In the wake of the bombing in Oslo and the shooting on Utoya Island in Norway, the spotlight has focused on confessed perpetrator Anders Behring Breivik. What drove the Norwegian citizen with extremist right-wing views to these mass killings? Although one of the terrorist's driving motives was anti-immigrant sentiment, he also killed fellow Norwegians belonging to his own ethnic group.

Why do human beings develop this kind of prejudice, and what makes it sometimes erupt into violence? Scientific American spoke with Steven Neuberg, a professor of psychology at Arizona State University in Tempe, about the psychology of anti-immigrant prejudice.

[An edited transcript of the interview follows.]

How would you define prejudice in psychological terms?
Prejudice is traditionally defined in social psychology as a negative feeling towards a particular group and its members. It turns out, though, that there are different kinds of prejudices and different prejudices towards different groups—and these prejudices have very different emotional components to them. For instance, towards some groups, the prejudice is characterized by disgust, others by anger, yet others by fear.

What underlies prejudice against foreigners?

We're highly dependent on people in our own groups. In fact, one could argue that our highly ultrasocial, interdependent form of group living may be the most important human adaptation. People tend to be invested in members of their groups, to have ongoing histories of fair exchanges and reciprocal relations, to treat one another reasonably well, to create and follow a set of agreed-upon norms, and thereby build up trust. Outsiders aren't going to have that same built-up investment in us or our group. Because of this, we tend to believe that people who are foreign to us are more likely to pose certain kinds of threats: We believe they may be more interested in taking our resources, more likely to cheat us in exchanges, to violate our norms and values, to take more than their fair share, and the like. These perceptions of threats are linked to negative emotions such as anger and moral disgust that contribute to anti-immigrant prejudices.

My colleague Mark Schaller at the University of British Columbia has explored an additional threat that people are likely to see in foreigners: People who come from faraway places, who live in somewhat different ecologies, carry different pathogens within their bodies—pathogens that their immune systems have had an opportunity to adapt to but that ours have not. Schaller's work shows that people perceived as being foreign—perhaps because they look different than us, speak different languages, eat different foods—automatically activate perceptions of disease threat. And groups who are perceived to pose disease threats activate prejudices characterized by physical disgust.

The alleged attacker in Norway, Anders Behring Breivik, had strong anti-immigrant prejudices. What was he feeling?
I can't tell you exactly what he was thinking, but as I mentioned, foreign groups coming into one's own society—immigrants—activate perceptions of a wide range of threats and elicit accompanying negative emotions such as anger, disgust and fear, which increases the likelihood of discrimination. If the perceived threats and emotions are strong enough, an individual may believe that he needs to rid his country of those who pose them. Moreover, anger and disgust, together, contribute to feelings of contempt, which we feel towards others we believe to be "less" than us, and can serve to motivate extreme actions.

It's useful to note a couple of things here. First, because immigrants are perceived to pose multiple kinds of threats, they are likely to be on the receiving end of especially pernicious prejudices and acts of discrimination. Second, such reactions to immigrants are nothing new—and we can look not only to current anti-immigrant sentiments throughout the world, but also to our own history in the U.S. Whether it was Italians or Irish, Poles, Jews, Germans, Chinese or whomever, each of these groups were initially perceived to pose a wide range of threats and consequently evoked powerful prejudices. It was only once people came to see these groups as nonthreatening, usually as they were seen to adopt "American" norms, that they were perceived as Americans.

Given his prejudice against immigrants, why did Breivik target ethnic Norwegians, his own people?
I haven't read his writings, but I hypothesize he was going after members of his group he saw as responsible for allowing the immigrant threat to exist. I think he saw the liberal politicians and government bureaucracy—whom he perceived as supporting Muslim immigration, cultural diversity and overall tolerance—as betraying the Norwegian people. Indeed, he attacked the liberal political class: The bomb was set off in a government center and the shootings took place at a camp for teenagers being educated in liberal politics. To Breivik, these folks may have been traitors because, to his mind, they were allowing immigrant Muslims to adulterate and contaminate his country. People seen as traitors are universally despised and stigmatized. Given how much humans, as social animals, invest in and depend upon their groups, betrayal of one's group is seen as one of the worst things one can possibly do. My guess is that Breivik saw the liberal politics of his country as a betrayal of his people, and so he attacked those politics and those engaged in them.

What makes someone like Breivik break and decide to use violence?
It's normal for people to over-perceive threats; our mind is designed to err in that direction. It's also normal for people, when confronted with the kinds of threats we've been discussing, to experience emotions like anger, disgust and fear. But just because we stereotype groups as posing certain threats, and hold certain prejudices against them, doesn't mean that we act on these stereotypes and prejudices in extreme ways. It just doesn't make sense to do so, and the normal mind typically weighs the consequences of engaging in such planned, extreme actions. I suspect that Breivik, and other extremists like him, possess a much lower threshold for perceiving others as threats and perhaps also a much more intense emotional reaction to those perceptions. Moreover, for someone like him, the ability to dive deeply into media that's like-minded, on the Web or otherwise, and to spend time with like-minded others, may significantly reinforce his sense of threat and his belief that something needs to be done about it. Like most rare, extreme behaviors, it takes a perfect storm—a psychological disposition shaped by genes and environment, in concert with current experiences, circumstances and opportunities.

What are some ways we can combat this kind of prejudice?

Prejudice against new immigrant groups is a natural aspect of our psychology. What's natural, however, isn't always good, and we can try to reduce inclinations to those prejudices we find morally problematic. Throughout history, immigrant groups that were once stigmatized very often end up accepted into society, because people come to understand that they aren't actually posing the threats they were once thought to pose. It helps when immigrant groups begin to adopt the norms and practices of their new homes, and the reduction of threat perceptions is furthered as people begin to form friendships across group lines.

How do friendships help?

Friendship entails interacting interdependently with another—sharing, taking turns, self-disclosing, and the like—and such actions reveal that many of the threats initially expected to exist may not be there after all. With friendship also comes a sense of "we," a sense that the person is like me and that we share something important and can trust them. Having a close friend that's a member of another group then provides a model that the group may not actually be as threatening as initially believed. As members of groups come to interact with one another more, the likelihood that they'll form friendships increases, and this will accelerate the reduction of prejudices.

Can we prevent prejudice from turning into violence?

I'm not very confident that we'll ever be able to eliminate the kinds of rare acts of violence we saw in Norway. I am, however, somewhat more optimistic that we'll be able to develop the behavioral and political "technologies" to reduce, or at least to manage, the more typical intergroup prejudices that characterize all of our everyday lives.