This paper uses a novel experimental design to study the contagion of pro- and anti-social behavior and the role of social proximity among peers. Across systematic variations thereof, we ﬁnd that anti-social behavior is generally more contagious than pro-social behavior. Surprisingly, we also ﬁnd that social proximity ampliﬁes the contagion of anti-social behavior more strongly than the contagion of pro-social behavior. Anti-social individuals are also most susceptible to the behavioral contagion of other anti-social peers. These ﬁndings paired with the methodological contribution inform the design of eﬀective norm-based policy interventions directed at facilitating pro-social behavior and reducing anti-social behavior in social and economic environments.