Here's a good article from the Wall Street Journal. (Note: the article has been reprinted on another website, so you don't have to be a WSJ subscriber to read it.)
Carriers Discover That Airlines With Simplest Operations Win08/14/2002
The article does state in part:
Finally, American has figured out what Southwest Airlines and others have known for some time: There is a cost to complexity. In fact, folks from American even called up Southwest Airlines and asked if they could come over -- they are based about 30 miles apart in the Dallas area -- and learn more about Southwest's operation. That's a stunning turn for an airline that seemed to regard Southwest as a pesky simpleton. And by some accounts, Southwest's people were surprised at how rudimentary the questions were from American. If you've done things one way through your airline career, it's hard to learn a new way.