I'm not sure what you mean by binary divide, but the reason why some is introverted versus extroverted fascinate me. It's a pretty basic question - learned behavior versus instinctual/genetic behavior. Maybe it's somewhere in the middle?

Here's what I think: as a young kid, you get exposed to things that shape your attitude. If your early experiences are negative in social settings, perhaps you grow up to be introverted. But what happened first - your negative experiences, or an attitude that pre-disposed you towards negative social experiences? Can little little kids have attitudes like that? Hard to day - how do we really know?

I don't care what physical science has to say on it regarding brain maturity, neuro-pathways, etc - unless we can communicate with infants, we don't know if they have pre-disposed attitudes or thoughts.