Remember from the ALCS at Yankee Stadium 2004: Riot police and chants of "Who's Your Daddy?" Not rascist, disgusting monkey sounds, but as ugly as it gets in the U.S. Quick anecdote: Sitting in the Bleachers of Yankee Stadium with a Red Sox jersey on is tantamount to a death sentence. I saw some local toughs, ugly sons-of-bitches, harrase and then attack a White Sox fan who was sitting alone. Cowardly and all done under the close eye of the NYPD who spent most of the game arresting someone for Drunk and Disorderly every half-inning. Surely not the same, but also indicative of the roots of fan violence: young, working class, men. Soft white guys from the suburbs, like me, don't want anything to do with it.
Maybe in the US, but in Europe it doesn´t really matter where you come from. I´ve seen quite a few kids from ´rich´ (a bit of an exaggeration) parents turn into something one wouldn´t quite expect. People can change and their roots or upbringing haven´t got that much to do with it. You can easily get accustomed to violence by experiencing it on a weekly basis.