While we're on the subject of mass delusions, the ideomoter response described here:
http://www.straightdope.com/columns/read/159/does-dowsing-for-water-really-work
is extremely cool. I always promised myself I'd read more about it but never did. It seems to explains a lot. Essentially it seems to bear the same relation to the widespread spiritualist scams of the 19th and early 20th century that the elementary "card force" bears to elaborate card tricks: the one key thing that everything else is built around.
Michael