Less Than Human is a book about dehumanization. It is widely recognized that dehumanization plays an important role facilitating acts of violence in genocide, war, and other forms of atrocity. Given this it is surprising to learn that scant attention has been paid to it in the scholarly literature. Scholarly literature on dehumanization is shockingly thin on the ground. I wrote Less Than Human to set this right, and to urge others to take the investigation of dehumanization seriously. When we dehumanize others, we conceive of them as subhuman animals. One widely-held misconception is that dehumanization is merely metaphorical. But the immense destructive force of dehumanization-its power to incite men and women to perpetrate the most extreme forms of violence and cruelty against one another-is a consequence of the fact that it is meant literally rather than metaphorically. Dehumanization isn't just name-calling. The Nazis really considered Jews to be less than human. It is also how Japanese invaders conceived of their Chinese victim during the Nanjing Massacre (they referred to their victims as chancorro, which means "subhuman"), and how Hutu genocidaires regarded their Tutsi compatriots during the 1994 Rwandan Genocide. This list could be extended indefinitely. I wrote Less Than Human in two stages. In the first, I combed through the literatures of anthropology, history, psychology, archeology, philosophy, and even primatology collecting nuggets information about dehumanization, and then stitched these pieces of information together into a narrative describing the crucial role that it has played in war, genocide, and slavery, from ancient times to the present. In the second stage, I set myself the task of theorizing dehumanization. Once one grasps that dehumanization is primarily a way of thinking rather than merely a way of talking, one is confronted with a formidable explanatory puzzle. What is it about the human mind that makes it possible for one group of people to think of another group as subhuman animals? When the Nazis consigned their victims to the ovens of Auschwitz they were, for the most part, slaughtering people who were physically indistinguishable from themselves. In fact, the ease with which many Jews could "pass" as Aryans evoked anxieties similar to those that haunted the minds of American racists when confronted with the possibility of blacks "passing" as white, and inspired costly and fruitless efforts to identify reliable physical markers of Jewishness. So, dehumanization isn't premised on how people look and how they behave. When we dehumanize others, something else is going on. What is it? The story that I present in Less Than Human proceeds along the following lines. Dehumanization is the upshot of two deep features of our psychology. The first is our penchant for essentialism. Psychologists have shown that even very young children carve up the natural world into kinds of things (philosophers call these categories "natural kinds"). Each of these kinds is imagined to possess an essence that is unique to it: a mysterious inner "something" that only and all members of the kind possess. According to this intuitive folk-taxonomy, to be human is to possess a human essence (or 'soul'), and this is what sets us apart from every other creature. Psychological essentialism opens up a gap between what a thing appears to be and what it really is. If you are an essentialist-as all of us are, deep down-then you will have no difficulty accepting that a being might outwardly appear to be one kind of thing while inwardly possessing the essence of another. This is how essentialism makes room for dehumanization. If essence and appearance can come apart in this way, it follows that a being might have a human appearance without a possessing a human essence. Such a being might look and behave exactly like human beings do without really being human. This way of thinking is a staple of horror and science-fiction plots involving creatures that have a human appearance but aren't human. The fact that we find this literary/cinematic trope congenial-or even intelligible-speaks volumes about our psychology. Psychological essentialism explains how it is possible for us to conceive of human beings as non-human animals. But what is it that accounts for conceiving of them as sub human? To explain this, we need to address an aspect of our moral psychology that has not been given the attention in the scholarly literature that it deserves. It is a psychological fact about human beings that we operate with the idea of intrinsic value. We believe that certain kinds of things have value in and of themselves, whereas other kinds of things have merely instrumental value. Of course, we do not think of everything with intrinsic value as having it to the same degree. Some kinds of things are thought to have much greater value than others. We tend to see intrinsic value as an objective feature of the items that possess it-to consider it a fact (for example) that human lives are worth more than the lives of mosquitoes. Now, judgments of intrinsic value come with moral obligations attached. The greater the value of a being, the more weighty the moral obligations that we feel we owe it. So, the moral obligations that we feel towards mosquitoes are at best minimal, whereas those that we feel toward our fellow human beings are immense. It follows from all of this that we are inclined to order natural kinds on a hierarchy of value. Those with the greatest intrinsic value are situated near the top of the hierarchy, and those with the least value languish near the bottom. Every other kind located at some intermediate rank. The lower the rank occupied by a natural kind, the less moral consideration owed to members of that kind. This scheme was formalized during the Middle Ages as the Great Chain of Being or scala naturae (although the idea of a cosmic hierarchy is vastly more ancient and widespread than many intellectual historians would have us believe). Traditionally, God was placed at the apex of the hierarchy, with humans a little further down, just below the angels. All the other kinds of creatures occupied ranks below that of human beings, arranged from what we still call the "higher" animals to the "lower" ones. These, in turn were ranked above the plants, as plants were ranked above minerals. In the most elaborate versions, each rank was subdivided into sub-ranks, which were in turn divided into sub-sub-ranks. For example, during the 18th century, the rank of the human was divided into races, with white Europeans modestly situating themselves at the very top and relegating sub-Saharan Africans and Native Americans to the bottom, just a hair's breadth above the great apes. The notion of a hierarchy of intrinsic value is required to make sense of the notion of subhumanity. To be subhuman is to be a member of a natural kind that is ranked below the human on the Great Chain of Being. Although the idea that the universe is hierarchically organized is a relic of a prescientific age, and is fundamentally at odds with a Darwinian picture of the biosphere, it remains stubbornly entrenched in our psychology. Moral creatures that we are, we simply can't get away from the idea of a hierarchy of value. Morality would probably be inconceivable without it. When we dehumanize others we conceive of them as having human appearance but the essence of a creature occupying a sub-human rank on the hierarchy of value. With this in mind it is easy to understand how dehumanization fulfills its function of disinhibiting violence. Recognizing another being as human entails having moral obligations towards that being. But if that person is seen as essentially subhuman, he or she can be killed, abused, or enslaved with impunity. Dehumanization thus enables us to reap the advantages of doing violence to our neighbors-advantages such as robbing them of their resources and exploiting their labor without compensation-without being burdened by moral reservations. At present, there is no research center anywhere in the world that is specifically concerned with investigating dehumanization and developing strategies for preventing it. If we want to fashion a future for humanity that is less hideous than its past, this has got to change. We need to make sure that the study of dehumanization is granted a prominent place on the research agenda, and that governments, non-governmental organizations, and universities devote serious resources to it. In the absence of such efforts, resolutions like "Never again!" are likely to be little more than empty talk.