The major expansion from current MBTI that I'm proposing would be that although a person has one type, which I refer to as the core personality type, the psyche is capable of learning to use more than one functional stack - the particular arrangement of functions associated with each of the 16 types.
One of the biggest issues this will help resolve is many people identifying with numerous types, some even believing that they have 'changed types' over the course of their life. This is in conflict with the professional stance that a person has one type that they're born with, and that it cannot be changed.
For ease of use, I'll refer to additional functional stacks by the type name that they're connected to.
There is a particular order and structure to the development of additional functional stacks, which serve to reinforce and expand the usage of each function.
The most noticeable influence on a person's behavior and actions will be the four types that differ from the core type by one letter. This doesn't mean that it'll be exactly like those other types, but some common points can be observed. Taking ISTP as an example:
1. ISTP: Core type a person is born with, which development is based upon. Those familiar with MBTI will already be aware of this.
2. ISTJ: Reversal of J/P. I refer to this as the 'backbone' type, as it eventually serves a major support role much like the backbone of a skeletal structure. The middle letters (ST) are the same as the core, and thus the dominant/auxiliary are from the same preferences as the core type, though in reverse order. ISTP has Ti-Se in the dominant/auxiliary position while ISTJ has Si-Te in those positions.
3. INTP: This third type shares the same dominant function as the core type. I view it as an 'expansion' of the core's abilities. It will eventually be used relatively seamlessly alongside the core type, though the extent varies from person to person and also depends on their development state. It can be a common source of typing confusion, as it can be difficult to determine whether a person is this type or the core type when considering MBTI from a single functional stack perspective. Especially if only considered at a glance, looking at surface behavior. A likely cause for many who believe/report that their type has changed over the years.
4. ISFP: The fourth type shares the same auxiliary function as the core type. I see it as a stabilizing 'anchor' for the psyche. The Fi dominant helps provide a balancing perspective so that the Ti dominant of the core doesn't get too carried away or lopsided. A balance against too much 'cold, hard logic' as it were, in this example, which ultimately leads to a better usage of the core's abilities.
5. ESTP: The 'capstone' of the most noticeable and influential types, helping to bring all the various areas together, and first foray across the E/I axis. It is currently quite rare for most people to reach this stage, though a number of influential people throughout history would've likely reached this stage, if not further. Further below, I've identified a few famous people who I believe made it past this point, to serve as potential illustrations.
Here's a link to a cheat sheet listing the functional stack for each personality type.MBTI Type functions chart.
The backbone (ISTJ for this example) also has a types 3-5 using the same dom/aux reinforcement principle, which develop alongside the core's 3-5, further strengthening the support structure in the background.
There are a few patterns that are followed as additional functional stacks are developed, in order to maintain a balanced progression.
1. From start to finish, it alternates between J or P for every new type.
2. After the first two types, the four main preferences (S, N, T, and F) are represented every two types.
3. There are loose pairings that combine to represent all eight functions, such as types 1 and 2, or types 3 and 4.
Broken into a grouping of 6, 4, and 5, here is how the progression would look for the ISTP example.
ISTP > ISTJ > INTP > ISFJ > ISFP > INTJ
The core and backbone, along with their respective third/fourth (expansion/anchor) types.
ESTP > INFJ > INFP > ESTJ
The capstone types of the core and backbone, and the two remaining introverted types, as per the above three patterns.
ESFP > ENTJ > ENTP > ESFJ > ENFP
The third/fourth types of the capstone and its backbone type, along with the final type. MBTI and Socionics are different systems that approach the psyche from different angles (currently with no consistent conversion method between the two - I've tried), but this final type would generally be considered the 'Dual' of the core in Socionics - the two types only sharing the final letter of their type name in common.
Sorted by the number of letters they differ from the core, all fifteen can be presented in functional stack format, with types further down having a less noticeable impact on average behavior:
Auxiliary: ISTJ, INTP, ISFP, ESTP
Tertiary: ISFJ, INTJ, INFP, ESTJ, ESFP, ENTP
Inferior: INFJ, ENTJ, ESFJ, ENFP
I currently believe that there may be subtypes, similar to the concept that exists in some areas of Socionics. Rather than label them by one of the eight functions, I find that it is more efficient to think of MBTI subtypes as being a dominant or auxiliary subtype. The difference would be what order types three and four are developed, as an auxiliary subtype above would develop ISFP and INTJ as the expansion types of the core/backbone while INTP and ISFJ would serve as the anchor types. This dominant/auxiliary preference would continue on the capstone side of development, and is one reason why I broke the above up into three groups.
The average person will generally have between 1 and 5 types active, with a higher number being more likely with age though there is no guarantee.Further Additions
To discuss the most likely trigger of a new functional stack, it is necessary to expand the current MBTI functional stack model from depicting four functions to include all eight. I'll use a term MBTI has sitting around and call the other functions 'shadow' functions which are part of the shadow functional stack. This second functional stack is ordered using the same preference order as the first functional stack.
Continuing the previous ISTP example, we would get this:
Shadow Dominant: Te
Shadow Auxiliary: Si
Shadow Tertiary: Ne
Shadow Inferior: Fi
A more collapsed version could be expressed thusly:
D: Ti -- Te
A: Se -- Si
T: Ni -- Ne
I: Fe -- Fi
The shadow tertiary and inferior, Ne and Fi in this case, will generally be the key point in activating any new functional stack because their starting state is the weakest of all eight functions. They are likely related to a person's 'hot button,' whether that be temper issues, major internal uncertainty and worry, or other weak areas. Viewing a type from that angle may also help narrow down what a person's core type might be.
The position of one or both functions in the new type, whether main or shadow functional stack, is likely to be the point that a person has trouble making a connection at. The largest hurdle is at the sixth type - the backbone's fourth type. From our above example, it would be INTJ. The shadow dominant/auxiliary is Ne-Ti. This is a large departure from the tried and true way of doing things. In type one, there was Ti-Se. In type two, Se-Ti in the shadow functional stack. In type three Ti-Ne serves to expand the natural Ti abilities. Ne taking the lead and Ti serving the auxiliary role is a very large hurdle to overcome. Without knowing to look in the shadow functional stack, the inability to connect the main Ni-Te pair may be all that is perceived.
In contrast, Fi serves as the tertiary of INTJ next to a Se inferior - a pairing that matches the dom/aux of ISFP. This makes Fi a significantly less likely cause of trouble when activating INTJ.
This essentially serves as a test of how well a person has made use of their 'expansion' in pursuit of being balanced, rather than relying on just their base strengths. A second major test for how much the other weak function (Fi, in this case) has been embraced comes when starting the third grouping - activating the capstone's type three.
This necessitates a few extra things. First is a definition list of the eight functions that can be used by any type, in any functional stack, in any situation. The biggest thing to note is that the F functions are not necessarily related only to emotions as commonly depicted, and are used even during logical problem solving. The reverse is also true, of course. Here's the current list that I've put together. It is by no means set in stone or some gold standard.
Si - what has occurred/worked previously - endurance - precedents
Se - immediate surroundings - whole environments - situations
Ni - what might work/be required in the future - possible improvements
Ne - details - specific parts of a whole - patterns
Ti - determine how to join multiple pieces together - efficiency
Te - constructs: order - structure - systems - planning
Fi - determine causes of positive and negative responses - provides vigor
Fe - harmony - smooth conveyance - minimizing conflict - provides warmth
Functions can go both ways, as suggested in the Fi positive/negative description. For example, Ti/Te can both put something together or disassemble it (such as when breaking something down to analyze it). Given the above needs of being fairly universal, I've tried to use rather adaptable terms.
The second need is a description of how the functional stack works - of how the functions interact with each other. My view is that they interact in pairs over the course of a few steps. The first member of the pair considers 'multiple' possibilities, which then culminate in a 'single' result in the second function. The process starts at the inferior and proceeds to the dominant. Along with the inferior > tertiary and auxiliary > dominant pairs, I believe that there may also be an auxiliary > tertiary pairing, as this third pairing helps smooth out the process.
There may be a few variations on this in regards to the shadow functional stack, which is a bit harder to determine. One would be that the process starts at the shadow inferior, goes up to the shadow dominant, then to the inferior before finally reaching the dominant of the main functional stack. A second would be that processing starts at both inferiors and proceeds up at the same time. A third would include a relation between the regular and shadow function of each position - a mixture of the two being used depending on the given situation being considered. There may be more than one correct answer. My intent is to provide information for others to consider that is as accurate and useful as I can make it, rather than provide a conclusive answer that precludes any alternatives.Example to Illustrate
An ENFP in my circle of friends is known for possessing several characteristics - two of those are luck and superstition. Years ago, when playing video games, we all noticed that she had a tendency to have beneficial items spawn right on top of her. She tried to tell us that she could sometimes sense a pattern to when and where they'd spawn, which we mostly ignored as implausible. We eventually attributed this continuing trend to her above average luck, for lack of a better explanation. One manifestation of her 'superstitions' is that when rolling dice she'll often stop to decide whether to use the right or left hand, or attempt to roll it in a specific way.
Both of these behaviors are completely logical, when consider via the ENFP functional stack.
In the first case, she would've started with past (Si) cases when items have spawned while playing the game, based on the game's coding (Te - though mentally noting it is coded a certain way isn't needed). She would then note times when she did or didn't (Fi) get the item based on her actions (Te) in that circumstance. Finally, she would overlap multiple cases of when she did get an item (Fi) until they line up and a single answer/pattern (Ne) can be extracted. When she noticed this pattern occurring again, she could then walk over to the perceived point she believed an item would spawn at a given time and attempt to collect her loot.
The second case, of superstitions when rolling dice, is similar. It would start from recalling past (Si) attempts, depending on how she rolled it (Te). She'd separate the results she liked and disliked (Fi) to focus on the mechanical method she had used in the past - such as whether she more commonly rolled a beneficial result with her right or left hand (Te). She would choose which version to try reproducing, such as using the right or left hand, (Fi) then roll the die in a specific way aiming to get a particular result (Ne). Fortunately for our board game sessions, her success in these attempts was not as pronounced as in the video game example.
When I discussed it with her, she said it was the first time somebody could describe what goes on in her head. She may not always consciously perform every step of the process, and often simply acts on a feeling such as 'there might be loot if I go over there.' More or less reading from the ENFP functional stack produced a rather accurate depiction of her thought process, as well as an explanation for her actions that had previously appeared rather illogical or random.
To add to that, we recently discussed some of her writer's block problems. She mentioned that she'd often rewrite various stories several times and would often see a similar, yet slightly different pattern in each. Her current trouble was trying to find a single theme to move forward with. Knowing her past actions fairly well, and also certain points that might represent when she'd activated a new functional stack, it sounded to me that she was facing the ENTJ road block - type four of her ENFJ backbone. The two initially weakest functions for an ENFP are Se and Ti. The shadow dom/aux of ENTJ is Ti-Ne - representing a need to use her core's dominant Ne in a very different way than the Fi pairing she is used to using.
A note that might help is that the shadow functional stack of a type is the same as the main functional stack of the backbone's capstone of that type. In the case of ENTJ, it would be INTP.Application
Thus, a similar structure can be used to gain a rough idea of what the hurdle in development might look like for whatever point of development that a person might be at. The situation representing their personal road block will vary from person to person, as everyone is different. There is a likelihood that some of the 'life changing' moments, that many speakers and people talk about experiencing, is related to when they activated a new functional stack. A significant event that differs from the norm of some kind is generally required, so that the situation is not something easily/fully resolved by the currently available functional stacks. Hopefully having an idea of what to look for will lower the average stress level that seems to often accompany overcoming the larger hurdles.
Determining current progress is something that has to be done on an individual basis. The problem/situation that might represent their current mental hurdle will also vary from one person to the next. Eliciting help from another person can prove quite useful.
A (very) rough yet quick test that I sometimes employ uses the type descriptions of the 16personalities website. Particularly the 'strengths' of the 'strengths and weaknesses' page for a given type (full description can be useful as well). An honest assessment of how many of the listed strengths seem to fit a person can give a ballpark idea of their current progress, though it is relatively subjective and imprecise. Examining how the person has changed over the course of their life may yield a clearer picture. Early childhood behavior can serve as something useful to examine when determining a core type.
The truly tricky part is actually overcoming each hurdle in a healthy manner - there's a reason so few people overcome the hurdle of activating that sixth type in particular. Being that I'm basically just a conceptual grease monkey, the best steps I can point to as preparation is to actively embrace the third and fourth types. Doing activities that'll make use of those two weakest functions so that the psyche can learn to better integrate them. In my case, this largely occurred when I read a lot of manga. Some of them were emotional (Fi of ISFP) and I chose to continue reading from that genre rather than avoiding it. The sheer quantity and variety of genres that I went through caused me to start to notice common patterns and formulas in the story structure (Ne of INTP). Eventually I started making a list of the series that stood out and impressed me (little bit of everything), meaning that I was actively looking for these things.
Different things will work for different people, but the main point is to pursue balance and a wider range of experiences, rather than always staying in a comfort zone.