Photo of protesters at the Capitol in Washington, DC on 9/12/09. The Tea Party gathering was in protest of Obama's healthcare plan and government spending.

CHICAGO - For the first time in American political history, Democrats and Republicans have sorted themselves into a perfect left-right split, a prominent political psychologist said this week, calling the result a "dangerous era" in U.S. politics.

Traditionally, political parties have been coalitions of broad groups of people, based more on industry, region and interest group than basic morals, University of Virginia psychologist Jonathan Haidt said here during a lecture at the annual meeting of the Association of Psychological Science. Since the 1970s and 1980s, however, Americans have increasingly sorted themselves by

liberalism and conservatism, resulting in two political parties that seem almost alien to one another.

We've never had a perfect left-right sort before, and now we do," said Haidt, author of "The Righteous Mind: Why Good People Are Divided by Politics and Religion" (Pantheon, 2012). This is troubling, he said, because people tend to cluster around their moral in-group and view outsiders with only suspicion, not understanding.

"You engage all these tribal moral dynamics," Haidt said. [The History of Human Aggression]

The basics of morals

The first psychologists to study the psychology of ideology and morals focused on two main issues: harm versus care, and fairness and judgment. Haidt and his colleages, however, have found evidence that humans base their moral code on far more than "does it hurt someone?" or "is it fair?"

In fact, Haidt has added four more moral dimensions to the mix, bringing the grand total of basic moral drivers to six. The first three — harm and care, fairness and justice, and liberty versus oppression — motivate both liberals and conservatives, he said. Liberals tend to care about harm and care most, and conservatives least, but everyone takes these issues into consideration.

How these issues manifest can depend a bit on ideology. Liberals, for example, worry more about inequality for inequality's sake. Conservatives worry more about proportionality, asking if everyone is putting in the work to get their benefits. Both are ways to think about fairness, but that doesn't mean the left and right can't have screaming fights over which is more moral.

Likewise, Haidt said, absolutely no one likes to feel oppressed. But the left tends to talk more about businesses and the rich as oppressors, as in the Occupy Wall Street movement, which protests the wealthiest "1 percent," while the right worries about government oppression, as in Tea Party protests festooned with "Don't Tread On Me" flags. [Rising Rancor: One Nation, Divisible By Politics]

What conservatives care about

Beside these three more-or-less shared values, Haidt has identified three more that matter only to conservatives. (In these studies, conservatism and liberalism refer to social beliefs, such as beliefs about gay marriage, not economic beliefs such as how much someone likes the free market. These social beliefs occur along a continuum, with the moral factors on a continuum of importance as well.)

The first conservative-only belief is loyalty and betrayal. People on the political right feel more strongly about group loyalty than people on the left, who tend to be ambivalent about groups, Haidt said. John Lennon's "Imagine," in which he sings about national borders melting away, is an example.

"It's because of these sorts of arguments that come from the left quite often that the right has a field day charging the left with treason," Haidt said.

The second conservative-only value is authority. Hierarchy and authority tend to be more important on the right — consider religious beliefs that "God is in charge" — while the left prefers to subvert authority. Leftist anarchists, for example, sometimes rally around the slogan "no gods, no masters."

Finally, conservatives worry about issues of sanctity, while liberals are more likely to take a "nothing is sacred" position. In the realm of sexual purity, for example, conservatives are much more likely than liberals to care about sexual sanctity and issues like remaining a virgin until marriage, Haidt said. Even flag-burning falls under the realm of sanctity. The best predictor of how much a conservative will hate flag-burning is how strongly he or she feels that some things are sacred, he said.

The danger of moralized politics

The danger, Haidt said, comes from humans' innate tendency toward tribalism. In the days of the Civil Rights movement and the Vietnam war, there were liberal Republicans and conservative Democrats. Today, that's hardly so. [The Awa: Faces of a Threatened Tribe]

Looking back at political polarization in Congress in the last century, "the bad news is that things get worse slowly and then they get worse quickly," Haidt said. "The good news is that the House [of Representatives] is now so polarized that it can't get any worse."

What that means is that major votes are now almost entirely along party lines. There is debate over whether the American public, not just the political elite, is increasingly polarized, but either way, the result has been a combative climate in Washington, D.C.

The mid-20th century period when polarization was low was an anomaly in history, Haidt said, and it's unlikely we'll get back to that point. But being as polarized as America is now is dangerous, he said. When politics are tied inextricably to morals, everything becomes sacred, from guns to flags to race. And when sacred values are threatened, people lash out. Debates are no longer disagreement, they're treason. And political opponents aren't just people with a different point of view.

"The worst person in the world is not your enemy," Haidt said. "It's the apostate or the traitor on your own team."

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.