The narcissism of small differences was a theory set forward by Freud, it is the idea that communities that are closer together are especially likely to engage in feuds because of hypersensitivity to details of differentiation. But such an idea lacks explanatory power. It simply tells us that what happens, but not why it happens. For the latter question, Girard’s philosophy provides a better answer.
If you have wondered why conflict between people of the same nation or family or ethnic group is so common, then you have made the intuitive assumption that conflict arises out of differences – The Clash of Civilizations thesis by Huntington tells us that conflicting paradigms are at the root of conflict. But Girard rejects this idea and provides us with his antithesis, namely, that conflict is a result not of differences, but of similarities.
Most conflicts occur within the same groups (civil war is much more common than wars between different countries, although less bloody) because members of the same group feel equally entitled to power. Democracy creates conflict because each world view is a viable alternative, and competition creates resentment because some people are excluded, not because of a natural or divine law, but because of a trivial contingency. Things could have been different. Girard would say that within the same group, the metaphysical distance has been refused, and thus, rivalry intensifies.
This does not mean that rivalry is inevitable, but it is far more likely.
According to Enzensberger, “it is generally the rule, rather than the exception, that man destroys what he most hates, and that is usually the rival on his own territory. There is an unexplained linkage between hating one’s neighbor and hating a stranger. The original target of our hatred was probably always our neighbor; only with the formation of larger communities was the stranger on the other side of the border declared an enemy.” He describes “cultivated war waged between nations and against external enemies” as an explicitly “recent development.
Girard’s Mimetic Theory
Siblings are often unconsciously aware of the closeness of their similarities. Thus, they grow to hate imitation and desire uniqueness, but since they both aspire to differentiate themselves from one another, they are even more alike. They are engaged in anti-mimetic mimesis. In the end, they are more alike that they will ever be comfortable to admit.
The point is to be aware of this psychological pattern lest you become a victim of it. Mimetic conflict is most probable when roles are exchangeable, and when minds are similar.