The reason it was believed that
masturbation causes
insanity is because during the
dark ages, the theory was that a
human being had 4 different
humours (fluids) within them:
blood,
yellow bile,
phlegm (which was believed to be the same as brain fluid), and
black bile.
These had to have the right mix and temperature for a person to function correctly (this is also why temperament is etymologically derived from temperature. Many names for different moods are derived from temperatures/humours).
Hence, if you masturbated, you messed up the mix and could therefore go insane. This is also why sneezing was bad, as you lost some of your brain-fluid.
See also: The Four Humours.
Audited October 6, 2001