Freud's Narcissism of small differences 1): the phenomenon that it is precisely communities with adjoining territories, and related to each other in other ways as well, who are engaged in constant feuds and ridiculing each other.
This theory applies to some other conflicts such as Hells Angels vs. Bandidos, Emacs vs. Vim, Sunni muslims vs. Shia muslims, Christians vs. Protestants etc.
Old joke (which you might well believe if you've heard Protestants talking amongst themselves/ourselves):
I was walking across a bridge one day, and I saw a man standing on the edge, about to jump off. So I ran over and said, "Stop! Don't do it!" "Why shouldn't I?" he said. I said, "Well, there's so much to live for!" He said, "Like what?" I said, "Well, are you religious or atheist?" He said, "Religious." I said, "Me too! Are your Christian or Buddhist?" He said, "Christian." I said, "Me too! Are you Catholic or Protestant?" He said, "Protestant." I said, Me too! Are your Episcopalian or Baptist? He said, "Baptist!" I said, "Wow! Me too! Are your Baptist Church of God or Baptist Church of the Lord? He said, Baptist Church of God!" I said, "Me too! Are your Original Baptist Church of God or are you Reformed Baptist Church of God?" He said, "Reformed Baptist Church of God!" I said, "Me too! Are you Reformed Baptist Church of God, Reformation of 1879, or Reformed Baptist Church of God, Reformation of 1915?" He said, "Reformed Baptist Church of God, Reformation of 1915!" I said, "Die, heretic scum!" and pushed him off.
This theory applies to some other conflicts such as Hells Angels vs. Bandidos, Emacs vs. Vim, Sunni muslims vs. Shia muslims, Christians vs. Protestants etc.
1) http://en.wikipedia.org/wiki/Narcissism_of_small_differences