When something involves a small percentage of the population, it's typically an individual issue/failure. When it becomes the vast majority of people, like in the case of obesity, you have to seriously consider that the issue is in large part systemic. I seriously doubt, for instance, that in the last 20-30 years, 80% of the American population suddenly woke up and lost all willpower when it comes to food, especially now that the obesity problem appears to be spreading to other countries.