Since I just read David Stewart's answer to What is the best question no-one has ever asked you before?
and Quora User
's comment to David about the coincidence that resulted in Christine asking this question, I find it interesting that I happen to be in the middle of a book about behavior AND I'm at a point where the author is discussing this exact topic.
Interesting book. Recommended. The Emotional Life of Your Brain: How Its Unique Patterns Affect the Way You Think, Feel, and Live-and How You Can Change Them: Richard J. Davidson, Sharon Begley: 9781594630897
Dr. Davidson used fMRIs as part of a study to test a previously held belief that turned out to be totally false — that children who start out as introverts or extroverts never change.
As it turns out, young extroverts turn into introverts (and vice versa) all the time.
According to the author, extroverts who turn into introverts often experience traumatic events that dramatically impact on their personalities (and the correlating brain activity). One example he used was a child who started out (I believe he was 3 years old) as the most outgoing and biggest extrovert in the study and later returned as a sedate introvert. The reason given was that, when the boy was eight years old, he witnessed his father's slow demise from cancer. Though not mentioned, my guess is that the trauma was exacerbated by the emotional and financial ramifications on the mother that often accompany medical crises.
BTW, I don't think trauma is the only factor that can turn an extrovert into an introvert. But it makes sense that traumatic experiences can knock the confidence and lack of concern out of impressionable and vulnerable children.
Just found the following:Susan Cain - The Power of Introverts by Ayush Jaiswal on Introvert support group
It includes an excellent TedTalk presentation — The Power of Introverts. Highly recommended.