A health educator who has spent decades trying to persuade teenagers not to smoke recently told me why she was close to despair. Hammering home the message that cigarettes cause lung cancer and heart disease seemed to have little or no effect, she said. The whole thing was tied up with fashion - with cultural dynamics and ideas of individual and group identity. And it was cyclical.

“As soon as smoking goes out of fashion in a school,” she said, “then a handful of trendsetters will take it up, precisely because nobody else is doing it. Then the whole thing will start again.”

Her words have an awful ring of truth to them. After all, that’s what happened with Hush Puppies. The brushed suede shoes had been all but dead as a brand. The manufacturer was even thinking of phasing them out. But sometime in the mid-Nineties, word started to get back that the shoes had become hip in the clubs and bars of downtown Manhattan.

Shortly after that, something extraordinary and quite unexpected occurred. Almost overnight, demand for Hush Puppies went through the roof. Suddenly, every fashion-conscious American wanted to be seen wearing them.

The brand, says Malcolm Gladwell, had reached its tipping point. We might not have heard it put quite that way before, but we know what he means almost instinctively. These things always start with a select few. They grow almost imperceptibly for a little while, and then suddenly it’s as though they have reached critical mass. Within a few months, we are all eating it, buying it, humming it or wearing it.

But how does it happen? How can we make it happen when we want it to? And how, when the consequences are undesirable, can we prevent it happening?

You might be forgiven for supposing that the processes involved are so complex as to defy analysis. Yet by applying a few simple principles, Gladwell seems able to dismantle them. What’s even more impressive is the way this staff writer for the New Yorker then lays out the parts and labels them in a language that is entirely accessible to the ordinary reader.

How did the makers of Sesame Street succeed in creating a learning epidemic among small children? What techniques did the makers of cult footwear for skateboarders employ when they set out to sell their trainers to millions of non-skateboarders? Why, in a remarkably short space of time, did New York shift from being one of the most dangerous cities in the United States to one of the safest?

Trends, concpts and products spread through populations in much the same way as viruses, says Gladwell. And provided that certain pre-conditions are met, then astonishingly small changes in circumstances can tip the process over the edge and trigger the sudden and massive growth which is evident in these and countless other examples.

This surprising and delicate mechanism explains how a school can suddenly run out of control, how teenage girls can all at once begin dressing like Barbie dolls and how young boys in Micronesia can one day begin to commit suicide in alarming numbers.

On the positive side, it explains how a policy of removing graffiti the instant it appears can help deter criminal behaviour, how a needle-exchange service can be utilised to educate drug users and even how students at a college can be persuaded to overcome their inertia and take part in a voluntary vaccination programme against tetanus.

When new ideas suddenly rage through a population with an apparent will of their own, certain types of individual have usually been involved at key stages along the way, says Gladwell. And he describes the Connectors (they know everybody), the Mavens (they know everything) and the Salesmen (they can sell anything). He introduces several of these exceptional characters in thumbnail sketches (have fun spotting friends and colleagues).

His explanation brings in the Law of 8020 (in any given situation, 20 per cent of people will put in 80 per cent of the work), the Law of 150 (it seems to be the maximum number of individuals with whom we are capable of having a genuine social relationship) and a variation of the principle of Six Degrees of Separation (whereby a very small number of people - the Connectors in our midst - can be linked to everyone else in a very few steps).

Much of what happens, says Gladwell, depends on timing. It depends on the disruption of finely-balanced systems, and on small but critical variations in circumstance. Above all, it is frequently counter-intuitive.

Take smoking. Why do teenagers do it? Certainly not through ignorance of the health hazards, he suggests. In tests, many smokers actually over-estimated the damaging effects of the habit on their health. So if dire warnings from adults, banning of advertising and draconian price increases only have the opposite effect, is nothing to be done?

Gladwell believes that in this as in other fields, relatively minor intervention in specific areas might achieve dramatic and unexpected results. It’s all a question of reaching that tipping point.

DAVID NEWNHAM