When the world was a simpler place, the rich were fat, the poor were thin, and right-thinking people worried about how to feed the hungry. Now, in much of the world, the rich are thin, the poor are fat, and right-thinking people are worrying about obesity. Evolution is mostly to blame. It has designed mankind to cope with deprivation, not plenty. People are perfectly tuned to store energy in good years to see them through lean ones. But when bad times never come, they are stuck with that energy, stored around their expanding bellies. Thanks to rising agricultural productivity, lean years are rarer all over the globe. Modernday Malthusians, who used to draw graphs proving that the world was shortly going to run out of food, have gone rather quiet lately. According to the UN, the number of people short of food fell from 920m in 1980 to 799m 20 years later, even though the world's population increased by 1.6 billion over the period. This is mostly a cause for celebration. Mankind has won what was, for most of his time on this planet, his biggest battle: to ensure that he and his offspring had enough to eat. But every silver lining has a cloud, and the consequence of prosperity is a new plague that brings with it a host of interesting policy dilemmas. As a scourge of the modern world, obesity has an image problem. It is easier to associate with Father Christmas than with the four horses of the apocalypse. But it has a good claim to lumber along beside them, for it is the world's biggest public-health issue today—the main cause of heart disease, which kills more people these days than AIDS, malaria, war the principal risk factor in diabetes heavily implicated in cancer and other diseases. Since the World Health Organisation labelled obesity an 'epidemic' in 2000, reports on its fearful consequences have come thick and fast. Will public-health warnings, combined with media pressure, persuade people to get thinner, just as they finally put them off tobacco? Possibly. In the rich world, sales of healthier foods are booming (see survey) and new figures suggest that over the past year Americans got very slightly thinner for the first time in recorded history. But even if Americans are losing a few ounces, it will be many years before the country solves the health problems caused by half a century's dining to excess. And, everywhere else in the world, people are still piling on the pounds. That's why there is now a consensus among doctors that governments should do something to stop them. The author write this passage mainly to ______.