The Collaborative International Dictionary of English v.0.48:

Proneness \Prone"ness\, n.
   [1913 Webster]
   1. The quality or state of being prone, or of bending
      downward; as, the proneness of beasts is opposed to the
      erectness of man.
      [1913 Webster]

   2. The state of lying with the face down; -- opposed to
      supineness.
      [1913 Webster]

   3. Descent; declivity; as, the proneness of a hill.
      [1913 Webster]

   4. Inclination of mind, heart, or temper; propension;
      disposition; as, proneness to self-gratification.
      [1913 Webster]

WordNet (r) 3.0 (2006):

proneness
    n 1: being disposed to do something; "accident proneness"