Coulrophobia [fear of clowns] is most commonly triggered by a traumatic experience in childhood, said Steven Luel, a psychologist in New York specializing in anxiety and phobias.
Indeed, that was the case with Wallace. At the age of 6, she met her first clown at the circus, an encounter she still remembers clearly 25 years later.
“A clown got right up in my face, and I could see his beard stubble under his makeup. He smelled bad and his eyes were weird,” she said. “I guess I never got over it.”
Enough said.
Link to article (with fantastic title) ‘Fear of Clowns: No Laughing Matter’ from INS News.
Mind Hacks 
Jack Hanley:
“To me, clowns aren’t funny. In fact, they’re kind of scary. I’ve wondered where this started and I think it goes back to the time I went to the circus, and a clown killed my dad.”