I've wondered about this.
It would be my observation that the advent of the low fat/cholesterol diet is the one major factor that occurred coincident with the population weight gain.
The fear/safety phenomena is another possibility, but I would tend to limit this more to the current generation of children.
TV watching and Fast Food existed for quite some time previously.
http://www.sciencenews.org/view/gene...nst_the_grains