Liberals and conservatives don't just vote differently. They think differently. Follow the money. As a young journalist on the political left, I often heeded this well-worn advice. If conservatives were denying the science of global warming, I figured, big fossil-fuel companies must be behind it. After all, that was the story with the tobacco industry and the dangers of smoking. Why not here? And so I covered the attacks on the established scientific knowledge on climate change, evolution and many more issues as a kind of search for the wealthy bad guys behind the curtain. Like many in Washington, I tended to assume that political differences are either about contrasting philosophies or, more cynically, about money and special interests. There's just one problem: Mounting scientific evidence suggests that this is a pretty limited way of understanding what divides us. And at a time of unprecedented polarization in America, we need a more convincing explanation for the staggering irrationality of our politics. Especially since we're now split not just over what we ought to do politically but also over what we consider to be true. Liberals and conservatives have access to the same information, yet they hold wildly incompatible views on issues ranging from global warming to whether the president was born in the United States to whether his stimulus package created any jobs. But it's not just that: Partisanship creates stunning intellectual contortions and inconsistencies. Republicans today can denounce a health-care reform plan that's pretty similar to one passed in Massachusetts by a Republican - and the only apparent reason is that this one came from a Democrat. None of these things make sense - unless you view them through the lens of political psychology. There's now a large body of evidence showing that those who opt for the political left and those who opt for the political right tend to process information in divergent ways and to differ on any number of psychological traits. Perhaps most important, liberals consistently score higher on a personality measure called "openness to experience," one of the "Big Five" personality traits, which are easily assessed through standard questionnaires. That means liberals tend to be the kind of people who want to try new things, including new music, books, restaurants and vacation spots - and new ideas. "Open people everywhere tend to have more liberal values," said psychologist Robert McCrae, who conducted voluminous studies on personality while at the National Institute on Aging at the National Institutes of Health. Conservatives, in contrast, tend to be less open - less exploratory, less in need of change - and more "conscientious," a trait that indicates they appreciate order and structure in their lives. This gels nicely with the standard definition of conservatism as resistance to change - in the famous words of William F. Buckley Jr., a desire to stand "athwart history, yelling "Stop!" " I call myself a liberal, so this description of openness resonates with me. But I think it's vital for everyone to understand, and it needn't be seen as threatening or a put-down; it seems to be part of the nature of politics.