According to some phobia studies, an estimated 20 to 30% of the entire population find clowns scary, or at least a little unsettling. A certain segment of that group finds them frightening enough to trigger panic attacks, anxiety, and other phobic reactions. The fear of clowns is called coulrophobia, from the Greek word for "limb," surprisingly enough. The association with clowns and apparatus such as stilts most likely inspired the name for the condition. There are a number of theories as to why people find them scary, including fears of the hidden face and frightening media portrayals.
One common theory involves a connection between the presence of clowns and a personal trauma suffered during early childhood. For a young child, a circus can create a sensory overload with all of its unusual sights, sounds, and smells. When professional clowns begin their routines, a child can easily become overwhelmed by the surreality of it all. Heavy makeup, colorful costumes, and over-sized prosthetics all help to mask the true emotions and intentions of the performers, which can become very unnerving to a child.
There is also the concept of what role clowns play in society. According to the established "rules," children have the right to behave as children and adults have the responsibility to act like adults. Some people find clowns scary because they are adults who are permitted, even encouraged, to act like children. If clowns are not restricted by the same social norms as their audience, they could be capable of doing other things besides entertaining crowds. Subconscious fears of molestation by a masked or disguised attacker may be one reason some people have this fear.
Clowns are often portrayed as emotionally unstable or even psychotic in many forms of media. The idea of a "killer clown" has been used in horror films and novels for decades, and photos of serial killer John Wayne Gacy as a clown have been published many times. Clowns are also often portrayed as emotionally conflicted, projecting a false impression of happiness to their audiences while hiding great personal pain. These images may instill a sense of dread or fear in impressionable children, which in turn could lead to coulrophobia later in life.
Not all people find clowns scary, and some just consider them to be annoying or unfunny. Much of a clown's humor is meant to be broad and physical, complete with pratfalls and pies in the face. The interaction with audience members, which often involve acts of humiliation, could be another reason some people don't like clowns. Feeling humiliated or ridiculed in public can be a very traumatic experience for some people, even if the performer's actions were done in the name of comedy. Many audience members dread the idea of becoming part of the act, so the association of public mockery and clowns could leave a long-lasting scar on a person's psyche.
There are actual treatment centers that specialize in treating coulrophobia and other unusual social phobias. Treatment usually involves intensive personal counseling or psychotherapy, along with group therapy and a supervised desensitization program involving real clowns. True phobia sufferers often experience the same social anxieties as agoraphobics, remaining indoors rather than risk an incidental encounter with a clown or seeing images of clowns in public areas.