I have linked before to Derren Brown videos in which he demonstrates how people can be hypnotized without their realizing it into some pretty remarkable hallucinations and delusion based behaviors.
http://www.youtube.com/watch?v=3Vz_YTNLn6w&feature=related
http://www.youtube.com/watch?v=XRilLMNRFPc&feature=PlayList&p=A1FDA903978D4998&index=0&playnext=1
Those same mechanisms can be triggered in people who are already socially 'primed' to believe in such things by accidental occurrences of phenomena they don't immediately recognize, be it strange noises or unusual glimmers of light in a darkened room. Under the right circumstances a little surge of fear or surprise caused by something weird you can't instantly account for, can trigger a person to start mentally shuffling through
paranormal explanations they have been primed to believe, rather than first sorting through ordinary possibilities. Without a good critical faculty in place, a startle can throw a person back to the level of a five year old reshaping a dimly seem coat hanging in a closet into a demonic face.
The 'priming' for paranormal experiences and visions of aliens is no mystery. We all 'prime' each other from a very young age to believe in ghosts and the paranormal, and that priming is constantly reinforced by popular entertainments; movies, TV, video games. Sharing ghost stories is a bonding experience much like sharing musical tastes and clothing trends, sex stories, and tales of adventure. Gathered around the campfire, or maybe just the intermittent glow from a bong in a darkened, funky room, people offer up their stories of their encounters with the unexplained along with mutual assurances that "Science doesn't know everything." It's fun and amusing to suppose experts are wrong, a way to vent general irritation with authority figures, and to bond with peers who suffer equally from being under the thumb of teachers, parents and "the man" in general. It's also fun to be scared (up to a point, of course), and guys who pay attention figure out that a properly spooked girl is receptive to being physically calmed and comforted with a protective arm around the shoulder. Or more.
All that priming is reinforced from a completely different direction in some families imbued with informal mystical traditions by stories of dying relatives speaking to the spirits of the previously departed on their deathbeds, of clocks stopping at the moment of death, of dead relatives appearing in dreams with important messages, and similar ones.
For some people the existence of ghosts is effectively proved by the prevalence of ghost stories alone: if ghosts are not real why do so many people claim to have seen them? Underlying that is a constant monitoring of what beliefs are in vogue, socially acceptable, socially desirable to hold: if everyone else seems to subscribe to a notion or trend, it’s a social gaff to dissent. By this mechanism people who don't necessarily believe in them pry their minds wide open in order to participate in the group dynamic in the same way they might open their minds to smoking pot or drinking, or to enjoying a new popular music group.