What's most disorienting is how elaborate motivated reasoning can become, especially among those who are knowledgeable or sophisticated. If we like to pretend that politics is rational and based on reason, it's in part because we can't believe that a think tank scholar with a Ph.D. could actually be arguing from emotions as he or she rattles of facts and statistics, or composes an entire book.

However, we should be suspicious of seeming brilliance most of all. Scientists are also starting to home in on a way of explaining the elaborate heights of our capacity for rationalization -- our argumentative creativity -- and just how floridly idiotic we can be. We're not only capable of being wrong; we make quite the show of it.

One team of thinkers -- philosopher Hugo Mercier of the University of Pennsylvania and cognitive scientist Dan Sperber of the Jean Nicod Institute in France -- have recently proposed that we've been reasoning about reasoning all wrong, trying to fix what didn't need fixing, if we'd only understood what its original purpose was. Contrary to the claims of Enlightenment idealists, Mercier and Sperger suggest human reason did not evolve as a device for getting at the objective truth. Rather, they suggest that its purpose is to facilitate selective arguing in defense of one's position in a social context -- something that, we can hardly dispute, we are very good at.

When thought about in the context of the evolution of human language and communication, and cooperation in groups, this makes a lot of sense. There would surely have been a survival value to getting other people in your hunter-gatherer group to listen to you and do what you want them to do -- in short, a value to being persuasive. And for the listeners, there would have been just as much a premium on being able to determine whether a given speaker is reliable and trustworthy, and should be heeded. Thus, everybody in the group would have benefited from an airing of different views, so that their strengths and weaknesses could be debated -- regarding, say, where it would be a good place to hunt today or whether the seasons are changing.

Considered in this light, reasoning wouldn't be expected to make us good logicians, but rather, good rhetoricians. And that's what we are. Not only are we very good at selectively cobbling together evidence to support our own case -- aided by motivated reasoning -- but we're also good at seeing the flaws in the arguments of others when they get up on top of the soap box, good at slicing and dicing their claims.

When lots of individuals blow holes in one another's claims and arguments, the reasoning of the group should be better than the reasoning of the individual. But at the same time, the individual -- or the individual in a self-affirming group that does not provide adequate challenges -- is capable of going very wrong, because of motivated reasoning and confirmation bias. Thanks to these flaws, the sole reasoner rarely sees what's wrong with his or her logic. Rather, the sole reasoner becomes the equivalent of a crazy hermit in the wilderness -- or, to quote the late Frank Zappa, the author of "that tacky little pamphlet in your Daddy's bottom drawer." And the unchallenged group member becomes like a cult follower.