PROJECT SUMMARY/ABSTRACT Youth externalizing psychopathology, such as attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder (CD), are highly comorbid with one another and are associated with many negative outcomes. Greater knowledge of etiology (e.g., cognitive, emotional, or motivational mechanisms) may improve interventions or facilitate prevention. However, the high rate of comorbidity and the popularity of investigating individual disorders has obscured relationships between symptoms and potential etiologies. While recognition of this problem has inspired more research on mechanisms of comorbidity, the extant literature suffers from two critical limitations. One, many studies implicitly treat diagnostic entities as distinct and independent by comparing disorders in group-based analyses or by utilizing separate factors for each disorder. These approaches often remove meaningful shared variance, and by treating diagnoses as unitary, do not account for the role of heterogeneity in comorbidity (i.e., heterogeneity within disorders may lead to higher comorbidity across disorders). Two, cross-sectional studies undermine our understanding of whether the externalizing pathway from childhood to adolescence represents the development of unique disorders or whether it represents different presentations of the same disease entity. Person-centered longitudinal approaches, such as Latent Transition Analysis, overcome both of these limitations. Aim 1 will establish the latent classes of externalizing symptoms present at three time points and the transitions between classes across time points. Aim 2 will examine cognitive, emotional, and motivational predictors of latent class status at each time point as well as the probability of transitioning between latent classes across time points. Exploratory aim 3 will evaluate the extent to which latent classes are correlated with functional impairments in order to examine if certain mechanisms are useful in predicting membership in, or transitions to, more impaired classes. Impact statement: This work will illustrate how key mechanisms are related to externalizing symptomatology over time, irrespective of DSM diagnosis. This will pave the way for greater etiological understanding, inform how knowledge of etiology can be utilized to refine our nosology, and improve risk identification. The proposed fellowship allows the applicant to build upon her foundational expertise in cognition to investigate other processes, such as emotion and motivation (training goal 1); advance her understanding of the theoretical and methodological approaches to examining comorbidity (training goal 2); and expand her statistical repertoire to include latent and longitudinal approaches (training goal 3). Lastly, the fellowship will provide opportunities for professional development and ethical training (training goal 4). PI Smith is a doctoral student in Clinical Science in Child and Ad...