The first problem is how to define them: Self-taught artists? Mavericks? Outsiders? Lynne Cooke, a curator at the National Gallery of Art, proposes a new term for American artists who have worked mostly outside the mainstream world of museum, galleries and patronage. She calls them "outliers."

"Outliers and American Vanguard Art" is not the first large exhibition to survey work made by isolated visionaries (Henry Darger and Martín Ramírez) or inspired folk artists (the quilters of Gee's Bend and retablos painters of the Southwest) or maverick figures who thrived by straddling boundaries (Horace Pippin and Jacob Lawrence). But it definitely feels like an institutional moment. If it isn't a benediction, it is certainly a resounding acknowledgment that something has changed in our understanding of these artists. "Outliers," Cooke argues, is a term without negative baggage and suggests a relationship more fluid than the black-and-white terminology of insider and outsider. Plus, Malcolm Gladwell has made it sexy.

The exhibition is enormous, engaging and sometimes upsetting. By focusing not just on outlier artists, but on three key moments when the boundaries between the outliers and the vanguard of the established art world became porous, the show documents influence and borrowings and ultimately the collapse of most distinctions between the schooled and the unschooled, the ironic and the naive, the conceptual and the obsessive. It includes some 250 works and makes the most extensive use yet of the concourse-level visiting exhibition galleries that were a major feature of the recent East Building renovation.

The three key moments begin with an almost two-decade period after the First World War when Americans began looking to folk art and early American artists for the creation of a new American nationalist art practice. With Depression-era programs that supported independent and folk artists, and a focus on "modern primitives" at the Museum of Modern Art in 1938, "insider" artists had access to a range of naive, direct and evocative forms of expression flourishing in places far from New York, and without pretense to being "high" art.

The second chapter, roughly coinciding with the civil rights movements of the 1960s and '70s, is devoted to outsider artists who directly challenged the centrality and gate-keeping function of the institutional art world. Artists of color, gay and lesbian artists and artists working in rural and isolated parts of the country emerged to speak on their own terms. The work of outsiders wasn't just a cornucopia of inspiration for mainstream painters and sculptors; it asserted the presence of new voices and new subjects and new techniques that had been traditionally excluded from the art world.

Finally, with a chapter beginning around 1998 and continuing into this decade, the lines seem to dissolve entirely. Eliminating hierarchies and the last vestiges of condescension from the relationship, the art world began to think of itself as flat, broad, and open, as if everyone was an outlier. In part, this was spurred by the internationalization of the art world, and an influx of artists from countries and cultures that were never part of the Western tradition.

But ultimately, it was the market that did most of the leveling. And so the world of outlier artists now has celebrities and art stars and a core canon of iconic works waiting for eager viewers to Instagram. Expect crowds near the scroll-like narrative paintings of Darger, the enigmatically spare works on paper of Bill Traylor, the squat, rough-hewed limestone carvings of William Edmondson and the intricately detailed maps of the psyche made by Ramírez. The easy headline summary of the exhibition for those who don't want to dig any deeper is that these works have now, finally, been accepted into the inner sanctum of the elite art world.

But this show raises deeper questions, and some of them are unsettling. The exhibition charts the traffic in ideas between established artists, such as Marsden Hartley, Charles Sheeler and Cindy Sherman, and outsider figures. But the questions it asks imply a "we," which comes from an institutional point of view: What did we learn from the outliers? How do we define them? Why and how did we exclude them? How can we open our practice to their insights and gifts?

One might, however, turn this around. How did the failings of the traditional art world create the space and need for outliers? What wants and cravings do outliers fulfill that the institutional art world cannot provide?

Once you start thinking about it that way, there are some surprising discoveries. First, and most pervasive, is the role of religion, which dominates the work of outliers through much of the show. This isn't polite, Sunday-school religion, but a passionate, Bible-quoting, often apocalyptic sense of spirituality. The hand-lettered signs painting on scraps of wood and metal that Jesse Howard made for his roadside display near Fulton, Mo., are full of elaborate biblical references; citations from the Book of Revelation are scrawled onto the naive drawings and paintings of Sister Gertrude Morgan, who spent years as a street preacher; religious carving and painting was central to the Hispanic artists of the Southwest who were "discovered" during the 1930s.

Religion isn't a universal subject, but it is dominant in so many works that it is strange not to confront the issue more directly. Perhaps there is a cause and effect: When the mainstream art world (and the larger culture) turned away from unselfconscious engagement with religion, people found ways to remedy the omission. This is part of a larger failure of the institutional art world, the question of what is art about. The turn to abstraction, and later minimalism and conceptual projects, limited for many viewers the range of what art seemed to take up as its subject. Much of the best of the outlier art, and most of the works that give the greatest pleasure, are distinctly about things in the world, often with narrative inscribed directly into the image. Clarity of expression and an almost cartographic way of depicting the world is a common thread from the 19th-century folk style of Edward Hicks through the wonderfully haunted rural images of James Castle, made with soot and saliva on found paper.

Near the end of the exhibition, works by James Benning take up in a sophisticated, art-world way, the larger themes of the show. Benning's "Two Cabins" project includes cabins constructed to represent Henry David Thoreau's dwelling on Walden Pond and the Unabomber Ted Kaczynski's hideout cabin in Montana. The artist has decorated the interiors of these cabins with handmade replicas of work by famous outlier artists, and the exhibition includes a film, "Stemple Pass," that focuses on one of the cabins, with a voice-over reading from Kaczynski's writings.

The work implies a continuum in American life, between the ecstatically spiritual and the psychotically paranoid, as if somewhere in our religious DNA there is a bad gene in the code, something that turns inward and destructive. One senses a similar aesthetic continuum, between the many works that are visually engaging, surprising and compelling in their idiosyncratic clarity and zeal and the true outliers, figures like Forrest Bess, who self-mutilated as part of a lifelong hermetic obsession with hermaphroditism. The old anxieties about the genuinely outsider art of mental illness linger on, for good reason, even if other barriers and hierarchies have been dissolved.

In the end, "Outliers and American Vanguard Art" becomes essentially a historical overview of the last century of American art. Outliers intersected with so many of the dominant currents — folk art, regionalism, surrealism, abstraction and even pop art — that to survey their contributions is to survey most of the larger field. But there are things missing, commercial art, and the emerging imagery of right-wing, pro-Donald Trump artists who are creating heroicized imagery of their idol. These are all missing by design because they are not part of the focus or the thesis. But it is probably premature to assume that there is a genuine rapprochement between the contemporary art world and outsider artists. And so we must ask, again, who isn't being well served? That is where one must look for the next generation of genuine outliers and one hopes they won't be inscribed in the contemporary art world before they have a chance to destabilize it.

Outliers and American Vanguard Art is on view at the National Gallery of Art through May 13. For more information visit

Correction: An earlier version of this article incorrectly identified a book of the Bible. It is Revelation, not Revelations.