The first thing that struck Susan Clancy during the weekend she spent with people who had been abducted by extraterrestrials was that they weren't that much odder than the folks at her family reunions.

It's not that Dr. Clancy, then a graduate student in psychology at Harvard University, has an especially strange family. But as she was drawn deeper and deeper into the world of "abductees," she realized that they tend to be respectable, job-holding, functioning members of society, normal except for their belief that short beings with big eyes once scooped them up and took them to a spaceship. What makes abductees stand out is something that is so common in American society it's a wonder there aren't more of them: an inability to think scientifically.

Reading the title of Dr. Clancy's new book, "Abducted: How People Come to Believe They Were Kidnapped by Aliens," millions of Americans probably figure the answer to the implicit question is obvious: People come to believe they were abducted by aliens because they were. Some 40 percent of Americans believe it possible that aliens have grabbed some of us, polls show.

Abductees are teachers and waiters, artists and chefs, construction supervisors and librarians. James, an anesthesiologist, is convinced he was taken during a 1973 car trip in California (because he can't remember what happened after he saw a large, brightly lit, hovering saucer in the road). Will, a massage therapist, was abducted repeatedly by aliens, he told Dr. Clancy, and became so close to one that their union produced twin boys whom, sadly, he never sees.

Numerous studies have found that abductees are not suffering from mental illness. They are unusually prone to false memories, she and colleagues found in a 2002 study, and tend to be unusually creative, fantasy-prone and imaginative, but so are lots of people who have never met a little green man.

Even the smartest abductees fall short, however, when it comes to scientific thinking. Dr. Clancy asked if they realize that memories elicited by hypnosis are unreliable. Yes, the abductees said, but they are really, really careful with hypnosis, so their recovered memories must be real. Do they understand that sleep paralysis, in which waking up during a dream causes the dream to leak into consciousness even while you remain unable to move, can mimic the weird visions and helplessness that abductees describe? Of course, they say, but that doesn't apply to them. As one abductee explained, she was taken not while she slept but when she was on the couch watching Letterman.

And do they understand that the most likely explanation of bad dreams, impotence, nosebleeds, loneliness, bruises or just waking up to find their pajamas on the floor does not involve aliens? Yes, they told Dr. Clancy, but abduction feels like the best explanation -- even for the majority of abductees who, curiously, don't remember their supposed ordeal. (Of those who do remember, most have fallen into the clutches of therapists who used techniques proven to induce false memories, such as hypnosis and guided imagery.)

Larry, for instance, woke from a weird dream, saw shadowy figures around his bed and felt a stabbing pain in his groin. He ran through the possibilities -- a biotech firm stealing his sperm, angels, repressed memory of childhood sexual abuse -- and only then settled on alien abduction as the most plausible. Sam blamed his impotence on aliens, not on his recent prostate surgery. He had read that stress can cause impotence, and alien abduction is stressful.

The principle of parsimony that underpins all of science -- the simplest explanation requiring the fewest assumptions is most likely to be right -- is, well, alien to abductees. So is the notion that "it feels right" doesn't make it so, and that exceptions to rules are, indeed, exceptions.

What an inability to think scientifically does not explain, however, is why many people believe this one weird thing, not weird things in general. In other words, why ET?

"Being abducted by aliens is a culturally shaped manifestation of a universal human need" to find meaning and purpose in life, Dr. Clancy writes. That need is stronger and more basic than any attachment to empiricism, logic or objective reality.

Most important, perhaps, is that alien abduction feels, to abductees, like the best explanation for their feelings and memories. It is transformative, giving their life meaning, reassuring them of their own significance. Will, the twins' dad, is happy he was "chosen," saying the abduction showed him there is "something out there much bigger, more important than we are." Through his twins, he can "have a part in it."

Dr. Clancy, raised as a Catholic, is aware of the human needs that religion fills -- and how belief in alien abduction fills them, too. "People get from their abduction beliefs the same things that millions of people the world over derive from their religions," she writes: "meaning, reassurance, mystical revelation, spirituality, transformation."

"Abducted," published by Harvard University Press, is reaching bookstores right in time for Halloween.