The fear of clowns is known as Coulrophobia, not being a sufferer myself I wondered what it is about clowns that scare people.
I thought that maybe people had a fear due to the fact that you cannot tell the emotion behind the clowns painted face and therefore making them unpredictable. This is supported by Micheal Quintons 2011 article on coulrohphobia. (http://www.worldwidewords.org/weirdwords/ww-col2.html )
Durwin (2004) says that psychologists believe that the phobia of clowns is caused by a traumatic childhood event. Durwin also believed that those who had not been exposed to clowns personally were exposed to them via the media, which created a hype against “Evil clowns”. A lot of middle aged people with a phobia of clowns can be to blame for Stephen king’s book/ film “IT” displaying pennywise the clown, having the same effect that jaws did to swimming in the water. Or the real life case of “John Wayne Gacy” a murderer and rapist who dressed as “pogo the clown”.
A lot of people I know that claim to suffer Coulrophobia cannot put their finger on why they are so scared of clowns, however you could argue that characters like Santa has the same ideology however not that many people have phobias of santa claus.
For all those coulrophobia sufferers out there here is a website full of sufferers and their stories http://www.ihateclowns.com/