From Slate Magazine on MSN.com:
Pain sensitivity varies genetically among people.
A gene called GCH1 regulates an enzyme that correlates with pain sensitivity in rats. In humans, having one copy of a particular version of the gene makes you less pain-sensitive
than other people; having two copies makes you highly insensitive to pain. Researchers' conclusions: 1) Maybe we can control chronic pain by blocking the gene. 2) People who seem more heroic than you may just be luckier and more comfortable