For all that it is a modern plague, as well as a ticking time-bomb, a drain on the health service and a chastening metaphor for western greed, a lot of people would miss obesity. Without it, for example, there might be no Olympic Games in 2012, since someone has to eat the mountains of Cadbury and drink the rivers of Coke that allow these fat-creators to imprint their brands on London's fitness legacy. To say nothing of the dieticians, personal trainers, big-pant makers, cellulite dissolvers, diet book publishers, health workers, gastric band professionals, gym magnates, low-calorie food producers, health farmers, liposuctioneers, electrical fat-wobblers, Special K advertisers (special congratulations to them), all of whom depend on gluttony for a living. Where would they be if everyone got thin?

Fortunately for all the above, peak fat seems very far off. More people are getting bigger, in more parts of the world and they are, with the help of fashion and celebrity culture, made to feel so miserable about it that they are supremely easy to fleece. Gym memberships they will never use – yes please! Untested surgical procedures – ditto. Diet and exercise regimes promoted by personalities who are, themselves, fat or even dead – why not? And the fat-fleecing industry is so young.

A former investment banker, Winton Rossiter, foresees "an emerging weight incentive industry", using cash rewards for successful slimmers. His company, Weight Wins, has been in the news after the NHS Eastern and Coastal Kent PCT announced the results of a joint trial. In a series of reports, Winton Rossiter declared the pilot a success. "We are absolutely thrilled with these results, which suggest that long-term financial incentives could be the best single weapon in the war on obesity," he said. "I urge the NHS and employers to adopt such schemes widely for the benefit of their patients and employees."

And if one were Winton Rossiter, one would certainly do the same, since the NHS paid his company £75,000 for a trial which resulted in three-quarters of the 402 participants dropping out. Of those who completed the course, the average loss was more than 25lbs, which would give them, I estimate, a cash reward of at least £70 – less than the £185 cost to the NHS of their participation. For the successful candidates, there can be no certainty, after so short a time, of sustained behaviour change. From the profit point of view, given that there is no supervision in the Weight Wins deal, you can see why Mr Rossiter would be thrilled. The publicity has also been fabulous. Even the Press Association's report introduced the trial as favourably concluded: "Giving obese people cash payments to shed pounds can lead to substantial weight loss, research suggests."

But it doesn't. NHS Eastern and Coastal Kent does not intend to repeat the experiment, describing the results as "mixed". Its assistant director of public health, Claire Martin, said: "There were high drop-out rates and so it is very difficult to interpret the results to show how successful this would be across our population." That is putting it mildly. You wonder why the trust ever got its hopes or its money up. If a high proportion of dieters was not destined to fail or to give up on his scheme, then Mr Rossiter would be running a charity, not a business. And if an NHS incentive scheme can make money, why (no offence) should it go to Mr Rossiter?

For many health professionals, who have their own financial incentives to consider, these results will be depressing. If only overweight patients could be more like dogs, albeit dogs who respond to tenners as opposed to Good Boy chocolate drops. Money already works, after all, with one-off bungs for things such as chlamydia screening, vaccinations, keeping appointments. And, where weight is concerned, it seems to do the trick with supermodels. In Kent, however, patients will not get out of bed for £70. The outcome of this pilot hardly challenges other public health studies suggesting that modest financial incentives do not achieve weight loss at 18 months and that, where permanent shifts in behaviour are concerned, cash prizes should be deployed, if at all, alongside possibly more respectful approaches such as personal support, education, sustained interest in an individual.

Too often, the King's Fund reported in its 2008 Kicking Bad Habits project, "financial incentives can help individuals achieve their personal behaviour change goals, but that people tend to fall back into former behaviour patterns when the incentive is removed". Which presumably explains why, in its first statement on public health, a nervous coalition assured GPs that their long-term financial rewards would be a priority: "We will give GPs greater incentives to tackle public health problems."

Given similar inducements, who knows how many of Kent's despairing dieters might somehow have persevered with personal behaviour change? Outside the bonus-receiving classes, however, there are anxieties, even when it works, about the ethics of paying people to behave responsibly. Thousands of students, for instance, now receive £30 a week in educational maintenance allowance, purely for turning up to lessons. This can enrage their unpaid peers, who may be equally deserving of additional clothes, drinks and electrical goods. Now the coalition has pledged to reform the chip'n'bin scheme along libertarian paternalist lines, so as to redefine another element of civic conduct as especially meritorious.

"The carrot is always better than the stick," states Caroline Spelman, who must be unaware of a widespread belief, among root vegetable sceptics, that bribes can have a perverse effect. In the case of household garbage, big producers will now be encouraged with vouchers, to get yet more stuff from, say, M&S. The biggest rewards, which are paid for by everyone, could go to the least responsible. Litterers and dog owners may soon be wondering if they, too, shouldn't get prizes for not leaving their crap on the pavement or park, the carrot being so much more agreeable than the stick.

If these bribes are not sustained, councils could find that British bin-rage is – almost unimaginably – exacerbated. One unwanted consequence of financial incentives, to judge by laboratory experiments, is the undermining of people's motivation when they stop. That officials still have absolutely no idea whether or not incentives are justifiable in public policy, either practically or ethically, does not seem to have dimmed the enthusiasm for trying them out, even when funds are scarce and all the talk is of volunteering. While one part of government gleefully proclaims Coca-Cola's sponsorship of the Olympics, another, in the shape of the National Institute for Clinical Excellence, has asked its "citizens council" to consider the "use of incentives to encourage healthy living". Though why any of these citizens should bother, with neither bung nor bonus in sight, I simply cannot imagine.