Abstract
Low responsiveness to sensory stimuli, particularly stimuli that are social in nature (i.e., social hyporesponsiveness), predicts expressive language in autistic children and in infant siblings of autistic children (Sibs-autism), who are at high likelihood for a future diagnosis of autism and developmental language disorder. However, our understanding of whether social hyporesponsiveness can be addressed via early intervention to improve expressive language outcomes of Sibs-autism is limited. This randomized controlled trial investigated whether Project ImPACT, a caregiver-implemented Naturalistic Developmental Behavioral Intervention (NDBI), has an indirect effect on expressive language outcomes by reducing social hyporesponsiveness. Sibs-autism were randomized into a Project ImPACT group (n = 23) for 12 weeks of intervention, or into a non-Project ImPACT control group (n = 23). Social hyporesponsiveness was measured immediately following intervention, and expressive language was measured three months after the end of intervention. Project ImPACT indirectly influenced distal expressive language outcomes through social hyporesponsiveness, but only for infants whose caregivers had high levels of education at study entry. Clinical implications of the results are discussed.