In 2014, there were a number of reported sightings, from Albuquerque to Fishers, Ind. Last year, a clown was spotted skulking around a Chicago cemetery. And just a few weeks ago, residents of Green Bay, Wis., reported that a clown was seen toddling around town clutching a bunch of black balloons. (Somewhere, there are probably plenty of stories about good clowns, too, but an email requesting comment from Clowns of America International, a trade group, was not immediately returned on Tuesday.)

Some locals made a sport out of spotting the Green Bay clown, whom they nicknamed Gags. But others pulled out their firearms, according to C.J. Guzan, a local actor who said that the Gags project was just a viral marketing stunt for a movie.

“It’s getting a little bit scary because people are starting to believe it a little bit more, and we’re starting to see some of those unsettling pictures on Facebook,” Mr. Guzan told a local ABC affiliate. “Not of the clown, but of people armed and preparing to defend themselves, saying, ‘I can’t go outside because I’m afraid of clowns,’ or whatever. That’s just a little too far.”

The pranksters, viral marketers and criminals may be taking advantage of a cultural fear of clowns, with examples including Mr. King’s “It,” and John Wayne Gacy, a serial killer who dressed as a clown. But Steven Schlozman, a child psychiatrist who teaches a course on the psychology of horror films at Harvard University, suggests that something more primal could be at work.

Humans are built to recognize patterns from an early age, and a clown’s exaggerated human features set off a primal warning bell from within our “crocodile brains,” Dr. Schlozman said in an interview on Tuesday.

“It has this kind of capacity to grab you emotionally before it grabs you cognitively,” Dr. Schlozman said. “That’s the key to making something viral online actually: to make people emotionally engaged before they’re intellectually engaged.”