When Rudy Giuliani was mayor of New York City, he had a zero-tolerance policy on graffiti, litter, and broken windows, on the assumption that small crimes like vandalism create an environment conducive to more serious crimes.
Now, a study in the Netherlands published in Science provides support for this theory. In an alley used to park bicycles, the experimenters set up two conditions, one in which the walls of the alley were freshly painted and one in which they were tagged with graffiti. Flyers advertising a bicycle shop were attached to the handlebars of all parked bicycles. In the graffiti condition, 69% of bicyclists dumped the flyer on the ground as litter; in the clean condition, 33% littered. They performed several other similar experiments, and in each case the test subjects were more likely to engage in anti-social acts such as littering, trespassing, and stealing in the condition of disorder as opposed to in the condition of order.
(Via The Economist, where you can read more details of the experiments.)