Approximately half of the world population currently resides in urban areas and over one billion are children who are highly susceptible to environmental adversities. Urban residence is a well-documented risk factor for various mental disorders, including externalizing psychopathologies (EPs) such as aggression. Drastic increases in crime and other antisocial behaviors in urban areas have prompted the WHO to declare violence as a major global public health issue. Genetics are an important cause of EP, but environment also accounts for 50% of its total variance. Previous studies in urban-dwelling populations have primarily focused on the social environments linked to EP, but the potential contribution of physical environments has been overlooked. The project's main objectives are to chart the trajectory of EP in urban-dwelling adolescents, identify a novel environmental contributor to urban mental health, and examine social-chemical interactions. The aims are: 1) to investigate whether traffic air pollutant (AP) exposure predicts aggressive behavior in adolescence and later development of antisocial personality disorder (ASPD) during young adulthood; 2) to examine whether traffic APs are more strongly associated with reactive (i.e. impulsive) versus proactive (i.e. planned) aggression; 3) to identify potential interaction effects of APs and social stressors/adversities on aggression. Supported by growing evidence on developmental neurotoxicity of traffic APs and motivated by critical knowledge gaps in the literature, these aims will be addressed using population data and exposure modeling tools from two USC longitudinal studies: the Risk Factors for Antisocial Behavior (RFAB) Study and the Children's Health Study (CHS). The RFAB includes over 750 twin pairs assessed at multiple time points from 9-10 to 19-20 years of age. Total aggression and trait aggression were measured using the Child Behavior Checklist and the Reactive and Proactive Aggression Questionnaire, and ASPD was assessed with the Mini International Neuropsychiatric Interview. The CHS will provide detailed exposure data and spatial modeling tools related to road traffic, the primary source of APs in urban environments. Using geographical information science (GIS) and exposure modeling tools, AP levels will be estimated at residential and school locations. Longitudinal data analyses combining structural equation and mixed-level modeling will be used to examine the hypothesized adverse neurobehavioral effects of traffic APs and their interaction with social stressors. My proposed training plan will advance my knowledge in developmental psychopathology, air pollution exposure assessment, and statistical analyses, which will prepare me for a career as an independent investigator conducting multidisciplinary epidemiologic research on population-based environmental neurosciences. This project addresses the NIEHS goal to understand individual susceptibility across the lifespan to environmental factors. The proposed work will increase understanding of urban mental health linked to AP exposure from early life to adolescence and help in targeting populations that would benefit from preemptive strategies addressing the adverse effects of APs.