PROJECT SUMMARY Children’s socioeconomic status (SES) is strongly associated with disparities in cognitive, linguistic, and academic development. Understanding the precise environmental and neurodevelopmental mechanisms that underlie these disparities is critical for developing maximally impactful interventions to reduce and ultimately ameliorate achievement gaps. This proposal will test an innovative developmental model in which specific features of children’s early language input engage the development of cascading frontotemporal neural networks that ultimately scaffold multiple aspects of social-cognitive development, including children’s language abilities as well as executive functioning and social cognition—critical school readiness skills that are known to contribute to SES achievement gaps in school. The proposed research will leverage multimodal assessments of children’s environments (real-world auditory language exposure and lab-based video-coding of parent-child interaction) and brain development (MRI measures of structural brain development, and fNIRS measures of brain function during domain-specific cognitive tasks) longitudinally across the preschool years (ages 3;0-4;11 years) to address three specific aims. Aim 1 will investigate the how three dimensions of early language experience (linguistic, social/interactive, and conceptual) influence structural and functional brain development underlying receptive and expressive language development. Aim 2 will extend this investigation beyond the language domain and evaluate whether these dimensions of language experience partially explain SES-related differences in executive functioning and social cognition, either directly or indirectly through developing language skills. Finally, Aim 3 will examine whether input-driven, longitudinal development of frontotemporal language-related brain networks also scaffold the development of neural circuitry underlying the emergence of executive functioning and social cognition. Results will help identify the precise components of early language experience that drive development across multiple neurocognitive domains, and will help determine the mechanisms by which exposures to specific social and environmental factors affect neural and cognitive plasticity during a critical period of brain development. By identifying the most impactful, malleable environmental factors that shape early neurocognitive development, this work has direct translational implications to inform the development of interventions to ameliorate income-related achievement gaps.