your dominant neurotransmitters is known as your bio-temperament. just because some one is an SJ doesn't mean that they are necessarily GABA dominant. they could be dopamine dominant for example. introversion and extroversion also have a direct link. take this with a grain of salt because i haven't looked into it for a while but im pretty sure dopamine has two forms, positive and negative which run parallel with extroversion and introversion, respectivly.
don't quote me on this, its worth buying if your really really interested and even as a diet book it goes far and beyond any book ive ever read.