I found Cain's book to be pretty well researched and very interesting. A key point is that there are a variety of extro/intro traits or tendencies, rather than a one or the other way of being overall (at least for most people). It helped explain why I thrive in groups of people I enjoy being with but hate big social gatherings with people I don't know. Also why I'm perfectly content to stay home after a day at work. On the other hand, DH really needs a lot of quiet time, but loves small talk and meeting new people.
