A case-control family study design, in which the current language-related a
bilities 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 disord
ers. 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 familie
s, Language Impairment (LI) occurred in 13.0% of offspring (excluding proba
nd) with neither parent affected, 40% of offspring with one parent affected
, and 71.4% of offspring in families in which both parents were language im
paired. Rates of impairment as determined in current testing were compared
directly to impairment rates estimated from family-history questionnaires c
ollected from the same families. Group data showed impairment rates estimat
ed from the family-history questionnaires to be similar to the rates based
on actual testing. Furthermore, both appeared in line with rates based prim
arily on questionnaire data as reported previously in the literature. Howev
er, case-by-case analyses showed poor intrasubject agreement on classificat
ion as language impaired on the basis of current testing as compared to his
tory information.