The percentage of overweight American children and teens has more than doubled in the past decade. Moreover, two-thirds of the adults are either overweight or obese, and at least 300,000 Americans die each year from obesity-related diseases. The reasons aren't hard to fathom. The U.S. food industry aggressively markets high-fat, high-sugar, super-sized foods. Modern communities encourage driving rather than walking. Physical education is being dropped by schools even as fattening snack foods are welcomed onto campus. And children are spending more time playing computer games than sports. The question for policymakers is whether the weight gain is just a matter of individual responsibility, or whether a society that makes it so easy to get fat should be retooled. At stake is not only the health of millions of Americans but also $117 billion in annual health-treatment costs.