Why do normal people sometimes do horrific things to other people? In his book, Less Than Human, Professor David Livingstone Smith explains that even ordinary people can demean, enslave, and kill other human beings. Based on Smith's research, it all starts with one important ingredient—the dehumanization of the victims.

Smith says: "Thinking about your enemies in subhuman categories is a way of creating mental distance, of excluding them from the human family. It makes murder not just permissive but obligatory. We should kill vermin or predators."

He goes on to explain that the early American settlers in Arizona characterized Native Americans as "savage beasts." The Nazis depicted Jewish people as "rats." The Japanese invaders of China called their victims "chancorro," which means something subhuman like a bug or animal. Prior to the 1994 Rwandan genocide, the Hutus who killed Tutsis routinely referred to them as "cockroaches." Americans fought barbarian "Huns" in World War I and godless "gooks" in Vietnam. When we slap a dehumanizing label on people, it's much easier to strip them of their dignity and mistreat them.