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.
   [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "instinctive":
   animal, animalian, animalic, animalistic, atavistic, automatic,
   beastlike, beastly, bestial, blind, bodily, born, brutal, brute,
   brutelike, brutish, coeval, compulsive, conditioned, congenital,
   connatal, connate, connatural, constitutional, dumb, forced,
   genetic, gut, hereditary, immediate, impulsive, in the blood,
   inborn, inbred, incarnate, indigenous, ingrained, inherent,
   inherited, innate, instinctual, intestinal, intrinsic, intuitional,
   intuitive, involuntary, irrational, libidinal, mechanical,
   mindless, native, native to, natural, natural to, nonrational,
   normal, organic, physical, primal, reflex, reflexive, regular,
   spontaneous, subconscious, subhuman, subliminal, temperamental,
   typical, unconscious, unintentional, unlearned, unmeditated,
   unpremeditated, unprompted, unthinking, unwilled, unwilling,
   unwitting, visceral, zoic, zooidal, zoologic