Clockwise, from top left: Eric Thayer for The New York Time; Daniel Borris for The New York Times; Joe Raedle, via Getty Images; Joe Raedle, via Getty Images

In the film version of “All the President’s Men,” when Robert Redford, playing the journalist Bob Woodward, is struggling to unravel the Watergate conspiracy, an anonymous source advises him to “follow the money.” It’s a good rule of thumb for understanding the behavior of politicians. But following the money leads you astray if you’re trying to understand voters.

Self-interest, political scientists have found, is a surprisingly weak predictor of people’s views on specific issues. Parents of children in public school are not more supportive of government aid to schools than other citizens. People without health insurance are not more likely to favor government-provided health insurance than are people who are fully insured.

Despite what you might have learned in Economics 101, people aren’t always selfish. In politics, they’re more often groupish. When people feel that a group they value — be it racial, religious, regional or ideological — is under attack, they rally to its defense, even at some cost to themselves. We evolved to be tribal, and politics is a competition among coalitions of tribes.

The key to understanding tribal behavior is not money, it’s sacredness. The great trick that humans developed at some point in the last few hundred thousand years is the ability to circle around a tree, rock, ancestor, flag, book or god, and then treat that thing as sacred. People who worship the same idol can trust one another, work as a team and prevail over less cohesive groups. So if you want to understand politics, and especially our divisive culture wars, you must follow the sacredness.

A good way to follow the sacredness is to listen to the stories that each tribe tells about itself and the larger nation. The Notre Dame sociologist Christian Smith once summarized the moral narrative told by the American left like this: “Once upon a time, the vast majority” of people suffered in societies that were “unjust, unhealthy, repressive and oppressive.” These societies were “reprehensible because of their deep-rooted inequality, exploitation and irrational traditionalism — all of which made life very unfair, unpleasant and short. But the noble human aspiration for autonomy, equality and prosperity struggled mightily against the forces of misery and oppression and eventually succeeded in establishing modern, liberal, democratic, capitalist, welfare societies.” Despite our progress, “there is much work to be done to dismantle the powerful vestiges of inequality, exploitation and repression.” This struggle, as Smith put it, “is the one mission truly worth dedicating one’s life to achieving.”

This is a heroic liberation narrative. For the American left, African-Americans, women and other victimized groups are the sacred objects at the center of the story. As liberals circle around these groups, they bond together and gain a sense of righteous common purpose.

Contrast that narrative with one that Ronald Reagan developed in the 1970s and ’80s for conservatism. The clinical psychologist Drew Westen summarized the Reagan narrative like this: “Once upon a time, America was a shining beacon. Then liberals came along and erected an enormous federal bureaucracy that handcuffed the invisible hand of the free market. They subverted our traditional American values and opposed God and faith at every step of the way.” For example, “instead of requiring that people work for a living, they siphoned money from hard-working Americans and gave it to Cadillac-driving drug addicts and welfare queens.” Instead of the “traditional American values of family, fidelity and personal responsibility, they preached promiscuity, premarital sex and the gay lifestyle” and instead of “projecting strength to those who would do evil around the world, they cut military budgets, disrespected our soldiers in uniform and burned our flag.” In response, “Americans decided to take their country back from those who sought to undermine it.”

This, too, is a heroic narrative, but it’s a heroism of defense. In this narrative it’s God and country that are sacred — hence the importance in conservative iconography of the Bible, the flag, the military and the founding fathers. But the subtext in this narrative is about moral order. For social conservatives, religion and the traditional family are so important in part because they foster self-control, create moral order and fend off chaos. (Think of Rick Santorum’s comment that birth control is bad because it’s “a license to do things in the sexual realm that is counter to how things are supposed to be.”) Liberals are the devil in this narrative because they want to destroy or subvert all sources of moral order.

Actually, there’s a second subtext in the Reagan narrative in which liberty is the sacred object. Circling around liberty would seem, on its face, to be more consistent with liberalism and its many liberation movements than with social conservatism. But here’s where narrative analysis really helps. Part of Reagan’s political genius was that he told a single story about America that rallied libertarians and social conservatives, who are otherwise strange bedfellows. He did this by presenting liberal activist government as the single devil that is eternally bent on destroying two different sets of sacred values — economic liberty and moral order. Only if all nonliberals unite into a coalition of tribes can this devil be defeated.

If you follow the sacredness, you can understand some of the weirdness of the last few months in politics. In January, the Obama administration announced that religiously affiliated hospitals and other institutions must offer health plans that provide free contraception to their members. It’s one thing for the government to insist that people have a right to buy a product that their employer abhors. But it’s a rather direct act of sacrilege (for many Christians) for the government to force religious institutions to pay for that product. The outraged reaction galvanized the Christian right and gave a lift to Rick Santorum’s campaign.

AROUND this time, bills were making their way through state legislatures requiring that women undergo a medically unnecessary ultrasound before they can have an abortion. It’s one thing for a state government to make abortions harder to get (as with a waiting period). But it’s a rather direct act of sacrilege (for nearly all liberals as well as libertarians) for a state to force a doctor to insert a probe into a woman’s vagina. The outraged reaction galvanized the secular left and gave a lift to President Obama.

This is why we’ve seen the sudden re-emergence of the older culture war — the one between the religious right and the secular left that raged for so many years before the financial crisis and the rise of the Tea Party. When sacred objects are threatened, we can expect a ferocious tribal response. The right perceives a “war on Christianity” and gears up for a holy war. The left perceives a “war on women” and gears up for, well, a holy war.

The timing could hardly be worse. America faces multiple threats and challenges, many of which will require each side to accept a “grand bargain” that imposes, at the very least, painful compromises on core economic values. But when your opponent is the devil, bargaining and compromise are themselves forms of sacrilege.

Jonathan Haidt is a professor of psychology at the University of Virginia and a visiting professor of business ethics at N.Y.U.’s Stern School of Business. Parts of this essay were excerpted from “The Righteous Mind: Why Good People Are Divided by Politics and Religion,” which was just released.