Source : Webster's Revised Unabridged Dictionary (1913)

Negativeness \Neg"a*tive*ness\, Negativity \Neg`a*tiv"i*ty\, n.
   The quality or state of being negative.

Source : WordNet®

negativeness
     n : the disagreeable quality of one who dissents [syn: {negativity}]
         [ant: {positiveness}]