Studies since the 1950s demonstrate the tendency of people to identify with whom they are grouped, no matter how arbitrary or even silly the group boundaries may be, and to judge members of their own group as superior.
This reminds me of the idea of high school where you have "the jocks", "the nerds", "the popular girls", "the other athletes" and "the nobodies". Where one group always sat themselves higher than individuals within one school. Looking at it from a bigger perspective of the world some religions, races, or sexes also find themselves superior to one another. I feel as if this issue however could have risen when society placed families in "social classes" based off their income.