Humans are, by nature, tribal and never more so than in politics. In the culture wars we all know the tribal stereotypes of what liberals think of con­servatives: Conservatives are a bunch of Hum­mer-driving, meat-eating, gun-toting, hard-drinking, Bible-thumping, black-and-white-thinking, fist-pounding, shoe-stomping, morally hypocritical blowhards. And what conservatives think of liberals: Liberals are a bunch of hybrid-driving, tofu-eating, tree-hugging, whale-saving, sandal-wearing, bottled-water-drinking, ACLU-supporting, flip-flopping, wishy-washy, namby-pamby bed wetters.

Like many other stereotypes, each of these contains an element of truth that reflects an emphasis on different moral values. Jonathan Haidt, who is a psychologist at the University of Virginia, explains such stereotypes in terms of his Moral Foundations Theory (see www.moralfoundations.org), which he developed “to understand why morality varies so much across cultures yet still shows so many similarities and recurrent themes.” Haidt proposes that the foundations of our sense of right and wrong rest within “five innate and universally available psychological systems” that might be summarized as follows:

Harm/care: Evolved mammalian attachment systems mean we can feel the pain of others, giving rise to the virtues of kindness, gentleness and nurturance. Fairness/reciprocity: Evolved reciprocal altruism generates a sense of justice. Ingroup/loyalty: Evolved in-group tribalism leads to patriotism. Authority/respect: Evolved hierarchical social structures translate to respect for authority and tradition. Purity/sanctity: Evolved emotion of disgust related to disease and contamination underlies our sense of bodily purity.

Over the years Haidt and his University of Virginia colleague Jesse Graham have surveyed the moral opinions of more than 110,000 people from dozens of countries and have found this consistent difference: self-reported liberals are high on 1 and 2 (harm/care and fairness/reciprocity) but are low on 3, 4 and 5 (ingroup/loyalty, authority/respect and purity/sanctity), whereas self-reported conservatives are roughly equal on all five dimensions, although they place slightly less emphasis on 1 and 2 than liberals do. (Take the survey yourself at www.yourmorals.org.)

Instead of viewing the left and the right as either inherently correct or wrong, a more scientific approach is to recognize that liberals and conservatives emphasize different moral values. My favorite example of these differences is dramatized in the 1992 film A Few Good Men. In the court­room ending, Jack Nicholson’s conservative marine Colonel Nathan R. Jessup is being cross-examined by Tom Cruise’s liberal navy Lieutenant Daniel Kaffee, who is defending two marines accused of accidentally killing a fellow soldier. Kaffee thinks that Jessup ordered a “code red,” an off-the-books command to rough up a disloyal marine trainee in need of discipline and that matters got tragically out of hand. Kaffee wants individual justice for his clients. Jessup wants freedom and security for the nation even at the cost of individual liberty, as he explains:

“Son, we live in a world that has walls. And those walls have to be guarded by men with guns.... You don’t want the truth because deep down, in places you don’t talk about at parties, you want me on that wall. You need me on that wall. We use words like honor, code, loyalty. We use these words as the backbone to a life spent defending something. You use ’em as a punch line. I have neither the time nor the inclination to explain myself to a man who rises and sleeps under the blanket of the very freedom I provide, then questions the manner in which I provide it.”

Personally, I tend more toward the liberal emphasis on individual fairness, justice and liberty, and I worry that overemphasis on group loyalty will trigger our inner xenophobias. But evolutionary psychology reveals just how deep our tribal instincts are and why good fences make good neighbors. And I know that ever since 9/11, I am especially grateful to all the brave soldiers on those walls who have allowed us to sleep under a blanket of freedom.

Note: This article was originally printed with the title, "Political Science."