Most existing studies have focused on analyzing peer effects in pro-social or anti-social behavior in isolation. Our principal contribution is the use of a novel experimental approach that enables us to analyze the contagion of behavior among individuals and groups in both domains and across different levels of social cohesion. Overall, anti-social behavior is found to be more contagious, while social cohesion reduces its impact. As a gloomy outlook, we find that a barrel of good apples is easily spoiled, while the reverse is particularly difficult to achieve and behavior quickly deteriorates to an overwhelmingly anti-social steady state. The results yield policy implications concerning designing effective nudges and interventions to facilitate (reduce) pro- (anti-)social behavior, in both social and economic environments.