Executive Summary Adolescent mental health in the United States has declined precipitously in the past decade. These declines in mental health have coincided with an increase in social media use, especially as accessed on smartphones. Because most existing research is correlational, however, it is unclear whether social media use causally impacts adolescent mental health. We will advance scientific and public knowledge by conducting the first longitudinal field experiment to examine the causal effects of social media on mental health in 11–14-year-olds. We propose a basic experiment in which we will recruit 500 adolescents aged 11 to 14, whose parents have decided to purchase them their first smartphone. Adolescents will be randomly assigned to use their smartphones without study-imposed restrictions on social media (naturalistic social media condition) or to use them without access to social media for three months (restricted social media condition). We are focusing on this population because research shows that most parents buy their children their first smartphones during this age range and the acquisition of one’s first smartphone is linked to greater exposure to social media. We hypothesize that adolescents in the naturalistic social media condition will report greater symptoms of anxiety and depression than adolescents in the restricted social media condition. Our experimental design will also allow us to provide evidence for the causal impact of social media on key mechanisms, providing targets for future interventions. We hypothesize that mobile access to social media will affect mental health by (A) increasing upward social comparison, (B) displacing sleep, physical activity, and in-person social interactions, and (C) interfering with the quality of in-person social interactions. Going beyond self-report measures of time spent on social media, we will use passive mobile sensing (EARS tool) to get objective, fine-grained measures of how adolescents use social media. Evidence shows that active social media (e.g., messaging, posting) use predicts better mental health, whereas passive social media use (scrolling, browsing) predicts worse mental health. We hypothesize that adolescents who use social media more actively will have better mental health outcomes. Finally, recent research suggests that while some people experience negative effects of social media use, others experience no or even positive effects. Individuals who already struggle to direct and sustain their attention should be particularly vulnerable to the distracting effects of mobile social media. We hypothesize that people in the naturalistic social media condition with greater deficits in attention at baseline will show greater increases in symptoms of anxiety and depression. We will also probe the moderating role of age, gender, and their interaction. To explore the role of a range of other moderators, we will employ an exploratory machine-learning approach, which will allow...