To chip in one last pair of cents on this subject, I can't help but feel that, at least with the origins of the middle finger, the anal intercourse threat must be a contributor. After all, the digit of preference for "getting your oil checked" back home in the Blue Ridge Mountians was of course the long probe of the middle finger. In fact, no other finger was really considered, making the "bird" an eminent threat of anal impalement. Now, to engage in some speculative armchair ethnography here: The Scots are Celtic people. Thus, they were at one time occupied by the Roman empire, and absorbed some of their culture. These Scots then settled the Appalachian Mountains, bringing those celto-roman folkways with them. Thus, there is a direct cultural lineage between the ass-banditry of Appalachian teens and the Digitus Impudicus