Australia is in the grips of “clown hysteria”, but the pranksters have big shoes to fill if the chilling reports are to be believed.

Clown Purge Australia, a Facebook page launched earlier this month, threatened to unleash 20,000 clowns across the country before a teenage girl was grabbed by men in costumes in Perth on Saturday.

The New York Times joined in the clowning and claimed the threat had “frightened Australians – a people who tend to pride themselves on a cheerful lack of fear”.

The Clown Purge Australia page has grown to more than 30,000 followers after the release of IT, a film adaptation of Stephen King’s 1986 horror novel starring creepy clown Pennywise.

“We’re coming back for you,” the group said in a Facebook post with more than 10,000 likes.

“We have now re-united and have over 20,000 anonymous clowns over Australia.

“Be prepared – we do not mean any harm, all of it is just for a scare and some laughs.”

The threat appears to breach the Eight Clown Commandments, listed on the Famous Clowns and Clowns of America International websites.

Commandment one stipulates: “I will keep my acts, performance and behaviour in good taste while I am in costume and makeup. I will remember at all times that I have been accepted as a member of the clown club only to provide others, principally children, with clean clown comedy entertainment.

“I will remember that a good clown entertains others by making fun of himself or herself and not at the expense or embarrassment of others.”

The threat also appeared to break the fourth commandment: “I will remove my makeup and change into my street clothes as soon as possible following my appearance, so that I cannot be associated with any incident that may be detrimental to the good name of clowning.

“I will conduct myself as a gentlemen/lady at all times.”

The psychology of creepy clowns

Psychologists have speculated on why clowns give us the heebie-jeebies or even “coulrophobia” – the irrational fear of clowns.

Dr Stephanie Lay at Open University in the UK researched “how people respond to things that are nearly but not quite human”, she wrote for The Conversation last year.

She said exaggerated features painted on their faces “means that the clown is unable to ever display a natural expression”. Dr Lay drew on research from Dr Paul Ekman, who found people did not trust others with contradictory facial expressions – like a smiling mouth but sad eyes.

Dr Frank T McAndrew found being a clown was considered the creepiest of all occupations.

He surveyed 1,341 people between the ages of 18 and 77, and hypothesised being “creeped out” was a response to unpredictability and uncertainty.

“When we are confronted with uncertainty about threat … we get the chills,” he wrote for The Conversation last year.

Canadian psychologist Dr Rami Nader hypothesised coulrophobia was driven by a clown’s disguised identity and hidden feelings.

And while clowns are hired for children’s parties, a UK study in 2008 found most children disliked them.

John Wayne Gacy was a clown in the 1970s, and painted pictures of clowns. The serial killer was behind the deaths of at least 33 people. Photo: Getty

Serial killer John Wayne Gacy performed as Pogo the Clown in the 1970s and also painted pictures of clowns.

The creepy clown trope came to life when authorities discovered he sexually assaulted and killed at least 33 people, most of whom were buried at his Chicago home.

Gacy was convicted of the attacks and was executed in 1994.

WA Police warned residents not to join in after a girl, 16, was spooked by three clowns in south-Perth on Saturday.

“Those who are dressing up and carrying out silly acts should think seriously about what they are doing – the distress caused to people is unnecessary and may lead to further harm,” police said in a statement.

“By participating in this craze, people may be committing criminal offences and if so they will be charged by police.”

The New Daily has contacted Clown Purge Australia, the Humour Foundation, and Clowns of America, International.

Gacy was executed in 1994. Photo: Getty