The Collaborative International Dictionary of English v.0.48:

Artlessness \Art"less*ness\, n.
   The quality of being artless, or void of art or guile;
   simplicity; sincerity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

artlessness
    n 1: the quality of innocent naivete [syn: artlessness,
         innocence, ingenuousness, naturalness]
    2: ingenuousness by virtue of being free from artful deceit
       [ant: artfulness]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "artlessness":
   absolute realism, authenticity, bluffness, bluntness,
   bona fideness, broadness, brusqueness, candidness, candor,
   directness, forthrightness, frankness, freedom, freeness,
   genuineness, honesty, inartificiality, ingenuousness, intactness,
   legitimacy, lifelikeness, literalism, literality, literalness,
   natural man, natural state, naturalism, naturalness, nature,
   openheartedness, openness, outspokenness, photographic realism,
   plain dealing, plain speaking, plainness, plainspokenness,
   pristineness, realism, realness, roundness, sincerity,
   state of nature, straightforwardness, true-to-lifeness,
   truth to nature, unadulteration, unaffectedness, unconstraint,
   unfictitiousness, unreserve, unrestraint, unspeciousness,
   unspuriousness, unsyntheticness, verisimilitude, virginity



The Devil's Dictionary (1881-1906):

ARTLESSNESS, n.  A certain engaging quality to which women attain by
long study and severe practice upon the admiring male, who is pleased
to fancy it resembles the candid simplicity of his young.