MBTI cognitive functions: how to read a function stack

Spend ten minutes anywhere people discuss personality types and the four letters disappear. In their place: Ni, Te, Fi, Se. Somebody is a "dominant Ne." Somebody else has an "inferior Fe" that is apparently causing trouble.

These are cognitive functions, and the ordered list of four that goes with each type is called a function stack. The notation looks like chemistry and is closer to bookkeeping. It can be worked out from any four-letter code in about a minute, and this page shows how. It also covers the part that usually gets skipped, which is how much of it has been tested.

Four functions, two directions

The idea is older than the questionnaire. In Psychological Types, published in 1921, Carl Jung described four mental functions and split them into two pairs.

Two are ways of taking information in. Sensation attends to what is concretely there. Intuition attends to what it suggests or connects to. Two are ways of reaching a decision. Thinking decides by principle and consistency. Feeling decides by worth and by the effect on the people involved. These are the middle two letters of a type: S or N, then T or F.

Jung added that any function can be pointed in one of two directions. Extraverted means aimed at the outside world of people and things. Introverted means aimed at the inner world of ideas and impressions. Four functions in two directions gives eight combinations, and the shorthand is a capital for the function and a small letter for the direction. Te is extraverted thinking. Ni is introverted intuition, with N standing for intuition because I was already taken.

A stack lists four of the eight in order of preference. The first is called dominant, the second auxiliary, the third tertiary and the fourth inferior.

How to read a stack from four letters

The rules came from Briggs and Myers, who needed a way to get from a questionnaire result to Jung's scheme. They fit in four steps.

1. Take the two middle letters. Those are the two functions the type prefers, one for taking in and one for deciding.
2. Look at the last letter. It tells you which of the two is shown to the outside world. J means the deciding function, T or F, is the extraverted one. P means the perceiving function, S or N, is. The other middle letter is introverted.
3. Look at the first letter. For an E, the extraverted function from step two is dominant and the introverted one is auxiliary. For an I it is the reverse.
4. Fill in the bottom. The tertiary is the opposite of the auxiliary and the inferior is the opposite of the dominant, where the opposite of S is N and the opposite of T is F.

Try it on ENFP. The middle letters are N and F. The last letter is P, so the perceiving function is the extraverted one: Ne, and feeling is introverted, Fi. The first letter is E, so the extraverted function leads. Dominant Ne, auxiliary Fi. The opposites fill the rest: thinking third, sensing last.

Now an introvert, because step three is where people slip. ISTP has S and T in the middle. The P says sensing is the extraverted one, Se, which leaves thinking introverted, Ti. But this is an I, so the introverted function leads. Dominant Ti, auxiliary Se, then intuition, then feeling.

Notice what happened there. An ISTP has a P at the end and a dominant function that is a judging one. For introverts the last letter describes the second function in the stack, the one other people get to see. It is the single most counterintuitive consequence of the system and the source of a great deal of confident mistyping.

What the stack claims that the letters do not

If the stack only restated the letters there would be no reason to learn it. It makes a stronger claim: that changing one letter can rearrange everything.

The standard example is INFP and INFJ. On paper they differ by a single letter and should be near twins. Run the four steps, give the bottom two the directions most charts give them, and the INFP comes out as Fi, Ne, Si, Te while the INFJ comes out as Ni, Fe, Ti, Se. The two lists have nothing in common. Every function in one appears in the other pointed the opposite way.

So the theory predicts that these two types, which share three letters, are built quite differently: one led by a private sense of what matters, the other by a private sense of where things are heading. Enthusiasts say the prediction matches what they see. Whether it does is a separate question, and it has an awkward answer.

The other claim is about the bottom of the list. The Myers and Briggs Foundation describes the inferior function as the one that receives the least energy and is the least conscious and developed. Function theory holds that this is what comes out, clumsily, when a person is worn down, and how each type changes under pressure describes what that looks like without the notation.

Where the experts disagree with each other

The third slot is unsettled even among people who teach this for a living. Most charts, including the one on each type page here, print the tertiary with a direction attached. The Myers and Briggs Foundation, on its own explanation of type dynamics, notes that the tertiary can be expressed as either an extraverted or an introverted process. When a stack says Te in third place, the T is the rule and the e is one school's convention.

A larger point sits under that one. No questionnaire measures the functions. The MBTI does not, and neither does the test on this site. Each asks about four preferences, produces four letters, and the stack is then derived from the letters by the steps above. Nobody observed your Ni. It was calculated.

How much of this has been tested

Less than the confidence of the conversation suggests.

The sharpest criticism came from inside the type community. In 2009 James Reynierse published a paper called The Case Against Type Dynamics in the Journal of Psychological Type, a journal put out by the Center for Applications of Psychological Type, which Isabel Briggs Myers helped found. He argued that the ordering of functions rests on anecdote, that it fails when checked against data, and that it should be dropped. His alternative was plain: treat the four preferences as four separate dimensions that each do their own work, with E and I describing E and I and nothing more.

That fits what outside researchers had already reported. When Robert McCrae and Paul Costa compared the MBTI with a Big Five questionnaire in the Journal of Personality in 1989, they found four fairly independent scales and concluded that Jung's theory was either incorrect or not adequately captured by the instrument. MBTI vs the Big Five has the details.

None of that proves the stacks are empty. It means the evidence for them is the kind a reader should weigh as testimony: many people find the descriptions recognizable, and recognition is cheap, for the reasons in why every type description fits.

What the functions are good for

I think of the stack as a second vocabulary laid over the first. It is useful when the four letters leave something out that you can plainly see: why two people with nearly the same code approach a decision from opposite ends, or why a normally careful person turns impulsive after a bad month.

It stops being useful when it turns into a way of overruling the result. If you answered the questions honestly and then argued yourself into another type because a function description sounded more like you, the description won an argument against your own answers. That is a poor trade.

MBTI is a trademark of The Myers-Briggs Company, which has no connection to this site. I run it by myself and I am not a psychologist, and nothing here is a clinical or diagnostic tool. Each of the sixteen type pages lists its stack so you can check your working, and the test gives you the four letters to start from.