Nature or nurture???

One of my favorite sounds in the world is listening to Hayden laugh, those laughs that are so uncontrollable that he can barely breathe, and the saliva starts dripping down his chin. What are the things that makes him laugh this hard? Tickling, and anything having to do with toots or poop. Hmmm, is it because he's a boy and it's in his nature, or because he's being raised by us and we've nurtured him that way? Unfortunately, probably a little bit of both.