IMAGINE a Briton's new year resolutions: he vows to stop smoking 20 cigarettes a day, forgo his daily bottle of claret and nightly whisky, and stop betting on football. Confronting his enlarging gut, he promises to make his ten-mile round-trip commute by bike, not car. He may even go walking at weekends.

What admirable goals. And since this gentleman's annual vice bill comes to around £7,500 ($11,750) he will be well-rewarded for his virtue even before considering the effect on his health. But the Treasury might rejoice a little less. More than half of that vice bill flows directly into government coffers. In the fiscal year 2010-11 nearly 10% of all taxes collected came from duty on alcohol, tobacco, gambling and fuel as well as from vehicle excise duty, a tax that falls most heavily on the least efficient cars.

Sin taxes have a long history as a fiscal wheeze: Parliament first introduced levies on beer and meat in 1643 to finance its fight against the Crown. Levies on alcohol have persisted: tax is now around 53p on a pint of beer, £2.18 per bottle of wine and £8.54 on a bottle of whisky. Tobacco was originally taxed as an imported luxury; today, duty on cigarettes accounts for about three-quarters of the price of a packet of cigarettes. Laziness is a harder sin to target, but one weapon against it is fuel duty: 23% of car journeys are of less than two miles, so walking or cycling are reasonable alternatives for at least some trips. Fuel taxes also target a greater ill—the exhaust fumes that contribute to global warming. Tax, including VAT, accounts for 63% of the price of petrol.

New year resolutions are notoriously short-lived. But the longer-run trend looks bad for the exchequer. Because many vices are in decline, so are receipts. That pattern will continue, predicts the Office for Budget Responsibility (OBR), the independent fiscal watchdog (see chart). Smoking rates have been falling for decades. Some 45% of people smoked in the mid-1970s; now 21% do. High taxes are one reason. So are public health campaigns, changing social mores and a smoking ban in workplaces introduced across Britain in 2007. The OBR predicts that tobacco receipts, now 0.6% of GDP, will supply half of that by 2030. The government could respond by increasing sin-tax rates. But when duties rise so do the incentives to get around them, by buying abroad or on the black market. This is particularly common with cigarettes, which are easy for individual smokers to import. In 2000 non-duty consumption reached a peak of 78%, according to the Tobacco Manufacturers' Association—a consequence of the weak euro as well as a sudden increase in taxes of inflation plus 5%. Declining gambling receipts reflect a similar problem: many operators have moved parts of their business offshore to escape high British levies. Petrol taxes are leaking more quickly. As with smoking, behaviour is changing: car and van mileage has fallen for four consecutive years, partly because petrol is so expensive. And new vehicles have better engines. In 2000 the average new car did 34.6 miles to the gallon. Last year it achieved 44. These trends, as well as the rise of electric and hybrid cars, are forecast to compress receipts from 1.8% of GDP in 2010 to just 1.1% in 2030. Vehicle excise-duty receipts are also expected to decline. Yet because oil prices have also pushed up pump prices, increasing the duty to compensate for greater efficiency is politically toxic. Under attack from newspapers and Conservative backbenchers, George Osborne, the chancellor, waived the 3p of rises planned for this financial year.

The exchequer cannot even rely on Britain's culture as a nation of boozers (see article). Drinking increased in the 1990s but has been going down since 2002, says the Office for National Statistics. Still, the OBR forecasts that alcohol duties, which accounted for £9.5 billion in 2010-11, will fall more slowly as a proportion of the tax take than tobacco or fuel duties.

There are, of course, advantages to Britons giving up their filthy habits. Smoking is the leading cause of preventable illness and premature death in Britain. It cost the National Health Service more than £5 billion a year in 2005-06, some 5.5% of its budget at the time, according to an Oxford University study. Giving up saves money as well as lives: there were 1,200 fewer emergency admissions for heart attacks in the first year after the smoking ban was introduced in England, according to a 2010 paper in the British Medical Journal. But any benefit to the NHS may be short-lived. Those who do not perish from diseases associated with smoking are likely to die more slowly of age-related illnesses.

Sin taxes are often a rather inaccurate weapon, too. Although high alcohol duties have helped to cut overall consumption, they have not stopped Britons from bingeing. Whereas smoking causes ill-health in the long-term, a single night of excess can land a boozer in accident and emergency. Alcohol-related hospital visits are typically shorter, cheaper and still less frequent than tobacco-related ones. But, worryingly, the number of drink-related admissions doubled between 2003 and 2010.

In moral terms, a decline in sin tax receipts suggests a job well done. But in fiscal terms, a hole is a hole. As the OBR sees it, falling Treasury income means Britons will be getting, in effect, an unannounced tax cut. Other taxes could therefore rise without leaving people worse off in aggregate. The maths makes sense. For the virtuous, though, being clobbered with new taxes may seem a rather poor reward.