Project Summary/Abstract The primary aim of the proposed Administrative Supplement is to examine the effect of a preventive intervention (IBP) on promoting early expressive language and reducing rates of late talking in young children during their second to third years of life. It capitalizes on our Parent R01 recruitment of young children from traditionally underserved and underrepresented backgrounds (i.e., ethnic/racial minority and low-income families) to examine the effect of a parenting intervention on promoting early expressive language and reducing rates of late talking (i.e., toddlers who acquire expressive language at a slower rate than their peers). Research shows that children identified as late talkers during their second and third years of life are more likely to show impairment in language and academic functioning at age 17 years. Additionally, young children living in poverty are twice as likely to have delays in expressive language, with evidence of the association between caregiver distress and dysregulated caregiver-child interactions with child language delays. However, research examining interventions for late talking is sparse and methodologically limited (e.g., no randomization, small sample sizes, lack of participant diversity). Additionally, most research has excluded important variables that places certain populations at increased risk, such as poverty and caregiver distress, and have not examined language before the fourth year of life, a critical developmental period when expressive language is first being used. The proposed supplement builds upon our pilot work, which was the first to examine the effect of an intervention targeting externalizing behavior problems, the Infant Behavior Program (IBP), on expressive language during the second year of life. We will replicate and extend these effects of the IBP to expressive language and late talking in a rigorously designed randomized controlled trial (RCT) using an active control condition and multimethod assessment of language and with a larger sample with children from predominately low-income and ethnic/racial minority backgrounds. We will also examine the theoretically-informed mediators (i.e., caregiver language input and caregiver-child interaction) and theoretically-informed moderator (i.e., caregiver distress) of intervention effects. This study will be the first to examine the efficacy of a preventive caregiver intervention on late talking in a larger, adequately powered RCT (n = 184), with participants from diverse ethnic/racial, socioeconomic, and linguistic backgrounds (94% ethnic/racial minority, 88% dual language learners), and using multimethod assessment of language (naturalistic recordings with LENA; videotaped caregiver-child interactions; and caregiver report on the MacArthur-Bates Communicative Development Inventories). This proposal falls within the scope of the Parent R01 and responds directly to NICHD’s Notice of Special Interest to increase underst...