I don't know if there is any point in being serious here. It is my experience that people have and instinct to form groups with like people. But they have to learn what the similarities and differences are.

To any or all of these can be the deciding factor, Politics, Income. religion, common interests, mutual dislike of others, etc. Different people put different weights on each factor. Also we can easily change our groups and loyalties. Take two otherwise identical young men, who meet in a bar, one a sailor, another a marine. Chances are there will be prejudice between them, even though just a few years before, nothing would have prevented them from becoming friends, unless they were in different schools or cheered for different football teams or came from rival towns.