HALLOWEEN is supposed to last for one night only. At Yale University (motto: “Light and Truth”) it has dragged on considerably longer. As happens at many American universities, Yale administrators sent an advisory e-mail to students before the big night, requesting them to refrain from wearing costumes that other students might find offensive. Given that it is legal for 18-year-old Americans to drive, marry and, in most places, own firearms, it might seem reasonable to let students make their own decisions about dressing-up—and to face the consequences when photographs of them disguised as Osama bin Laden can forever be found on Facebook or Instagram. Yet a determination to treat adults as children is becoming a feature of life on campus, and not just in America. Strangely, some of the most enthusiastic supporters of this development are the students themselves.