Northern European Mutt?

What am I supposed to say when asked about my ethnicity? I don't find "American" to be a satisfying response, freighted as it is with nativist connotations, but for those of us who cannot trace our origins to any single immigrant group and who keep few or no traditions associated with a particular "old country" culture, are we supposed to just respond with befuddled silence? That is itself problematic, in that it presumes a white-bread default state. I suppose I am mostly Scotch-Irish, although my stepfather's Japanese background was probably a more obvious influence when I was growing up than anything else.