The human brain is a funny machine. Imperfectly designed by natural selection, it finds itself in an environment that has little resemblance with the one it evolved in. Gone is the savannah in which our ancestors had to guard themselves from fierce creatures. Instead, we live in a complex and ever expanding social milieu, our neighborhood now encompassing the whole planet. Is it any wonder that our poor brains are not doing so well in this brave new wired world?

Our brains seem to fail to grasp reality, as demonstrated by the fact that a majority of Americans don't "believe" in evolution (whatever "believing" in a scientific theory means), while a sizable percentage is ready to accept the existence of an imaginary all-powerful god, as well as of the devil, hell, and a slew of angels. Why is it so difficult to be a reasonably skeptical person? What is it that makes so many apparently intelligent people so gullible about things that their brains clearly have the power to master? And-perhaps most importantly for the skeptic-how do we get people to change their minds in an informed way on so wide an array of irrationalities?

Obviously, I am not going to present the reader with the magic bullet that can answer these questions, but a starting point is being provided by recent research in neurobiology. It turns out that lately we have learned a lot about how the brain works and why it makes mistakes while interpreting reality. Since our most powerful tool doesn't come with an owner's manual, it may pay off to spend a little time thinking about how we think.

Perhaps one of the most dramatic ways we are learning about the brain is by studying patients who literally have a split one. The brain is made of two hemispheres, joined by a structure called the corpus callosum which contains nerve fibers that continuously exchange signals between the right and left hemisphere. Some individuals have suffered more or less complete damage to the corpus callosum, either because of a stroke or because of a surgical operation. These subjects are invaluable to neurobiologists because it is possible to interrogate the right and left hemispheres separately, see how differently they think, and then piece this information together to reconstruct the thought patterns of normal individuals. The problem with attempting to "talk" to both hemispheres is that language is controlled by the left one, the only hemisphere that can articulate things. Fortunately, the right side can still "respond" to interrogations by virtue of its control over the motor functions of the left half of the body, including the arm and hand.

Perhaps the most astonishing thing neurobiologists have discovered from split-brain patients is that the left hemisphere, which normally "dominates" the right one, is literally in charge of our view of the world. And it fights hard to preserve it. In a wonderfully elegant experiment, a group of researchers led by Michael Gazzaniga at Dartmouth College showed pictures to the right and left hemispheres of a split-brain patient and then asked each hemisphere to pick another picture to accompany the one originally presented. The right side was shown (through the left half of the visual field) a house with snow and, logically enough, it picked a shovel. The left hemisphere was shown a chicken leg (through the right half of the visual field), and it picked a chicken head-also quite logically. The experimenters then verbally asked the patient to explain his choices. The left hemisphere was the only one that could articulate an answer, but remember-it did not know why his right counterpart had chosen a shovel, since the information about the house with the snow did not cross the severed corpus callosum. The patient's answer was as astounding as illuminating: "Oh, that's simple. The chicken claw goes with the chicken [which was true], and you need a shovel to clean out the chicken shed [which was coherent, but completely false]." In other words, the left hemisphere acted as an interpreter of the worldview of the individual and fabricated a just-so story to fit all the available data!

These sort of experiments have shown that the left hemisphere is in charge of our worldview, of the paradigms we currently hold about a variety of aspects of reality. In normal patients, these paradigms are constantly evaluated against external evidence, gathered by both hemispheres through a suite of sensorial inputs. The left interpreter has the all-important function of making sense of the world, and it does a reasonably good job at it. However, when the incoming data is insufficient, or when some piece of evidence contradicts the currently held view, the left hemisphere either rejects the unfit information or it distorts it so to make sense of it. This process of "rationalizing" the world goes on up to a certain point. If the degree of conflicting information is too high (i.e., there is too much dissonance between what one believes and what one perceives) then that most stupendous phenomenon suddenly occurs: we change our minds (literally)!

The problem that rational people face, then, is twofold. On the one hand, the brain has evolved a powerful mechanism to avoid to change its mind too often, which means that people will stubbornly continue to believe all sorts of nonsense because it is less painful than to radically alter their worldview. On the other hand, we know that the problem is all the more insurmountable when the data fed to the subject is poor, and unfortunately most of what modern human beings are exposed to by the media is pure garbage.

However, there is no need to despair just yet. Understanding the problem is a necessary (though by all means not sufficient) step to solve it. Realizing where people's stubbornness (and sometimes our own) comes from will help not getting unduly irritated or downright nasty when facing patent irrationality in our fellow human beings. And empathy is one important step toward connecting with anybody. The second message of modern neurobiological research is perhaps an old one, but which now comes with the weight of evidence: education is our (slow) way out. What we need to do is to keep educating people, to feed good information to the brain's interpreter. If neurobiologists are correct, most brains will come to understand reality if properly nurtured. It is ignorance which provides the necessity for just-so stories, with all the tragic consequences that follow when people defend a flawed worldview at all costs.

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