‘Outsider Art’ is used to refer to some self-taught artists. Many of these ‘outsiders’ have mental health problems, live in institutions or isolation, or are in some other way removed from wider society. They are typically hermits, religious obsessives and loners. It is a genre heavily steeped in the so-called ‘art of the insane’ and its predecessor is French artist Jean Debuffet’s genre Art Brut (“raw art”), a collection of work by asylum-dwellers, children and prisoners.

These artists operate outside the confines of visual arts education, societal norms and expectations. But one of the criticisms often levied at the terminology is that ‘Outsider Art’ is patronising or ‘othering’. That it in some way hints at a primitive compulsion to create art that is closer, perhaps, to our ancestors who painted on cave walls than to the art which hangs in Tate Modern. It suggests that ‘outsiders’ paint without the 'concepts' that drive contemporary art. That theirs is pure impulse – physical, even – rather than intellectual.

This tension will be interestingly, intriguingly explored in New York later this month. To coincide with the 25th Outsider Art Fair ever held in the Big Apple another rather surprising show is making an appearance and providing another answer as to why human beings create art. It is called ‘Known/Unknown: Private Obsession and Hidden Desire in Outsider Art’ and is at Manhattan's famous Museum of Sex (MoSex). It is believed to be the first time a collection of erotic artworks by outsider artists have been brought under one roof.

Morton Bartlett, Untitled, c. 1950s, gelatin silver print. Courtesy Marion Harris, New York

The show is set to shock. Sure, there are images of genitalia and sexual overtones in plenty of commercial and public galleries. But, there’s something unbridled, blatant and overt about the raw expressions of desire set to go on show at MoSex. Why? Because these artworks, for the most part, were never intended to be seen. They quite literally and unrestrainedly express sexuality, but without the self-consciousness of a Jake and Dinos Chapman who (arguably) want to leave the Mary Whitehouses in the gallery in tears. Almost all the artists exhibited at MoSex, and more broadly in at the Outsider Art Fair, are long dead and never sought public exhibition of their work. Put simply: they did it for themselves.

“What is compelling about the pieces in Known/Unknown is that at first sight the work appears to be relatively straightforward,” says MoSex director of exhibitions Mark Snyder. “However, on a closer look, the images in the show are often complicated by an artist's traumatic or psychologically-driven event that shifts them away from reality, and makes the viewer’s encounter with the pieces all the more intimate and challenging.”

In some cases in Known/Unknown the artworks appear to reveal dark, possibly deviant, desires which, if played out in reality, would lead to arrest. But there are also examples of intense passion that underpins functional marriage or the erotica that fills a void of grief.

Eugene von Bruenchenhein, Untitled (Portrait of the Artist’s Wife Marie), c. 1940s, Gelatin silver print. Courtesy Lewis and Jean Greenblatt, Chicago.

On display will be carnal sculptural figures by Steve Ashby who did not make art until after the death of his wife in 1960. Works, including Rocking Bed Cunnilingus Whirlgig and Masturbating Man with Hand Under a Woman's Blouse, give animated reign to Ashby's sexual fantasies. The Virginia-born son of a freed slave worked most of his life as a waiter and gardener. But after his retirement in the 1950s had plenty of time on his hands, even more so after the death of his wife. In the 20 years between her death and his in 1980 he produced a number of striking works that are a paean, if a strange one, to her loss – including a lifesize replica of her, which he dressed and moved around the house.

Similarly, Eugene Von Bruenchenhein's erotic photographs of his wife Marie are a mixture of titillating and profound. A baker and cake decorator by day, Von Bruenchenhein would arrive home from work at 4am, sleep a little and then spend his days getting his beautiful wife to pose like a pin-up, sometimes wearing a crown made from an old coffee tin and Christmas decorations. The topless and nude images private and only began to be sold when Marie was left an impoverished widow following Von Breunchenhein’s death.

More disturbing are the grainy photographs taken of unsuspecting women by recluse Miroslav Tichý, considered the master of the stolen image in his hometown of Kyjov, Czech Republic. Tichý’, who crudely constructed of his own homemade cameras out of shoeboxes and toilet rolls, took pictures of women’s legs, or other body parts displayed on the beach. The ultimate voyeur, his work makes for unsettling viewing.

￼￼￼￼Ike Morgan, Untitled, 2014, Acrylic on pasteboard. Courtesy of Webb Gallery, Waxahachie TX

And then there’s Morton Bartlett. A Harvard-educated graphic designer, on paper Bartlett is not your typical outsider artist. But in the middle of his life, when he lived alone, he constructed a family of fifteen anatomically correct dolls: three young boys and twelve prepubescent or pubescent girls. His photographs of these dolls are probably the most disconcerting thing on display. He made clothes for them, learning to knit and sew for this specific purpose, and posed them in various tableaux, sometimes entirely innocently and at other times very suggestively.

“There is a lot of speculation about what is going on with Morton Bartlett,” Known/Unknown curator Frank Maresca says. “There are those who say he studied anatomy and wanted to produce the accurate figures. Which means including genitalia, otherwise you’re just making Barbie dolls. But these are not the kinds of figures that you normally find.”

“I’m a photographer myself and I think that Bartlett, who by all accounts was an ethical, smart person, had certain desires.... By creating the dolls he worked out his desires in a legal, moral, ethical way. It was his private obsession.”

Steve Ashby, Untitled (Woman with Articulated Knees and Magazine-Cutting Face), Mixed Media. Collection of Robert A. Roth

Henry Darger, probably the most famous artist exhibited at MoSex, is another controversial figure. His watercolour, At Sunbeam Creak/At Wickey Lansinia, is full of images of little girls, sometimes being horribly tortured — a theme that is possibly related to the artist's own childhood traumas growing up in an orphanage, and later an asylum. This piece was among 15,000 pages of text and hundreds of drawings and watercolors discovered by Darger's landlord shortly before the artist's death.

There is a growing market for the Outsider Art, particularly in the US where Christie's New York now hosts an annual sale coinciding with the Outsider Art Fair with the top lot, William Edmondson’s Boxer, setting a new record at auction of $785,000 last January. In the UK the first commercial gallery selling these works, the Gallery of Everything, opened in Marylebone last September.

“I was intrigued at first by the fact that this [an exhibition about outsider artists and sex] had never been done,” Maresca says. “The bottom line is that there’s nothing more instinctual than sex. We can intellectualise it, but it is instinctive, animalistic. With self-taught and outsider artists, they work in instinctual ways, anyway. It just seemed incredibly natural.”