In the Pragmatic Programmer book [1], to help make the point that if you keep your code clean, it is more likely to stay that way, the authors cite the "broken windows" theory [2]. They mention a case where police observed an abandoned car lying untouched on a dark street for days, but once a window was smashed, vandals destroyed the car within hours [3]. I don't know much beyond that, but the theory rings true to me and I try to do my bit to stop it, in code and in real life.
Thanks for the links. The theory rings true to me as well. I've coerced sloppy colleagues into writing cleaner code when working on my sections of the project. Conversely I find that I relax my normally high standards when I work on spaghetti code.