I recently learned about DDT as a potential culprit. Turns out that the grandchildren of folks who were exposed are significantly predisposed towards obesity, which would be a significant impact today. It doesn't 'cause' obesity, per se, but it's much harder for affected people to keep the weight off. Speaking as somebody who can sit on the couch eating ice cream for a month without gaining a pound, I used to think that diets were silly and vain... turns out I got lucky on the (epi)genetics.