[JeffJenkins]

Introversion and extroversion are obviously a spectrum. The reason that having that as an axis when discussing personalities rather than two different axes because people do tend to fall somewhere on the spectrum and many of the traits are opposites. Knowing that someone is, say, "a little introverted" is a decent heuristic to use when describing someone's personality.

I wonder of the author has been Myers Briggs stuff where people are often slotted as one or the other (although a good assessment will still show where on each axis someone is)