“That led to a funny result,” Mr. Manjoo writes. “People in the study became polarized.”

A fair reading of the two studies might have led the students to question whether any strong conclusions could be drawn about deterrence, and thus to tone down their views on the death penalty. But the opposite happened. Students on each side accepted the evidence that conformed to their original views while rejecting the contrary evidence  and so afterward students on both sides were more passionate and confident than ever of their views.

That’s what we seem to be seeing in the Democratic primaries. Even though the policy differences between the two candidates are minimal, each camp is becoming increasingly aggravated at the other. A Washington Post poll published Wednesday found that more than one-third of Democrats say that they may not support their party’s nominee if it is not their own choice.

Another challenge is the biased way in which we gather information. We seek out information that reinforces our prejudices. One study presented listeners with static-filled recordings of speeches that they believed they were judging on persuasive power. Listeners could push a button to tweak the signal, reducing the static to make it easier to understand. When smokers heard a speech connecting tobacco with cancer, they didn’t try to improve the clarity to hear it more easily. But they pushed the button to get a clearer version of a speech saying that there was no link between smoking and cancer. Nonsmokers were the exact opposite.

This resistance to information that doesn’t mesh with our preconceived beliefs afflicts both liberals and conservatives, but a raft of studies shows that it is a particular problem with conservatives. For example, when voters receive mailings offering them free pamphlets on various political topics, liberals show some interest in getting conservative views. In contrast, conservatives seek only those pamphlets that echo their own views.

Likewise, liberal blogs overwhelmingly link to other liberal blogs or news sources. But with conservative blogs, the tendency is much more pronounced; it is almost a sealed universe.

The situation isn’t hopeless. Similar psychological processes govern our perceptions of race, yet we’ve made great progress in revising our views and reducing prejudices. The same is true of attitudes towards gays.

The only solutions I see are personal ones, to work out daily to build our mental muscles. Just as we force ourselves to nibble on greens and decline cheesecake, we should seek an information diet that includes a salad bar of information sources  with a special focus on unpalatable rubbish from fools. The worse it tastes, the better it may be for us.

If that’s why you’re reading this, congratulations! And thanks!