Pic c/o Gems sty
You might remember the Broken Windows theory from Gladwell's Tipping Point: the idea that the way to stop big crimes is to clamp down on small ones.
How it works is this: 'broken windows', litter, graffiti and other signs of social disorder are believed to induce or encourage the other crimes, both petty & more serious ones, because they give the message that other most of the people we share a given space with aren't following the social norms that prohibit such things (what other folk do being our favourite shorthand to decision making). It's a Herd thing, of course.
While many of us feel the theory is really plausible, the evidence to support it has been – at best – debatable (Freakonomics for example suggested the clean-up of NYC worked because of a demographic change in the crime fighting and not because of the BWT strategy adopted)
So this piece by Kees Keizer in Science which details 6 field experiments makes interesting reading: Download Keizer_etal_2008_Disorder_spread
"There is a clear message for policy-makers and police officers: early disorder diagnosis and intervention are of vital importance when fighting the spread of disorder. Signs of inappropriate behaviour like graffiti or broken windows lead to other inappropriate behaviour (e.g. litter or stealing), which in turn results in the inhibition of other norms (i.e. a general weakening of the goal to act appropriately). So once disorder has sprea, merely fixing the broken windows or removing the graffiti maynot be sufficient any more. An effective intervention should now address the goal to act appropriately on all fronts"
Hat tip to Alex