The Collaborative International Dictionary of English v.0.48:

Instinctive \In*stinc"tive\, a. [Cf. F. instinctif.]
   Of or pertaining to instinct; derived from, or prompted by,
   instinct; of the nature of instinct; determined by natural
   impulse or propensity; acting or produced without reasoning,
   deliberation, instruction, or experience; spontaneous.
   "Instinctive motion." --Milton. "Instinctive dread."
   --Cowper.
   [1913 Webster]

         With taste instinctive give
         Each grace appropriate.                  --Mason.
   [1913 Webster]

         Have we had instinctive intimations of the death of
         some absent friends?                     --Bp. Hall.
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   Note: The terms instinctive belief, instinctive judgment,
         instinctive cognition, are expressions not ill adapted
         to characterize a belief, judgment, or cognition,
         which, as the result of no anterior consciousness, is,
         like the products of animal instinct, the intelligent
         effect of (as far as we are concerned) an unknown
         cause. --Sir H. Hamilton.

   Syn: Natural; voluntary; spontaneous; original; innate;
        inherent; automatic.
        [1913 Webster]

WordNet (r) 3.0 (2006):

instinctive
    adj 1: unthinking; prompted by (or as if by) instinct; "a cat's
           natural aversion to water"; "offering to help was as
           instinctive as breathing" [syn: natural, instinctive]