Origins and Implications of the J/P switch (wikisocion)
Some maintain that judging introverts in MBTI map to irrational ("perceiving") introverts in socionics, and perceiving introverts to rational ("judging") introverts, with extraverted types remaining the same. The rationale for this is based on an equivalence between the two theories' functional models. According to MBTI theory, the leading function of judging introvert is an introverted perceiving function (Si or Ni), while perceiving introverts' leading function is a judging function (Fi or Ti). Myers and Briggs maintained that introverts make contact with the world through their second, extraverted function, and decided to define judging or perceiving through the second function rather than the first, as in socionics. According to MBTI theory, if the 1st function is judging, then the 2nd is perceiving, and vice versa. For MBTI extraverts, it is the leading function that determines perceiving or judging.
Therefore, if we look at the functional structure of an MBTI INFP, we actually see the functions of a socionics EII (or INFj):
INFP = (1) introverted feeling, (2) extraverted intuition
The official theoretical MBTI definition of J/P is thus identical in structure to the Dynamic/Static Reinin dichotomy, again on the assumption of functional equivalence. For example, an LII or "Ti Ne" which is classified as static (introvert*rational), would be expected to test as an INTP, whereas an ILE, which is extrovert*irrational, and predicted to test as ENTP, would be Static as well. In Socionics an LII (along with LSI, EII, and ESI) is commonly called a j type (lowercase is preferred) because it is rational, yet the term "static" applies nonetheless. The converse, that MBTI J corresponds to Dynamic, is implied as well.