It is thought the fear itself comes from a revulsion of things similar to human form, yet exaggerated, or not perfect. Plus, clowns are just damned creepy...
Fear of Clowns
As for the fear itself… It is thought that a person’s revulsion of a clown comes along the same path as a person’s revulsion of humanoid robots, humanoid aliens or even such things as corpses and zombies. It is the differences between a perfect human form, as perceived by the observer, and the exaggerations or disfigurements that cause this revulsion. A clown, being somewhat a caricature of a person, although highly exaggerated or disfigured, can cause an antipathy towards it, and this antipathy can, in turn, grow to anxiety and fear.
- Killer Clowns from Outer Space
- House of 1,000 Corpses
- Dead Clowns