Herein you find chapter one of Eric Alterman's "What Liberal Media?", a critical study of why the myth of the "Liberal Media" was created and how it was perpetuated.
"Social scientists talk about "useful myths," stories we all know are not necessarily true, but that we choose to believe anyway because they seem to offer confirmation of what we already know (which raises the question, if we already know it, why the story?). Think of the wholly fictitious but illustrative story about little George Washington and his inability to lie about that cherry tree. For conservatives, and even more many journalists, the "liberal media" is just that: a myth, to be certain, but a useful one. If only it were true, we might have a more humane, open-minded, and ultimately effective public debate on the issues facing the nation. Alas, if pigs could fly. . . ."