When people dehumanize others, they think of them as subhuman creatures or inanimate objects rather than as human beings. European settlers in the New World thought of their African slaves and indigenous Americans as less than human, and more recently Rwandan genocidaires characterized their victims as cockroaches and snakes, and members of the German Nazi party conceived of Jews as dangerous creatures akin to vermin (to give only three of very many historical examples). Even though it is widely accepted that dehumanization plays an important role in genocide, war, and human rights abuses, there has been surprisingly little research into the nature of the phenomenon, its causes, psychological dynamics, and social functions. In particular, there has been practically no work integrating of psychological research with scholarly work on dehumanization by historians, philosophers, and anthropologists. Dehumanization has often played a role in “negative” eugenics—the sterilization or killing of people deemed to be members of genetically inferior groups.

History of dehumanization
The term “dehumanization” was coined in the early 19th century, and has acquired a wide range of meanings since then. These include: treating certain people in degrading ways (for instance, merely as means to an end), referring to them as non-human animals or as inanimate objects, denying that they possess distinctively human characteristics, treating them in degrading ways that cause them to experience themselves as less than human, denying that they have mental states, conceiving of other people as less human than oneself, (7) conceiving of them as inanimate objects, and (8) conceiving of them as subhuman animals.

These meanings are not equivalent, and for the most part do not entail one another. For example, it is possible to refer to people as subhuman animals (by using such language to denigrate them) without conceiving of them as subhuman animals. Although any and all of these senses of “dehumanization” are pertinent to the treatment of supposedly inferior groups by eugenicists, almost all of the psychological research into dehumanization is concerned with (7) and (8), so this article is mainly focused on dehumanization understood in these ways.

There is evidence that people have dehumanized others at least since the beginning of civilization. There are references to enemies and foreigners as subhuman creatures in the literatures of ancient Egypt, Mesopotamia, and China, and it is also present in the cultural practices of indigenous people all over the world.

Dehumanization and psychology
Dehumanization seems to have been first described as a psychological phenomenon by the 17th century Anglican clergyman Morgan Godwyn, who wrote that slaveholders believed that Africans are “Creatures destitute of Souls, to be ranked among Brute Beasts, and treated accordingly”. The Scottish philosopher David Hume also mentioned our tendency to dehumanize enemies in his 1738 Treatise of Human Nature.

Moving on to more recent work, few psychologists investigated dehumanization prior to Jacques-Philippe Leyens in the 1990s. Leyens studied attitudes toward what psychologists call “in-groups” and “out-groups.” A person’s in-group consists of all of the people that he or she includes as “one of us,” and a person’s out-group consists of all the people that she or he regards as “one of them.” Leyens demonstrated that we tend to attribute only “primary” emotions to members of our out-group (primary emotions are very basic emotions such as anger and fear that we also attribute to many non-human animals), and he found that we are more inclined to attribute “secondary” emotions (more cognitively complex, distinctively human emotions) to in-group members. Leyens interpreted these findings as showing that we are disposed to think of outgroup members as “less human” than in-group members. He called this phenomenon infrahumanization to distinguish it from explicit characterizations of other people as nonhuman animals.

Nicholas Haslam extended Leyens’ work in several key respects. Most importantly, he argues that we operate with two distinct conceptions of what it is to be human, each of which corresponds to a distinctive form of dehumanization. Human nature traits are traits that we share with non-human animals and which distinguish both human beings and nonhuman animals from inanimate objects, whereas uniquely human traits are traits that set us apart from other animals. When we deny that others possess human nature traits, we think of them as machines or other inanimate objects, which Haslam calls this mechanistic dehumanization. When we deny that others possess uniquely human traits we think of them as non-human animals rather than as inanimate objects, which Haslam calls this form of dehumanization animalistic dehumanization. Haslam’s work on dehumanization is very influential, and most contemporary psychological work on the topic draws on it.

Violence, moral disengagement, and psychological essentialism
Virtually all researchers agree that dehumanization is intimately bound up with mass violence and human rights abuses. Most hold that this is explained by the fact that dehumanization causes people to become morally disengaged from those whom they abuse. Conceiving of other people as inanimate objects or nonhuman animals makes it morally permissible to treat them in ways that it is impermissible to treat fellow human beings.

There are three broad views on the relationship between dehumanization, moral disengagement, and human rights abuses. On one view, dehumanization has the function of disabling inhibitions against harming, killing, or exploiting others. According to this view, we dehumanize others in order to make it easier for us to harm them. Another view is that dehumanization has the consequence of facilitating harm, but this is not its function (that is, we do not dehumanize others for the purpose of harming them). A third approach is that dehumanization justifies or excuses violence post hoc rather than being a cause of it.

Much of the psychological literature on dehumanization draws on the theory of psychological essentialism. This theory states that people tend to think of living things (including human beings) as divided into kinds (for example, biological species), and think of each of these kinds as having a causal essence. The causal essence of a kind is supposed to be something that only and all members of the kind possess, which makes it the case that they belong to that kind, and which is, in some sense, “inside” them—for example, in their genes or in their blood. According to some researchers, when we dehumanize others we believe that they do not have a human essence or hold that they have a non-human essence. However, there are controversies about how the notion of psychological essentialism should be used in the context of theories of dehumanization. The eugenics movement was explicitly married to a version of essentialism, in which genes played the role of essences. Some groups were regarded essentially (genetically) inferior, and were to be prevented from transmitting their undesirable essence to future generations, and others, who were regarded as essentially (genetically) superior were encouraged to proliferate their desirable essence by producing children. In reality, equating genes with essences is scientifically erroneous.

Conclusion
When we dehumanize other, we conceive of them as subhuman animals or inanimate objects. This way of thinking enables us to treat the dehumanized population in ways that it would be morally impermissible to treat other human beings—for example, killing them, sterilizing them, or otherwise violating their basic human rights. Dehumanization has often played a role in eugenics, by stigmatizing those who are regarded as genetically inferior as less than human. Psychological research into dehumanization began in the latter part of the 20th century, and there is now a robust literature on it by social psychologists. Recently, philosophers have begun to take an interest in the subject, and are beginning to contribute to our understanding of it. The increasing attention being paid to this phenomenon by psychologists and philosophers, after centuries of relative neglect, promises to transform our understanding of it in years to come.

-David Livingstone Smith

• Bain, Paul G. et al. (eds.), 2013, Humanness and Dehumanization. New York: Psychology Press.

• French, Shannon E. and Jack, Anthony I., 2014, “Dehumanizing the enemy: the intersection of neuroethics and military ethics.” In Whetham, David, Responsibilies to Protect: Different Perspectives. Leiden: Martinus Nijhoff.

• Haslam, Nicholas, 2013, ‘What is dehumanization?’ In Bain, Paul, et. al. (eds.) Humanness and Dehumanization. New York: Psychology Press, 34-48.

• Kelman Harold. C., 1973, “Violence Without Moral Restraint: Reflections on the Dehumanization of Victims and Victimizers”, Journal of Social Issues 29, pp. 25-61.

• Jack, Anthony I. et al., 2013, “Seeing Human: Distinct and Overlapping Neural Signatures Associated with Two Forms of Dehumanization”, Neuroimage 79, pp. 313-328.

• LeMoncheck, Linda, 1985, Dehumanizing Women: Treating Persons as Sex Objects. Totowa, NJ: Rowman and Allenheld.

• Leyens, Jacques-Philippe, et al, 2000, “The Emotional Side of Prejudice: The Attribution of Secondary Emotions to Ingroups and Outgroups”, Personality and Social Psychology Review 4, pp. 186–197.

• Mikkola, Maria, 2011, “Dehumanization”, In Brooks, Thom (ed.), New Waves in Ethics. New York: Palgrave-Macmillan, pp. 128-149.

• Smith, David L., 2011, Less Than Human: Why We Demean, Enslave, and Exterminate Others. New York: St. Martins Press.

• Smith, David L., 2014, “Dehumanization, Essentialism, and Moral Psychology”, Philosophy Compass 9, pp. 814-824.