>Neuroticism might be one of the best psychometrics, with most people experiencing only a small decrease over a life time, with some experiencing a very large one.

Would you say this is more of a nature or nurture effect? I.e. some people naturally decrease in their neuroticism levels as they mature and most people don't, OR a small percentage of people stabilize their lives and statuses in their respective societies, which presumably leads to a decrease in their neuroticism levels.