with rates based primarily on questionnaire data as reported previously in the literature. However, case-by-case analyses showed poor intrasubject agreement on classification as language impaired on the basis of current testing as compared to history information.