Ever want to explain your villain's—or your hero's—cold-blooded with something other than "he had a bad "? More and more research is suggesting that you can do just that, because some people are just biologically primed to be more aggressive. And specific kinds of training and pressure can encourage people to indulge that tendency.

Biologically, one factor that's getting a lot of is a mutation that causes low activity in the MAO-A . Some people refer to this form of the gene (somewhat romantically) as the "warrior gene," because research suggests that people with that gene may be more aggressive.

MAO, or monoamine oxidase, exists in each (nerve cell) in the brain, and it acts like a recycling factor for neurotransmitters (brain chemicals). In people who have the "warrior gene," less MAO is produced, which means that less of the neurotransmitters serotonin, norepinephrine, and are broken down. As noted above, higher levels of these brain chemicals seem to equal higher levels of aggression. These people feel less for others (if they feel any), and are more willing to harm others on a whim.

Environment or "nurture" is still important, however. A 2010 Scientific American article cites anecdotal evidence from several sources suggesting that killing others isn't as easy as pulling the trigger. Soldiers, for example, must typically be encouraged to kill by "intensified training, direct commands from officers...and propaganda that glorifies the soldier's cause and dehumanizes the enemy." In other words, mind games must be played to break through an innate revulsion—at least with most people.

Here are a few of the psychological principles behind those mind games:

Ethnocentrism is the belief that one's own group is superior. There's a natural tendency to believe that—ask any sports fan whose team is the best!—and a related tendency to view others in the same group with you more positively.

Commitment to one's own group can be strengthened by investment and the insufficient justification effect. The investment model states that our commitment to something depends on how much we have invested. The more time, energy, and suffering one invests, the more committed one becomes to the group, cause, or person. Hazing, which we can loosely define as harassment of prospective members by forcing them to complete physically and/or psychologically meaningless, difficult, or humiliating tasks, strongly increases one's belief in the value of and investment in a group. Because there is not sufficient justification for putting ourselves through such tasks (i.e. no one literally has a gun to our heads), we convince ourselves the group itself must be worth it.

Outgroup homogeneity is the tendency to see the members of an outgroup—a group other than one's own—as all alike. Slurs and epithets—calling a woman by the "b" or "c" word, an African American by the "n" word, or a gay or lesbian individual by the "f" word, for example—strip people in another group of individual names. As English professor Roy Fox writes in his essay Salespeak:

"Names [are] sacred: they communicate the essence of our , not just to others but to ourselveeone of her name [is] to appropriate her identity, to deny her existence."

Ethnophaulism takes things even farther, deliberately creating negative cognitive images of an outgroup. This usually involves portraying the enemy as inferior, stupid, evil, or monstrous.

Most people have a strong tendency to obey when the person giving the orders is in an authoritative position. A famous study by Stanley Milgram demonstrated that ordinary people would shock another person to death if they were pressured by the experimenter. (For the record, the participants weren't really shocking anyone, but they believed they were. Some of the people actually begged to be allowed to stop the shocks, but continued when asked to do so.)

The expectations of a setting can also be extremely powerful. Another study, this one by Phil Zimbardo, demonstrated that when given the opportunity to act as "prison guards" in a mock prison, some participants quickly became sadistic, reveling in their power, and in humiliating the "prisoners." (It should be noted that participants were randomly assigned to either the "guard" or "prisoner" roles, and that the aggression truly seemed to arise in response to the opportunities provided by the situation. Zimbardo dubbed this "The Lucifer Effect.")

Finally, Dave Grossman, author of the 1995 book On Killing, cites the use of video-game-like shooting environments that allow service people to practice their marksmanship and in high- situations. Anyone who hasn't checked out modern video games would probably be astonished by the gory realism of first-person shooter games. (Based on Grossman's assertion that these games both reduce inhibitions and improve shooting accuracy, if there is ever a real zombie apocalypse, my only real hope is that my brother, who's a master at these games, is armed to the teeth...and on my side.)

© 2011 Carolyn Kaufman, Psy.D.

For more information about heroes, villains, and getting the psychology right in your story, read The Writer's Guide to Psychology: How to Write Accurately About Psychological Disorders, Clinical Treatment and Human Behavior.