Having kids, I can say that my kids have changed surprisingly little since they were babies. My oldest has learned how to deal with the parts of his personality he's never liked, but it's all still in there.
For me, though, I think the balance is somewhat more even between nature and nurture. Sure, a person might be born rebellious, but the trick is in teaching them how to direct that energy. It's like a jet engine. Once you fire that puppy, it's going to start generating heat and energy, but until you strap it onto a plane and point it somewhere, you won't get anywhere.
So it goes with my characters. I need to know where they have been and what they've done, and through this process, I learn more about who they are. Incidentally, this figures into my outlining process ... Ideally, the characters' histories are what drives the story forward, and as they are revealed as the people they are, we discover a hidden world of meaning in everything that went before.