Introvert or Extravert, which will he be?

What makes human beings introverted or extraverted, I wonder? I've read statistics that 25% of people are introverts and I find myself pondering about what type of personality my child will have. Some say that people are born with that trait, others, that it is forms in childhood. The heritability of introversion and extraversion appears to be polygenic, much like intelligence, and I find myself wondering if this child of two introverts will be introverted as well.

The baby's father is extremely introverted. He was shy and contemplative as a child, and while not really shy anymore, he's a thoughtful, almost reclusive, adult. He has a big, beautiful mind, one that fascinates me, and he's constantly feeding it. He prefers his own company and that of a few good friends, and often cannot wait to get back home, even if it is only from a day at the office. Being social is a big stretch for him, and usually requires not only recovery time, but prep-time. He will turn down social engagements well over 90% of the time if they are sprung on him last-minute. He's not even interested in internet-based socializing, even though he's worked in the internet-realm for over a decade. To him, things like internet messaging are tools for achieving an end-goal, rather than a way to keep in touch with people -- he has an analytical, transactional view of social exchange. He can enjoy social interactions while they are happening, but rarely misses relationships much when they are inactive. In fact, with him and some other introverts I know, a relationship doesn’t really exist when it is inactive.

I am either an outgoing introvert, or an introverted extravert. It is difficult to say. I score nearly 50/50 on the Myers-Briggs tests, with a slight edge given to 'E'. Reports from adults who knew me as a child state that I was a solemn child, but I could also be quite talkative. I recall being a bookworm -- I preferred my own company or the company of books to social interaction, unless I was interacting with adults. Adults were interesting. I had few friends, mainly because I had little in common with children my age -- I lived on a completely different plane, mentally and emotionally, from my peers.  As an adult, people who know me socially sometimes argue with me if I claim to be an introvert. I can be quite social as long as I have time to prepare and meet my minimum required amount of me-time. I have a handful of deep, intimate friendships and a wide circle of more casual friends. I am uncomfortable being the center of attention and feel anxious speaking to groups -- for example, the thought of a baby shower makes me nervous. I've had to learn how to actively maintain relationships with people I don't interact with on a daily basis, because most of my friends are introverts as well, and their internal lives are so rich they don't notice when relationships go dormant.

In many ways, life is easier for extraverts (or extroverts, as some people like to spell it). In our American culture, people tend to think there is something wrong with a person who is quiet rather than social, and introverted children are invariably given a hard time at school. But introverted children tend to be better behaved and self-entertaining than their extraverted counterparts. I worked with kids for years, and I can't say I have a favorite personality type. I expect that which ever trait our son exhibits, we'll be fine with it. I'm just hoping that if he does end up being an introvert I can teach him how to be social while still respecting his inner needs for solitude and social down-time. If I learned how, so can he. I hope.