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group were given a profile of an introvert (shy, timid, quiet), while subjects in another group were given a profile of an extrovert (sociable, talkative, outgoing). When asked to make a personality assessment, those subjects who were told that the person would be an extrovert tended to ask questions that would lead to that conclusion; the introvert group did the same in the opposite direction.3 In a 1983 study, psychologists John Darley and Paget Gross showed subjects a video of a child taking a test. One group was told that the child was from a high socioeconomic class while the other group was told that the child was from a low socioeconomic class. The subjects were then asked to evaluate the academic abilities of the child based on the results of the test. Even though both groups of subjects were evaluating the exact same set of numbers, those who were told that the child was from a high socioeconomic class rated the child’s abilities as above grade level, and those who thought that the child was from a low socioeconomic class rated the child as below grade level in ability.4 This is a striking indictment of human reason but a testimony to the power of belief expectations.

The power of expectation was displayed in a 1989 study by psychologists Bonnie Sherman and Ziva Kunda, who presented a group of subjects with evidence that contradicted a belief they held deeply, and with evidence that supported those same beliefs. The results showed that the subjects recognized the validity of the confirming evidence but were skeptical of the value of the disconfirming evidence.5 In another 1989 study, by psychologist Deanna Kuhn, when children and young adults were exposed to evidence inconsistent with a theory they preferred, they failed to notice the contradictory evidence, or if they did acknowledge its existence, they tended to reinterpret it to favor their preconceived beliefs.6 In a related study, Kuhn exposed subjects to an audio recording of an actual murder trial and discovered that instead of evaluating the evidence first and then coming to a conclusion, most subjects concocted a narrative in their mind about what happened, made a decision of guilt or innocence, then riffled through the evidence and picked out what most closely fit the story.7

The confirmation bias is particularly potent in political beliefs, most notably the manner in which our belief filters allow in information that confirms our ideological convictions and filters out information that disconfirms those same convictions. This is why it is so easy to predict which media outlets liberals and conservatives choose to monitor. We now even have an idea of where in the brain the confirmation bias is processed thanks to an fMRI study conducted at Emory University by Drew Westen.8

During the run-up to the 2004 presidential election, while undergoing a brain scan, thirty men—half self-described “strong” Republicans and half “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 of the candidates, Republican subjects were as critical of Kerry as Democratic subjects were of Bush, yet both let their own preferred candidate off the evaluative hook. Of course. But what was especially revealing were the neuroimaging results: 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, and the anterior cingulate cortex—our old friend the ACC, which is so active in patternicity processing and conflict resolution. Interestingly, once subjects had arrived at a conclusion that made them emotionally comfortable, their ventral striatum—a part of the brain associated with reward—became active.

In other words, instead of rationally evaluating a candidate’s positions on this or that issue, or analyzing the planks of each candidate’s platform, we have an emotional reaction to conflicting data. We rationalize