Our political predilections are a product of unconscious confirmation bias

Michael Shermer in Scientific American:

During the run-up to the 2004 presidential election, while undergoing an fMRI bran scan, 30 men--half self-described as "strong" Republicans and half as "strong" Democrats--were tasked with assessing statements by both George W. Bush and John Kerry in which the candidates clearly contradicted themselves. Not surprisingly, in their assessments Republican subjects were as critical of Kerry as Democratic subjects were of Bush, yet both let their own candidate off the hook.

The neuroimaging results, however, revealed that the part of the brain most associated with reasoning--the dorsolateral prefrontal cortex--was quiescent. Most active were the orbital frontal cortex, which is involved in the processing of emotions; the anterior cingulate, which is associated with conflict resolution; the posterior cingulate, which is concerned with making judgments about moral accountability; and--once subjects had arrived at a conclusion that made them emotionally comfortable--the ventral striatum, which is related to reward and pleasure.

More here.