"The decline of a civilization has long been linked, anecdotally, to less moral censure and a decline in manners (manners being self, usually-moral censorship)." (Brock Adams, commenting an earlier question).
My question is: what is the historical pedegree of this notion? Brock Adams has suggested "Cato, Mark Twain, Heinlein, Bismark (I think), and Churchill" but I am not sure about Twain. Cato, for certain. Gibbon also naturally comes to mind.
(To clarify my meaning, J.B. Bury had written two books about the history of ideas: "History of the Freedom of Thought (1914)" and "Idea of Progress (1920)"; I am looking for a similar history of the idea of moral decline).