Jacques Valee has a number of books on this subject. He's a bit on the spooky rather than skeptic side of the fence though, and argues for some sort of common u

Jacques Valee has a number of books on this subject. He's a bit on the spooky rather than skeptic side of the fence though, and argues for some sort of common underlying phenomenon. I think he might be right to some extent -- even if it's a psychological phenomenon, that fits the bill for a common underling cause.