In a recent tweet, said that Democrats want illegal immigrants to "to pour into and infest our Country." Talk of infestation is an escalation in his already extreme rhetoric against illegal immigrants. Previously, Trump had called them criminals, drug dealers, and rapists, but the term "infest" is usually applied to swarms of insects and animals that cause damage and disease. Then immigrants are less than human.

David Livingstone Smith's book, Less Than Human, documents and analyzes numerous cases of dehumanization, the practice of depicting groups of people as lacking in the essence of human beings. Europeans and Arabs viewed Africans as subhuman in order to justify enslaving them. The Nazis depicted Jews as rats and vermin in order to encourage their extermination. In Rwanda, the Hutus branded the Tutsis as cockroaches in order to mark them as deserving elimination.

What are the psychological processes that drive dehumanization? The cognitive processes include categorization, imagery, and metaphor. The dehumanized group is classified, not as members of the human species, but as non-human animals. The categories used are not just verbal, but carry with them potent images such as long-nosed rats and swarming cockroaches. Saying that immigrants are infestations is not literally true, but metaphorically has substantial impact.

The impact is emotional. The point of the categorizations, images, and metaphors that are applied to dehumanized groups is to generate the same kinds of emotions that people normally apply to non-human agents that produce damage and disease. Dehumanization depends on emotional analogies that transfer the negative feelings that go with vermin to the group that the speaker wants to attack. Marking immigrants, Jews, Africans, or Tutsis as systematically similar to insects transfers the emotions that apply to vermin to the scorned group of people. Characterizing people as akin to animals that are unclean, prey, or predators carries over the emotions that go with those categories.

The transferred emotions include disgust, , hatred, and . These form a hideous package that can be used to inspire and justify extreme measures against despised groups, ranging from separating children from their parents to slavery to gassing. Dehumanizing groups of people produces a kind of emotional Gestalt shift, replacing the respect and compassion that normally go with recognizing people as human, with a different emotional package that applies to threatening subhuman species. Propaganda campaigns were used by the Nazis, Hutus, and other aggressive parties to bring about this kind of emotional shift.

How can dehumanization be fought? One basic tool is , which is also a kind of emotional analogy. Putting yourself in someone else's shoes helps you to see others as analogous to yourself, and therefore deserving of the same kind of human rights. In turn, rights are not based on some kind of abstract human essence, but on the fact that all human beings have the same fundamental needs. These include physical needs for food, water, shelter, and health care, but also psychological needs for relatedness to other humans, autonomy, and competence. Separating children from their parents dramatically deprives them of their ability to satisfy their psychological needs. There is no such thing as an infestation of children.