Why do we kill?

There's a branch of science called evolutionary biology that suggests many, if not most, of our behaviors come to us from our prehistoric ancestors. According to this line of thought, the reason we kill is because our ancestors killed. By killing, our ancestors removed rivals and ensured the survival of their offspring. In other words, we're violent because all the peaceful ancestors to humans were killed off by the violent ones. We've inherited our nature from our predecessors.

This view is by no means universal. Scientists from different disciplines have criticized evolutionary biology, saying that it oversimplifies human behavior and serves as a genetic excuse for bad behavior. While there is scientific consensus that the human brain is the product of evolution, there's a gap between those who think our brains are in Stone Age mode and those who say the brain is much more flexible than evolutionary biologists admit.

One counterargument to evolutionary biology states that our minds are adaptive and evolve far faster than evolutionary biology can explain. Differences in cultures around the world suggest there is no universal human nature -- the environment and our adaptation to it means that each culture has its own unique nature [source: Begley].

On a superficial level, it seems like the explanation for why we kill boils down to another nature-versus-nurture argument. The nature side suggests that we are inherently a violent species and it should come as no surprise that we sometimes kill one another. The nurture side says that we are an adaptive species and that our environments -- including everything from family structure to political influences -- shape our behaviors. The truth is probably that we're a product of both. Ignoring one set of influences while concentrating on the other is missing the story.

If we're the product of both inherited traits and environmental influences, what would give us the reason to kill? Many answers boil down to survival. In some cases, it's as simple as access to resources. Whether it's a conflict between two people or multiple nations, the reason to kill may be linked to the fact that one party wants what the other party possesses. That might motivate people to kill in order to take or protect those resources. The intellectual and emotional need for those resources is often greater than the reluctance to kill.

Not all violent conflicts are over resources, though. What else makes us kill?

For most people, killing another person -- or many other people -- isn't a trivial matter. But there are times when one person ends the life of another that seem to defy reason. What makes that happen?

A person with antisocial personality disorder feels no empathy toward others. This psychological designation includes people we call psychopaths and sociopaths. They feel very little emotion at all and may seek out dangerous or thrilling situations to get an emotional response. They tend to be deceitful and feel no shame or guilt for misleading others. While they may recognize right from wrong, they may not care about the distinction.

Knowing right from wrong is important in the legal world. It separates sane people from the insane. An insane person, by legal definition, is one incapable of distinguishing reality from fantasy or isn't able to control his or her own actions [source: Hill and Hill]. Just because a person demonstrates sociopathic tendencies doesn't mean that person is insane.

Whether antisocial personality disorder is chiefly an inherited trait or the result of environmental influences is a matter of debate. It's likely a product of both, and not all individuals who suffer from antisocial personality disorder will display violent tendencies. But the lack of empathy and the drive to seek out thrills can lead to violent confrontations. Many serial killers and mass murderers fall into this designation -- they kill because they lack the inhibitions and empathy the rest of us possess.

What about acts of genocide? How do societies justify wiping out an entire subsection of people? According to a hypothesis posed by Ervin Staub, genocide is a result of a combination of environmental hardships and psychological coping. Staub suggests that when times are hard, people look for an excuse or scapegoat. That can include identifying a subsection of the population as being responsible for the hardship the community experiences. Wiping out that population is a way to cope with the hardship. It's a means to solve a problem, even though the solution and problem aren't necessarily connected in reality. Staub points out this process is complicated and takes time -- it's generally not a spontaneous reaction.

What about the rest of us? What could drive us to kill? Since our decisions are based upon both emotions and reason, we can sometimes favor one over the other. In emotionally charged situations, we may allow ourselves to act impulsively, ignoring rationality. These so-called crimes of passion can happen between people with strong emotional bonds. According to the U.S. Bureau of Justice Statistics, 30 percent of all female murder victims were killed by their spouses. Another 18.3 percent were killed by ex-spouses. Only 8.7 percent of all female victims were killed by a stranger [source: Bureau of Justice Statistics].

The different reasons for crimes of passion are numerous. Common motives include jealousy, revenge, fear and anger. These feelings may be conscious or unconscious. The act of killing may be spontaneous or premeditated. Depending upon the circumstances, the killer may be charged with murder or manslaughter -- or could be exonerated if the court finds the killer acted in self-defense. The killer may claim that the crime of passion was an act of temporary insanity -- a condition that's difficult to prove in court.

Humans kill because we're not dispassionate, robotic beings. We have wants and needs and possess the ability to pursue them. We may never know the full explanation of why we behave the way we do, but as we learn more we may find ways to improve ourselves and make murder a thing of the past.

Learn more about violence and psychology by following the links on the next page.