Objectives: To examine whether social-interactive skills at 6 months, nonverbal communication at 12 months, and language development from 12 to 36 months may be disrupted in siblings of children with autism who go on to show signs of the disorder.

Methods: Data was drawn from a larger sample of siblings who met clinical cutoffs on the ADOS at least once at 18/24/36 months (ever-affected), siblings who never met ADOS cutoffs (never-affected), and typically-developing infants (control). Frequencies and durations of infants’ socially-directed gaze, smiles, and vocalizations during a free-play interaction with the caregiver were coded at 6 months (n: ever=10, never=40, control=40). The ESCS was administered at 12 months to examine rates of joint attention and requesting behaviors (n: ever=21, never=77, control=56). Overall language abilities were measured at 12/18/24/36 months using the Mullen (n: ever=43, never=158, control=97).

Results: Infants who went on to show signs of autism did not differ from never-affected siblings and control infants in the extent to which they directed gaze, smiles, and vocalizations toward their caregivers at 6 months. By 12 months, the ever-affected infants exhibited lower rates of high-level joint attention (pointing, showing) and requesting behaviors, and language delays that averaged 1.8 months. The ever-affected infants also had lower rates of language growth between 12 and 36 months.

Conclusions: We found no behavioral markers of autism at 6 months of age among siblings who went on to show signs of the disorder. However, deficits that cut across both verbal and nonverbal communication emerged by 12 months of age, with language delays becoming more pronounced across the 12 to 36-month period.