Over the past 70 years, obesity has increased significantly, as have heart disease and several other related diseases. If someone wants to argue that obesity is caused by low willpower (as the above poster at least partially claims)

The hard part about losing weight isn't knowing what to eat, it's having the willpower to eat less (or exercise more). Of course, you can't sell a book by telling everyone "we're fat because we have no willpower". "We're fat because we've been lied to", however, will sell.

then I want to see some explanation for why willpower has declined over that time period. If someone tells me diet changes have caused obesity and they point to diet changes over the past N years and rising obesity over the past N years, the pieces of their story fit together. So I’m asking (somewhat skeptically) what’s the mechanism for the willpower story? How does it fit the observed trend?