This is from an article - "Wonderfully Curious" - that Mike Yates wrote for Musical Traditions:

During the Late 1940's and 50's a number of labour historians began to reassess Sharp's definition. Were folk songs, they asked, really the exclusive domain of the rural class (or peasantry to use the term that Sharp preferred), or could folk songs also be found in urban societies? And, if so, were these songs still passed from generation to generation in the manner that Sharp had suggested? And, why only study the so-called folk songs, when singers were singing many other types of songs? In 1967 A L Lloyd published his influential book Folk Song in England which contains many urban - Lloyd called them industrial - songs, thus leading the way for the American folklorist and historian Archie Green to come up with the all-embracing phrase vernacular songs, thus eliminating the need for discrimination when it comes to the repertoire of folk singers.