In order to get belly button lint, you need a hairy body and some suitably soft fabric such as a cotton t-shirt. The hairs rub on the inside of the shirt as the guy moves and they collect lint... with all that chest hair pointing down, the lint travels and gathers itself into a nice soft pill nestled snugly into this convenient hollow. The more chest hair, the better the lint collection (at least in my observations). Most of the males I know wear cotton t-shirts or undershirts. I don't know any man who regularly wears silk underthings... although long johns and other thermals out of silk are fairly common in cold climates. A couple decades ago when dating an astrophysicist, my finger started orbiting his belly button. I asked, "is the event horizon of a black hole spherical?" He responded, "it depends upon the speed of rotation", confirming my surmise. Hmm... either I'm going to have to start dating again (find hairy male and supply him with silk) or somebody else is going to have to investigate whether silk and polypro also produce lint. This is a matter of empyricism over guesswork! Carolly the Scientifically-Minded ;^)