Here is another excerpt from Chris Mooney's Republican Brain, this one looking at the science of partisanship and how motivated reasoning ("thought and argument that seems rational and dispassionate, but really isn't anything of the sort") is built into the human brain. Mooney writes that we are "driven to interpret information in a biased way, so as to protect and defend our preexisting convictions:"
...motivated reasoning might perhaps best be thought of as a defensive mechanism that is triggered by a direct attack upon a belief system, physically embodied in a brain...the individual -- or the individual in a self-affirming group that does not provide adequate challenges -- is capable of going very wrong, because of motivated reasoning and confirmation bias.
Along those lines, this image struck me as mordantly amusing (h/t: Weekly Sift):
I would cut a slice from "liberal media bias" to add their "let's agree to disagree" dodge, but otherwise it's fairly comprehensive.