I'm sure much scholarly work has been done on this topic, and I'm probably
reinventing a wheel in a slightly oval shape, but this page is for my
musings, so I'm not going to go researching the work of others for this. :)
On the one hand, people are inclusive. We form groups easily, we're very social. On the other hand, our comfort level for groups seems to be fairly small, a legacy of our tribal past or maybe a reason why we stayed at the tribal size for so long before someone invented agriculture and cities and stuff. So we look for reasons to split the teeming throngs of humanity into "us" and "them" in a bunch of ways. Sadly, one of the simplest unconscious shorthands for enforcing this sort of division is to not think of every member of Homo sapiens as being "truly" human.
I think that the way we view others of our species can be broken up into five rough bins, a Humanity Scale. Everyone has at least a few people in every group, if only extreme edge cases for some people.
It's important to note that for most people, this sorting task is done without conscious thought. If asked, they might claim to believe that everyone (except demonstrated monsters) is truly human and deserving of all human rights...but their actions and reactions likely demonstrate otherwise. "It's a shame the prisoners left behind will die in the hurricane...some guy will have to clean that mess up," is a statement that indicates the speaker places prisoners in category 4 or 5, for instance.
Category 1: Truly Human
This category almost always includes oneself, loved ones and very close friends. You know you're human, and you know these people well enough to vouch for them. True humans deserve all rights and responsibilities designated to human beings. Their deaths should be mourned, and if you believe in souls you know they have souls and hope those souls go to one of the good afterlives.
Category 2: Probably Truly Human
This category is for people you maybe don't feel you know well enough. Maybe they're monsters who are just good at fooling people. But you're not dubious enough to deny their humanity. In terms of your actions, you treat them the same as the Truly Human, but won't feel as bad if you have to put your own needs or the needs of category 1 people over the needs of a category 2 person.
Category 3: Technically Human
A sort of second-class human. The laws protect them, even if they don't necessarily deserve it. If you get to know them, they might rise into category 2 or even 1 eventually, but they're definitely Others. Always "them" and not "us" until proven otherwise. You'll observe the proper forms when dealing with Technical Humans, but it's fairly easy to keep your conscience untroubled if their rights get trampled on in the service of some other cause like Security or The Economy. If you believe in souls, you may say you think category 3's have souls, but you might not really believe it.
The more "stacks" of privilege you have, the more likely you are to not even realize that you've placed huge swaths of humanity into category 3. It's not about consciously looking down your nose at Those People, but privilege tends to breed distance from the unprivileged. You have to make an effort to look beyond that privilege to truly accept Others into your definition of humanity. If raised in a middle-class white neighborhood in a mostly white city, an urban hispanic moves and acts and talks in ways that don't match your learned definition of human.
Category 4: Talking Bipedal Animal
Here's where it starts to get pretty ugly, and if you ever catch yourself thinking of anyone as being in that category, you need to sit down and seriously examine your motives and beliefs, because you are now excluding a member of your own species from consideration as a person. While you will treat them as a human under the law, it takes very little provocation to decide you can ignore that law for a good enough reason. You can justify all sorts of inhumane treatment for category 4 subhumans: deport them, pack them ten to a cell, shoot 'em if they step one foot onto your property, make them fill out a dozen forms in line at the grocery store just to feed their kids, etc. Things you would never countenance being done to any True Humans are okay, because these are just animals who look and sound human. Heck, you might even accept treatment of them that you wouldn't allow for a pet dog or a dairy cow. As for the soul issue...they're animals, what soul? Maybe they have an inferior sort of animal soul, or one doomed to Hell.
While everyone is at least somewhat racially prejudiced, if you're okay with inhumane treatment of someone purely because of the color of their skin, you're definitely a full-on no-doubts racist.
Category 5: Monster In A Human Suit
This isn't a human. This is a monster, and killing them or letting them die is morally right and proper. However, many of these monsters have fooled people into thinking they're actually human, so killing them is against the law. If you thought you could get away with it, you'd shoot this sort of "person" on sight and claim it was self-defense. While you might not weep if a category 4 gets killed, you will cheer (if only in private) at the news of a dead monster.
Some Homo sapiens really are objectively monstrous. Mass-murderers or rapists, sociopaths who manipulate nations into war or famine, people who deliberately infect innocents with fatal diseases. And even these wastes of carbon can be seen as Technically Human by someone with a big enough heart, but most of us have a short list of monsters we'd love to see die painfully.
Oh, and anyone who places an entire group in category 5 because of the color of their skin, their religion, their sexual or gender identity, or any other demographic quality? They're the real monsters in skin suits. Bigots and racists beyond the bounds of humanity.
This essay was inspired by discussion of what happens to prison inmates in the path of hurricanes. Being put on trial for a felony tends to drop most people by a category in the minds of Americans, and being convicted can drop someone by two categories. For a nation with so many prison inmates, America sure has an aggressively negative view of anyone who ever spent time in prison. It's as if every conviction brings a revocation of one's status as a human being. Combine this with the fact that our prisons are disproportionately full of non-whites, and it's easy to find an American who will cheer at news of a flood sweeping through a prison and killing all the inmates. "Too bad for whoever has to clean up the mess," will be their only pang of regret.