Is the GOP becoming a dysfunctional chatroom? In economics, Gresham’s law on currency markets holds that “bad money drives out good.” That principle also applies to the comment sections on online sites. In comments sections — including such mega-versions like Twitter — the nastiest commenters post more, and more obnoxiously, than the decent ones until, eventually, the decent folk just decide not to hang out anymore. The only remedy for this is comment moderation, where grown-ups in charge try to thwart the trolls lest they lose their more valuable customers. In Tim Miller’s book, “Why We Did It,” the former...