"Nature", absolutely.
While we are in some ways inseperable from our culture, which is to say, we either seem to embrace it or rebel against it, in either fashion being shaped by it. This is in most cases, I'll avoid going all Sith and using absolutes, but my basic premise here is, no matter how much a francophile one might be, they will still generally have more in common with a fellow American (or whatever) than they will someone from France. It just can't be helped. You're aware of the same general set of political issues, food, movies and literature, and so on, as the rest of your culture. Which is not to say you aren't also aware of a whole slew of French cultural stuff, but even if you move to France and get citizenship and immerse yourself fully, you will always recognize Ren and Stimpy.
That said, nature still wins hands down. No matter how different British food is from Thai food, a Brit and a Thai both eat and shit food, and their identities are formed heavily by this. A large part of us, of the respose we offer to stimulus, is based on need. The fundamental needs of humans shape a tremendous part of who we are. This is in many ways invisible to us, because, as we all possess these characteristics, they've blended into the backround, and we forget that they are a point of commonality. That is to say, if everyone you ever see wears a red shirt, it's difficult to realize you're all red shirt wearers, never having seen a blue shirt wearer.
A child who's grown up in poverty as compared to a child reared in wealth? Both are born, respirate, eat, excrete, fight, fuck, and die. Both would like, to some degree, to feel that their lives are meaningful. Both have, to some degree, a concept of asthetics and beauty. Both have, in whatever twisted and permutated way, a desire to be loved. Both stop wriggling if you make all their blood fall out.
However, an earlier poster made an excellent point. The question itself becomes a tautology when examined in detail. If you accept Darwinism, (and lets, for the sake of argument), then our nature is in itself nothing more than the result of our species nurturing. We nurture the way we nurture because it is in our nature to do so. And indeed, our versatility as a species, which is to say, our ability to come out on top despite often sub par nurturing, is a facet of our nature.
If all that sounds like a dodgy copout or an excuse to talk a lot, the put me down for 48/52, cause I'm a goddamned rebel.
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