In 1963, an engineering student named Robert was driving home to Chicago after visiting his girlfriend when he got lost unaccountably on a familiar road, saw strange lights and his car stopped working.

The next thing he remembered was waking up, driving into Chicago two hours later. When Robert took a shower he noticed an odd mark on his shoulder, and the experience left him with strange dreams.Although Robert thought his experience may have involved an extraterrestrial phenomenon, it occurred before other alleged alien abductions gained widespread publicity and he tried to shrug it off.

But in 1981, after nearly two decades of anxiety and personal problems, he underwent hypnosis, and an investigation that involved conscious recall and questioning of acquaintances.

Under hypnosis, Robert said that after his car stopped, a little creature appeared in the back seat. The alien - small, with a round, hairless head, big, black eyes and a skin-tight uniform - led Robert into a UFO, where he was subjected to a physical examination and shown strange, movie-like images.

The aliens, who Robert said communicated by telepathy rather than speech, eventually walked him back to his car, saying it would be to his benefit if he could not recall the encounter.

Robert's experience is typical of at the least 300 instances of alien abduction that have been reported worldwide, primarily in the United States, England and South America.

But one controversial theory, proposed recently by a psychologist, holds that Robert and the others were not the victims of weird little creatures from outer space, but rather are victims of "fantasy-prone" personality, a still poorly understood psychological state.

"Fantasy-prone people are not psychotic, not crazy, not even neurotic. They're just a little different than the rest of us," said Robert Baker, a psychology professor at University of Kentucky.

"What lies behind most of so-called alien encounter reports is a problem with people who are suggestible being told these things," said Baker.

Psychological researchers Heryl Wilson and T.X. Barber were among the first to characterize the fantasy-prone personality, reporting in a 1983 study that such individuals could comprise as much as 4 percent of the population.

Such people become actively involved in the events they fantasize - reporting sights, sounds and smells. In addition, researchers say such people enter hypnotic-like trances when they are deeply involved in a fantasy.

"In the past, before the technological age, such people saw angels and said the angels carried them up to heaven, or they were abducted by fairies, leprechauns or other little folk," said Baker, adding that in order to maintain a facade of normality such people learn "to be highly secretive and private about their fantasy lives."

The difference between normal people with vivid imaginations and those with fantasy-prone personalities is the latter group's difficulty in distinguishing between reality and fantasy, according to Baker.

Writing in The Skeptical Inquirer, the psychologist said although most abductees appear to be mild-mannered, unassuming citizens, if they were given intensive psychological tests, "It is highly likely that many similarities would emerge, particularly unusual personality pattern categorized as fantasy-prone."

Baker, who deems all the reported alien abduction cases "highly suspect" due to lack of physical evidence from outer space, anticipates a rash of new reports in the wake of highly popular, "non-fiction" books on such sagas, including Whitley Strieber's "Communion" and Budd Hopkins' "Intruders".

But others question the personality theory.

Mark Rodeghier, scientific director of J. Allen Hynek Center for UFO Studies in Chicago who investigated Robert's case, said the fantasy-prone theory "may or may not explain some abduction cases."

"As far as abduction claims themselves, we are on the fence. I think they should be taken seriously because a number of people who reported them are traumatized by the experience, whatever it was," said Rodeghier, a psychology lecturer at the University of Illinois.

Although UFO investigator Hopkins concedes Baker's fantasy-prone theory "certainly may apply in a number of cases," he argued that Baker did not conduct research to back up his claims.

It is also difficult to apply the concept to incidents in which several people reported being abducted simultaneously, Hopkins argued.

He cited the case of a registered nurse and her two sons who reported an unexplained two-hour time lapse after their car was approached by a UFO. Under hypnosis all gave similar accounts of abduction by aliens.

Hopkins said alien abduction could be a pattern that starts in early childhood, with extraterrestrials "more or less tagging humans like elk or deer and following them through the years." Such experiences would likely create an unusual psychological makeup, which might be labeled fantasy-prone, he said.

Baker challenges abduction stories elicited through hypnosis, claiming this encourages and rewards the fantasy-prone.

"There's currently no way even the most sophisticated hypnotist can tell the difference between a memory that is real and one that's created. If a person is hypnotized and highly suggestible, and false information is implanted in his mind, it may get embedded even more strongly," Baker writes.

Rodeghier acknowledged that an unethical or inexperienced hypnotist could nudge a suggestible person into "recalling" extraterrestrial encounters, but said faulting hypnosis techniques is not the same as debunking alien abductions.

Many claims people make about extraterrestrials do not require hypnosis, with about one-third of the cases involving "a fair amount of conscious memory," Rodegheir said.

In his article, Baker attributed some extraterrestrial accounts to hallucinations that occur as people are waking or drifting off to sleep. In such "waking dreams", a person feels awake, but paralyzed or floating, and may "see" creatures.

The abduction skeptic calls Whitley Strieber's best-seller, "Communion", "a classic textbook description" of such a hallucination, despite Strieber's insistence his abduction by aliens was real.

"The point cannot be more strongly made that ordinary, perfectly sane and rational people have these hallucinatory experiences and that such individuals are in no way mentally disturbed or psychotic"

Hopkins conceded Strieber's recollections are "not a very strong case" compared to many other accounts, but said one experience - in which Strieber said he was carried out of his cabin in upstate New York and into a spaceship - "definitely has the marks of a real abduction."

Baker said similarities in abduction stories - such as little grey humanoid creatures and flying saucers - show they are based on common UFO tales people have seen in newspapers, books, movies and television.

"Fantasy-prone abductees' stories would be much more credible if some of them, at least, reported the aliens as 8-foot-tall, red-striped octapeds riding bicycles and intent on eating us for dessert," he wrote.

But Hopkins believes the connection works the opposite way, with authors basing stories on witnesses' sightings. He said these accounts had led to a switch in science fiction from rockets to flying saucers and from monster-like to humanoid aliens.

Dr. Rima Laibow, a Dobbs Ferry, N.Y., psychiatrist who has talked with six abductees and reviewed many other cases, also doubts Baker's theories.

"The notion of a fantasy-prone personality does not seem to hold water in the clinical analysis of abductees. We all have fantasies; we do not all produce the same details in our fantasy," Laibow said.

She said testing and clinical observations "make clear these people are not of one personality type."

Although she does not know if external or internal factors are responsible for the reports, Laibow said most abductees display symptoms of post-traumatic stress disorder.

"If this phenomenon is not caused by an external event, it would be the first instance of post-traumatic stress I know of without a physical cause," she says.

The stress disorder is a form of anxiety stemming from a tragic event, such as an airplane crash, physical torture or war. Symptoms include insomnia, headaches, nightmares and repeatedly reviewing memories of the event.

Laibow said abductees' versions of reality should be respected by psychologists. But Baker said reinforcing alien abduction stories can lead people to believe they actually have been victims.

"It upsets them and turns them into neurotic persons if they weren't disturbed before," Baker said.

"Instead of convincing fantasy-prone people that the abduction really did happen, pscyhologists should help them realize it didn't happen so the people will not feel like aliens can come back at any time at take them away."