PROJECT SUMMARY / ABSTRACT The objective of this proposal is to understand the neural and environmental mechanisms of socioeconomic (SES) differences in 36-month-old children’s language development. In older school-aged children, the quantity and quality of parental language input statistically mediates effects of SES on brain development, which in turn impacts children’s language outcomes. Although these findings are well-characterized in older children, they have yet to be examined in children under age three, the time when brain maturation is most rapid. The primary objective of this proposal is to examine the relation between SES, parental language input, and brain and behavioral indices of child language during the first three years of life. Efforts to examine environmental and neural mechanisms of language in young children have been hindered by small samples and methodological challenges. For instance, it is not clear whether approaches to automatically estimate parental language input produce reliable estimates in non-White and socioeconomically diverse populations. This proposed study will overcome previous challenges by employing secondary data analysis on an existing longitudinal dataset of socioeconomically and racially diverse mothers and children (N=253). Our first aim is methodological and will examine reliability between automatically generated and manually coded measures of parental language input. Aims 2-3 involve additional transcription and manual coding of parents’ language input to test the longitudinal association between SES, language input, structural and functional brain measures, and language outcomes. We hypothesize that SES effects on child language at 36 months will be mediated by parental language input and the structure/connectivity of brain regions at 30 months. The proposed work is important because SES-based differences in child language ability have long-term implications for decreased educational attainment, lower occupational status, poorer physical and mental health, and increased risk for all-causes of mortality. By identifying the proximal experiences that mediate effects of SES on child language outcomes at age three, we may identify actionable targets for preventative and early intervention efforts to improve child and family wellbeing.