This paper develops a theoretical model to investigate the effect of airlin
e alliances on market outcome for fairly general demand and cost specificat
ions. Two typical alliance types are examined: complementary and parallel a
lliances. The complementary alliance refers to the case where two firms lin
k up their existing networks so as to feed traffic to each other, while the
parallel alliance refers to collaboration between two firms who, prior to
their alliance, are competitors on some routes of their networks. Our model
predicts that a complementary alliance is likely to increase total output
whereas a parallel alliance is likely to decrease it. The results of an emp
irical test utilizing trans-Atlantic alliance routes for the 1990-1994 peri
od confirm the theoretical predictions on partners' output and total output
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