PROJECT SUMMARY/ABSTRACT Sluggish cognitive tempo (SCT) is a set of behavioral symptoms characterized by excessive daydreaming, slowed thinking, and mental confusion and fogginess. It is now established that SCT can be reliably measured across parent, teacher, and self-report ratings and is distinct from other psychopathology dimensions including ADHD and internalizing symptoms. A rapidly growing body of research also demonstrates SCT to be strongly associated with functional impairment, above and beyond other psychopathologies. However, SCT remains absent from current models of psychopathology, in large part because the field lacks rigorous longitudinal research examining SCT in relation to other psychopathologies. In cross-sectional studies, SCT symptoms are consistently and strongly associated with internalizing symptoms. Preliminary findings also document associations between SCT and increased suicide risk. Importantly, our pilot data show SCT predicts increased internalizing symptoms rather than the reverse. Further, SCT symptoms uniquely predict internalizing problems and not externalizing behaviors, suggesting that SCT may be a unique factor in understanding the development of internalizing problems specifically. Yet studies linking SCT to internalizing symptoms in youth are limited in several ways, including: (a) use of cross-sectional designs that preclude establishment of temporal associations, (b) using convenience samples (e.g., ADHD) rather than a sample enriched for SCT specifically, (c) failing to examine possible mechanisms or vulnerabilities linking SCT to internalizing symptoms, and (d) focusing on school-aged children even though SCT symptoms and internalizing problems sharply increase in adolescence. This study will address these limitations by using a prospective longitudinal, multi-informant, multi-method design across the developmentally sensitive period of early adolescence to examine SCT symptoms as a predictor of diverse internalizing outcomes and to test mechanisms and vulnerabilities linking SCT to internalizing symptoms in a community sample enriched for SCT symptomatology. Specifically, a community-based sample of 330 young adolescents (ages 10-12 years) enriched for SCT symptomatology will be recruited and assessed at three timepoints one year apart. Consistent with the NIMH Research Domain Criteria (RDoC) initiative and a developmental psychopathology framework, a multi-informant, multi-method battery that cuts across physiological, behavioral, and self-report units of analysis will be used. We will examine dimensional SCT symptoms as a predictor of internalizing psychopathology change over time, test mechanisms of the longitudinal relation between SCT and internalizing psychopathologies, and explore vulnerabilities (physiological reactivity, punishment sensitivity) that exacerbate these longitudinal relations. Findings establishing longitudinal effects and identifying mechanisms and vulnerabilities that cut across...