People who claim to have been kidnapped by aliens have a tendency to believe in fantasies and suffer disturbing experiences in their sleep, scientists have found. But the researchers say "abductees" also believe in their experiences so deeply that they display real stress symptoms similar to those of traumatised battlefield veterans.
The latest research on the "taken" phenomenon was unveiled at the annual meeting of the American Association for the Advancement of Science in Denver.
"This underscores the power of emotional belief," Professor Richard McNally, from Harvard University, told the BBC.
I've had several encounters with alien craft and I've had an alien implant removed from my body
"If you genuinely believe you've been traumatised and recall these memories, you'll show the same psycho-physiologic emotional reactions as people who really have been traumatised."
A group of abductees told the BBC about their experiences on Saturday. One of them said: "I've had several encounters with alien craft and I've had an alien implant removed from my body."
It was typical of the stories they all had to relate. It is thought there are about four million Americans who believe they have been abducted by extraterrestrials.
Scientists believe this clearly is not true, so why do abductees believe they have been taken?
Professor McNally has found that many of them share personality traits and sleep disorders.
"Most of them had pre-existing new-age beliefs - they were into bio-energetic therapies, past lives, astral projection, tarot cards, and so on," he said.
"Second, they have episodes of apparent sleep paralysis accompanied by hallucinations."
These frightening experiences usually prompted the individuals to visit therapists, who would frequently suggest alien abduction as a cause - an explanation which the abductees readily accepted, he said.
Professor McNally has come up with a rational explanation of alien abduction experiences which was endorsed by other psychologists in Denver. He said the individuals conformed to a "common recipe".
But the researcher stressed that many of the people really did believe what they were saying.
In laboratory experiments, individuals were asked to relate their experiences. These stories were played back to them and their physical responses recorded.
"When a Vietnam vet has his experiences played back to him in the lab of some combat event, his heart rate goes up and you see an increase in sweating. If you don't have post-traumatic stress disorder, you don't react that way.
"The heart-rate responses and sweating responses were at least as great in the alien abductees when they heard their memories of being taken and molested by space aliens and subjected to experiments as those of people with genuine traumatic events."
As someone who regularly suffers from sleep paralysis, I can see why many sufferers mistake the experience with that of Alien abductions. Paralysis combined with visual hallucinations and feelings of intense fear are common symptoms of this condition. As I am sceptical of aliens and the paranormal I do not relate my experiences with an abduction, however I do see a spirit like entity. I know it is not real, but it seems to be there. If I had believed in aliens before I started having these episodes, I can well imagine that I would be another alien abduction statistic.
Henry Coleman, UK
Aliens do not exist, and until they do we shouldn't worry
Timothy Howard, London, UK
I am curious to hear if their stories are similar in content. Did they see and experience the same things? Nicole, USA
The story says "therapists" suggest to distressed people that they may have been abducted by aliens, but offers no insight as to why therapists would say such a thing. I find this therapy every bit as curious as abductee belief.
ML Titius, USA
I believe these people are not fabricating the experiences they relate.
Tish Payne, USA
I think this thing of being abducted by aliens is totally crazy and people who claim this has happened to them need help.
I love to hear people like Professor McNally try to explain events that fall outside the accepted realm of what scientists allow themselves to believe or understand. Before I go any further, I have to say that I have never been abducted or claimed to have been abducted. But I find it outrageous to label another person's experience a "hallucination" just because it falls outside of my experience or the expected "norms" of most people's experiences.
Bill Eck, USA
I used to experience sleep paralysis and I used to see little figures scuttling at the bottom of my room. at first I thought it was devils or Martians, but then I grew up and realised it was a dream.
Usually when someone suffers from PTSD it's due to real experiences. If a Vietnam veteran suffers from PTSD, no one questions whether his experiences were real. However, when an abductee reports similar experiences, they are automatically disregarded as fantasy. Most abduction research indicates that the "fantasy" theory doesn't explain abductions. The balance of evidence points to a real, albeit strange, phenomenon.