Sanctioning political speech on social media is driven by partisan norms and identity signaling.

Social media is marked by online firestorms where people pile-on and shame those who say things perceived to be offensive, especially about politically relevant topics. What explains why individuals engage in this sort of sanctioning behavior? We show that two key factors help to explain this pattern. First, on these topics, both offensive speech and subsequent sanctioning are seen as informative about partisanship: people assume that those who say offensive things are out-partisans, and those who criticize them are co-partisans. Second, individuals perceive that such sanctioning is an injunctive norm and believe that their fellow co-partisans approve of it-sanctioning someone allows them to signal their partisanship by adhering to that norm. Using three original experiments, we show strong support for this argument. Sanctioning this type of offensive speech is as informative about perceived partisanship as explicit partisan electioneering. Further, people perceive that a wide variety of sanctioning behaviors are (partisan) group norms. We also show that while people are reluctant to be the first to criticize someone online, they are quite willing to pile-on to others' criticisms, which helps to explain why this behavior spreads so rapidly in online firestorms. Our results have implications for online social dynamics, as well as partisanship and partisan animosity more broadly.