Why Are Some People Afraid Of Clowns?

Coulrophobia is a fear of clowns. For some, clowns are the stuff of horrific nightmares. The fear can be so strong that a simple picture of a clown can induce a panic attack.

Often used for entertainment at birthday parties and circus acts, clowns are supposed to do just the opposite - they're supposed to create a fun, humorous and happy environment. So why are some people deathly afraid of clowns?