It's something I've observed personally.

One example that stands out: I was playing pool in a quiet bar in a sleepy village. The next thing I know, a sixteen year old walks in and attacks another 16 year old. They are at each others throats, smashing cues on each other and throwing pool balls at each other. They are absolutely battered and bleeding by the end of it. One of the kids was the barman's son. As they finish their fight and are dragged out, another barman says to me "there's no need for that in here" and adds "you know what it's all about - who owed who money for drugs".

Just one example, but I've seen it a few times.