PROJECT SUMMARY/ABSTRACT Each year, violence in this country exacts staggering personal and financial costs. This burden is disproportionally born by adolescents, for whom violent behaviors linked to peer-based rejection are a leading cause of death. This may be related to the fact that peer relationships become highly salient in adolescence. Peer-based aggression is also a transdiagnostic symptom of several mental health disorders with high adolescent onset rates, and predicts risk for bipolar disorder, anxiety, and depression. Because adolescence is a sensitive period for neural plasticity, it is critical to identify neural mechanisms that bias youth towards perpetrating aggression before habitual antisocial tendencies are instantiated. Traditional intervention programs that target those at risk for perpetrating aggression have had only limited success. This may be because aggressive behavior is likely influenced by a complex cascade of neural responses that unfold across a social interaction. However, no study has examined these relations in adolescents, the group most impacted by peer-based aggression. Progress towards this goal has been hindered by the limited availability of ecologically-valid fMRI-based social interaction tasks that delineate temporal stages within a social interaction, such as anticipation and receipt of peer feedback, and contemplating aggression. The proposed work addresses these limitations by using a novel fMRI-based paradigm, the Virtual School and Aggression (VSA) task, to identify brain regions engaged at each temporal stage of a social interaction that predict aggressive behavior in adolescents (N=50; 11-15 years). I will also use time lapsed network interaction analyses, a novel form of functional connectivity analyses, to examine how the threat and cognitive control networks in the brain interact across distinct temporal stages as social rejection elicited aggression unfolds. Aims of this study are consistent with the NIMH strategic plan [(Objective 1): defining the mechanisms of complex behaviors, in this case, how social experiences and neural mechanisms influence peer-based aggressive behavior. The proposed study will determine both when during a social interaction regional brain function promotes aggressive behavior, and also how neural networks interact to promote social rejection elicited aggression. Such findings will provide novel treatment targets for aggressive behavior in youth. The proposed training plan, which consists of workshops, experiential learning, and mentorship, is designed to develop the applicant's expertise in the neurobiology of aggression, advanced neuroimaging methods, and the development and implementation of interventions for aggressive youth.] The proposed study will occur within Temple University's clinical psychology program, which has a successful track record of conducting impactful NIMH-funded research and training research scientists.