(from
http://www.boingboing.net/2008/04/01/poltergeists-and-qua.html)
"The researchers--Pierro Brovetto, formerly of the Institute Fisica Superiore, and his colleague Vera Maxia--hypothesize that female neuronal changes at puberty can cause quantum mechanical disturbances."
For a while, I've thought about the possibility of ghosts being related to quantum phenomenon - heck, I even
wrote a story on the same subject. While I don't know if it's something that can, with our current understandings, be properly researched, but it remains an interesting possibility nonetheless.