[Athenian200]

I would say that it influences sociability, but not completely. Ease of psychological arousal would definitely play a role as well. I believe that the best test of Introversion/Extraversion would be whether you feel more relaxed and comfortable alone/with less than four good friends, or around lots of other people you may or may not know. This may not work for everyone, but it seems to be the best deciding factor I've come up with.

I would also consider the following criteria:

• Reactivity to stimuli. E's have a higher threshold/need for stimulation, I's are more aware of and affected by typical things.
  
  
• Focus. E's prefer to focus on what their actions achieve/effect outwardly, towards others/reality, and I's prefer to focus on what their actions achieve/effect inwardly, towards their inner idea.
  
  
• Mental Energy. E's tend to feel mentally stimulated by being in the company of other people, and I's feel mentally stimulated by contemplating/dealing with something alone or with only a few other people.
  
  
• Evenness of relationships. Extraverts tend to have more friends, but aren't usually much closer to a smaller number of them than to others. Introverts tend to have fewer friends, but have markedly defined, smaller inner circles of very close friends. (Extraverts also have inner circles, they just aren't as sharply defined.)