Our constellation of moral instincts arose because it helped us to cooperate. It helped us, in unprecedented speed and fashion, to dominate our planet. Yet the same moral reaction also means we exist in a state of perpetual, nasty political disagreement, talking past each other, calling each other names.

So Mr. Haidt explains in “The Righteous Mind: Why Good People Are Divided by Politics and Religion,” undoubtedly one of the most talked-about books of the year. “The Righteous Mind” spent weeks on the hardcover best-seller list. Mr. Haidt considers himself mostly a liberal, but his book has been especially popular in the conservative blogosphere. Some right-leaning intellectuals are even calling it the most important book of the year.

It’s full of ammunition that conservatives will love to throw out at cocktail parties. His research shows that conservatives are much better at understanding and anticipating liberal attitudes than liberals are at appreciating where conservatives are coming from. Case in point: Conservatives know that liberals are repelled by cruelty to animals, but liberals don’t think (or prefer not to believe) that conservatives are repelled too.