It is not often that Boris Johnson and Gordon Brown see eye to eye, but the Labour prime minister and the new Conservative mayor of London seem to have found something in common. Brown declared earlier this year that “our message to thugs must be zero tolerance”. And Johnson is embarking on what has widely been described as a zero tolerance approach to crime in the British capital.

Both men have been inspired by the startling drops in American crime rates recorded during the past decade by a number of urban police forces that have based their strategies on the theory that cracking down on minor offences has a knock-on effect that reduces crime.

The statistics are unarguable. In 1990 there were 2,245 murders in New York; last year there were 494 — the fewest in the city since reliable police statistics became available in 1963. By cracking down on seemingly minor villains — such as subway fare-dodgers and street-corner squeegee bandits — Rudolph Giuliani, the former mayor, and William Bratton, his police chief, appeared to engineer one of the greatest social turnarounds in American urban history.

So dramatic has been the change in New York’s fortunes that some of the city’s crime novelists, who previously relished the gritty lawlessness of Harlem and the Bronx, have had to write about terrorism and Wall Street skulduggery instead. A recent collection of New York essays worried that the once raucous metropolis was being tamed by the ultimate horror — “the forces of suburbanisation”.

With London reeling from a wave of knife crime and other antisocial behaviour, it should not be surprising that Johnson and Brown are looking to America for advice. Yet a curious transatlantic disconnect is beginning to emerge between British enthusiasm for zero tolerance and American studies that are casting serious doubts on the importance of its role in combating crime.

“There’s no detectable evidence that\mattered at all,” claimed Professor Jens Ludwig, co-director of the University of Chicago Crime Lab. “The drop in US crime was experienced by many cities that never changed their policing methods.”

Some of America’s leading criminologists have been publicly feuding over who or what was most responsible for the plummeting US crime rate — and how much credit should be attributed to the so-called “broken windows” theory that inspired Giuliani and Bratton.

It was in 1982 that George Kelling and James Wilson published a groundbreaking article in Atlantic Monthly magazine entitled Broken Windows: The Police and Neighbourhood Safety. Their argument was compellingly simple: if a broken window was left unrepaired, it would suggest that no one cared or no one was in charge.

Soon someone would break into the house. Eventually that neighbourhood would have a crime problem.

Wilson, now 77, has retired, but Kelling, a professor at Rutgers University in New Jersey, remains a passionate advocate of focusing on smaller offences in order to prevent bigger ones. He now runs a private consultancy that advises police forces on crime prevention techniques.

Yet when I spoke to Kelling last week he had little good to say about zero tolerance, which he regards as a dangerous misnomer that misrepresents his original theory.

“The point about broken windows is that the police should be highly discretionary,” he said. “It’s not just a case of arresting everyone and throwing them in jail. You need to educate some offenders, warn others, arrest a few. You need vigilance, not stridency. Zero tolerance is not realistic and not good policing.”

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