Good thread Jay!
To chime in, I'd say between some & most of it.
I personally don't think I can change my personality, but I do think I can control what I do with it>how to limit it.
To make an example, I personally get aggrevated-angry-negative about certain trivial things fairly quickly, it's just the way I am, and it's just the way my thought proccess works, that is not something I can control or change, how I respond to such situations on the other hand I can control to a certain degree.
To give a slightly skewed example:
I am pretty confident I could actually kill certain people without the slightest regret if I knew I wasn't ever going to get caught for it.
On the other hand, I am intelligent enough to understand the laws, to understand the concept of morals, and I know that just randomly killing someone is wrong-bad-unjustified, and this keeps me from doing such actions.
So I kind of agree with both you and kain.
But to note, medical conditions-changes-defects before, during or after birth can also change your personality I believe.