Sydney Morning Herald:

A flyer for a non-existent restaurant was attached by a rubber band to the handlebar of every bicycle parked there. The question was what the bicycle owner would do with the flyer once he or she returned to their machine. (The flyer had to be taken off to ride the bicycle away, and there was no litter bin in the alley.) Would they drop it on the ground, or take it with them?

The first experiment was conducted when the walls of the alley were clean, and again when they were liberally daubed with graffiti by the researchers. Broken-window theory predicts that the graffiti would induce some bike riders who might otherwise pocket the flyer to drop it on the ground. This is what happened. Where there was no graffiti, 33 per cent dropped the flyer. But when the graffiti was added, this number went up to 69 per cent.

In another study, the graffiti was replaced by fireworks being let off illegally in the distance. Once again, littering increased significantly, against the background of illegal activity. In other experiments, forbidden activity increased a lot when other people's bikes were illegally chained to a fence, and (this one in a car park) when supermarket trolleys were left standing around and not returned to the proper place. In each case, the existence of some form of disorder seemed to encourage a hefty proportion of people to breach standards of behaviour that would otherwise have been observed.

The final experiments involved leaving an envelope hanging out of a mailbox with a €5 note visible in the window where the address normally appears. When the mailbox was clean, 13 per cent of passers-by stole the envelope. When graffiti was applied to the box, this proportion went up to 27 per cent. When the graffiti was removed and litter placed around the box, the proportion of thefts was 25 per cent.

The researchers conclude: "The most likely interpretation of these results is, as before, that one disorder (graffiti or littering) actually fostered a new disorder (stealing) by weakening the goal of acting appropriately."

If crime didn't encourage more crime, there wouldn't be any such thing as looting nor mob violence.