Vanity Fair.

Other than that, I have been reading magazines this week: Vanity Fair, New Yorker, Atlantic Monthly as well as a crop of others. The letters pages in these magazines interest me. How are letters selected? There seems to be a preponderance of professors in the pages of correspondence, are they selected according to the perceived importance of the correspondent, and are some, perhaps, selected as they give the author of the original article the opportunity for a smart rejoinder and demonstrate his or her superior intellect?