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]

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.