The Collaborative International Dictionary of English v.0.48:

Propensity \Pro*pen"si*ty\, n.; pl. Propensities.
   The quality or state of being propense; natural inclination;
   disposition to do good or evil; bias; bent; tendency. "A
   propensity to utter blasphemy." --Macaulay.
   [1913 Webster]

   Syn: Disposition; bias; inclination; proclivity; proneness;
        bent; tendency.
        [1913 Webster]

WordNet (r) 3.0 (2006):

propensity
    n 1: an inclination to do something; "he felt leanings toward
         frivolity" [syn: leaning, propensity, tendency]
    2: a natural inclination; "he has a proclivity for exaggeration"
       [syn: proclivity, propensity, leaning]
    3: a disposition to behave in a certain way; "the aptness of
       iron to rust"; "the propensity of disease to spread" [syn:
       aptness, propensity]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "propensity":
   a thing for, affinity, an ear for, an eye for, animus, aptitude,
   aptness, bent, bias, capacity for, cast, character, conatus,
   conduciveness, constitution, delight, diathesis, disposition,
   eagerness, eccentricity, fancy, fascination, favor, feeling for,
   felicity, flair, genius for, gift for, grain, idiosyncrasy,
   inclination, inclining, individualism, innate aptitude, kidney,
   leaning, liability, liking, make, makeup, mental set, mettle, mind,
   mind-set, mold, mutual affinity, mutual attraction, nature,
   partiality, penchant, predilection, predisposition, preference,
   prejudice, probability, proclivity, proneness, readiness,
   sensitivity to, set, slant, soft spot, stamp, strain, streak,
   stripe, susceptibility, sympathy, temper, temperament, tendency,
   tropism, turn, turn for, turn of mind, twist, type, warp, weakness,
   willingness