Symptom or Culprit? Social Media, Air Pollution, and Violence

Abstract This paper provides the first causal evidence that hostile activities online trigger physical violence. Given the recently documented relationship between pollution and social media, I exploit exogenous variation in local air quality as an instrument for online aggression. An event study analysis shows that air pollution increases by 5.8 standard deviations (SD) when refineries experience unexpected malfunctions. On pollution spike days, surrounding areas see 0.11 SD more aggressive tweets and 0.24 SD more crimes; geographically distant but socially networked regions see aggressive tweets increase by 0.008 SD and crimes by 0.015 SD. My findings highlight the impacts of online hostility and contribute to the debate on cyberspace regulation.