Have you ever heard of the broken window effect?
There's this theoretical neighbourhood--it's clean and pretty and everyone's friendly. But then, through some accident, a window gets broken. It's just a little crack, and the homeowners can't afford to fix it, so they leave it be. What harm can it do? But then the next door neighbour sees it and lets his grass get a little too long--after all, it doesn't look so bad next to the broken window. Then the neighbour on the other side doesn't bother to fix her cracked driveway, because what does it matter if her neighbours haven't cut their grass or fixed their windows? Pretty soon the whole neighbourhood spirals out of control. We've seen it happen.
But did you know that the broken window effect also works in reverse?