PROJECT SUMMARY/ABSTRACT Adolescents & young adults (AYA) accounted for >400,000 new HIV infections in 2021 worldwide. Heavy alcohol use is a prevalent and strong modifiable risk factor for HIV transmission risk behavior in Uganda. Studies from in high-income countries show that AYA often misperceive peer norms around health and health risk behavior and that these misperceptions increase risk behavior and impede health behavior. While these perceptions are modifiable risk factors, few studies of AYA in southern & eastern Africa describe how misperceptions of peer norms drive alcohol use and HIV/STI transmission risk and prevention behavior and associated health outcomes nor address how to change misperceived norms. The central hypotheses of this application are that AYA in rural Uganda frequently misperceive peer norms about alcohol consumption and HIV/STI prevention; that these misperceptions affect their own propensities to engage in HIV/STI transmission risk behavior; and that perceived norms are malleable when AYA receive information about actual peer norms that conflicts with personal perceptions and behavior. These hypotheses have been formulated based on preliminary data showing that AYA in rural Uganda misperceive norms about alcohol use and HIV/STI transmission risk behavior, a finding that is consistent with other preliminary data demonstrating similar phenomena among adults in this context. The preliminary data are also consistent with published trials from the U.S., UK, and elsewhere showing that AYA misperceptions can be altered through “personalized normative feedback” and other norms correction interventions wherein they are provided with correct information about actual norms. The team has engaged local leaders & educators in Mbarara, Uganda to propose a population- based cohort study, in which they will embed a randomized controlled trial, of AYA in 3 secondary schools to achieve three specific aims: 1) Use qualitative methods to explore perceived norms around alcohol, HIV/STI transmission risk behavior, and HIV/STI prevention among AYA in Mbarara, Uganda; 2) Estimate the causal effects of perceived norms around alcohol use and HIV/STI transmission risk behavior on HIV/STI prevention among AYA; and, 3) Conduct a mixed-methods, randomized controlled trial to test the preliminary efficacy and mediating mechanism of a personalized normative feedback intervention on behavioral intentions, heavy alcohol use, and HIV/STI prevention among AYA. The key innovation is that this study will be the first to rigorously estimate the causal influence of perceived peer norms around alcohol and HIV/STI transmission risk behavior on HIV/STI prevention among AYA in Africa. The study will have significant public health impact by laying the foundation for strategies to focus on changing misperceived norms to influence HIV/STI prevention uptake among AYA throughout eastern & southern Africa.