This chapter explores a case study in contemporary fanaticism. We adopt Katsafanas’s conceptualization of fanaticism to make possible an in-depth discussion of and evaluation of a diffuse but important social movement — the anglophone manosphere. According to Katsafanas, fanatics are fruitfully understood as members of a group that adopts sacred values which they hold unconditionally to preserve their own psychic unity, and who feel that those values are threatened by those who do not accept them. The manosphere includes several social (...) movements, including incels, men’s rights activists, and men-going-their-own-way. We show that many members of these groups are well-characterized by Katsafanas’s theory of fanaticism, and that they have forged their fanatical group identity using tools that philosophers have tended to associate with more benign social movements and the task of “conceptual engineering.” Indeed, we think that manosphere groups have in many cases been more successful than more palatable liberationist movements, and that this case study in the dark side of conceptual engineering may be both sobering and informative. (shrink)