“I really tortured others. At night we went out and raided villages. We killed whomever we saw. If we happened to see a woman, we raped her.... Fighting is all there is in the life of a man. Whenever I hear guns go off, I want nothing more than to fight. This thirst lies deep within me.”

The young man who is describing these unbelievable horrors has a gaunt but friendly face. We are in Goma, a bleak city at the eastern edge of the civil war taking place in the Democratic Republic of the Congo. Our team of German researchers from the University of Konstanz and an aid organization, called vivo international, sits shivering in our encampment.

The United Nations houses former child soldiers at this site. They are brought here by truck or helicopter from hundreds of miles away in the bush, and they are nervous. Many of them have been taking drugs daily and are going through withdrawal symptoms. One young man is suffering from pneumonia. We hear the rattling in his chest every time he takes a breath, and he groans from fever and pain. Others have arrived with gunshot wounds that are now infected.

First they are interrogated by U.N. peacekeepers, who are cold and suspicious. “What unit were you in? Who is your commander? How large is your unit? Where is it located? And don't try to lie to us, buddy!” After the questioning, they finally get some food and, if they are lucky, some painkillers. Then they go talk with the whites—the psychologists from Germany—assuming they are willing. As it turns out, they all want to talk.

We have learned from hard experience in war-torn areas that research into violence must involve the perpetrators, and so we are interviewing hundreds of fighters. First, we listen to the sufferings that these Congolese warriors have had to endure. Some of them were not even born when the First Congo War, in 1996, sent the country spiraling into a humanitarian crisis. The realities of war in this enormous, inaccessible region in the heart of Africa can seem completely foreign to many of us: primitive at times, often brutal and sadistic, and impossible to reconcile with any conventional notion of morality. Our challenge is to gain insight into how people respond in the face of constant brutalization.

Fear is a pure form of stress; we all try as best we can to avoid this emotion. Yet fear is key to survival. When we recognize we are in acute danger, our brain triggers a cascade of physical alarms. Our senses sharpen to soak up information about prospective threats, blood rushes to our muscles and the body releases chemicals that suppress pain. At this point, all animals, including humans, are ready to run for cover or stand and fight. An aggressive move is often enough to make an enemy back down.

Lashing out in self-defense is a familiar response, one that researchers refer to as facilitative or reactive aggression. There is another form of violence, however, an “evil” form, which we have called appetitive aggression. It arises from the thrill of the hunt. Even the act of planning an attack can arouse intense excitement.

What is truly frightening is that appetitive aggression is a surprisingly common phenomenon. The more that young men perceive that violence gives them feelings of superiority and pleasure that are otherwise lacking in their daily lives, and the more frequently they engage in aggressive acts, the more they will seek the stimulation that hunting and killing affords. Inflicting pain on our fellow humans is part of our fundamental repertoire of behaviors, every bit as much as caring for the sick and injured. The question is: Why?

Stress and Survival

A few million years ago our hominin ancestors began to hunt and consume meat. For the first time, the human diet had access to a concentrated source of energy that was lacking in our earlier all-vegetarian diet. According to one popular theory, eating meat allowed our energy-intensive brains to grow bigger and more complex, giving us some of the cognitive edge that has helped our species dominate the planet. The most successful hunters could feed more offspring, attract more sexual partners and attain higher status in a group.

Humans do not only hunt other animals, however. When conflicts arise, we also hunt members of our own species. Culture is what constrains us from wanton violence by delineating who is friend and who is foe. We are socialized into a code of conduct that frowns on antagonizing those who belong to our own group.

Little attention has gone to understanding what happens when that code of conduct gets violated on a grand scale. Insights into the psychology of war are crucial if we are to help societies rebuild after periods of conflict. One robust finding in trauma research, for example, is that the greater a person's exposure to life-threatening events, the greater his or her likelihood of developing post-traumatic stress disorder (PTSD). This connection has been observed in soldiers and civilians alike. Yet we wondered if the story were really so simple. In our evolutionary past, the killing of our own kind could easily have evolved as an inborn strategy for securing greater reproductive success. Violence might not always lead to trauma.

Our first opportunity to examine this question was with 269 Rwandan prisoners who were accused or convicted of crimes relating to the genocide in 1994. We asked them about the types of traumas they encountered and crimes they committed. We also assessed the severity of their PTSD symptoms and probed whether they had discovered a taste for violence. Two such queries were “Once fighting has started, do you get carried away by the violence” and “Once you get used to being cruel, do you want to be crueler and crueler?”

About a third of men answered “yes” to the two questions above; significantly fewer women agreed. Two other questions elicited affirmative responses from more than half the men in our survey—they agreed that they tended to get carried away by the violence of a fight and that defeating an opponent was more fun when they saw blood. For women, 30 and 40 percent said yes, respectively. Although women were less likely to develop an appetite for aggression, they were not immune to it.

We noticed some other trends in the data we collected from Rwanda and, later, from Ugandan child soldiers and South African criminal offenders. The more violent events a fighter had witnessed or committed, the higher his or her rating on our appetitive aggression questionnaire. These higher scores also predicted fewer symptoms of PTSD. Having a taste for violence benefited their mental health.

Further, our results show that observing atrocities and engaging in them predicts the enjoyment of cruelty. As much as these findings conflict with our deeply held sense that violence is morally repulsive, they might help us comprehend how conflicts perpetuate—and how so many people can perish so quickly in an event such as the Rwandan genocide.

In 2011 military historian Sönke Neitzel and social psychologist Harald Welzer published excerpts from the transcripts of World War II Allies who had eavesdropped on captured soldiers from the German Wehrmacht. The recordings revealed a fascination with violence and the hunting down of humans among some of the fighters. The revelations caused an uproar, especially among former participants in the war, who feared being labeled as monsters. Yet these transcripts are consistent with our studies.

Another urgent question that we are beginning to investigate is what happens when combatants return to society. Our research hints at a “perpetrator mode” that may wear off once a fighter reenters civilian life. Of the soldiers we interviewed, those who had spent more time in society between the end of the genocide and their arrest scored lower on aggression but showed more trauma symptoms than those who were rounded up sooner. Violence may be perceived differently by a person in perpetrator mode than by someone exposed to the tempering effects of culture.

What all these data suggest to us is that the thrill of the kill is neither a sign of mental illness nor is it uncommon. Our ancestors on the hunt were not so different from contemporary combatants: both groups experience great hardships. Sometimes they must track their prey for days. They must suppress their fear of being injured or snuffed out entirely. For us to be willing to endure these conditions, behaving brutally must somehow become rewarding—potentially even pleasurable.

This is not to say that human beings enjoy murder. Only child soldiers, who are recruited by force before reaching puberty, sometimes describe their first kill in glowing terms; virtually everybody else experiences extreme stress. But incessant battle can break down moral inhibitions and alter our perception of our own actions. We should not be amazed by the brutality of soldiers observed at places such as Abu Ghraib or Guantánamo. War changes everyone.

Altered Values

Half a year after our visit to Goma, we again met the young men we had tested before. Too many of those who had scored high for appetitive aggression had returned to the bush—to armed groups that, though impoverished, hungry and often riddled with disease, nonetheless provided them the opportunity to fight, rape and kill.

The men who remained at the camp reported an overwhelming sense of isolation. They told us about the heroic deeds that other fighters understood but that civilians rejected out of hand. As one of them recounted, “I miss being a soldier. I miss the power! Sometimes I killed people just for the fun of it. Others did as well. The sight of blood can really get you going. So much so that it is impossible to stop killing.”

Because most of us struggle to comprehend what happens in war, soldiers often lack adequate social support once they return to civilian life. They may struggle with emotions that conflict with the values of their surrounding culture. They may fail to adjust to new opportunities and ways of life. Only by understanding the experience of combatants in the heat of war can we start to counsel them, integrate them and curb the immense tragedies brought on by violence.