Do you think the average American's willpower has radically plummeted since the 70s? And the rest of the developed world nosedived with them? It's gotta be calories-in/calories-out and discipline to a degree, but things have shifted so rapidly and decisively that it's worth entertaining environmental explanations.

Plus there's little harm in kicking this hypothesis around. A fat person who is otherwise unmotivated by traditional health advice might perk up once you appeal to his inner conspiracy theorist and tell him he's being fed industrial waste.