Smartphone use, gender, and adolescent mental health: Longitudinal evidence from South Korea

•We use longitudinal data and FE-IV technique to identify causal effects of smartphone use on adolescent mental health.•We find that longer smartphone use causes increased depressive symptoms and higher suicidal ideation for girls, not for boys.•Social and active smartphone usage such as communicating with friends and family predicts lower depressive symptoms.•Personal history of depressive symptoms and the timing of puberty moderate the effect of SUT on depressive symptoms.•Addictive smartphone behavior is frequent among adolescents, and more prevalent among girls than boys.