The “tendency for people to behave in a particular way can be strengthened or weakened depending on what they observe others to be doing.”
This makes intuitive sense to me. I even see it with dirty dishes. If a few dishes accumulate in the sink then we are more likely to let more accumulate there. Before you know it, leaving a couple of dirty dishes in the sink has lead to a sink full of dirty dishes.
When applied to policing, the idea is that by getting on top of petty crimes like graffiti, littering police can prevent other crimes. It makes intuitive sense but had not actually been proven – until now.
Read the article for details on the experiments (which were quite clever).
The first experiment involved observing people passing through an alley. Some littering was observed when the alley was freshly painted but littering was more than doubled in the same alley when the walls were tagged with graffiti.
Another experiment showed that letting off fireworks during a time when they are prohibited increased littering substantially.
In the final experiment people were twice as likely to steal €5 from a postbox if the postbox was covered in graffiti. The same increase was even observed when the postbox was clear but there was litter around it!