Objectives: Investigate cognitive and language development in infant siblings of children with autism (“high-risk infants”; HR) to determine whether: 1) infants who go on to develop ASD (“ASD infants”) perform below unaffected HR infants, and 2) unaffected HR infants nonetheless exhibit a broader phenotype, i.e., perform below infants from families without autism history (“low-risk infants”; LR).

Methods: Assessments administered between 6 and 36 months included: the Mullen Scales of Early Learning, which measures gross motor, fine motor, visual reception, receptive language, and expressive language, the Ages & Stages Questionnaire (ASQ), which measures communication, gross and fine motor, problem solving, and personal-social development, and the MacArthur-Bates Communicative Development Inventory (CDI), which measures gesture and language skills.

Results: On the Mullen, the three groups (LR: n=140, HR: n=35, ASD: n=8) were indistinguishable at 6 months, but by 24 months, ASD infants underperformed HR infants on all subscales (with no differences observed between LR and HR infants). On the ASQ, ASD infants underperformed HR infants by 10 months in communication, personal-social and problem-solving behaviors. At all ages tested, HR infants underperformed LR infants. On the CDI (10-14 months, Words & Gestures), in all categories, ASD infants underperformed HR infants, who underperformed LR infants. On the CDI (18-36 months, Words & Sentences), the underperformance of the ASD infants continued; however, the HR infants caught up with the LR infants in language skills.

Conclusions: The differences observed between LR and HR infants support an early broader phenotype in ASD, while those observed between HR and ASD infants provide potential early markers for developing ASD. Such findings complement those from other laboratories using this HR infant approach.