Coulrophobia is the fear of clowns.
The term is of recent origin, probably dating from the 1980s, and according to one analyst, "has been coined more on the Internet than in printed form because it does not appear in any previously published, psychiatric, unabridged, or abridged dictionary." However, the author later notes, "regardless of its less-than-verifiable etymology, coulrophobia exists in several lists."
The prefix coulro- may be a neologism derived from the Ancient Greek word κωλοβαθριστής (kōlobathristēs) meaning "stilt-walker."[nb 1] Although the concept of a clown as a figure of fun was unknown in classical Greek culture, stiltwalking was practiced. The Online Etymology Dictionary states that the term "looks suspiciously like the sort of thing idle pseudo-intellectuals invent on the Internet and which every smarty-pants takes up thereafter".
According to a psychology professor at California State University, Northridge, young children are "very reactive to a familiar body type with an unfamiliar face". Researchers who have studied the phobia believe there is some correlation to the uncanny valley effect.
A study conducted by the University of Sheffield found that the children did not like clown décor in the hospital or physicians' office settings. The survey was about children’s opinions on décor for an upcoming hospital redesign. Dr Penny Curtis, a researcher, stated "We found that clowns are universally disliked by children. Some found the clown images to be quite frightening and unknowable." In other studies playing with therapeutic clowns reduced anxiety in children and improved healing in children with respiratory illness. 
It is argued that notorious clown figures in literature (Pennywise in It) and real life (John Wayne Gacy) have contributed to adults being averse to clowns. Additionally, the fact that much clown behavior is "transgressive" (anti-social behavior) can create feelings of unease.
In popular culture
Coulrophobia can be a plot device used to show how the protagonist must overcome his fears in order to vanquish the enemy. Examples include the 2009 film Zombieland, in which a character must overcome his fear in order to kill a zombie clown.
It can also be used for humor. In The Simpsons episode "Lisa's First Word", Homer tries to build a bed for Bart after he outgrows his crib, and fashions the headboard into a clown. However, instead of being the "fun friend" Homer intends it to be, it triggers insomnia in Bart, who keeps repeating "can't sleep, clown will eat me." The phrase became an Internet meme and inspired the Alice Cooper song "Can't Sleep, Clowns Will Eat Me".
In Stephen King's It, the eponymous menace is a being that preys on, and by unknown mechanism, manifests his victims' worst fears. When in the presence of multiple other beings, It takes the form of a clown, suggesting that the clown is the "lowest common denominator" for the irrational fears of all the protagonists.
In American Horror Story: Freak Show, John Carroll Lynch portrays Twisty the Clown. He is a murderous clown with a mask and overall dirty appearance who is killing people, leading the authorities of Jupiter, Florida to believe that the troupe of freaks are committing the crimes. Lynch says that Twisty has "strangely pure" motives in the show. Creator Ryan Murphy says that the crew of American Horror Story wanted to create the "scariest clown of all time"
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