Radical flank effects (RFEs) are interactive processes involving radical and moderate factions of social movements and third parties outside those movements. They result in detrimental and/or beneficial impacts of radical group actions upon the reputations and effectiveness of more moderate collective actors – typically social movement organizations. The relative “radicalism” or “moderation” of actors is generally defined in terms of the degree of legitimacy that is imputed to their objectives, rhetoric, and tactics by relevant external audiences. Radical flank effects were first studied systematically by Haines in his investigations of the American civil rights/Black Power movements, but other scholars had made reference to RFE-like phenomena in earlier works on the civil rights, feminist, labor, and anti-nuclear movements.