That's right. About one of seven of us are deathly afraid of clowns.
SWEET MERCIFUL CRAP! KILL IT WITH FIRE!
There's a number of reasons behind this. Pediatric research at the University of Sheffield and at California State University, Northridge found that even as kids, we hate clowns. We hate clowns, we hate clown decor, we hate pasty white faces with permanent grins when we can SEE the actual emotion behind the makeup and we hate seeing a familiar shape with an unfamiliar face.
A lot of this has to do with what filmmakers and scientists call the Uncanny Valley. When a human figure looks and acts almost but not quite human, we get repulsed by it. This is one of the reasons Blade Runner Replicants don't exist, CGI hasn't replaced real actors, and The Polar Express continues to haunt the nightmares of millions of children. The problem here is that you've got a person who is not only painted in such a manner that their face is not recognizeably human, their body shape, while definitely humanoid has usually been altered by clothing and props to not fit into a general "human" mold and they're behaving in ways that a normal human would not.
Plus, they breed in the sewers.
Given the above, it's really not hard to see why clowns, mimes and ventriloquist dummies are, as TVTropes puts it, the "trifecta of vaude-villainy". There's just something about someone forcing that unmoving grin at you that's creepy when you know that there's something lurking behind it that isn't quite right. Intellectually, we know that it's just a dude in face paint. Emotionally, though, this asshole's got something to hide and I don't like it.
Horror LOVES this trope but it doesn't have a clear origin. The Man Who Laughs (1928) is probably the first instance approaching it on a film level and the DC Comics villain The Joker was based on Conrad Veidt's performance but the film is actually a romantic melodrama. No, the trope really took off thanks to serial killer and rapist, John Wayne Gacy. He was known for performing at children's parties in his persona of Pogo and was referred to by the media as "The Killer Clown". And I'm sure that much therapy has come from that realization.
26 bodies in the basement. Just sayin'.
But here's WHY horror loves this trope. Not only do you have the "seemingly innocent with something to hide" angle but because you're covering something up anyway, why not make that "something" completely and utterly evil. As pictured above, Pennywise (from Stephen King's IT and played BEAUTIFULLY by Tim Curry) appears as a clown to children because that is what children's minds want him to be. In reality (and do NOT bitch about spoilers, here, the book is 25 years old), Pennywise is a Lovecraftian cosmic horror that lives on fear.
Now, I don't know about YOU but I'm pretty sure that any parent that hires a clown for their child's birthday party is doing one of two things.
A) They're actively working to ensure years of tears, nightmares and therapy because torturing children is fun and makes them stronger people (or so Grampa says).
B) They are fattening children up with cake and ice cream for sacrifice to the great Clown God, Bozothulhu who requires massive amounts of sugar to keep him from destroying the world with madness and balloon animals. Party games just make them too tired to resist.
It's a close call. What do YOU think?