During the Holocaust, Nazis referred to Jews as rats. Hutus involved in the Rwanda genocide called Tutsis cockroaches. Slave owners throughout history considered slaves subhuman animals. In Less Than Human, David Livingstone Smith argues that it's important to define and describe dehumanization, because it's what opens the door for cruelty and genocide.

"We all know, despite what we see in the movies," Smith tells NPR's Neal Conan, "that it's very difficult, psychologically, to kill another human being up close and in cold blood, or to inflict atrocities on them." So, when it does happen, it can be helpful to understand what it is that allows human beings "to overcome the very deep and natural inhibitions they have against treating other people like game animals or vermin or dangerous predators."

Rolling Stone recently published photos online of American troops posing with dead Afghans, connected to ongoing court-martial cases of soldiers at Joint Base Lewis-McChord in Washington state. In addition to posing with the corpses, "these soldiers — called the 'kill team' — also took body parts as trophies," Smith alleges, "which is very often a phenomenon that accompanies the form of dehumanization in which the enemy is seen as game."

But this is just the latest iteration in a pattern that has unfolded time and again over the course of history. In ancient Chinese, Egyptian and Mesopotamian literature, Smith found repeated references to enemies as subhuman creatures. But it's not as simple as a comparison. "When people dehumanize others, they actually conceive of them as subhuman creatures," says Smith. Only then can the process "liberate aggression and exclude the target of aggression from the moral community."

When the Nazis described Jews as Untermenschen, or subhumans, they didn't mean it metaphorically, says Smith. "They didn't mean they were like subhumans. They meant they were literally subhuman."

Human beings have long conceived of the universe as a hierarchy of value, says Smith, with God at the top and inert matter at the bottom, and everything else in between. That model of the universe "doesn't make scientific sense," says Smith, but "nonetheless, for some reason, we continue to conceive of the universe in that fashion, and we relegate nonhuman creatures to a lower position" on the scale.

Then, within the human category, there has historically been a hierarchy. In the 18th century, white Europeans — the architects of the theory — "modestly placed themselves at the very pinnacle." The lower edges of the category merged with the apes, according to their thinking.

So "sub-Saharan Africans and Native Americans were denizens of the bottom of the human category," when they were even granted human status. Mostly, they were seen as "soulless animals." And that dramatic dehumanization made it possible for great atrocities to take place.

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