Far,far,FAR more likely that those claiming to have been "visited" have *very* serious psychiatric,or neuropsychiatric,conditions.
BTW, Mack (from 1980 to 1986) was the chairman of the executive committee of the Department of Psychiatry at Harvard Medical School.
There is a condition called "sleep paralysis" which can account for many of the experiences of "abductees." It causes people to get stuck between sleeping and waking, where they cannot move but they can see. This state is often accompanied by vivid hallucinations and even out-of-body experiences. The hallucinations seem to take the form of popular culture; sleep paralysis sufferers hundreds of years ago would see demons and succubi, while modern sufferers might see aliens.
I would bet that the majority of people describing abductions actually had an episode of sleep paralysis.
I've had bouts of sleep paralysis for as long as I can remember, though the frequency has dropped since I reached adulthood. I always hallucinated ghosts. I won't go into details, but the hallucinations are very vivid and terrifying.