When the "enemy" is different, an outsider, it's easier to draw quick conclusions, to develop stereotypes. It's simply human nature: There is "us," and there is "them." But what happens when the enemy looks like us - from the same tradition and belief system?
That is the conundrum in the case of Norway and Anders Behring Brevik, who is being called a "Christian extremist" or "Christian terrorist."
As westerners wrestle with such characterizations of the Oslo mass murder suspect, the question arises: Nearly a decade after 9/11 created a widespread suspicion of Muslims based on the actions of a fanatical few, is this what it's like to walk a mile in the shoes of stereotype?
"Absolutely," said Mark Kelly Tyler, pastor of Mother Bethel African Methodist Episcopal Church in Philadelphia. "It clearly puts us in a position where we can't simply say that extreme and violent behavior associated with a religious belief is somehow restricted to Muslim extremists."
"It speaks to cultural assumptions, how we are able to understand something when it (comes from) us," Tyler said. "When one of us does something terrible, we know that's not how we all think, yet we can't see that with other people."
Psychologists say stereotypes come from a deeply human impulse to categorize other people, usually into groups of "us" and "them."
"Our brains are wired that way," said Cheryl Dickter, a psychology professor at the College of William & Mary who studies stereotypes and prejudice.
When Dickter examined brain waves, she found that people process information and pictures about their "us" group differently compared with information about "them" groups. People remembered information better when it reinforced their stereotypes of other groups, she said, and when information didn't fit their stereotype, it was often explained or simply forgotten.
"That's how stereotypes get maintained in the face of all this (contradictory) information," Dickter said.
So during the first reports that someone had detonated a car bomb and then opened fire at a youth camp in Norway, many assumptions clicked into place.
"In all likelihood the attack was launched by part of the jihadist hydra," Thomas Joscelyn, a senior fellow at the Foundation for Defense of Democracies, wrote within hours on the Weekly Standard website.
The massacre was actually committed, police say, by a blond Norwegian whose photo would not seem out of place in an American college directory. As Breivik's 1,500-page manifesto emerged, calling for violence to rid Europe of non-Christians and those he deemed traitors to Christian Europe, some seized on the religious aspect of his delusions.
Mark Juergensmeyer, editor of the book "Global Religions: An Introduction" and a sociology professor at the University of California, Santa Barbara, wrote an essay likening Breivik to Timothy McVeigh, the American who killed 168 people in the 1995 Oklahoma City bombing. It was the deadliest terrorist attack on U.S. soil until 9/11.
McVeigh and Breivik were both "good-looking young Caucasians, self-enlisted soldiers in an imagined cosmic war to save Christendom ... and both were Christian terrorists," Juergensmeyer wrote.
In a column for Salon.com, Alex Pareene said Breivik is not an American-style evangelical, but he listed other connections to Christianity. "All of this says 'Christian terrorist,'" Pareene wrote.
Such claims drew strong resistance. "Breivik is not a Christian. That's impossible. No one believing in Jesus commits mass murder," Bill O'Reilly said on his Fox News show.
That makes sense to Joyce Dubensky, CEO of the Tanenbaum Center for Interreligious Understanding. She said it also makes sense that "millions of Muslims say Osama bin Laden is not a Muslim, that no one who believes in the prophet Muhammad commits mass murder."
"We need to hear Bill O'Reilly, but we also need to hear and understand the voices of the overwhelming Muslim majority around the world who condemn those who are terrorists in the name of their faith," she said.
People have a hard time seeing extremism in their own religion.
For Christians who think of their faith as preaching peace, how to explain the faith-sanctioned killing of the Crusades? For Muslims, what about the thousands of jihadists now following violent interpretations of Islam?
Or consider the Ku Klux Klan's burning crosses. If those were the actions of a misguided minority, shouldn't the same be said of the 19 men who hijacked airliners on 9/11?
Art Markman, a psychology professor at the University of Texas at Austin, said research shows that when people are asked to describe someone else's behavior, they focus on personal characteristics - who that person is. But when asked to describe their own behavior, people focus on their individual situation.
"If you're a Christian and you see this Norway murderer, you say, I have these teachings and I haven't murdered anyone, so the teachings can't be the problem," Markman said. "But if you're talking about the 'other,' it's different. And if you don't know what the actual Muslim teachings are, it seems like a plausible explanation."
Some Christians say they do know the Muslim teachings, and that they are the problem. "There is a lot of text to justify the link between Islam and terrorism," said Michael Youssef, founder of the Evangelical-Anglican Church of the Apostles in Atlanta. "In the Quaranic text, and in the tradition that was written by the followers."
Many Islamic scholars say violent interpretations are wrong, and Youssef acknowledges that. However, "If your role model is Jesus, then nonviolence will be the way you change things. If your role model is somebody who waged war and killed people, then you say, 'I can do that,'" said Youssef, who was born in Egypt to Christian parents.
But Arsalan Iftikhar, an international human rights lawyer and author of the upcoming book "Islamic Pacifism: Global Muslims in the Post-Osama Era," said the Norway attacks "proved that terrorism can be committed by a person of any race, nationality or religion."
Iftikhar, who is Muslim, said one effect of the tragedy would be "to restart a debate on the term terrorism, and who and when the term should be applied."
"Sadly, the last ten years, the term has been co-opted in public discourse and only applies to Muslims," he said. "Now here we have a right-wing Christian extremist who has committed an act of terror, and many people don't know how to react."