ABSTRACT The goal of the parent R01 MH121364 investigation is to validate the effectiveness of a new automated online screening tool based on parent report—the Social Communication (SoCo) CheckUp—which incorporates empirically-tested screening items for communication delay and autism spectrum disorder (ASD). The parent investigation is recruiting 4,000 infants who join Baby Navigator by 9 months of age. All families of children with a positive autism screen at each SoCo Checkup, and a comparison group with a negative screen, are invited to upload a video home observation that is coded using the Systematic Observation of Red Flags of ASD (SORF). The parent grant is addressing the following research aims: Aim 1. Study psychometric features of the SoCo Checkup cut-off and composite to diagnostic outcome; Aim 2. Study the trajectories of the SoCo surveillance and screening to predict severity of autism diagnostic features; and Aim 3. Construct a risk algorithm based on multiple parameters to develop an efficient, cost-effective mobile screening application for use in infants that is readily deployable in the general population. The expected outcomes of this validation study will demonstrate the effectiveness of an online surveillance and screening system beginning in the first year of life that will be ready for immediate, rapid, scalable, and sustainable deployment across the US. In response to NOT-DC-23-005, this administrative supplement will build upon our previous research on late talkers and capitalize on the evaluation of children recruited for the parent grant to contribute to elucidation of early social communication trajectories and unique language outcomes of late talking children in this sample. Of the children recruited and screened in the parent grant, 1,768 will be 2-4 years of age during the supplement project period. We will invite 300 children to participate in the supplement follow-up evaluations at 2-4 years of age. The evaluations in the parent grant will be enhanced by coding new early measures based on observation of social communication and language (OSCL) and observation of responsive parenting strategies (ORPS) as part of the supplement using the collected home observations from the parent grant. This supplement will address the following research aims: Aim 1. To study associations among new measures coding the OSCL and ORPS from the home observations and the collected measures in the parent grant—the SoCo CheckUp and SORF from 9-24 months; Aim 2. To study trajectories of the SoCo CheckUp, SORF, OSCL, and ORPS from 9- 24 months in predicting new measures of language outcomes at 2-4 years of age; and Aim 3. To examine sociocultural demographic indicators as predictors of social communication trajectories and language outcomes of late talkers. The early social communication measures and sociocultural indicators together with these new measures of language at 2-4 years will enhance data available to characterize differential ...