From WordNet (r) 3.0 (2006) [wn]:
instinctively
adv 1: as a matter of instinct; "he instinctively grabbed the
knife"
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Instinctively \In*stinc"tive*ly\, adv.
In an instinctive manner; by force of instinct; by natural
impulse.
[1913 Webster]