Seeing ghosts is staggeringly common. 60% of widowers admit to having seen a ghost of their dead spouse. ~40% of UK and US citizens believe in ghosts. And about 18% of all Americans claim to have seen a ghost themselves, with even higher percentages in other cultures.
The obvious question arises: what is driving all of these ghost sightings?
For widowers, the answer seems to be something called a "bereavement hallucination". It's a common way of dealing with grief, and ghosts in these hallucinations are generally extremely beneficial. By seeing the ghost, the widower is able to feel the presence of their loved one again, and reduce their overall suffering.
What about people who are not suffering from tragedy? Here the answer is even more interesting. Scientists believe one of the leading causes for seeing ghosts is low frequency sound, also known as infrasound.
This is sound from electrical machinery, severe weather, and other normal phenomena. These sounds are below our range of hearing, but they do affect another body part: our eyes. Infrasound will literally vibrate your eyeballs to create ghostly apparitions!
There are other useful explanations - electromagnetic fields, sleep paralysis, and carbon monoxide poisoning among them. But vibrating eyeballs is easily the coolest.
As you probably guessed, I don't believe in ghosts. But after learning about the causes of ghost sightings, I do believe other people when they say they've seen a "ghost".