The narcissism of small differences (German: der Narzissmus der kleinen Differenzen) is the thesis that communities with adjoining territories and close relationships are especially likely to engage in feuds and mutual ridicule because of hypersensitivity to details of differentiation. The term was coined by Sigmund Freud in 1917, based on the earlier work of British anthropologist Ernest Crawley. In language differing only slightly from current psychoanalytic terminology, Crawley declared that each individual is separated from others by a taboo of personal isolation, a narcissism of minor differences.
I heard Christopher Hitchens mention this on a YouTube video I had on in the background yesterday. I had never heard of the term before but I had long been aware of the phenomenon. When we did Eat Your Way Around the World in London all those years ago, we developed a rule of thumb that if the food from two different national restaurants was, to our naive palates, indistinguishable the two countries (India and Pakistan or Greece and Turkey) would be at daggers drawn.