Treat enough people like scum and people will act like scum. Plan B’s new record about last year’s riots is well worth a listen on its merits and as social commentary.
Creating or reinforcing a positive or negative stereotype about someone will cause them to live up or down to that stereotype. Chris points us towards the Oak School experiments that showed pupils arbitrarily deemed to have high IQ subsequently did better at school. Other examples, again cribbed from Chris, show that American blacks (pdf) or low-caste Indians can be primed to live down to their negative stereotypes.
So in answer to Plan B‘s question: Why do some kids not care if they get criminal records? Why risk an easy life for some new trainers? To a degree, this is because they’ve been told that people just like them don’t care if they get a criminal record and that all they care about is a new pair of trainers (preferable hooky). Constantly demonise the working class as chavs and you will end up with part of the working class acting like demons.