Mr. Hemphill's true love was Folk Art but he was aware by the time this book was written, that he was acquiring more and more work of a more intense personal and transformative nature. For example, he already had at least nine American artists; Bochero, Castle, Evans, Sister Gertrude, Jones, Podhorsky, Ramirez, Wentworth and Yoakum who we would consider Art Brut today yet when he wrote his book he was convinced that even though the term Folk Art did not really work for these more intense artists, oddly enough, the term Folk Art itself had become an umbrella term so he decided to use it.
Hemphill never acquired any work which was not American. On the other hand, he began to use the term Outsider Art to refer to work made by the artists mentioned above. This is a term coined by Roger Cardinal in 1972 as a translation of the term Art Brut and as the title of his book by that name written in that year.
"Art Brut" itself was a term invented by the French artist Jean Dubuffet to mean art without precedent or tradition, art made outside of history without any learning or planning.
Of course all the artists in Dubuffet's collection were Self-Taught by definition and even by the early seventies some folks (including myself and, shortly thereafter Victor Musgrave and Roger Cardinal) could identify artists like Martin Ramirez, Joseph Yokum, Henry Darger (to name just three Americans included in the Outsider Art show in London in 1979) as Outsider Artists or to state it more clearly, I think, as Art Brut artists.
Nobody, then or now, finds the term, Outsider Art helpful and, in fact, it seems only to confuse because of its other and ordinary uses. (It is unfortunate how the British avoid using French terminology.) It is clear that both Folk Art and Art Brut are by definition Self-Taught. Those of us who are fascinated with this phenomenon (as I have been since 1970) can identify serveral Art Brut artists who are American.