Using M.R.I. scanners, neuroscientists have now tracked what happens in the politically partisan brain when it tries to digest damning facts about favored candidates or criticisms of them. The process is almost entirely emotional and unconscious, the researchers report, and there are flares of activity in the brain's pleasure centers when unwelcome information is being rejected. Researchers have long known that political decisions are strongly influenced by unconscious emotional reactions, a fact routinely exploited by campaign consultants and advertisers. But the new research suggests that for partisans, political thinking is often predominantly emotional. It is possible to override these biases, Dr. Westen said, but you have to engage in ruthless self reflection, to say, 'All right, I know what I want to believe, but I have to be honest.' " He added, "It speaks to the character of the discourse that this quality is rarely talked about in politics."

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