Want a shiver of horror as you get ready for Halloween? Watch the trailer for American Horror Story: Freak Show, which premieres tomorrow on FX. As you might expect from the name, it’s a bit dark and twisted. Set amid one of the last freak shows in the 1950s, it follows people like sword-swallowers, bearded ladies—and worse—scary killer clowns.

It begs the question: What exactly is it about clowns that freaks us out?

For starters, coulrophobia is the proper term for an abnormal fear of clowns. And if you don't like clowns, you have plenty of company. Researchers from the University of Sheffield interviewed 250 children aged 4 to 16 about including clowns as part of hospital decor, and they found that clowns were overwhelmingly disliked by ALL those surveyed. Children found them "frightening" and "unknowable." (Note: you may want to rethink booking a clown for your kid's birthday party.)

Some professionals say the fact that clowns mask their emotions behind white makeup and a fake nose is what really scares us, while others say that we're frightened by the tricksters' ability to get away with things that the rest of society can't do. (Terrifying tibdit from a Smithsonian magazine feature on the history of scary clowns: serial killer and registered clown John Wayne Gacy allegedly told investigators, "You know… clowns can get away with murder.")

The theories are that people who are afraid of clowns view them as sinister, impish characters hiding their real faces (and possibly displaying fake emotions), so we don't trust them. Much like wearing an actual mask, their exaggerated get-up acts as a barrier and it's the fear of the unknown that scares us.