Project Summary/Abstract For the millions of bilingual parents around the world, and in the US specifically, one of the most pressing questions in the face of the Autism Spectrum Disorder (ASD) diagnosis is how to reconcile it with bilingualism. The choice is most frequently framed in terms of whether to maintain the home language or to switch to the community language (English in the US). Existing studies on bilingual children with ASD take the approach of comparing them to monolingual children with ASD in order to establish whether bilingualism carries additional risks to the development of an autistic child10,22,28,62,110,116,121,122,126,131,141,143,152. However, comparisons between bilinguals and monolinguals can be deeply biased, and equally importantly, such a group-comparison approach says little about the best way to support dual language development in young bilingual children with autism. In this proposal we ask: What are the patterns of dual-language development in young bilingual children with ASD, and how does dual-language input shape their language outcomes? An especially significant and innovative aspect of this proposal is its combination of tightly-controlled experimental tasks testing early English and Spanish skills via highly sensitive eye-gaze measures and ecologically-valid measures of language input, obtained via parent-child interactions. Our participants are young children (18-36 months) exposed to Spanish in the home, with and without ASD. We will provide the diagnosis of ASD in-lab, using the family’s preferred language, thus shortening the evaluation timeline, and yielding an immediate benefit to Hispanic and Latine families that are drastically underserved, both in terms of service access and in terms of research participation. Aim 1 is to examine the development of Spanish and English skills in young bilingual children with and without ASD, defining trajectories of dual-language development. Aim 2 is to examine whether dual-language input can be optimized in young bilingual children with and without ASD. Studies under this Aim will offer first evidence for the need to separate dual-language input into distinct streams to optimize language outcomes in bilingual children with ASD. Aim 3 is to characterize input parameters in the linguistic environment of bilingual children with and without ASD over time, and to test the effects of dual-language exposure on language outcomes. This will be the first prospective longitudinal study to examine how families adapt their language practices when they receive a diagnosis of ASD and to investigate how dual-language input shapes language outcomes in bilingual children with and without ASD. Proposed studies are highly innovative in their focus on very young children, use of dynamic processing measures that can capture language skills in children with varying levels of ability, and longitudinal approach. Recruitment of a neurotypical group that is distribution- matched to children ...