The Dunning-Kruger Effect

Or, as MyDD puts it, "why the uninformed scream the loudest":

an example of cognitive bias in which "...people reach erroneous conclusions and make unfortunate choices but their incompetence robs them of the metacognitive ability to realize it". They therefore suffer an illusory superiority, rating their own ability as above average. This leads to a perverse result where people with less competence will rate their ability more highly than people with relatively more competence. It also explains why competence may weaken the projection of confidence because competent individuals falsely assume others are of equivalent understanding. "Thus, the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others."

In other words, people who can't get it think they actually get it better than everone else and people who do get it think everyone else can too. It is the affliction of those whose arguments have been completely destroyed and are left with no evidence, and yet think they won the debate anyway - like the birthers. It is also why the smart ones don't understand the failure to communicate and keep pressing. If this theory sounds overly simplistic or arrogant, it's worth pointing out that it's based on a study by two Cornell professors called "Unskilled and Unaware of it." It certainly explains a lot about the national discourse.