PROJECT SUMMARY/ABSTRACT Despite misconceptions that young children do not experience self-injurious thoughts and behaviors (STBs), data from samples of youth that include children under age 9 suggest rates of suicidal ideation between 11%–19%, non-suicidal self-injury at 6.2%–21%, and suicide attempts between 2.4%–3.5%. Although rare, rates of suicide death in young children are increasing. However, the development of STBs in young children is vastly understudied. Our team’s research demonstrates a robust link between threat exposure history and risk for STBs in youth. Moreover, data support that threat experiences increase risk for STBs possibly through heightened emotional reactivity and reactive aggression. Much of the work examining the impact of threat exposure on the development of STBs focuses on adolescent youth, long after many children have experienced threat and after STBs have developed for some youth. The purpose of the current study is to test key components of our conceptual model positing that a history of exposure to threat initiates develop- mental pathways toward earlier risk for STBs in young children through its influence on emotional reactivity and reactive aggression. Moreover, youth may be at greatest risk for STBs in the days after future threat and non-threat negative events occur, highlighting the need to understand potential short-term risk (i.e., over days) for STBs among young children. Aim 1 of this proposal will investigate the extent to which more severe and multiple threat exposures contribute to earlier emergence of STBs in young children ages 5–9. Aim 2 will examine emotional reactivity and reactive aggression as predictors of STBs, including short-term (days) and longer-term (months) predictors, and as mechanisms through which threat may influence STBs. Aim 3 will examine the extent to which future threat and non-threat negative events amplify short- and long-term risk for STBs among young children with emotional reactivity and reactive aggression. We will conduct exploratory analyses testing our full model positing that threat influences STBs through heightened emotional reactivity and reactive aggression, particularly when future threat or non-threat events occur. We will also explore sex and race differences in STBs in young children. We will enroll 300 5- to 7-year-olds with a history of a range of threat exposure and their caregiver in a racially diverse catchment area and follow youth longitudinally for 2 years (until ages 7–9). Youth and caregivers will complete an in-home baseline assessment composed of multiple methods, including clinical interviews; questionnaires; and observational, physiological (i.e., respiratory sinus arrythmia), and behavioral measures within developmentally relevant negative emotional contexts (e.g., social-evaluative stressor). Caregivers and youth will complete three 1-week periods of daily diary assessments spaced 1 month apart to assess short-term STB risk. Remote follow-u...