This is a response to Vol 16 Drury 11 ("Yes, but")...
What an interesting question! I have been thinking about this quite a bit over the last few years.
It's rather depressing to think that we "solidify" in high school and then settle into lifelong ruts. Many people adopt this belief and use it as an excuse for giving up on other people. Why bother trying to reform criminals or reverse deeply held prejudices?
On a more personal level, I think we often "give up" on other people prematurely in our relationships. We hear or tell ourselves "Don't waste your time on him" or "She's no good." The most deadly problem with this attitude is that every time we give up on another person we give up on a part of ourselves. We eventually come to believe that it's too late for us to change or grow.
So I don't want to believe in a fixed personality, but I'm becoming increasingly convinced that there's something to it. Drury and I both went to the same high school reunion and I have to admit that for the most part the lack of change was stunning. A room full of thirty year olds immediately coalesced into precisely the same cliques they formed in high school. There was the popular crowd and the dangerous crowd and the nerd crowd. It was scary.
Another thing I've noticed is that most people I've talked to claim to be about twenty years old. That is, on the outside they are thirty or forty or fifty or whatever, but INSIDE they still feel like twenty year-olds. This is interesting when you consider that the human brain stops growing at about age twenty.
Longitudinal studies of personality as measured by various personality tests such as the MMPI also tend to show a stability in adults over time. There are some subtle changes in certain traits, but for the most part people's scores on these tests don't tend to change that much.
So I think there is something about the essence of a person that forms by age twenty and remains more or less fixed thereafter. But I think there is also still room for much growth within the constraints of each personality. That is one thing I liked about the Enneagram book I read a few months ago (see book review). The author posits a system in which personalities solidify by age 20, but each personality is allowed to swing along a wide arc of different but characteristic behaviours. So we can still change, but each of us changes in a characteristic way.
What do you all think about this? Do you still feel 20 "inside"? Are personalities fixed or is the word "personality" so vague as to be meaningless? What are the implications of a fixed personality?