This reaction could be adaptive, something humans have evolved to feel, with being "creeped out" a way to maintain vigilance during a situation that could be dangerous.
Why clowns set off our creep alert
In light of our study's results, it is not at all surprising that we find them to be creepy.
Rami Nader is a Canadian psychologist who studies coulrophobia, the irrational fear of clowns. Nader believes that clown phobias are fueled by the fact that clowns wear makeup and disguises that hide their true identities and feelings.
This is perfectly consistent with my hypothesis that it is the inherent ambiguity surrounding clowns that make them creepy. They seem to be happy, but are they really? And they're mischievous, which puts people constantly on guard. People interacting with a clown during one of his routines never know if they are about to get a pie in the face or be the victim of some other humiliating prank. The highly unusual physical characteristics of the clown (the wig, the big red nose, the makeup, the odd clothing) only magnify the uncertainty of what the clown might do next.