It sounds like it ought to be a relic of 17th-century puritanism, but gay conversion therapy is alive and well across the modern United States.

From Seattle to southern Florida, schools offering to help youngsters "pray the gay away" have resisted attempts at prohibition, forcing youngsters to disavow their homosexuality.

Opposed by authorities from the American Psychological Association to former president Barack Obama, these centres offer counseling, support groups and, in extreme cases, drugs, hypnosis and even surgery.

It is a practice which has gone under the radar to some extent, but a new movie on the issue starring Chloe Grace Moretz has been shocking audiences and delighting critics at the Sundance Film Festival.

"The (Trump) administration actually completely believes in conversion therapy," Moretz told AFP on the red carpet for Monday's world premiere of "The Miseducation of Cameron Post" at the annual event in Park City, Utah.

"(Vice President) Mike Pence tried to get it state-funded when he was senator, so it is a very real problem."

Moretz puts in what some reviewers are calling a career-best performance as the titular character -- a teenage girl who is caught having a sexual encounter with the prom queen in the mid-1990s.

"You know, 45 out of our 50 states in this country have it to be legal to practice gay conversion therapy," Moretz added.

"And actually two weeks ago New Hampshire voted against banning it in their state."

In the movie, Cameron is forced into a therapy centre by her conservative aunt and uncle, her legal guardians since her parents perished in a car crash.

'QUEER JOHN HUGGES' MOVIE

She falls in with rebels Adam (Forrest Goodluck), a Navajo "two-spirit male" and his one-legged friend Jane Fonda -- presumably a nom de guerre, although we never know for sure.

The three smoke pot together and swap sardonic anecdotes about their adult instructors, acutely aware that these religious zealots and amateur psychologists have no idea what they are doing.

By turns laugh-out-loud-funny and deeply tragic, "Miseducation" is described by sophomore feature director Desiree Akhavan ("Appropriate Behavior") as a "queer John Hughes" movie.

As with the legendary late filmmaker's output -- a string of 1980s hits including "The Breakfast Club," "Weird Science" and "Ferris Bueller's Day Off" -- "Miseducation" has been credited with articulating the teenage experience with a rare clarity.

"This film deals with a lot of deep issues -- homophobia, sexism, racism, all the 'isms' -- and brings to light prejudices and biases that are hidden under the rug in this country and countries all around the world," said Goodluck, last seen as Leonardo di Caprio's son in "The Revenant."

"It's a film that is attempting to scream at the top of its lungs that everybody should be accepted, find their own path and be confident of their identities."

"Miseducation" is based on a novel of the same name by Emily M. Danforth, who told AFP gay conversion therapy was "absolutely something that I think we should be talking about, and worldwide."

The therapist who heads up the centre in the film, played by Jennifer Ehle, is a devout Christian who got into gay conversion after using her brother as her guinea pig.

'DEEPLY MISGUIDED'

She imposes various tasks on her young charges as a means of convincing them that their homosexuality is the result of gender confusion, and is an affliction no different to drug addiction.

"It was quite a (task) to try to find a way to play her without demonizing her, to really focus on how much she really does feel she cares and really does believe she is helping," Ehle ("Zero Dark Thirty," "Detroit") told AFP.

She added that while people involved in conversion were "deeply misguided and dangerous," they thought they were saving youngsters from damnation.

John Gallagher Jr. ("10 Cloverfield Lane") said he, too, realised through his role as the centre's pastor, Reverend Rick, that the people running these places are not straightforward villains.

"He himself has gone through the therapy so now he's teaching the kids, saying, 'You can have this happy life too if you just follow the steps like I did," he told AFP.

"I think it's important for this movie to be out there right now. I think it's going to raise awareness to a thing that people don't even realise is still happening."

As with many in the cast, Gallagher says he had no idea that dozens of gay conversion centres operated across the US.

"It's good for a film to get out there and remind people that this is a thing that is still happening in the shadows and we have to bring it out in the light, talk about it, and hopefully eradicate it for good."