sircallen wrote:I wonder how much of personality is nurture vs nature?
Nature is mostly working through temperamental dimensions, which are based on genetics (and are measurable).
Temperament gets fixed in the first years of life. Personality is more flexible and may evolve in time, but it's anchored in temperament (which is not changeable).
There are lots of theories on personality, but this Myers-Briggs thing is not a scientific one. It's a self-reported test, based on a theory on personality dimensions written by a psychoanalyst (Jung). So, having two people in the same category doesn't really say that much, their personalities might actually be very different (because the test doesn't allow for much granularity of measurement and the personality dimensions are very simplified).
People use this test like a social game, mostly. It's fun to check which "type" are you, if your "type" is similar with X or how different is from X's..