How, exactly, did clowns go from lovable children's entertainers to the bewigged, bone-chilling incarnation of evil? The answer is complicated, and spans a period of almost 200 years, even if the current trend of coulrophobia seems to have peaked with the ascent of online media.

Traditionally clowns are anarchic figures who defy the boundaries of normal social conduct, even before Heath Ledger's Joker just wanted to watch the world burn. In Edgar Allan Poe's 1849 story Hop-Frog, a physically deformed court jester who's consistently the butt of practical jokes encourages the king and his court of noblemen to dress as orangutans covered in tar, at which point he sets them all on fire. The unpredictable nature of a clown's behavior, and his or her tendency to transgress acceptable standards of behavior (by, for example, throwing pies in each others' faces, or squirting water on an innocent bystander with a trick buttonhole flower), probably makes us wary of what other lines they might cross.

The makeup, too, is a factor. Traditional clown face paint—a white base, with exaggerated red lips and cheeks—was pioneered by Joseph Grimaldi, a popular entertainer in the early 19th century, and can be manipulated to create a face that is either grinning in an absurd rictus or tragicomically sad. "At its roots, clownaphobia springs from the duplicity implied by the frozen grins and false gaiety of clowns," writes cultural critic Mark Dery in his 1999 book The Pyrotechnic Insanitarium: America on the Brink. "The clown persona protests too much; its transparent artificiality constantly directs our attention to what's behind the mask." The frozen smile of a clown makes his or her true expression impossible to read—yet another factor that leads us to ponder whether or not they can be trusted.

Despite all this, clowns were typically viewed in a positive light for most of the 19th and 20th centuries, even though Leoncavallo's aforementioned 1892 opera, Pagliacci, told the story of a clown who murders his unfaithful wife and her lover with a knife. (Se il viso è pallido, è di vergogna, the clown sings, or, If my face is white, it is for shame.) The turning point, culture-wise, appears to have been the arrest of Gacy, dubbed "the Killer Clown" by the media, whose grisly string of sexual assaults and murders contrasted so vividly with his alternate clown persona. As Pogo, Gacy performed at parades, parties, and charitable events, even meeting First Lady Rosalynn Carter in 1978 thanks to his role as director of Chicago's Polish Constitution Day Parade. While on death row, he painted a number of portraits of clowns, many depicting himself as Pogo, claiming that he wanted to use the paintings "to bring joy into people's lives."

The national shockwave following the exposure of one of the most prolific serial killers in American history may have forever traumatized the country as far as clowns were concerned. In 1980, Gacy was sentenced to death. Two years later, Tobe Hooper and Steven Spielberg released Poltergeist, a movie in which evil forces terrorize a household, in part by bringing young Robbie's Clown Doll to life and having it pull him under his bed and attempt to throttle him to death. In 1986, Stephen King published IT, a horror novel about the murderous Pennywise the Clown, who stalks children, terrifying them and occasionally ripping off their limbs. In 2009, King talked about the idea for Pennywise with Conan O'Brien. "As a kid, going to the circus, it'd be like 12 full-grown people piling out of a little car, their faces were dead-white, their mouths were red, as though they were full of blood," he said. "They were all screaming, their eyes were huge: What's not to like?"