I would say, that based on my experience on growing in Poland (I was born in 1998), the people become richer and suddenly everybody now seems to be minding own business. So after you fill some economic needs, the will to pick on people decreases. But that will is surely dependent on how much you trust others are think similar (and won't rob you).
Anyway, the parents' commenter point was > near perfect "ethnical cohesion", doesn't stop crime or increase trust in any way
I can't see why should it be true. Perhaps it doesn't stop crime completely, but perhaps it actually increases trust. Based on the argument above we can't know.