IT HAS long been the subject of speculation among the police and criminologists: what would happen if all the officers who now spend so much of their time taking statements, profiling criminals and moving pieces of paper around were suddenly put on the streets? Crime figures released by London's Metropolitan Police this week provide the best answer yet.

Following the bombings of July 7th and 21st, thousands of police officers materialised on London's pavements, many of them sporting brightly coloured jackets. Drawn from all over the city, they were assigned to guard potential targets such as railway stations. The police presence was especially heavy in the bombed boroughs: Camden (which was struck three times), Hammersmith and Fulham, Lambeth, Tower Hamlets, Westminster and the City of London.

The show of force did not just scare off terrorists. There was less crime in July than in May or June, which is unusual: the warmer month tends to bring out criminal tendencies, as windows are left open and alcohol is imbibed alfresco. But the chilling effect was much stronger in the six boroughs that were targeted by terrorists. There, overall crime was down by 12% compared with July 2004. In inner London as a whole, crime fell by 6%. But in outer London, where the blue line was thinner, it went up slightly.

Simon Foy, who tracks such trends at the Metropolitan Police, says that crime fell particularly steeply on the days of the attacks, partly because of the overwhelming police presence and partly because “even criminals were watching their televisions”. What is significant is that crime barely rose thereafter. That was a change from the aftermath of September 11th 2001, when crime quickly soared just about everywhere—possibly because officers were deployed only in the very centre of London.

“The received wisdom among criminologists is that marginal changes in visible patrolling have little or no effect on crime,” says Mike Hough, a criminologist at King's College London. July's experiment should put that argument to rest. Even if offenders do not make rational calculations about the odds of being caught—which was low both before and after the bombings—they will be moved by a display of overwhelming force.