This post will discuss some of the more intermediate and advanced parts of MBTI, so for those who are not familiar with the system, I recommend visiting the MBTI Foundation and reading their basics first.
Myers-Briggs personality typing is based on the work of Carl Gustav Jung, and is one of the main systems for personality types being used in corporate environment the other two major options being D.I.S.C and Big 5. As an empiricist, my major contention with MBTI is that it does not stand up very well to scientific rigor, people often score different types upon being tested again and various other issues . However, as a probabilistic heuristic it serves a different purpose, the goal is not that it is repeatable the goal is that it works for the purpose you are employing it.
The MBTI system consists of 4 psychological functions, extroversion vs introversion, intuition vs sensing, thinking vs feeling and judging vs perceiving. In addition each of these functions are either directed out into the world or towards your inner self (objective vs subjective).
For instance, the feeling function directed outwards tend towards maintaining social structures, emotional care-taking of others, and making judgements based on how it will affect the social cohesion and group unity on an emotional level. The thinking function directed outwards tends to be focused on efficiency, pragmatism, judgements based on facts and impersonal measurable criteria. 
On the functional level
The cognitive functions exist in four levels, dominant, auxiliary, tertiary and inferior, where your dominant is your strongest function and the inferior as the weakest function. A great analogy is that of a family consisting of Mom, Dad, a 7 year old and a 4 year old. Your dominant function is the dad, and the family is fine as long as he drives, if he gets tired, mom can drive as well without many problems. However, the trouble comes when the 7 year old or 4 year old takes the wheel and crashes the car.
Over your life you will develop all your functions to a reasonable level, but the order cannot change. During the first 15 – 20 years of your life you will develop your dominant function to a high level use, to the point where it is “just how my mind works“. You will develop your secondary function to a reasonable level, where you can use it effectively and in a targeted manner, however it will not be as ingrained as your dominant. As you enter your 20s your tertiary function will start to develop, which can be a traumatic events. For the INTJ who has spent most of his life using introverted intuition and extroverted thinking his whole life, to suddenly start asking the questions “What do I want?”, “What is the purpose of what I have been doing with my life?” As his nascent introverted feeling starts to scream from the back seat can rapidly undermine their normal convictions and characteristic certainty.
Function development can take different paths than Dominant > Auxiliary > Tertiary > Inferior. Female thinkers who are fairly rare, will often have more developed feeling functions than their male counterparts. Feelers who are interested in science and engineering will often develop their thinking function earlier than outlined.
The necessity of functional development comes in that they all work together and the more developed they are the better they work together. In the ISFJ and ISTJ, their inferior function, Extroverted Intuition will tend towards making the person imagine every upcoming scenario ending as a catastrophe. In the same note, inferior Introverted Feeling in ENTJs and ESTJs, can result in uncharacteristic aggressive bursts of emotions.
The dominant function is the one we start to develop shortly after birth. Nobody really knows if types are genetic or not, but neuroscientists claim to have identified unique patterns that correspond to the types and the functions . Your dominant over time will become the driver of your personality, and is a major reason for how you end up seeing the world.
For an ISTJ, their dominant introverted sensing infuses their past with the subjective, so that to them a bowl of soup is not merely a bowl of soup, it is their grandmother’s kitchen on a cold winter’s day and represents security. Dominant introverted sensing grounds people in their past, in both positive and negative ways. On the positive side, the ISFJ who labors to ensure that their children also get to experience the family traditions. On the negative side the ISTJ who insist on double and triple checking calculations done by Excel with a calculator.
For an ESTP, their dominant extroverted sensing keeps them in the moment, able to observe the world as it really is, and to grab life by the balls. To them a car is simply a cool car or a sexy woman is just a sexy woman. Dominant extroverted sensing keeps ESFP and ESTP in the present moment, always aware of what is happening around them. On the positive side this makes them the ultimate salespeople and athletes. On the negative side, if you have an ESTP or ESFP daughter, you will be very familiar with the phrase “We were just going to….”
For the ENTP their dominant extroverted intuition keeps them in a permanent realm of possibilities, being able to see many options for everything. To them a paperclip can be a spacecraft, a lock pick and many other things. On the positive side, dominant extroverted intuition keeps the ENTP and ENFP in a world of possibilities. On the negative side, they have trouble sticking with anything longer than about 5 minutes without getting distracted.
For the INFJ their dominant introverted intuition gives them a perspective on how the future will become, meaning they do not carry their past or present with them. To a dominant Ni user, everything is cycles and patterns repeating for eternity. The only thing that every changes is our perception of events in context of the pattern. On the positive side, this makes an INTJ and INFJ able to re-frame and re-perceive any situation. On the negative side, they may find themselves slowly withdrawing from the world and into their own world.
For the ESTJ, their dominant extroverted thinking makes it clear what is real, what is actionable and what is a waste of time. Extroverted thinking is on a constant lookout for what does not work within the system and will then move heaven and earth to fix it. On the positive side, this keeps ENTJ and ESTJ grounded in empiricism and reason. On the negative side, they may find themselves dismissing many opportunities for growth in their search for efficiency.
For the ENFJ their dominant extroverted feeling keeps them in touch with the values and perspectives of their social group, and what makes the group work. Extroverted feeling looks to improve groups, and enforce emotional and relational values. On the positive side this keeps the ENFJ and ESFJ in the role of social butterfly, the person who ensures that everyone feels seen and valued. On the negative side, they may find themselves engaging in copious amounts of emotional manipulation and reasoning to justify the actions they take to keep a group cohesive.
For the ISTP, their dominant introverted thinking allows them to rapidly categorize and break down anything into it’s constituent parts to then use leverage points to get the maximum effect. Introverted thinking classifies, categorizes and organizes everything according to an internal logical framework and seeks to understand the world as related to that framework. On the positive side, this gives INTP and ISTP an ability to understand any complexity through disassembly. On the negative side, they often struggle to explain their own complex system of categories, rules, and definitions.
For the INFP, their dominant introverted feeling lets them remain aware of their own emotional states and values at all times. Introverted feeling seeks to understand the subjective emotional reactions to the external and internal world. On the positive side, this keeps ISFP and INFP in constant contact with their own motivations and purpose in life. On the negative side, they project their own internal states out into the world and seek to right wrongs where none exist through a crusade.
The auxiliary functions are the main supporters of your dominant function. If you are a dominant judger this will be a perception functions, if you are dominant perceiver this will be a judgment function. These two functions work in tandem where one function dictates what type of information you prefer and how it is perceived. The other makes judgements based on information that has already been judged.
For an INTP, their main source of information is their expansive auxiliary Ne, which is tempered with the categorical, structured and rational introverted thinking. Introverted thinking takes the ideas presented by extroverted intuition and views it through a lens of “does it make sense?”.
For the ISFJ, their main source of information is their grounded introverted sensing, which is translated through extroverted feeling which seeks to create the same sensing experience in the external world. For instance, if an ISFJ has made a connection that large family dinners are a source of security and happiness, then they will seek to recreate this feeling by having their own large family and hosting dinners.
The Inferior Function
The inferior function is the least developed function in the functional stack of a person, and this is what it remains throughout the life of a person. Throughout life, it tends to be the fuel added to the fire when a person is under stress. What happens under stress is that a person will attempt to resolve a problem in the manner they normally do, utilizing their dominant and auxiliary function. When this fails to work, they often get discouraged and end up “In the Grip”, which is a phrase that represents a person controlled by their inferior function.
Inferior Se (INTJ and INFJ): When healthy Se adds a “groundedness” to the present moment, an ability to observe things just as they are, and to take action in the world. When it manifests in INTJ and INFJ, it often results in an addiction to immediate sensory gratification and over-indulgence, with no regard of the consequences. An INFJ may have been on a diet for months, then goes on a 3 week ice-cream binge.For Ni users their inferior extroverted sensing also serves in a different role, which is to constantly feed information to their introverted intuition, that introverted intuition can then process in the background.
Inferior Si (ENTP and ENFP): Dominant and healthy Si, adds a certainty through the connection with the past, and attention to detail. When the inferior manifests in ENTP and ENFP it tends to be a pseudo Si, where they become sticklers for peculiar details. An ENTP may focus intently on font sizes in a presentation, or an ENFP on the perfect way to arrange a bookshelf.
Inferior Ni (ESTP and ESFP): Dominant and healthy Ni adds a certainty, an ability to engage in extremely long term planning, and to “just know“. Inferior Ni, in ESFP and ESTP takes them away from their normal mode of being in the moment, and draws them into a dark abyss where they obsess over the future.
Inferior Ne (ISTJ and ISFJ): Dominant and healthy Ne allows for envisioning all the possibilities in the world and to entertain novel thoughts or ideas. When the inferior manifests in ISFJ and ISTJ, it will replace their normal certainty and steadfastness with uncertainty. They will feel as if there is a disaster around every corner.
Inferior Te (ISFP and INFP): Dominant and healthy Te allows a person to make judgements according to external, objective systems and correct deviations from the planned and the observed. Inferior Te when it erupts in ISFP and INFP comes across as bossy and demanding.
Inferior Ti (ESFJ and ENFJ): Dominant and healthy Ti allows a person to categorize ideas, facts and theories according to an internal logical system, and evaluate all new information according to how it fits in that system. When ESFJ and ENFJ are in the grip of their inferior Ti, they will attack people using their own subjective brand of logic, that in reality is only logical to themselves.
Inferior Fe (INTP and ISTP): Dominant and healthy Fe maintains groups, takes care of the people who make up a group and enforces group rules. When inferior Fe manifests in INTP and ISTP, it does with quite a bit of force, as they tend to have emotional outbursts and frequently accuse other people of sabotaging them. They may also engage in obvious attempts at emotional manipulation.
Inferior Fi (ENTJ and ESTJ): Dominant and healthy Fi gives a certainty to a person’s beliefs, values and morals. When inferior Fi manifests in ENTJ and ESTJ, it often results in emotional outbursts, frequently targeting themselves.
The main reasoning behind being aware of your inferior function and how it manifests is to find ways to control and channel it towards more productive ends, than binges (inferior Se), emotional self-bashing (inferior Fi) or imagining that an asteroid is about to hit Earth (inferior Ne). Your tertiary function can be a great asset in getting out of the grip.
The tertiary function
The role of the tertiary function is one that has rarely received much focus in MBTI. The role of the tertiary function is as a form of mediator in many cases. For instance, the In the ENTJ their tertiary extroverted sensing is what gives them their love of competition, and their characteristic tendency to drive forward regardless of the obstacles. In an ESTJ their tertiary extroverted intuition adds creativity and an ability to engage with novel ideas. Even though they frequently view it as a waste of their time.
The tertiary function can also serve to allow your Dominant or Auxiliary function to “take a break“, as a moderating influence or as a supporter. For instance, the ESTP is often a great salesman because his tertiary extroverted feeling serves to connect him to his target and gives him insights into social groups and behaviors. An INTJ without a moderating Fi, will often engage in long term planning without taking into consideration the consequences for other humans or whether or not his plan is truly congruent with what he desires.
Functions in conflict
The functions themselves can conflict with each other in multiple ways. The first major form of a functional conflict is in a time horizon. Si is focused on the past, and infuses past experiences with a personal connection. Ni is focused on the future and infuses the future visions with a personal connection. Si wants to keep everything the same as this is the source of their security. Ni wants to change everything to adhere to the vision. Fe prefers tact over facts, Te has very little patience for tact.
The other major form of conflict is between the subjective and objective versions of the same cognitive function. Si is measured, conservative and risk averse. Se is impulsive, progressive and loves risk. Te is empirical, prone to generalizations and heuristics, Ti is rationalistic, exacting and definite. Extroverted intuition is focused on all the possible possibilities and generating more possibilities, Ni wants to narrow many possibilities down to a single possibility.
A very common conflict is ISTJ and INTJ where the ISTJ is n a leadership position over the INTJ. In this situation the INTJ will constantly be coming up with suggestions to improve efficiency, processes, and strategies. When he or she brings this to the ISTJ they will be dismissed as the ISTJ is seeking to maintain stability and their sense of security above all else. This causes the ISTJ to perceive the INTJ as a constant troublemaker, and the INTJ to perceive the ISTJ as a backwards obstructionist.
Telling functions apart heuristics for a heuristic
There are no hard and fast rules, and the following observations are not empirically validated, but are observations I’ve made as a long time practitioner of the system.
Fe from Fi: Fe takes shape from the people and groups around it, whereas Fi is shaped to fit the values of each individual. The key to telling them apart is that Fi is usually quirky, and the values of Fi can deviate a lot from the dominant values of a group.
Te from Ti: Te is empiricism, Ti is pure reason. A theory that is developed by a Te user will conform to the known and relevant facts, whereas a theory developed by a Ti user will omit or dismiss facts if they do not fit with the theory. Te users (both dominant and auxiliary) will tend to speak very bluntly whereas Ti users are less prone to hurting people’s feelings.
Ne from Ni: Ne seeks an increasing amount of possibilities while Ni constantly tries to narrow it down to just one. The major way to tell them apart is that Ne users are perfect brainstormers and love to come up with novel idea after novel idea, while Ni users would rather work with a single idea and get to the bottom of it.
Se from Si: Hedonism as opposed to the most boring person who ever lived. It was an Si user who made that statement “I’m planning to be spontaneous on Wednesday“. Se users on the other hand live for both impulsive and excitement.
Statistics on the prevalence of types
Statistically speaking the types vary in prevalence and have different gender distributions as demonstrated by the table below:The majority of people fall within the SJ category, ESTJ, ISTJ, ESFJ and ISFJ, that are often referred to as the Guardian temperament, with introverted sensing, extroverted thinking and extroverted feeling being the most common functions. This follows as SJs are the people who ensures the stability of a group or organization through among others being detail oriented, and highly aware of the past. David Keirsey refers to the SJ grouping as the Guardians.
The above table shows the distribution of cognitive functions between the genders in a theoretical population of 1 million people. For instance, out of 1 million, 105.000 prefer to use extroverted thinking as their dominant function, out of those 105.000, 66% are male, 34% are female. The table below shows the distribution of a given cognitive function in the dominant role in the entire population.
From the above, we can see that the most common dominant perception functions are Si and Se, and the most common judgment functions are Fe, Fi and Te. This follows as the SPs (Sensing Perceivers) and SJs (Sensing Judgers) are the most common types in the population according to the statistics.
Summary and Conclusions
The major reason why I would recommend some baseline knowledge of MBTI to most people is that it helps you understand how to be communicate to a person to get the end result you are after. If you are communicating to a room full of ISTJs and ESTJs, you want to avoid speaking in many theoretical and abstract terms. You want to make sure things are actionable and appear highly pragmatic.
If you are working with a group that has many extroverted perceivers (ENTP and ENFP especially), spending two hours outlining the rules, procedures, and regulations they have to follow, will ensure that they are both bored, and in the case of the ENTPs a great position to troll you.
Find yourself in a situation where you want to open up a set of girls, if you can identify the ISFJ, you have identified the most likely cockblock/mother hen. Getting ready to bounce her to a second location and have identified her as an ESFP, offer her something that feels impulsive and exiting.
A second major reason for learning MBTI is that by understanding your own patterns and modes of thinking you can supercharge your self-development. The types and functions all have a tendency towards certain patterns of self-destructive/non-efficient behavior.
ENTP – Tendency to lose focus and bounce from activity to activity. “So Dad, I know I said I wanted to play football, but I changed by mind and I’m now going to be the world’s first superbike riding fire-juggler”
ESTP – Tendency to engage in highly impulsive behavior without regard for future consequences. ” Officer, I have no idea how old she is, or who owns that car, if you find a bag full of white powder, that is mine.”
ENTJ – Tend to bulldoze through all obstacles without checking if there is a minefield. “What do you mean collusion and insider trading?”
ENFJ – Often engage in highly manipulative behavior. “I heard from Cheryl in accounting that your wife called asking for your financial information, apparently she’s filing for divorce. By the way, the board wants to hear your presentation now.”
ESFJ – Enforces conformity, resulting in a more well functioning group that nobody wants to be a member of. “My lord Darla, do you not realize that a frozen dinner for your husband because you were out gallivanting in a skirt that barely covers your ankles makes you a horrible wife?”
ENFP – Flighty in personal relationships. “Yes, I know I said I loved you and wanted to spend the rest of my life with you, but that was last night!”
INTJ – Tendency to engage in enormous amounts of research and planning and never putting that plan into action. “If only I can find 4 more books, and 8 lectures on this subject, I could finally start planning to plan my master plan”
INTP – Will often sulk and become excessively pedantic and thus spend much time debating things they agree with. “Hmmf, no I’m not sulking, sulking involves stomping feet, I’m seething!”
INFP – Seeks to ensure that nobody gets hurt, and in doing so suppresses free speech. “YOUR FACTS SHOULD CARE ABOUT MY FEELINGS!”
INFJ – Can often end up living in their own fantasy world unconnected from reality. “So for this cake, I need some unicorn powder, 4 eggs, and … how did that light-bulb end up in the mixer?”
ISFJ – So obsessed with the needs and comfort of others that they end up neglecting their own needs completely. “Ok everyone, dinner is served, I have a dessert in the fridge, I put on the coffee, your laundry is drying, if it’s OK with you, I’ll go get treated for my heart attack now”
ISTP – The least expressive of all types. “I haven’t said “I love you” in 25 years? I married you, that says it all”
ISTJ – Can get overly stuck on the past. “I know cars exist, but the internal combustion engine has only been around a few hundred years, how do we know that it’s safe?”
Was That Really Me? By Naomi Quenk A great book that explores inferior functions in depth.
Building Blocks of Personality Type by Leona Haas and Mark Hunzinger A very in depth overview of MBTI and personality typing.
Personality Type an Owner’s Manual by Leonore Thompson A book that is helpful if you are still struggling to identify your type.