The whole "Scots-Irish" thing is, IMO, really overdone. It MIGHT be there if you really want to reach for it, but I honestly don't think that anything Rosen is talking about is especially "unique" to Americans. If anything I might take the position that our reputed bellicosity has its roots in the (possibly perceived) existence of outside threats to the existence of the nation and from emulating the example of the British in many ways.