I haven't read deeply into this thread but I thought I'd toss in my experiences if it helps to get a wider sample.

I am the most introverted person most people have ever met. Many times in my life people have commented that they find me incredibly frustrating because they just can't figure out who I am, like they can with others. I will lock myself away in my apartment for 7+ days straight, doing nothing but working on projects in solitude, and a great week is one where I am not once forced out of that solitude.

My life has been comparatively easy. My parents were wonderful, loving and caring, the only thing I could possibly fault them for is sheltering me too much. I had a big family and we all get along very well. I've experienced death but not in my immediate family. We were certainly not wealthy but we were never in any trouble, and we lived in a very safe and nice town. I can't think of anything that has happened to me that others would consider a trauma, unless I'm blanking something out, which seems pretty unlikely. That said, I find that I am hypersensitive to many things, and it's possible some things in my life have affected me in a traumatic way, that would not have had any affect on an average person... But nothing like what some others in this thread have mentioned.

I feel pretty strongly that my introversion is all nature. When I look back to my behavior even as a toddler, the signs were there. I had incredible anxiety over going to school or summer camps, even prior to grade school. My siblings were all raised the same and I am far more introverted than them. My parents had me join sports teams and other clubs and groups, like a normal kid, and I always hated it, I only did it because I was a kid and figured I had no choice. If anything their nurturing should have made me LESS introverted, but there was nothing to be done, it's just the way I am.

Now all of that said, though I have never been diagnosed, I sometimes suspect I may be somewhere on the mild end of Asperger's. So if that is actually true, then I suppose it explains why it's so much more nature over nurture for me.