For decades, the major United States airlines have raised passenger fares through coordinated fare-setting when their route networks overlap, according to the United States Department of Justice. Through its review of company documents and testimony, the Justice Department found that when major airlines have overlapping route networks, they respond to rivals’ price changes across multiple routes and thereby discourage competition from their rivals. A recent empirical study reached a similar conclusion: It found that fares have increased for this reason on more than 1000 routes nationwide and even that American and Delta, two airlines with substantial route overlaps, have come close to cooperating perfectly on routes they both serve.
Baker, Jonathan , "Oligopoly Coordination, Economic Analysis, and the Prophylactic Role of Horizontal Merger Enforcement" (2021). Articles in Law Reviews & Other Academic Journals. 1946.