Background: Autism and specific language impairment (SLI) are developmental disorders that share language as a deficit. Past research suggests that there is an increased incidence of language difficulties in families of children with autism and SLI. Few studies have directly compared autism to SLI, and the exact relationship between the two disorders remains unclear.

Objectives: To examine the language and reading abilities of children with autism, children with SLI, and their first-degree relatives.

Methods: Participants were 51 autistic children, 34 children with SLI, their siblings, and their parents. The autism group was divided into two groups: those with and without language impairment (ALI, N=31; ALN, N=20). Participants were tested and compared on a battery of IQ, language, and reading assessments, including the WISC/WAIS, CELF-III, CTOPP, PPVT, WJ-R, and non-word repetition. Relationships to severity of autistic symptoms were also examined.

Results: ALI and SLI probands performed similarly on most measures while ALN probands scored higher. CELF-III and non-word repetition scores in the autism group were not correlated with symptom severity on any of the algorithm domains of the ADI-R or ADOS. SLI family members performed poorly on most measures, and over half scored in the LI range on CELF-III or non-word repetition. Similarly, the percentage of ALI relatives scoring in the LI range on these measures was significantly higher than in ALN relatives, especially in mothers, despite a lack of significance on group comparisons.

Conclusions: These findings suggest considerable overlap in ALI and SLI beyond basic language abilities and that language difficulties in the autism group are not related to autism severity. Also, although as a group relatives of autistic children performed better than those of SLI children, a significant proportion did perform poorly on clinically relevant language assessments.