Just like the wicked, there is going to be no rest for Conservative MPs this summer. Courtesy of David Cameron, they have been issued with a reading list of 37 heavyweight books. Top of the list, of course, is Nudge, by the American academics Cass Sunstein and Richard Thaler, the text that Cameron and George Osborne have been enthusing about in recent weeks.

As with most of the new crop of ideas books, Nudge focuses on the foibles and idiosyncrasies of human behaviour and on how, with a little discreet encouragement, we can usually be ushered in the right direction. The problem is, for all the Tory lather, it and ideas like it have been around for some time, and are beginning to look very shaky indeed.

The notion that our behaviour can be nudged in a more propitious direction was given an enormous fillip by the publication in 2000 of Malcolm Gladwell's bestseller The Tipping Point. One of the most memorable vignettes in the book offered an intriguing answer to a puzzle that had been exercising the minds of America's social scientists. In New York there had been an abrupt and vertiginous collapse in serious crime, and no one could figure out why. Gladwell noted that the New York police department had implemented a novel approach to policing called "broken windows" theory, which floated the idea that if steps were taken to curb minor manifestations of disorder like graffiti, fare-dodging, drunken behaviour and littering, the bigger crimes would take care of themselves.

What policymakers had hitherto failed to realise was that crime moves through society like a virus - and, like most epidemics, its spread could be traced to a point of origin among relatively few people. By concentrating initiatives in specific areas and among a small core of potential offenders, he argued, the NYPD had sent a signal that more serious crimes would not be tolerated. In so doing, the police had "tipped" the problem in the right direction and turned the tide.

The idea that crime or fashion trends zip around like viruses was an arresting one and Gladwell, an expert storyteller and synthesiser of ideas, told it marvellously well. Before long the idea had itself "tipped" into the public consciousness, and tipping points were turning up everywhere from the property market to the war in Iraq. All this activity around a single idea is all the more surprising because, for all that it was talked up, there was no real evidence that it worked. Take the fight against crime. A report commissioned by the American National Research Council in 2004 found no strong support for the proposition that strategies focused on minor misdemeanours make any headway against serious crime. In the same year, the economist Steve Levitt pointed out that crime went down in the 1990s even in areas of New York where "broken windows" policing had not been tried.

In an exhaustive survey of the evidence for a 2006 article in the University of Chicago Law Review, the academics Bernard Harcourt and Jens Ludwig concluded that there "appears to be no good evidence that broken windows policing reduces crime, nor evidence that changing the desired intermediate output of broken windows policing - disorder itself - is sufficient to affect changes in criminal behaviour".

The reason for the drop in crime, the pair found, was much more likely to stem from something called "mean reversion" - those areas of New York which suffered the greatest hikes in crime in the 1980s were bound to experience declines at some point - what goes up must eventually come down.

Another of Gladwell's examples told the story of the sudden rebirth of Hush Puppies shoes as a New York fashion item after a small group of young people decided to adopt them as the height of hipster cool. The result was a torrent of so-called "viral" marketing campaigns aimed at using small groups of trendsetters to "tip" their products into the public imagination. While there have been a few notable successes, the embarrassing truth is that most of them didn't even get off the ground. The reason - as Duncan Watts, professor of sociology at Colombia University, has shown in a series of experiments - is that Gladwell overestimated the role of the small group of "connectors" he deemed crucial in influencing the spread of trends. The mundane answer why crime fell and Hush Puppies sales rose was not because of the efforts of a few influential criminals or shoe fanatics, but because changes in the culture made New Yorkers less likely to commit crime, and more likely to dress in retro chic.

The real lesson is that, while it is entirely possible to isolate the moment at which a small group turn their back on the temptations of crime, or a product bursts its way into public consciousness, it is devilishly difficult to reproduce that effect. The reason why quietly nudging things in a favourable direction seems such a good idea to those in authority is that it promises a magic bullet for social problems - at the margins and on the cheap. The tragedy is while they have been busy doing many little things in the hope that some of them might make a difference, they could just as well have been rolling up their sleeves and doing something big.

· James Harkin is the author of Big Ideas: The Essential Guide to the Latest Thinking

jamesh@ica.org.uk