Joe Moran:

Yet introversion is not the same as shyness, as Cain is careful to point out, although the two do often overlap. Introverts are people whose brains are overstimulated when in contact with too many other human beings for too long — in which case I am most definitely a shy introvert. If I’m in a noisy group of people for more than about an hour, my brain simply starts to scramble like a computer with a system error, and I end up feeling mentally and physically drained. Introverts such as me need to make frequent strategic withdrawals from social life in order to process and make sense of our experiences.
Shyness is something different: a longing for connection with other people which is foiled by fear and awkwardness.

I’m not quite sure how they supposedly overlap. As a certified introvert, that brain-scrambling sensation is nothing pleasant, and certainly nothing I long for. But while I’m skeptical that introversion and shyness could occupy the same psyche at the same time, I do think they can occupy different points on the same life cycle. I mean, I was painfully shy as a kid. Quiet and withdrawn, but completely lacking in confidence and assurance. I would have loved being popular among my peers; I would have been terrified of standing out from the crowd and attracting critical attention. It wasn’t until adolescence that I started making tentative steps toward individuality, and it probably wasn’t until my thirties that I was able to honestly say that I knew my own mind and character enough to be self-contained and largely indifferent to the madding crowd’s ignoble strife. There’s a great line in a James Kavanaugh poem: “I do not like many people, love; they bore me, or attack me, or talk too much when there is nothing to say.” Yep. I was born shy, but I achieved introversion.

Another trait peculiar to introversion: Introverts never have to drink water. They can get all the water they need from reading books.