Good question...and one that's really hard to find an answer for. I know that personally my own sense of humor came from living inside my head a lot, so I started coming up with things that entertain me and some of it just turned out to make other people laugh too. In my case, at least, it's really hard to make an argument for environment and upbringing, because pretty much everyone I know enjoys juvenile, slapstick humor that really has to jump out at them before they think it's funny. I, on the other hand, have a pretty dry sense of humor and until recently very few of the people around me got it...now I've been told that I have a very British sense of humor, even though I've never lived outside Wyoming and have never visited any other countries or had much exposure to people from elsewhere. Maybe it's just something in how our individual personalities are wired, but again exactly where the wiring comes from is hard to pinpoint.