Why? Why did they do it? Why would two teenagers deliberately plan and methodically carry out a murderous attack on their classmates? Why didn't anyone see this coming? Why didn't anyone intervene and prevent them from killing? How can we prevent anything like that from happening again?

Experts in crime, mental health, education, and social sciences have all been trying to understand the pathways to school violence. A few common observations emerge. The first, and most disturbing, is that human beings, like few other species, are pervasively aggressive, violent and murderous to each other. The major predators of humans are other humans.

The second important point is that all violence is not the same. Some violence is due to impulsive behavior, some due to the disinhibition by drugs or alcohol, some due to serious mental illness, some to hate, revenge, or retribution. How any individual comes to kill is a complex combination of circumstances, and it is almost impossible to know exactly "why" for any given act of violence. We will never really know the full answer to "why" for the murders at Columbine.

Yet this should not stop us from trying to understand and prevent violence. We know that not all humans kill. And some societies are more violent than others. So what do we know about the conditions that increase violence? What observations are common across cultures and through history when violence emerges?

1. When we become desensitized to death or killing, violence increases. When death and violence surrounds someone, the value of human life can diminish and the horror of violent death can decrease. In Europe during the Black Plague, up to half the people in a village could die in a few months. The survivors often migrated to medieval cities and, soon afterwards, the rates of violence and murder skyrocketed, exceeding the rates of killing in modern New York. Pervasive death desensitizes. Pervasive violence desensitizes. In the United States, while we have been spared the horrors of war in our land and plague, we will self-expose ourselves to remarkable violence. We will watch 200,000 graphic violent acts on TV by age 18. Too many of us have become desensitized to violent acts, not realizing the true effects of a bullet passing through a human body.

"That's so cool. Look at his head explode." Spoken by a nine-year-old boy watching TV. His aggressive behaviors in school were so disruptive that he was placed in a special classroom.

Being part of the solution: Don't watch so much violence. It is everywhere, but try to watch less. Certainly if you are watching and someone younger is in the room, turn the channel, get them out and help younger children see less violence. You may be able to understand something is "just television," but a young child cannot. Try to learn something about the real impact of violence. Listen to the mother of a murdered child. Find a classmate who has lost a parent or sibling to violence — maybe they can tell you what violence is really like. Your community may have a Survivor of Violence group; see what they can tell you. Try to see what a bullet really does. A little research can teach you more about violence than a lifetime of TV or movies.

2. When we become more detached from each other and from common unifying beliefs, violence increases. Without being connected to others, we care less for their welfare. When we share common bonds of belief and value with others, we are less likely to be aggressive or violent to others in our community. When individuals become isolated, marginalized, and without some connection to those around them, violence increases.

After seeing the crying parents of the girl he had beaten, strangled, and stabbed to death, an 18-year-old murderer muttered, "I don't know why they're crying — I'm the one going to jail."

Being part of the solution: Be part of something — at school or outside. Spend time with friends, in structured and non-structured activities. Talk, listen, laugh and be together. Time with friends, family, teammates, and classmates promote healthy social or emotional relationships. Along the way, identify isolated or marginalized kids — you know who they are. Reach out and include them in something. Look them in the eyes; talk to them between class; sit with them at lunch. You will be surprised at how much you both can grow up.

3. When we allow hateful ideologies to make groups or classes of people to be viewed as different, bad or even less than human, violence increases. All too often, violence is linked to hate. Hateful beliefs such as racism, anti-Semitism and misogyny allow whole groups to be dehumanized. The more any group is misunderstood, the more the unknown can fuel fear and misunderstanding. In high schools, this can happen when cliques form — jocks, preps, geeks. Fear and misunderstanding can lead to hateful words and violent behaviors.

"They were just camel jockeys. They don't belong in this country anyway. I don't see what the big deal is. It's not like we robbed a priest." Comments from an interview with a fifteen-year-old boy who participated in an armed robbery at a convenience store run by a family from Lebanon.

Being part of the solution: Be intolerant of intolerance. Learn more about other religions, cultures, and worldviews. Be wary of individuals with hateful beliefs. Prevent degrading, humiliating, or bullying behaviors. Don't laugh at jokes that use hateful ideas — and certainly don't repeat them. Don't be afraid to call someone on a hateful or degrading comment about another group, religion, or culture. These hateful beliefs are like a cancer; they are never benign. They can spread, invade, and destroy. Stop them before they spread. At the heart of this tolerance is respect. If we treat each other with respect, we will be enriched by each other's beliefs rather than diminished.

4. When we are under the influence of alcohol or drugs, violence increases. Alcohol makes all people stupid and some people violent. A huge percentage of impulsive violence takes place under the influence of alcohol or other drugs. This is a particular problem with first-time or inexperienced drinkers. With little experience measuring the effects of alcohol, youth often drink too much, too often, and in the wrong places.

"I don't remember why it even started. We had a lot of beers and smoked some reefer. I didn't think he would die." A comment from a 17-year-old-boy who was one of three who beat a classmate to death at a party after a fight broke out — apparently about a parking place.

Being part of the solution: Stay away from alcohol and drugs. And if you won't, be moderate in your use, and be with people you know and trust in places that are safe. Stay off the roads. Don't ever pressure someone else to drink or use. Let them make up their own choices. And be prepared to live with the consequences of your choice. Grown-up behaviors have grown-up consequences. Hundreds of youth die each year due to the influence of alcohol or other drugs.

Over the last few generations, two new observations have emerged. There are some unique properties to our recent wave of violence in the United States.

5. We have become more efficient and practiced at killing behaviors. Five thousand years ago, a drunk, isolated, hateful person could try to kill, but would be limited by the means at hand. There were no handguns, no automatic weapons, and no explosives. They could kill one or two in an incident. Today, in a single fit of rage and hate, one person with automatic weapons can kill dozens of people. Today, one hateful person can bomb a building and kill hundreds. We have more available and efficient means to kill. And we are practicing. In the games we play — paintball, video games, and simulated war games — we are becoming practiced in the behaviors required to kill.

"It was pretty strange. I just raised up the rifle and shot. Just like I had a million times when I was a kid. It was just a little pop. And he just looked at me. And then slumped down. I was just trying to warn him. I didn't think it would kill him." From an interview with a 13-year-old boy who killed another youth.

Being part of the solution: Decrease the amount of time spent playing violent video games or practicing lethal behaviors. If you see younger children "playing" at killing, see if you can help them find other ways to channel their energies.

6. We have easy access to handguns. There is ready availability of lethal weapons in our society. Handguns, rifles, automatic weapons are all easily purchased — legally and illegally. Children and youth can get guns. When someone is angry, drunk or hateful, the gun allows him or her to act in lethal ways. What may have been a fistfight becomes a murder. The availability of guns increases the probability of lethal violence.

"My dad just kept it in the drawer by his bed. I wanted to scare these guys that were messing with me at school. So I put it in my backpack and took it to school." From an interview with a nine-year-old child who took a loaded handgun to school.

Being part of the solution: Don't play with guns. Use guns with supervision. Never take guns to school. Never mix drinking and shooting. Don't carry a weapon. And if your family has a gun, help your parents come up with a safe place to keep it.

While we may never understand Columbine, we do know that we can help prevent more violence. We are not helpless. We know that acting in these six areas can decrease violence. Each of us plays a role. We are all part of a solution to school violence.

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Note: This is a fuller version of Dr. Perry's article, "Why?" which appeared in Scholastic Scope, vol. 48 (15).

Dr. Bruce D. Perry, M.D., Ph.D., is an internationally recognized authority on brain development and children in crisis. Dr. Perry leads the ChildTrauma Academy, a pioneering center providing service, research and training in the area of child maltreatment (www.ChildTrauma.org). In addition he is the Medical Director for Provincial Programs in Children's Mental Health for Alberta, Canada. Dr. Perry served as consultant on many high-profile incidents involving traumatized children, including the Columbine High School shootings in Littleton, Colorado; the Oklahoma City Bombing; and the Branch Davidian siege. His clinical research and practice focuses on traumatized children-examining the long-term effects of trauma in children, adolescents and adults. Dr. Perry's work has been instrumental in describing how traumatic events in childhood change the biology of the brain. The author of more than 200 journal articles, book chapters, and scientific proceedings and is the recipient of a variety of professional awards.