PROJECT SUMMARY/ABSTRACT Language delays are common in autism and ADHD. Prior studies on early language development in infants developing autism or ADHD have examined each condition separately and are largely incompatible with a transdiagnostic framework, limiting our understanding of whether similar or different mechanisms contribute to language delays in these populations. One proposed mechanism of early language learning is absent reference, which is conceptualized as a three-way connection between words, imperceptible physical entities, and mental representations. To better understand the developmental course of absent reference in typically and atypically developing samples, longitudinal studies beginning early in life are needed. Additionally, while social contingency in the infant-toddler period appears to be important for the development of language, it is unknown whether absent reference mediates this association. This F31 proposal will leverage the primary mentor’s longitudinal sample of n=163 infants at elevated and typical familial likelihood for autism and ADHD, with data collected at 6/9, 12, 18, 24, and 36 months of age. At 36 months, clinical best estimate outcomes (e.g., Autism, ADHD Concerns) are determined. Using a novel eye-tracking task which I developed, I will first examine trajectories of absent reference over the first two years of life, including comparisons of absent reference trajectories between typically and atypically developing infants (Aim 1). Then, I will use micro-analytic behavioral coding to capture the degree of temporal coordination (contingency) during infant-caregiver interactions and a standardized measure of receptive and expressive language, probing absent reference as a potential mediator of the association between early social contingency and later language abilities, and exploring the possibility of moderated mediation by outcome group (Aim 2). Strategic training goals and activities have been crafted as part of the F31 proposal to fill important knowledge, skill, and experience gaps in my training through robust mentorship from experts in longitudinal studies using transdiagnostic frameworks (Miller), cognitive and language development (Xu, Thurman), dyadic interactions (Schwichtenberg), and advanced statistical procedures (Iosif). Overall, the proposed training and research plans will facilitate progress towards my long-term goal of securing a tenure-track faculty position that bridges developmental and clinical science and will lead to advancements in the fields of language development, neurodevelopmental conditions, and developmental psychopathology. Results from the proposed study will have implications for efforts aimed at detecting language difficulties earlier and for the development of targeted interventions to support language development.