The atom is like a solar system, with electrons whirling around the nucleus like planets orbiting a star. No, actually, it isn’t. But as a first approximation to help us visualize something that is so invisible, that image works as a metaphor.

Science traffics in metaphors because our brains evolved to grasp intuitively a world far simpler than the counterintuitive world that science has only recently revealed. The functional activity of the brain, for example, is nearly as invisible to us as the atom, and so we employ metaphors. Over the centuries the brain has been compared to a hydraulic machine (18th century), a mechanical calculator (19th century) and an electronic computer (20th century). Today a popular metaphor is that the brain is like a Swiss Army knife, with specialized modules for vision, language, facial recognition, cheating detection, risk taking, spirituality and even God.

Modularity metaphors have been fueled by a new brain-scanning technology called functional magnetic resonance imaging (fMRI). We have all seen scans with highlighted (usually in red) areas where your brain “lights up” when thinking about X (money, sex, God, and so on). This new modularity metaphor is so seductive that I have employed it myself in several books on the evolution of religion (belief modules), morality (moral modules) and economics (money modules). There is a skeptical movement afoot to curtail abuses of the metaphor, however, and it is being driven by neuroscientists themselves. The November 11, 2007, edition of the New York Times, for example, published an opinion piece entitled “This Is Your Brain on Politics,” by neuroscientist Marco Iacoboni of the University of California, Los Angeles, and his colleagues. The writers presented the results of their brain scans on swing voters. “When we showed subjects the words ‘Democrat,’ ‘Republican’ and ‘independent,’ they exhibited high levels of activity in the part of the brain called the amygdala, indicating anxiety,” the authors note. “The two areas in the brain associated with anxiety and disgust — the amygdala and the insula — were especially active when men viewed ‘Republican.’ But all three labels also elicited some activity in the brain area associated with reward, the ventral striatum, as well as other regions related to desire and feeling connected.” So the word “Republican” elicits anxiety and disgust, except for when it triggers feelings of desire and connectedness. The rest of the conclusions are similarly obfuscating.

In a response befitting the selfcorrecting nature of science, Iacoboni’s U.C.L.A. colleague Russell Poldrack and 16 other neuroscientists from labs around the world published a response three days later in the Times, explaining: “As cognitive neuroscientists who use the same brain imaging technology, we know that it is not possible to definitively determine whether a person is anxious or feeling connected simply by looking at activity in a particular brain region. This is so because brain regions are typically engaged by many mental states, and thus a one-to-one mapping between a brain region and a mental state is not possible.” For example, the amygdala is activated by arousal and positive emotions as well, so the key to interpreting such scans is careful experimental design that allows comparison between brain states.

Additional skepticism arises from knowing that fMRI measures blood-flow change, not neuronal activity, that the colors are artificially added in order to see the blood-flow differences and that those images are not any one person’s brain but are instead a statistical compilation of many subjects’ brains in the experiment. “Some of the claims made by neuroscientists sound like astrology,” Poldrack told me in an interview. “It’s not the science itself that is the problem. It’s taking a little bit of science and going way beyond it.” For example, there is the problem of reversing the causal inference, “ where people see some activity in a brain area and then conclude that this part of the brain is where X happens. We can show that if I put you into a state of fear, your amygdala lights up, but that doesn’t mean that every time your amygdala lights up you are experiencing fear. Every brain area lights up under lots of different states. We just don’t have the data to tell us how selectively active an area is.”

University of California, San Diego, philosopher of the mind Patricia S. Churchland told me with unabashed skepticism: “Mental modules are complete nonsense. There are no modules that are encapsulated and just send information into a central processor. There are areas of specialization, yes, and networks maybe, but these are not always dedicated to a particular task.” Instead of mental module metaphors, let us use neural networks.

The brain is not random kludge, of course, so the search for neural networks associated with psychological concepts is a worthy one, as long as we do not succumb to the siren song of phrenology.

During an early episode of the über-pyrotechnic television series MythBusters, Adam Savage was busted by the camera crew for misremembering his predictions of the probability of an axle being ripped out of a car, à la American Graffiti. When confronted with the unmistakable video evidence of his error, Adam sardonically rejoined: “I reject your reality and substitute my own.”

Skepticism is the fine art and technical science of understanding why rejecting everyone else’s reality and substituting your own almost always results in a failed belief system. Where in the brain do such belief processes unfold? To find out, neuroscientists Sam Harris, Sameer A. Sheth and Mark S. Cohen employed functional magnetic resonance imaging to scan the brains of 14 adults at the University of California, Los Angeles, Brain Mapping Center. The researchers presented the subjects with a series of statements designed to be plainly true, false or undecidable. In response, the volunteers were to press a button indicating their belief, disbelief or uncertainty. For example:

Mathematical

(2 + 6) + 8 = 16.
62 can be evenly divided by 9.
1.2⁵⁷ = 32608.5153.

Factual

Most people have 10 fingers and 10 toes.
Eagles are common pets.
The Dow Jones Industrial Average rose 1.2% last Tuesday.

Ethical

It is bad to take pleasure at another’s suffering.
Children should have no rights until they can vote.
It is better to lie to a child than to an adult.

The findings were revealing. First, there were significant reaction time differences in evaluating statements; responses to belief statements were significantly shorter than responses to both disbelief and uncertainty statements (but no difference was detected between disbelief and uncertainty statements). Second, contrasting belief and disbelief in the brain scans yielded a spike in neural activity in the ventromedial prefrontal cortex, associated with decision making and learning in the context of rewards. Third, contrasting disbelief and belief showed increased brain response in the left inferior frontal gyrus, the anterior insula and the dorsal anterior cingulate, all associated with responses to negative stimuli, pain perception and disgust. Finally, contrasting uncertainty with both belief and disbelief revealed elevated neural action in the anterior cingulate cortex, a region associated with conflict resolution.

What do these results tell us? “Several psychological studies appear to support [17th-century Dutch philosopher Benedict] Spinoza’s conjecture that the mere comprehension of a statement entails the tacit acceptance of its being true, whereas disbelief requires a subsequent process of rejection,” report Harris and his collaborators on the study in their paper, published in the December 2007 Annals of Neurology. “Understanding a proposition may be analogous to perceiving an object in physical space: We seem to accept appearances as reality until they prove otherwise.” So subjects assessed true statements as believable faster than they judged them as unbelievable or undecidable. Further, because the brain appears to process false or uncertain statements in regions linked to pain and disgust, especially in judging tastes and odors, this study gives new meaning to a claim passing the “taste test” or the “smell test.”

As for the neural correlates of belief and skepticism, the ventromedial prefrontal cortex is instrumental in linking higher-order cognitive factual evaluations with lower-order emotional response associations, and it does so in evaluating all types of claims. Thus, the assessment of the ethical statements showed a similar pattern of neural activation, as did the evaluation of the mathematical and factual statements. People with damage in this area have a difficult time feeling an emotional difference between good and bad decisions, and they are susceptible to confabulation — mixing true and false memories and conflating reality with fantasy.

This research supports Spinoza’s conjecture that most people have a low tolerance for ambiguity and that belief comes quickly and naturally, whereas skepticism is slow and unnatural. The scientific principle of the null hypothesis — that a claim is untrue unless proved otherwise — runs counter to our natural tendency to accept as true what we can comprehend quickly. Given the chance, most of us would like to invoke Adam’s Maxim because it is faster and feels better. Thus, it is that we should reward skepticism and disbelief and champion those willing to change their mind in the teeth of new evidence.

The human understanding when it has once adopted an opinion … draws all things else to support and agree with it. And though there be a greater number and weight of instances to be found on the other side, yet these it either neglects and despises … in order that by this great and pernicious predetermination the authority of its former conclusions may remain inviolate.

—Francis Bacon, Novum Organum, 1620

Pace Will Rogers, I am not a member of any organized political party. I am a libertarian. As a fiscal conservative and social liberal, I have found at least something to like about each Republican or Democrat I have met. I have close friends in both camps, in which I have observed the following: no matter the issue under discussion, both sides are equally convinced that the evidence overwhelmingly supports their position.

This surety is called the confirmation bias, whereby we seek and find confirmatory evidence in support of already existing beliefs and ignore or reinterpret disconfirmatory evidence. Now a functional magnetic resonance imaging (fMRI) study shows where in the brain the confirmation bias arises and how it is unconscious and driven by emotions. Psychologist Drew Westen led the study, conducted at Emory University, and the team presented the results at the 2006 annual conference of the Society for Personality and Social Psychology.

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.

“We did not see any increased activation of the parts of the brain normally engaged during reasoning,” Westen is quoted as saying in an Emory University press release. “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.” Interestingly, neural circuits engaged in rewarding selective behaviors were activated. “Essentially, it appears as if partisans twirl the cognitive kaleidoscope until they get the conclusions they want, and then they get massively reinforced for it, with the elimination of negative emotional states and activation of positive ones,” Westen said.

The implications of the findings reach far beyond politics. A jury assessing evidence against a defendant, a CEO evaluating information about a company or a scientist weighing data in favor of a theory will undergo the same cognitive process. What can we do about it?

In science we have built-in self-correcting machinery. Strict double-blind controls are required in experiments, in which neither the subjects nor the experimenters know the experimental conditions during the data-collection phase. Results are vetted at professional conferences and in peer-reviewed journals. Research must be replicated in other laboratories unaffiliated with the original researcher. Disconfirmatory evidence, as well as contradictory interpretations of the data, must be included in the paper. Colleagues are rewarded for being skeptical. Extraordinary claims require extraordinary evidence.

We need similar controls for the confirmation bias in the arenas of law, business and politics. Judges and lawyers should call one another on the practice of mining data selectively to bolster an argument and warn juries about the confirmation bias. CEOs should assess critically the enthusiastic recommendations of their VPs and demand to see contradictory evidence and alternative evaluations of the same plan. Politicians need a stronger peer-review system that goes beyond the churlish opprobrium of the campaign trail, and I would love to see a political debate in which the candidates were required to make the opposite case.

Skepticism is the antidote for the confirmation bias.