[Ratufa]

I think that the overall causes of obesity are fairly complex --- there's not some simple cause one can point to that is "the problem". But, threads like this one and the recent HN thread about Western diets remind me of this set of photos on "what the world eats":

http://www.time.com/time/photogallery/0,29307,1626519_137366...

Which provides some evidence that how Americans get their carbs in 2011 is likely very different from how they got their carbs in 1914. For example:

http://www.time.com/time/photogallery/0,29307,1626519_137369...

and

http://www.time.com/time/photogallery/0,29307,1626519_137374...