Haidt isn’t just scolding liberals, however. He sees the left and right as yin and yang, each contributing insights to which the other should listen. In his view, for instance, liberals can teach conservatives to recognize and constrain predation by entrenched interests. Haidt believes in the power of reason, but the reasoning has to be interactive. It has to be other people’s reason engaging yours. We’re lousy at challenging our own beliefs, but we’re good at challenging each other’s. Haidt compares us to neurons in a giant brain, capable of “producing good reasoning as an emergent property of the social system.”

Our task, then, is to organize society so that reason and intuition interact in healthy ways. Haidt’s research suggests several broad guidelines. First, we need to help citizens develop sympathetic relationships so that they seek to understand one another instead of using reason to parry opposing views. Second, we need to create time for contemplation. Research shows that two minutes of reflection on a good argument can change a person’s mind. Third, we need to break up our ideological segregation. From 1976 to 2008, the proportion of Americans living in highly partisan counties increased from 27 percent to 48 percent. The Internet exacerbates this problem by helping each user find evidence that supports his views.

How can we achieve these goals? Haidt offers a Web site, civilpolitics.org, on which he and his colleagues have listed steps that might help. One is holding open primaries so that people outside each party’s base can vote to nominate moderate candidates. Another is instant runoffs, so that candidates will benefit from broadening their appeal. A third idea is to alter redistricting so that parties are less able to gerrymander partisan congressional districts. Haidt also wants members of Congress to go back to the old practice of moving their families to Washington, so that they socialize with one another and build a friendly basis on which to cooperate.

Many of Haidt’s proposals are vague, insufficient or hard to implement. And that’s O.K. He just wants to start a conversation about integrating a better understanding of human nature — our sentiments, sociality and morality — into the ways we debate and govern ourselves. At this, he succeeds. It’s a landmark contribution to humanity’s understanding of itself.

But to whom is Haidt directing his advice? If intuitions are unreflective, and if reason is self-serving, then what part of us does he expect to regulate and orchestrate these faculties? This is the unspoken tension in Haidt’s book. As a scientist, he takes a passive, empirical view of human nature. He describes us as we have been, expecting no more. Based on evolution, he argues, universal love is implausible: “Parochial love . . . amplified by similarity” and a “sense of shared fate . . . may be the most we can accomplish.” But as an author and advocate, Haidt speaks to us rationally and universally, as though we’re capable of something greater. He seems unable to help himself, as though it’s in his nature to call on our capacity for reason and our sense of common humanity — and in our nature to understand it.

You don’t have to believe in God to see this higher capacity as part of our nature. You just have to believe in evolution. Evolution itself has evolved: as humans became increasingly social, the struggle for survival, mating and progeny depended less on physical abilities and more on social abilities. In this way, a faculty produced by evolution — sociality — became the new engine of evolution. Why can’t reason do the same thing? Why can’t it emerge from its evolutionary origins as a spin doctor to become the new medium in which humans compete, cooperate and advance the fitness of their communities? Isn’t that what we see all around us? Look at the global spread of media, debate and democracy.

Haidt is part of this process. He thinks he’s just articulating evolution. But in effect, he’s also trying to fix it. Traits we evolved in a dispersed world, like tribalism and righteousness, have become dangerously maladaptive in an era of rapid globalization. A pure scientist would let us purge these traits from the gene pool by fighting and killing one another. But Haidt wants to spare us this fate. He seeks a world in which “fewer people believe that righteous ends justify violent means.” To achieve this goal, he asks us to understand and overcome our instincts. He appeals to a power capable of circumspection, reflection and reform.