IN 1937 George Orwell suggested that "changes of diet" might be more important than "changes of dynasty or even of religion." Now he is being proved right in a way he might not have expected. Having spent millennia worrying about not having enough food, mankind's main concern is now eating too much (see our special report on obesity). The story of human health in the past few decades is a broadly encouraging one. Life expectancy has increased - globally, by 12 years for women and 11 years for men from 1970 to 2010. But greater longevity means that people spend more years chronically ill (see article). Obesity makes things worse by raising the risk of diabetes, heart disease, strokes and some cancers. In much of the world, being too fat is now the single largest driver of sickness. In 2008 obesity rates were nearly double those of 1980. One in three adults was overweight, with a body-mass index (BMI) of 25 or more (at least 77kg for a man 175cm tall); 12% were obese, with a BMI of at least 30. In America, ever the world leader, about two-thirds of adults were overweight in 2008. But Britain lumbered close behind, with six in ten too fat. The problem is not confined to rich countries. Thanks to economic growth, people around the world are eating more food. Workers burn fewer calories at their desks than in the fields. Even in China, one in four adults was too fat in 2008. In Brazil more than half were. Obesity rates in Mexico, Venezuela and South Africa matched those of America. The Pacific islands and Gulf states are home to some of the world's fattest people. For those (like this newspaper) who believe that the state should generally keep its nose out of people's private affairs, obesity presents a quandary. "A millionaire may enjoy breakfasting off orange juice and Ryvita biscuits," Orwell pointed out; "an unemployed man doesn't...You want to eat something a little bit tasty." If people get great pleasure from eating more than is good for them, should they not be allowed to indulge themselves? After all, individuals bear the bulk of the costs of obesity, quite literally. They suffer at work, too: their wages are often lower and, in America, some employers also make fat workers pay more for health insurance. Yet in most countries the state covers some or most of the costs of health care, so fat people raise costs for everyone. In America, for instance, a recent paper estimated that obesity was responsible for a fifth of the total health-care bill, of which nearly half is paid by the federal government. And there are broader social costs. The Pentagon says that obesity is shrinking its pool of soldiers. Obesity lowers labour productivity. And state intervention is justified where it saves people from great harm at little cost to themselves. Only zealots see seat-belt laws as an affront to personal liberty. Anti-smoking policies, controversial at first, are generally viewed as a success. Whose fault is fat? Obesity is, at its heart, the result of many personal decisions. But the rise of obesity - across many countries and disproportionately among the poor - suggests that becoming fat cannot just be blamed on individual frailty. Millions of people, of all cultures, did not become lazy gluttons at the same time, en masse. Broader forces are at work. The government can try to influence them by discouraging overeating. Drugs and surgery can help in the most extreme cases. They do not, however, offer a solution to the wider problem. Economists, faced with behaviour they don't like, tend to favour imposing "sin" taxes. But eating fatty and sugary foods is not a "sin," even in the fiscal sense, for unlike cigarettes, fatty foods are not uniformly unhealthy. Moreover, since poor people spend a higher proportion of their income on food than rich people do, such a tax would be regressive. It would also be an administrative nightmare, as the fat content of each item of food would have to be measured. Denmark, which imposed a fat tax in 2011, abandoned it after a year. In the absence of a single big solution to obesity, the state must try many small measures. Governments, some of which already intervene a lot in the first few months of people's lives, should ensure that parents are warned of the dangers of overfeeding their babies. Schools should serve nutritious lunches, teach children how to eat healthily and give them time to run around. Urban planners should make streets and pavements friendlier to cyclists and pedestrians. Taxing sugary fizzy drinks - which unlike fatty foods have no nutritional value - and limiting the size of the containers in which they can be sold may work. Philadelphia and New York, for example, have implemented a range of such policies, and have seen child-obesity rates dip ever so slightly. There is a limit, however, to what the state can or should do. In the end, the responsibility and power to change lie primarily with individuals. Whether people go on eating till they pop, or whether they opt for the healthier, slimmer life, will have a bigger effect on the future of the species than most of the weighty decisions that governments make.