the ones I've witnessed were usually initiated by someone wearing not-so-cheap clothes and jewellery. (Maybe rich people are more spoiled, i.e. more used to getting their way, and therefore more likely to get resentful when they don't, or something.)

Where I live, in my experience, the most dangerous neighbourhood (for young men) is the one occupied by many middle-class teenagers. The dangerous ones are the ones who have something to prove about how tough they are. They also make a lifestyle out of pretending to live in american ghettos, and simultaneusly pretending to be wealthy. I was friends with these people growing up. They are entertaining and scary.

We don't have american-style violent ghetto-dwellers here, though.