There are many reasons you may want to make your contributions to public debates anonymously, and there are many reasons you may want to act in solidarity with others. Why might people engaged in social movements want to do both at the same time? “Anonymous solidarity”—symbolized by a great many protestors wearing one and the same iconic Guy Fawkes mask—signifies not only solidarity (“we are as one”) but also multiplicity (“we are many”) and interchangeability of each for the other (“for every one of us who falls, ten more will take our place”). The latter two features make a movement more likely to succeed, the former by rendering it stronger and the latter by rendering it more robust. A raft of evidence shows that people are more likely to participate in collective action that is more likely to succeed, even if their own participation is in no way essential for its success.