PROJECT SUMMARY/ABSTRACT Youth with anxious solitude (AS), the most common form of social withdrawal, remain alone in familiar social contexts (school) more often than peers due to social anxiety. Youth AS forecasts profound impacts on adult mental health (anxiety, depression) and delays in adult developmental milestones for participation in family and society (participation in higher education/work; achieving a career, income, romantic partnership, procreation). We will investigate these patterns in a young adult follow-up study of the 230 participants from the NIH- supported longitudinal study Multiple Trajectories in Anxious Solitary Youth. This sample was drawn from US public schools and will be age 23-26 at the time of the proposed study. This sample is unique because 1) half of participants were originally selected for AS and the other half were demographically-matched comparison children, 2) AS was assessed with school-based peer reports. Our investigation will connect 5 yrs of existing data on psychosocial development in 3rd-7th grade to new multi-informant data in young adulthood (age 23–26). Aim 1: Test youth AS trajectories (3rd-5th grade) as predictors of young adult adjustment (1 wave: 23-26). We hypothesize youth high and increasing AS trajectories will predict young adult maladjustment, including poor quality relationships, affective-behavioral continuity, social anxiety, delayed developmental milestones. Aim 2: Test gender differences in the likelihood that youth AS trajectories predict young adult adjustment. We hypothesize that youth high and increasing AS trajectories will predict pervasive detriments to young adult adjustment (specified in Aim 1) for both sexes (no gender moderation), in contrast to previous research downplaying detriments to women's societal participation. However, marriage and procreation delays may occur only in men with youth high-increasing AS trajectories. Aim 3: Test interpersonal processes as links between youth AS trajectories and young adult adjustment. We hypothesize that youth peer exclusion will link (mediate) youth high and increasing AS trajectories and young adult maladjustment because it limits opportunity for interpersonal learning processes critical to healthy development. We will also test as mediators additional forms of youth and young adult interpersonal adversity (relationship quality), related interpersonal learning and self-development processes. Results promise to 1) update knowledge of adjustment in American young adults with youth histories of AS; 2) facilitate recognition of developmental challenges in young women with youth AS, counteracting the tendency to downplay impacts on women's societal participation; and 3) generate new knowledge of interpersonal processes that link youth AS to adult adjustment. Knowledge of interpersonal processes promises to inform prevention efforts and enable families, educational institutions, and workplaces to foster the inclusion, health, and devel...