Thats the Nature vs. Nurture arguement. Scientist have been argueing it for years now. Its a huge issue in child development. There have been some interesting studies involving indentical twins where they follow a set that was seperated by birth (not by them but by adoption agencies) and compare what is the same and what is different about them. The answer always seems to be a bit of both for nature vs nurture. So now the arguement is just how much of each. Is it 20/80, 50/50, or maybe even 70/30? We just don't know.
Its an interesting debate and worth reading up on.