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A PLACE that is covered in graffiti and festooned with rubbish makes people feel uneasy. And with good reason, according to a group of researchers in the Netherlands. 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. They found that they could, by a lot: doubling the number who are prepared to litter and steal.

The idea that observing disorder can have a psychological effect on people has been around for a while. In the late 1980s George Kelling, a former probation officer who now works at Rutgers University, initiated what became a vigorous campaign to remove graffiti from New York City's subway system, which was followed by a reduction in petty crime. This idea also underpinned the “zero tolerance” which Rudy Giuliani subsequently brought to the city's streets when he became mayor.

Many cities and communities around the world now try to get on top of anti-social behaviour as a way of deterring crime. But the idea remains a controversial one, not least because it is often difficult to account for other factors that could influence crime reduction, such as changes in poverty levels, housing conditions and sentencing policy—even, some people have argued, the removal of lead from petrol. An experimental test of the “broken windows theory”, as Dr Kelling and his colleague James Wilson later called the idea, is therefore long overdue. And that is what Dr Keizer and his colleagues have provided.

The writing's on the wall

Dr Kelling's theory takes its name from the observation that a few broken windows in an empty building quickly lead to more smashed panes, more vandalism and eventually to break-ins. 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 does not necessarily mean that people will copy bad behaviour exactly, reaching for a spray can when they see graffiti. Rather, says Dr Keizer, it can foster the “violation” of other norms of behaviour. It was this effect that his experiments, which have just been published in Science, set out to test.

His group's first study was conducted in an alley that is frequently used to park bicycles. As in all of their experiments, the researchers created two conditions: one of order and the other of disorder. In the former, the walls of the alley were freshly painted; in the latter, they were tagged with graffiti (but not elaborately, to avoid the perception that it might be art). In both states a large sign prohibiting graffiti was put up, so that it would not be missed by anyone who came to collect a bicycle. All the bikes then had a flyer promoting a non-existent sports shop attached to their handlebars. This needed to be removed before a bicycle could be ridden.

When owners returned, their behaviour was secretly observed. There were no rubbish bins in the alley, so a cyclist had three choices. He could take the flyer with him, hang it on another bicycle (which the researchers counted as littering) or throw it to the floor. When the alley contained graffiti, 69% of the riders littered compared with 33% when the walls were clean.

To remove one possible bias—that litter encourages more litter—the researchers inconspicuously picked up each castaway flyer. Nor, they say, could the effect be explained by litterers assuming that because the spraying of graffiti had not been prevented, it was also unlikely that they would be caught. Littering, Dr Keizer observes, is generally tolerated by the police in Groningen.

The other experiments were carried out in a similar way. In one, a temporary fence was used to close off a short cut to a car park, except for a narrow gap. Two signs were erected, one telling people there was no throughway and the other saying that bicycles must not be left locked to the fence. In the “order” condition (with four bicycles parked nearby, but not locked to the fence) 27% of people were prepared to trespass by stepping through the gap, whereas in the disorder condition (with the four bikes locked to the fence, in violation of the sign) 82% took the short cut.

Nor were the effects limited to visual observation of petty criminal behaviour. It is against the law to let off fireworks in the Netherlands for several weeks before New Year's Eve. So two weeks before the festival the researchers randomly let off firecrackers near a bicycle shed at a main railway station and watched what happened using their flyer technique. With no fireworks, 48% of people took the flyers with them when they collected their bikes. With fireworks, this fell to 20%.

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.