This topic has been written about numerous times before, but I will add a few more comments of my own.
Myers-Briggs Typology has thoroughly sided with the current psychological terms "extraversion and introversion" -- which are based not on Jung's work but on Eysenck's -- rather than their original Jungian meanings. Basically, what took place between Jung and Eysenck is that Jung's terms were qualitative, while Eysenck's were quantitative. In the process of quantifying Jung's original concepts, Eysenck "slid" from the original intention to what was most readily measurable, causing a drift in meaning. Here is a brief summary of the terms from Wikipedia:
The trait of Extraversion-Introversion is a central dimension of human personality. Extraverts (sometimes called "extroverts") are gregarious, assertive, and generally seek out excitement. Introverts, in contrast, are reserved, deep in thought, and self-reliant. They are not necessarily asocial, but they tend to have few true friends, and are less likely to thrive on making new social contacts.
Psychological introversion correlates highly with IQ and moderately with socionics introversion, logic, and intuition. For instance, many highly intelligent people behave like the following:
An introverted person is likely to enjoy time spent alone and find less reward in time spent with large groups of people (although they may enjoy one-to-one or one-to-few interactions with close friends). They prefer to concentrate on a single activity at a time and like to observe situations before they participate.
-- because the dominant culture of most groups is foreign to them, and so they have difficulty getting involved and obtaining recognition in the group. However, if you put them in a group that is a better match for them mentally or where mental powers are perceived positively, many or most of them will behave more extravertedly.
Now this aspect of psychological extraversion and introversion seems most like the introverted intuiter types in socionics:
An introvert is energized when alone. Introverts tend to "fade" when with people and can easily become overstimulated with too many others around. Introverts tend to think before speaking. When given the chance, an introvert will sit alone and think rather than talk with someone else.
It is no accident that 75% of Americans are extraverts according to the MBTI. From my socionics perspective, they have taken most reasonably sociable socionic introverts and put them in the 'extravert' category and have taken many or most highly intelligent extraverts and put them in the 'introvert' category. Basically, all high-IQ people who are not in the entertainment industry are Myers-Briggs introverts. By my observations, approximately 50% of Americans are socionic introverts.
Example: the Myers-Briggs INTJ
(See INTJ page at Wikipedia)
Forming just 1% of the population, the MBTI INTJs are a very special lot. What makes them different from the population at large is their independence of thought, creativity, and ability to go against the grain. They are strongly motivated to express themselves creatively and to elaborate complex concepts and intellectual designs. They are "acutely aware of their knowledge and abilities," which leads to great confidence, making them natural leaders.
As you can see, all these traits are highly correlated to high IQ.
The list of "distinguished INTJs" includes great philosophers, statesmen, scientists, and generals. Of the people in the list I know something about or know how socionists usually type them, here are the probable socionic types:
Friedrich Nietzsche -- possibly EIE
Stephen Hawking -- ILE
Niels Bohr -- ILE or ILI
Peter the Great -- SLE or ILE
Ayn Rand -- probably ILE
Isaac Newton -- ILI
Osama bin Laden -- ILE or EIE
Donald Rumsfeld -- LSI or LSE
General Colin Powell -- SLE
Arnold Schwarzenegger -- LSE
Thomas Jefferson -- LII
Ulysses S. Grant -- probably SLI
(Interestingly, most of these people are probably socionic extraverts!)
Based on the INTJ description shown above, it makes sense why each of these might be considered an INTJ. Each of them "did things very differently." What the MBTI has done in the INTJ's case is make high intelligence the essential characteristic of the type rather than functional operation. From a socionics perspective we can see that the direct, action-oriented Schwarzenegger has a completely different functional makeup than the theoretically-minded Isaac Newton. Schwarzenegger , for instance, had no need to conceptualize and lay out all his ideas, and Newton had no need to keep his body in continual motion. For socionics, intelligence and uniqueness are secondary traits whose effects are to be studied within the framework of their functional makeup.