Most research on trends in physical aggression has shown declining levels among adolescents during the past two decades. However, few studies have attempted to explain such time trends. Based on two representative cross-sectional surveys of students in the final year of high school in 2007 (N = 6631; 58.8% girls) and 2015 (N = 4145; 60.3% girls), this study reports a substantial decline in physical aggression among Norwegian adolescents. Moreover, mediation analyses show that declining levels in problematic alcohol use and family violence during the same period are plausible explanations for some of this reduction. The results are discussed in light of contemporary changes in socialization of adolescents, and implications for violence prevention are presented.