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