Hmmm... But there has always been information glut when you think about - everyday life experience is loaded with information - a glut that we learn to deal with by employing simple heuristic mechanisms. What is different today is not volume but kind of information - it is not information given off by the nature anymore, but manufactured by humans. People do not travel and observe anymore, but zoom through space without noticing their surroundings to get in front of their idiot-boxes that spit the digested informational pulp at them. I guess the digested form of that information discourages any digesting on one's own and thus contributes to stupidity - but I do not think it is its major source.
As far as the root causes of stupidity are concerned, I think that family and its hierarchical structure is a major source but not the only one. I think it is the source religious superstition by imprinting in young mind the notion of parent-like "supernatural" figures that controls one's life, and protect it from things that overpower the child. If that notion were not imprinted in the young impressionable mind, religious would be nothing but poorly written laughable bunch of crock. Also, as Else Frenkel-Brunswick argued in her contribution to _Authoritarian Personality_, parenting practices that stress rigid conformity to conventional norms and expectations also produce behavior that may qualify as "stupid."
I think that the hierarchical family structure "primes" people for stupidity, while hierarchies of power continue instilling it by creating orthodoxy that delegitimizes alternative modes of knowledge. But there is also another important source - social pressure to conform to conventional norms which produces "learned helplessness" - i.e. people being informally sanctioned and punished for making their own mind instead of flowing the conventional wisdom, which teaches people not think or try on their own (this is well documented in psych literature).
So to summarize, there are three mains sources of learned stupidity: hierarchical family structure, power hierarchy, and social pressure to conform. Since all three are present in every human society, this explains the ubiquity of stupidity.
Wojtek