This study suggests an interesting reason why that may be the case:
The investigators used functional neuroimaging (fMRI) to study a sample of committed Democrats and Republicans during the three months prior to the U.S. Presidential election of 2004. The Democrats and Republicans were given a reasoning task in which they had to evaluate threatening information about their own candidate. During the task, the subjects underwent fMRI to see what parts of their brain were active. What the researchers found was striking.
“We did not see any increased activation of the parts of the brain normally engaged during reasoning,” says Drew Westen, director of clinical psychology at Emory who led the study. “What we saw instead was a network of emotion circuits lighting up, including circuits hypothesized to be involved in regulating emotion, and circuits known to be involved in resolving conflicts.” Westen and his colleagues will present their findings at the Annual Conference of the Society for Personality and Social Psychology Jan. 28.
Once partisans had come to completely biased conclusions — essentially finding ways to ignore information that could not be rationally discounted — not only did circuits that mediate negative emotions like sadness and disgust turn off, but subjects got a blast of activation in circuits involved in reward — similar to what addicts receive when they get their fix, Westen explains.