Belief in ghosts is probably related to a powerful bit of human social cognition called theory of mind, the intuitive sense that other people have minds, complete with beliefs, desires, and internal states. On the subject, Steven Pinker has written,
Perhaps the ubiquitous belief in spirits, souls, gods, angels, and so on, consists of our intuitive psychology running amok. If you are prone to attributing an invisible entity called “the mind” to other people’s bodies, it’s a short step to imagining minds that exist independently of bodies. After all, it’s not as if you could reach out and touch someone else’s mind; you are always making an inferential leap. It’s just one extra inferential step to say that a mind is not invariable housed in a body.
So it is, partly, a misfiring of a basic adaptive faculty. That is why believers (and I was once one) tend to be convinced more by feeling and intuition than evidence or careful consideration of the implications. There are many nonsensical aspects of the concept of disembodied spirits and haunters oft written about. Here are a few of mine that I have never seen mentioned. Useful if you ever want conjure some serious cognitive dissonance in the minds of believers you know, do some haunting of your own.
1. Why aren’t the places where humans have lived (and died) the longest the most haunted?
There have been permanent human settlements in places in present-day Egypt and Iraq for at least 11,000 years. If ghosts accumulate in recently-founded cities just centuries old, then surely cities 100 times older have 100 times as many ghosts (per capita). Cairo should have 1000 times more ghosts than it has people. They ought to be everywhere, all the time. Why aren’t they? This is to say nothing of animal ghosts… animals have been living and dying for hundreds of millions of years. There should be trillions of animal ghosts around us all the time. It will be a great boon to the paranormal industry when the “ghost hunter” sect cottons on to the idea of dinosaur ghosts, roaming the earth, ruing the unfairness of their meteoric demise.
2. Why can’t humans interact with them directly, physically?
There are at least three good reasons to think this should happen. One, ghosts stand on the floor or ground. Hard to do that if you can’t interact with physical stuff. Two, people and cameras, allegedly, “see” ghosts. But cameras, retinas and visual cortices are physical things made of matter that can only interact with matter (and photons). Why can a ghost interact with a cell in a retina, but not a cell in the skin? Three, and most curious of all, humans are ghosts. Just ones that still have a shell. I presume most believers think that upon death the physical body gives up its spirit. But wait, that means the spirit was always there. Why can’t the spirit part of our being always interact with the spirit that is the ghost? It’s the same type of stuff, right?
Of course, many would say they can interact, but then the question is why would it only be sometimes and not all the time?
3. Why should natural selection favor corporeal bodies at all?
It seems like a sucker bet. A ghost can 1) have a complete mind, 2) interact with matter, and 3) reproduce*, and all without having to worry about eating, sleeping, extinction, predators, and so on. Having a body is a stupid move. It is needless, and only has downsides.
4. Why can Stephen Hawking write entire books while being able to do little but twitch his eye, but ghosts who can slam doors, name letters, and create visuals not tell us anything important? If you died and learned the secrets of death, wouldn’t you do your best to communicate the astonishing truth to loved ones?
5. Why does death turn people into insufferable jerks?
As noted in #4, ghosts could reveal answered to some of the biggest questions humans have about their existence, but instead choose to spend eternity slamming doors, making floors creak, and fiddling about with lights. Even if you no longer felt connected to humanity, why would you care? Why would you bother? If you did care, why not haunt someone like Kim Jong Un instead?
This is sometimes explained as a result of rage or anger over an untimely, unfair, violent, or murder-type death. But this doesn’t wash with observations of how people really respond to trauma, including life-threatening experiences in which they came close to death. They tend to get over it, and get on with their lives without any radical shift toward sociopathy. If anything, the reality of an afterlife would make being murdered much more bearable. It would mean it wasn’t the end of the line for you, which, whatever the other details about it are, is probably better than the alternative.
* We know this because, for example, there was once just 10,000 humans and today there are more than 7 billion. Where did the extra souls come from? They must have reproduced somehow.