A family aggregation study: the influence of family history and other risk factors on language development.

Substantial evidence continues to accrue for familial transmission of specific language impairment (SLI). The incidence in families with a history of SLI is estimated at approximately 20%-40%, whereas in the general population the estimated incidence is about 4%. Typical aggregation studies compare data on the speech and language status of parents and siblings of individuals with SLI (the probands) to similar data from family members of control individuals with no speech or language disorder history. In the present study, family aggregation of SLI was examined for a unique sample of children who were ascertained before 6 months of age and thus did not have SLI, but were born into a family with a positive history of SLI (FH+). No study to date has examined the pattern of affectance in families of children ascertained at such a young age. In addition, the ratio of boys to girls born into such families was investigated, as previous studies have suggested alterations in the expected gender ratios. Consistent with prior research, SLI was found to aggregate in families; the average affectance rate in FH+ families was 32%, with significantly more boys (41%) reported as having SLI than girls (16%). A comparison of FH+ and control families (FH-) on sociodemographic factors and medical history revealed differences in the overall rate of autoimmune diseases; FH+ families reported a significantly higher incidence (35%) compared to FH- families (9%). Finally, the 3-year language abilities of a subset of 32 children from FH+ families were compared with those of 60 children from FH- families. Children from FH+ families scored significantly lower on standardized measures of language and were more likely to fall below the 16th percentile (28%) than children from FH- families (7%). These results provide converging evidence that children from FH+ families are indeed at greater risk of developing language delay compared to children from control families.