The Collaborative International Dictionary of English v.0.48:

Contributive \Con*trib"u*tive\, a.
   Contributing, or tending to contribute. --Fuller.
   [1913 Webster]

WordNet (r) 3.0 (2006):

contributive
    adj 1: tending to bring about; being partly responsible for;
           "working conditions are not conducive to productivity";
           "the seaport was a contributing factor in the growth of
           the city"; "a contributory factor" [syn: conducive,
           contributing(a), contributive, contributory,
           tributary]