Not one of the reasons listed suggests that it could be our educational system...the most obvious place to begin looking. Definitely a lack of intelligence on the part of those doing the study, I’d say.
It is instructive to read the letters of a Civil War soldier and hold them in comparison to the writings of the average American today. The infantry privates of that era, educated in simple one room schoolhouses on a diet of The Bible, Shakespeare, Coleridge, Keats and Sir Walter Scott displayed a degree of literacy which surpasses the vast majority of scribblings, tweets, and blogs offered up by today's typical college grad - myself, especially, included.
http://www.civilwararchive.com/LETTERS/letters.htm