Immensely useful research from the PubMed online research database:
The hypothesis presented herein says that abdominal hair is mainly responsible for the accumulation of navel lint, which, therefore, this is a typically male phenomenon. The abdominal hair collects fibers from cotton shirts and directs them into the navel where they are compacted to a felt-like matter. The most abundant individual mass of a piece of lint was found to be between 1.20 and 1.29 mg (n=503). However, due to several much larger pieces, the average mass was 1.82 mg in this three year study. When the abdominal hair is shaved, no more lint is collected.
(Hat tip: NCBI ROFL, which collects interesting papers from PubMed for your amusement. E.G.: Optimizing the sensory characteristics and acceptance of canned cat food: use of a human taste panel.)