Edit: The following is mostly a non sequitur. My response was based in part on a misreading of the article's arguments. They're concerned primarily with the "consolidation" of obesity since about 1980 without any obvious changes for why. The changes for why were in the previous decades and the chickens should have come home to roost by 1980. Yet we are indeed much more obese than in 1980 but not more sedentary. So... why? This I have to say, is in fact somewhat mysterious.

I have a contention with the "obesity hasn't been reversed in any population" claim. Here in Canada, child obesity has in fact decreased in recent years, after several decades of continual increases. Given that childhood obesity, and obesity in general, has been a major social issue everyone is aware of and has been working hard on (I assume), the small decrease in proportion to the amount of energy invested, is rather depressing. Still, maybe cause for some hope.

> Something seems to have changed. But surprisingly, we don’t seem to have any idea what that thing was.

We don't? While exactly how much this is a factor is contended, I thought it was near-consensus at this point that it's because we are less physically active. The timeline certainly fits - beginning about 1890 and ending about 1975 -- that is the period from the beginning to universal adoption of the car.

Between home appliances, powered vehicles and industrial automation, many modern people don't actually have to do anything physical. Hauling their corporeal form between various chairs is literally all the exercise many people get.

This would be unfathomable to my great-grandparents. They walked to the store then hauled the groceries home. They walked a few miles every Sunday to go to church. They walked to work. They tended the fields largely with manual labour and a horse. They mowed the garden and yard, by hand. Every nail in the home, driven by hand. Ground meat for dinner? Minced by hand. And by then, industrialization had already completely changed things for them, to their own grandparents, giving them what they must have thought an amazing amount of free time and leisure. (My great-grandparents clothes were almost certainly machine-woven, for example.)

If we're eating as many calories, or slightly more, as our grandparents did, while barely moving in comparison to them, is it any surprise we all got really fat? In fairness to the paper, I suppose one could view this social change as a mimetic contagion of a sort.