It was supposedly derived from Carl Jung's work; however, I've seen very little real evidence of this, past the attitude/function type theory that Rocky has linked to several times.

I haven't actually seen the development process of the theory myself. All I know is that it originated in Russia somewhere.

I think most of it relies on common sense; but of course, the crosstypes control that. (?)

I could say the same about my crosstype extensions to the theory. People ask me, "where is your proof?" Well, I don't see much "proof" coming from the "official" socionics side either. Certainly there is no mathematics available with which to describe it. I mean, someone can explain to you that "under socionics this function modifies this function this way, etc.", but that really doesn't prove the theory past people's desire to believe in it.