Communication and Emotional Well-Being in Adolescent Best Friend Dyads: An Experience Sampling Method Study - ABSTRACT Adolescence is a critical period for the onset of depressive symptoms. During the transition from childhood to adolescence, friends become increasingly salient conversation partners and a primary source of social support. Notably, evidence now suggests that adolescent friends’ levels of depression can predict change in each others’ levels over time, in a process called ‘depression contagion’. The current project aims to advance knowledge on the process of depression contagion in adolescence by studying granular, and potentially modifiable interpersonal processes that may contribute to depressive contagion, as well as factors that may moderate risk for contagion, with an eye toward informing development of preventative interventions. Using an experience sampling design in the natural environment of best friend dyads, we propose four aims: (1) examination of momentary contagion of affective states between best-friend dyads, with a focus on dysphoric and positive affect, as those most strongly relate to depressive conditions; (2) examination of friends’ communication in both online and offline contexts as mechanisms driving contagion, considering communication in the context of negative (co- rumination) and positive (co-dampening) affect and experiences as mediators of the contagion process; (3) investigation of individual difference characteristics as potential moderators (sex, empathetic distress, empathetic joy) of affective contagion, to identify factors which create vulnerability; and (4) to conduct a preliminary examination of the extent to which affective contagion may mediate change in depressive symptoms within a brief longitudinal design. To achieve our aims, we will first do a depression screening in classrooms in grades 9-12 to obtain a sample with varying levels of depressive symptoms. Based on screening levels, one hundred dyads (n = 200) of true best friends in grades 9-12 will be signaled on their smartphone daily for 14 days to answer questions about thier affective states (depressed and happy affect) since their previous report, contact with best friend since previous report, and in instances in which contact occurred, context of contact and ratings of co-rumination and co- dampening since previous report. They will further complete two assessments (baseline and follow-up after 7 weeks). A rigorous dyadic approach is used in both the design and analysis plan. Specifically, data will be analyzed using longitudinal actor-partner interdependence model, allowing the capture of state-level, momentary changes in both friends due to contagion. This project has the potential to offer unprecedented insights which will be crucial to better characterize the process of depression contagion. Furthermore, it can provide more precise and nuanced understanding of interpersonal friendship behaviors for tailored, “just in time” prevention and intervention efforts.