What makes a killer?

There's a somewhat controversial approach to psychology that says some, if not many, of our behaviors are hardwired into us. According to this science, humans evolved with certain behavioral traits that they passed down to subsequent generations. Over time, these behaviors can change. The approach falls under the umbrella of evolutionary psychology.

There's a lack of consensus about evolutionary psychology even among its proponents. According to David Sloan Wilson, part of the reason there's such controversy around the subject is because of an early popular theory posed by Leda Cosmides and John Tooby [source: Huffington Post]. Wilson states this view of evolutionary psychology is narrow and misleading. Among his complaints with their work include the way he says Cosmides and Tooby limited the environment of evolutionary adaptedness as a series of environments in which people lived over the course of their evolutionary process. As a result, Wilson said, Tooby and Cosmides say the human mind developed many pieces designed to solve particular problems encountered in those environments. Wilson believes Cosmides and Tooby oversimplify elements of the human experience with their theory that there are two basic human natures -- one for men and one for women. In addition, he's also critical of the scope of their theory, saying that they didn't take into account variation across populations and the brain's rapid evolution over a relatively short time. Wilson points out there are many other approaches to explaining human behavior from an evolutionary point of view.

In their chapter on evolutionary psychology as it relates to homicide in the book "Evolutionary Psychology and Violence," David M. Buss and Joshua D. Dunley hypothesize that aggression is a trait inherent in humans that dates back to the prehistoric era. People had to compete with one another for resources. Sometimes this competition became violent. As a result, humans who survived developed parallel sets of skills. One set helped people survive aggression. The other set helped humans inflict greater harm on competitors.

According to this hypothesis, people became more adept at both avoiding and inflicting harm over generations. From a high-level perspective, you could say that based on this hypothesis we are all capable of killing. Many proponents of evolutionary psychology, including Buss and Dunley, don't deny that other factors play a role in turning someone into a killer. They acknowledge that a person's circumstances will influence him or her as well. But at our core, we are all killers.

Critics of evolutionary psychology say that our minds aren't as hardwired as these psychologists suggest. They point to how quickly the mind has evolved since our prehistoric days -- the brain is much faster than the popular presentation of evolutionary psychology can justify. Critics like David Brooks say that humans are too complicated and adaptive for evolutionary psychology to hold much meaning [source: Brooks].

What's the flip side of the coin? What is it that could condition a person into becoming a killer?

Assuming we aren't all killers who refrain from causing harm due to social or psychological restrictions, what could create a killer? According to soldiers like retired U.S. Army Lt .Col. Dave Grossman, it's a process involving four strategies: brutalization, classical conditioning, operant conditioning and role modeling [source: Grossman].

Brutalization is the process in which you lose a sense of your own worth as an individual. Within the military, this is a highly regimented process. New recruits undergo difficult and sometimes humiliating training experiences to wear down a sense of individuality. It helps the military chip away at the resistance most people have to the notion of ending someone else's life.

The military also conditions soldiers to become effective. In classical conditioning, the goal is to associate a desired behavior with a reward. Grossman says this isn't used often in American military training because it seems morally reprehensible to associate violence with rewards. In operant conditioning, soldiers train in simulated environments to develop an automatic response to stimuli. An example is firing at human-shaped targets.

The role model in the military is the drill sergeant. It's the drill sergeant's job to demonstrate aggression while maintaining discipline. Assuming the process works the way the military intends it to, soldiers will look to the drill sergeant as the model for behavior. This combined approach, in theory, will create soldiers capable of killing an enemy in combat.

It's possible that killers who were never soldiers -- including most serial killers or mass murderers -- had experiences similar to those of a military recruit. The major difference is that with these killers, the exposure wasn't in a controlled environment. The backgrounds of many killers show a history of brutalization. In several cases, the killers began to enact violence on weaker creatures as a way of asserting control or demonstrating behaviors learned through being brutalized.

Look into the backgrounds of serial killers, and you'll start to notice some common elements. Many people who eventually become serial killers had traumatic childhoods and were themselves victims of abuse. It's a gross oversimplification to suggest a traumatic childhood is the chief contributing factor to a serial killer's behavior, but there does appear to be a strong correlation.

Many killers have expressed feelings of alienation and have shown evidence of harboring violent fantasies before they commit to killing. In some cases, the killer suffers from a mental disorder or brain damage that either inhibits or prevents the social and psychological restrictions humans have toward killing other people.

Whether we're repressing urges that are more than several millennia old or we're naturally averse to ending the life of another person, it's clear that in most cases we need a catalyst to push us to killing. Identifying and understanding the elements that can change a person at risk into a killer may help us treat and prevent tragedy in the future.

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