Did anyone bother checking their enyclopedia(s)?...or would opening one be too eccentric, these days, CB? Once more, what I found...
"Folk song is usually melodic, not harmonic" (The Hutchinson Encyclopedia); Folk "musical structure is the simple repitition of a tune (with or without chorus)" (Philip's Essential Encyclopedia).
(And is it not the case that such encyclopedias consult these experts in their field that several of you mentioned above....or why have encyclopedias?)