Why are clowns scary to most of the people?

In Medieval, comedic clowns were a popular entertainment. It was a character made to make people happy and laugh and they wore bright patterned uniforms. 

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"Andrew Stott, an English professor who socializes in clowning culture, says that fools always had a tenuous grip on life and society. 'The medieval fool was continually reminding us of our mortality, our animal nature, of how unreasonable and ridiculous and petty we can be.' "
During the Middle Ages, Jesters got paid to make the king laugh. But if they couldn't, they just get paid with small amount of money. So in order to make people smile and laugh, they would have their muscles cut which enabled the mouth to frown.
The comedic clowns entertainment continued to the 16th century and in Shakespeare's works, jesters were often related to dark truths and death.
"King Lear's fool wanders around reminding everyone that they're not as clever as think they are while talking in contorted double speak to undermine our sense if what we think is going on," says Scott.
"Clowns have always been associated with danger and fear, because they push logic up to its breaking point," he adds. "They push out understanding to the limits of reason and they do this through joking but also through ridicule." (Goldhill, 2014)

There are many horror films or TV series which involved creepy and evil clowns. On below, I have explained the reason why at first the clowns image is innocent and meant to bring people happiness and then turn into a horror and evil clown character.

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" People are typically frightened by things which are wrong in some way, wrong in a disturbingly unfamiliar way;" Prof Salkovskis says. (Rohrer, 2008) Due to the popularity of the TV movie, 'It' in 1990, it affects more people scare and afraid of clowns as it is about a sadistic and murderous clown named 'Pennywise the Dancing Clown' who kills children. A lot of horror movies also use evil clowns as the title and for example, such as ' Fear of Clowns' in 2004, 'Mr Jingles' in 2006, etc. These actually helps to spread the ideas of clowns are scary and evil to people and make them think clowns are one of the characters in horror. There is even a symptom about causing fear of clowns. "Search for fear of clowns on the internet and the results include plenty of sites referring to 'coulrophobia'." (Rohrer, 2008) Coulrophobia is a kind of specific phobia and it relates and describes a person who has got an irrational fear of clowns. This phobia can cause a person sweat, feeling sick, panic, fear and nervous, difficult to breath and irregular heartbeat. 

"a University of Sheffield study of 250 children for a report on hospital design suggests the children find clown motifs 'frightening and unknowable." (Rohrer, 2008)

"The place of the scary clown in mainstream popular culture can be seen in The Simpsons with Bart's intonation of 'can't sleep, clown will eat me'. And the real-life serial killer John Wayne Gacy, who often dressed as a clown for neighbourhood parties, provides an unpleasant undertone to the motif." (Rohrer, 2008) John Wayne Gacy is a real American serial killer and he is known as the Killer Clown. Because this is a real case, this add to the clowns character being horrifying and creepy to people.

'Harvard Medical School psychiatrist and horror- film fan Steven Schlozman explains the concept to Vulture.
The uncanny explains a lot of horror tropes, where you look at something and it's not quite right - like a human face that's decomposing. It's recognizable, but just enough away from normal to scare you.' (Goldhill, 2014)

Website references:
http://news.bbc.co.uk/1/hi/magazine/7191721.stm
http://www.telegraph.co.uk/culture/halloween/11194653/Why-are-we-so-scared-of-clowns.html
http://www.vulture.com/2014/10/expert-explains-why-youre-scared-of-clowns.html