<p>Lots of professors agree that new editions are not usually justified. There have actually been polls on this subject. However, when assigning textbooks they often don’t have the option of ordering an older edition. They can, of course, tell their students that “editions 8, 9 and 10 will all work.” There are problems with that (it’s annoying because everything is on a different page number, for example), but they are probably manageable.</p>
<p>Publishing houses like to have a new edition every 3-4 years because it keeps their revenue stream up. The prof who wrote the book can refuse to produce a newer edition, but in many cases the publisher can then take it over and publish a new edition of the book using revisions done by someone else. That’s a pretty powerful incentive to profs to keep coming out with new editions; they don’t want to lose control of their work.</p>
<p>I think it’s safe to say your profs aren’t in cahoots with the publishers. I doubt the administration is either. There might be a cozy relationship between the college bookstore and the publishers, but I don’t have any evidence that there is.</p>