For a primer on Myers-Briggs Type Theory see the previous post. For an overview of Jungian/Myers-Briggs type-dynamics theory see the post here.
Myers-Briggs (MB) type theory and the Myers-Briggs Type Indicator (MBTI - the 'four letters') were developed in isolation from both evolutionary theory and brain architecture. It was also developed in isolation from academic personality theory but there has been recent convergence with the dominant 'five-factor model', particularly as the Myers-Briggs community is developing a new scale for their missing dimension (five-factor's 'emotional stability -- neuroticism' dimension).
In this note I want to consider how to understand the conventional four MB dimensions against the triune brain architectural model, which posits the human brain as a human-specific thinking-reasoning neocortex wrapped around a mammalian limbic system implicated in social, emotional, affiliative behaviour, which in turn surrounds the 'reptilian' R-complex brain core which accounts for instinctual survival behaviours. We start by considering the somewhat unworldly 'Rationals'.
1. The NT combination ('Rationals')
In MB type theory NTs are scientific, detached intellectuals. This would correlate with neocortical dominance emphasising its conceptual-creative and systemic implication-processing functions. With neocortical dominance comes inhibition of the limbic and R-complex functions leading to the controlled and repressed-emotions characteristic of 'Rationals' (cf. Mr Spock).
2. The NF combination ('Idealists')
Here we see a combination of limbic system affiliative drives ('F') combined with neocortical associational function ('N') to produce the idealistic champion of causes. NFs are relationship-oriented, although if the 'F' is introverted this may be in a 'behind the scenes' values-oriented way. The 'T' cortical function which tests concepts for consistency, completeness and coherence with other ideas is much less dominant in Idealists.
The concrete types
We now come to a puzzle for MB type theory. According to MB type dynamics, the relevant personality types should be ST and SF. The former are concrete, tough-minded individuals, the latter concrete but more relationship-oriented, affiliative people.
However, many people have observed that the empirically most useful distinction seems to be between SJ 'Guardians' who are authority/institution minded and SP 'Artisans' who are individualistic/performance oriented. An example of the former would be a bank manager, of the latter an athlete or 'maverick' go-getter.
We start with the orthodox MB approach.
MB theory has the 'S' (sensor) designation focusing on the immersion of the 'S' person in the situational-concrete. This is an R-complex level of behaviour shared of course by all animals. In 'S' people that situational primacy is supported and given context by the knowledge embodied in higher-layer limbic and neocortical functions.
The ST/SF distinction now enters as to whether the response pattern is driven primarily out of the R-complex or the limbic system.
The R-complex tends to drive robust, aggressive, hierarchy-maintaining behaviour (pecking order) which in MB theory gets classified as 'T' behaviour. Notice this isn't the same T as in NT where it's better understood as 'Thinking' (= a striving for logical consistency); the ST 'T' by contrast is raw 'tough-minded'. For reasons no doubt due to political correctness, the MB community does not properly engage with this distinction.
If the limbic system is used to engage with the sensor's situational awareness, then the SF is inclined to use affiliative, diplomatic and negotiating methods to deal with their social environment.
So far, so Jungian/MB.
To resolve the puzzle described above, we note that situational awareness 'S' is always social: human beings could not survive in isolation in the environment of evolutionary adaptation (EEA). All personality types are therefore acutely aware of the dangers of social rejection and isolation.
The 'J' designation in S(T/F)J according to MB type-dynamics means that the 'S' is introverted and that the extraverted (T/F) function presents to other people. The introversion of the social-situated mode of the sensor creates a social context for their behaviour.
So, if they're STJ they will do what they take to be objectively right for the benefit of the collective - a watchdog function if you like - based on pre-existing social norms. That's why so many STJs are police/military and in general pillars of the community. To repeat, their 'toughness' translates into doing what's right for the concrete community (not for an abstract, possibly innovative ideal: that would be NF or NT according to whether the ideal was affiliatively or intellectually founded).
On the other hand SFJs with their affiliative approach ('F' = 'friendly') will labour in a more convivial, helpful, diplomatic, host/hostess-type way for the (conventional) general good.
The 'P' designation in S(T/F)P according to MB type-dynamics means that the 'S' is extraverted and the 'T' or 'F' introverted. As the 'S' is concrete and specific rather than interiorised and abstracted, SPs luxuriate in their personal immersion in the wider (social) world. Their 'T' or 'F' functions determine whether that dynamic social immersion is focused on achievement of their more primal status-oriented goals (STPs with R-complex drives) or more affiliative objectives (SFPs with limbic system dominance).
It is well known that people exhibiting antisocial personality type disorders have the personality classification of ESTP, as are mavericks of all descriptions, sociopaths and some very successful movers-and-shakers.
In conclusion it appears that the success of the MB approach to personality type classification and interactional dynamics is a consequence of the fact that it has quite a good fit to an evolutionarily well-founded theory of human brain architecture, factoring in the extreme sociality of the human animal. I think there is more scientific utility lurking within MB type dynamics than the academic empiricist tradition of personality research accepts at the moment: give it time and I expect further convergence.
Note for physicists: physicists sometimes discover MB type theory. They soon discover they're NTs and as they seldom know any other type of person, rapidly come to the conclusion that's it's all just a trivial novelty.
So to flag that it's not entirely trivial, note that psychological type is evaluated through the subjects answering a detailed inventory of normed multiple-choice questions which are scored: a great deal of psychological research is done this way. Over a statistically-valid population of subjects the classification (i.e. the proportion of the tested population assigned to each type category) are consistently distributed and individually-repeatable. Type-categorisation, like IQ, is an effective predictor of 'fit' to occupational category in which capacity it is widely used in business.
In academic research the population score is taken to be normally distributed in each of the five personality dimensions: in MB theory the questionnaires return a bimodal distribution. My view is that the underlying brain-physiology variance which underpins psychological diversity is indeed normally distributed, as is IQ, and that the MB approach focuses more on outliers. Business is most interested in talent identification = outliers and has embraced the MBTI for this reason.
It is worth noting that when a person takes the MBTI, they don’t get back simply a four letter classification. The degree to which a person has scored along each access is also returned. People with extreme scores tend to exhibit their four-letter type characteristics more clearly, showing we have an outlier effect. More here.