If everyone gets fat all at once, there has to be more of an explanation than lack of willpower (though I think cultural issues/cultural change also play a part, very much so).

This study demonstrates that people during the 1980s had an easier time not getting fat while eating the same amount of calories.

Though I think the study is onto something, a caveat: I can’t access the paper and I’m not paying $31.50 to do so, but the people involved in the National Health and Nutrition Examination Survey all self-reported their calorie counts. This is notoriously unreliable (vast, vast majority of people under-report, a few people like me over-report) — but that doesn’t invalidate the study. I don’t know if they controlled for it or anything. Just wanted to bring it up as I like having all of the evidence, even if it proves wrong (or right) everything I thought I knew.

That said, I suspect the study is onto something because as I noted above there is no real explanation for why obesity blasted off like an Atlas V around 1990 or so.

My guesses about why this happened, in order of contribution:

1) Some change in environment — chemical exposure being most likely.

2) Food getting cheaper overall.

3) Changing cultural norms around obesity combined with identity politics (complete cultural triumph of neoliberalism in every sphere around that time, asserting the primacy of individual and their preferences).

4) Increasing societal precariousness causing willpower transfer to more difficult things, away from controlling food intake.

5) Better medical treatments allowing older obese people who would’ve formerly died early to live longer, thus reflecting more obese people in the numbers.

Change the order as you like. I suspect there is no single factor, and I’d really like to see some scientific investigation into why obesity is so much more prevalent, combined with the FA people shutting their traps and actually helping with that effort.