I’ve been thinking of writing horror a good bit. More than once, when I’ve talked to students about writing horror, the topic of their fear of clowns arises. The technical word for this phobia is coulrophobia. The word, as well as many articles on the topic of “why” we are frightened of clowns is all over the net. I found one really good article on the topic of coulrophobia at this link: 

Go there and learn why we fear the funny face.  Writers in the genre of horror must understand why we fear the clown.