In "The Psychological Cost of Learning to Kill in War and Society," Lt. Colonel Dave Grossman, a former Army psychologist and professor at West Point, writes that as humans we have an aversion to intraspecies killing. He notes an analysis of battlefields reveals that most of the killing is done by a few soldiers. In battles of long duration in which the majority of soldiers hesitated in killing the enemy, Colonel Grossman points out that a majority of deaths on the battlefield result when one side turns its back and flees. Soldiers feel less squeamish about shooting someone with whom they do not have eye contact.
He writes: "If one does not have to look into the eyes when killing it is much easier to deny the humanity of the victims." This explains why, for example, Nazi and Communist executions were done with a bullet to the back of the head.
Colonel Grossman reports that the willingness to kill can be increased with advanced training techniques making killing a conditioned response. He notes that during the Vietnam War, firing rates increased from 15 to 20 percent in World War II to more than 90 percent because of the advance training these infantrymen received.
Turning to increased acceptance of violence in America, Colonel Grossman writes that a similar conditioning to killing has resulted in "an epidemic, a virus of violence" that has been unleashed in America. His theory is that violence in movies, video games and television have had the same effect as the young in America on the advanced training had on soldiers sent to Vietnam.