The concept of dehumanization is something that can be found across all of human culture and throughout human history. It's so common that it's amazing no one has seriously studied and written about it, but apparently that's the case — at least until now. But it is definitely a subject that we all need to learn more about because it has played such a significant (albeit negative) role in human affairs.
David Livingstone Smith writes in Less Than Human: Why We Demean, Enslave and Exterminate Others:
In this book, I will argue that dehumanization is a joint creation of biology, culture, and the architecture of the human mind. Grasping its nature and dynamics requires that we attend to all three elements. Excluding any of them leaves us with a hopelessly distorted picture of what we are trying to comprehend. ...
Dehumanization is too important a topic to be left to the experts, so I've tried to make this book appealing and accessible to a broad general readership while at the same time addressing the concerns of specialist scholars in several disciplines.
And what is dehumanization, anyway?
Think of the word dehumanization. It literally means something like "removing the human-ness." Now, take someone and imagine that their humanity has been stripped away from them. What's left? When the founding fathers dehumanized their slaves, what remained of them?
When European colonists dehumanized Native Americans or Nazis dehumanized Jews, what remained? In their eyes, what was left was a creature that seemed human—had a human-looking form, walked on two legs, spoke human language, and acted in more-or-less human ways—but which was nonetheless not human.
As I will explain in detail later on, dehumanization is the belief that some beings only appear human, but beneath the surface, where it really counts, they aren't human at all. The Nazis labeled Jews as Untermenschen ("subhumans") because they were convinced that, although Jews looked every bit as human as the average Aryan, this was a facade and that, concealed behind it, Jews were really filthy, parasitic vermin.
Of course, Jews did not wear their subhumanity on their sleeves. They were regarded as insidiously subhuman. Their ostensible humanity was, at best, only skin deep.
Dehumanization happens all around us all the time; David Livingstone Smith's book doesn't address every sort of dehumanization but rather specific manifestations of it. Nevertheless, his work will still help anyone who wants to understand the subject better, even if they adopt a broader perspective. Dehumanization happens far too often and in far too many circumstances for us to ignore. We have to better understand it because it's the only way we'll be able to fight it — fight it in ourselves and in others.