I'm glad this thread came up because something has been on my mind for a while...
The Yanks! Looking at films etc. (as have never been there), it seems they are the politest people in the world. 'Yes/No Sir' and 'Yes/No Maam' to elders, and the like, and no doubt doing all the good things mentioned in this thread, BUT...
They have one of the highest incidences of gun crime in the world!! The bit in 'LA Story' where Steve Martin is at an ATM and someone toting a gun comes up to him and politely introduces themselves as his 'mugger' and relieves him of his cash is humourously indicative of this sharp polarity...
Perhaps it is sincerity that has gone out of the window, rather than manners