The Collaborative International Dictionary of English v.0.48:

Impulsiveness \Im*pul"sive*ness\, n.
   The quality of being impulsive.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impulsiveness
    n 1: the trait of acting suddenly on impulse without reflection

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "impulsiveness":
   abruptness, alacrity, capriciousness, desultoriness, deviability,
   eccentricity, erraticism, expeditiousness, feverishness,
   fickleness, flightiness, forwardness, freakishness, furiousness,
   haste, hastiness, hurriedness, impatience, impetuosity,
   impetuousness, impulse, inconsistency, inconstancy, instability,
   irregularity, mercuriality, moodiness, overhastiness, precipitance,
   precipitancy, precipitateness, precipitation, precipitousness,
   precociousness, precocity, prematureness, prematurity, promptness,
   quickness, rashness, recklessness, restlessness, rush, shiftiness,
   speed, suddenness, swiftness, uncertainty, undependability,
   unfixedness, uninhibitedness, unpredictability, unreliability,
   unrestraint, unsettledness, unstableness, unsteadfastness,
   unsteadiness, untimeliness, variability, variation, variety,
   wantonness, waywardness, whimsicality