Why being abducted by aliens can seem so real

A bizarre, dream-like paralysis experienced when people wake from a deep sleep can cause some sufferers to believe they have seen a ghost or even to have been abducted by aliens, despite being otherwise mentally normal.

Susceptible people experience the sleep paralysis (when they are semi-conscious yet incapable of moving) and sometimes hallucinate so vividly they genuinely believe they are victims of a supernatural event, Richard McNally, professor of psychology at Harvard University in Cambridge, Massachusetts told the annual meeting of the American Association for the Advancement of Science in Denver. His study of people claiming they had been kidnapped by aliens found all had suffered sleep paralysis combined with "hypnopompic hallucinations", when vivid, wakening dreams of intruders, flashing lights and an alien presence can seem real.

The same psychological phenomenon can also explain other supernatural beliefs. "It is part of believing in ghosts," Professor McNally said. "In Newfoundland it's called being visited by the old hag, in the southern United States its being ridden by the witch, in Europe in the Middle Ages it was the incubus and the succubus, agents of the devil. In Cambridge, Massachusetts, it's space aliens."

Ten adults convinced they were alien abductees were asked to relive their experiences which were taped and played back to them while they were monitored for stress. They were compared with ordinary people who said they had never been abducted and another group who had suffered from a genuine experience and been diagnosed with post-traumatic stress disorder. Heart rates, skin sweat levels and other signs of stress were similar to those of the alien abductees, Professor McNally said. The type of people likely to believe in alien abduction have common patterns that Professor McNally called a "recipe" for an alien abductee. "First, most had New-Age beliefs, [and] they were not only more open to alien visitation, they are interested in astral projection, tarot cards, telling the future, bioenergetic therapies and so forth.

"Second, they scored high on absorption, a trait associated with fantasy proneness and vivid imagery. Third, and crucially, they had episodes of apparent sleep paralysis accompanied by hypnopompic – upon waking – hallucinations," Professor McNally said.

"Sleep paralysis occurs in about 30 per cent of the population and about 5 per cent of people will have the paralysis plus the hallucinations. It can be a terrifying experience."