Kees Keizer and his colleagues at the University of Groningen deliberately created such settings as a part of a series of experiments designed to discover if signs of vandalism, litter and low-level lawbreaking could change the way people behave.
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The most dramatic result, though, was the one that showed a doubling in the number of people who were prepared to steal in a condition of disorder. In this case an envelope with a €5 ($6) note inside (and the note clearly visible through the address window) was left sticking out of a post box. In a condition of order, 13% of those passing took the envelope (instead of leaving it or pushing it into the box). But if the post box was covered in graffiti, 27% did. Even if the post box had no graffiti on it, but the area around it was littered with paper, orange peel, cigarette butts and empty cans, 25% still took the envelope.

The researchers’ conclusion is that one example of disorder, like graffiti or littering, can indeed encourage another, like stealing. Dr Kelling was right. The message for policymakers and police officers is that clearing up graffiti or littering promptly could help fight the spread of crime.