If you have any friends who are parents, you can see how kids from the same family can have very different personalities from the time they are babies. And while nurture starts from the time a kid is born, and even within the same family, the parents may unconsciously relate to different babies differently, it seems likely that some of the differences were there from birth.
I don’t know what percentage is nature and which is nurture, and I’m not sure that its possible for anyone to know. I’m sure that there have been many studies done, but I don’t put that much trust in studies. It’s nearly impossible to do a flawless study.
And I think this topic is one which it would be pretty impossible to prove one way or another. Part of the reason for this is that nature probably contributes greatly to nurture in very subtle ways that we are not even aware of. The two things are very connected and influence each other a lot, and it is hard to know which is influencing the other.