INFJ, INTJ, ISFJ & ISTJ Types: Similarities, Differences & the Brain

By Dr. A.J. Drenth

What do introverts (I) and judging (J) types have in common? For starters, both tend to be reflective and deliberative, taking care to “look before they leap.” In contrast to extraverted (E) perceivers (P), introverted judgers (“IJs” for short) are reluctant to act on impulse or partake in unduly risky behavior.

IJs are also inclined to conserve their physical energy. In contrast to EP types, IJs aren’t brimming with impulsive energy that demands frequent release. In fact, it’s not uncommon for IJs to have limited physical energy, prompting them to conserve and judiciously utilize it. Thus, IJs tend to be more naturally sedentary than other types, requiring less in the way of regular physical exertion. Of course, this isn’t to say that IJs, like everyone else, can’t benefit from an exercise regimen, even if they’re not overly enthused at the prospect.

I’ve always considered “The Tortoise and the Hare” fable an apt portrayal of the respective IJ and EP approaches. Impatient with preliminary research and planning, the hare (i.e., the EP type) is prone to start out fast, but may fail to properly pace herself. If the EP hare ends up prematurely exhausting all her energy and resources, the IJ tortoise, who proceeds at a slow but steady pace, will usually come out ahead.

In addition to the commonalities among the four IJ types, we also know that they differ in important ways. And once we introduce intuition (N) and sensing (S) into the mix, things get a lot more interesting. Hence, for the remainder of this post, we will compare INJs (i.e., INFJ & INTJ), along with their dominant function—Introverted Intuition (Ni), to ISJs (i.e., ISFJ & ISTJ) and their dominant function—Introverted Sensing (Si). We will also discuss the relationship of these functions to the two brain hemispheres.

INFJ / INTJ vs. ISFJ / ISTJ

As discussed in my book, My True Type, both Ni and Si are introverted perceiving functions. I realize this may be a bit confusing considering that IJs are classified as judging types, but it stems from the fact that the J-P label was largely intended to describe a type’s outer (i.e., extraverted) presentation. So even though IJs inwardly lead with a perceiving function (i.e., Ni or Si), outwardly they come across as judgers. Their J presentation is partly due to the convergent nature of their perceptive process, which we will soon discuss, as well as their use of an auxiliary judging function, either Extraverted Feeling (Fe) or Extraverted Thinking (Te).

Precedent (Si) vs. Insight (Ni)

The inner perceptions of Si commonly involve recollections of the past. This is why memories—along with the habits, traditions, and keepsakes that enhance and substantiate them—are usually of great significance to ISFJs and ISTJs.

In my post on the function roles, I suggest that the primary role of Si is one of preserving. Not only do ISJs preserve physical energy, as we saw earlier, but also the beliefs, behaviors, and relationships they’ve come to embrace over time. Generally speaking, the more ISJs are exposed to something, the more preferable it becomes. Thus, Si convictions about truth and morality can often be traced back to the traditions in which the ISJ was raised. While childhood is a formative period for all personality types, the retrospective propensities of Si makes it particularly relevant for ISJs.

Ni perceptions aren’t recollective per se, but they do inspire a similar sense of allegiance and conviction as those of Si. Once an Ni intuition or vision has been embraced by an INFJ or INTJ, it’s not easily uprooted, at least not in its entirety. The following excerpt from J.L. Heiberg’s Autobiographical Fragments illustrates the powerful and enduring effects of an Ni revelation:

Pondering constantly over what was still unclear to me, it happened one day that, sitting in my room… I was gripped by a momentary inner vision, as if a flash of lightning had suddenly illuminated the whole region for me and awakened in me the hitherto central thought. From this moment the system in its broad outline was clear to me and I was completely convinced that I had grasped it in its innermost core. I can say in truth that that strange moment was the most important juncture in my life, for it gave me a peace, a security, a self-awareness which I had never known before.

Heiberg’s experience illustrates some key features of Ni: unexpected, illuminating, awakening, engendering clarity and conviction. Granted, the majority of Ni’s insights aren’t this intense or life-changing, but we can nonetheless understand why INJs, like ISJs, might exhibit strong convictions (J).

Considering that both Si and Ni are related to ideational convergence and persistence, J.E. Sandoval has collectively described them as “worldview” functions. As we’ve seen, Si builds its worldview steadily by way of repeated exposure to certain beliefs, lifestyles, and traditions. Ni worldview development is less dependent on environmental circumstances, owing largely to the fact that it perceives different sorts of phenomena than Si does. Namely, Ni attunes to deep underlying patterns, whether scientific, social, spiritual, or psychological. While ISJs may enjoy studying these topics, they aren’t discovering them first-hand through intuition and are thus forced to rely on other factors, such as past precedent, to evaluate their veracity. By contrast, INJs may experience a deep sense of knowing something in an instant, as Heiberg’s account demonstrates.

“Represented Experience”

In her book, Personality Type: An Owner’s Manual, Lenore Thomson suggests that IJs are largely oriented to “represented experience,” that is, to “words, facts, numbers, signs and symbols: the kind of data that can be acquired or explored in the mind.”

Thomson also characterizes IJs, along with their EJ counterparts, as predominantly “left-brained” personality types. This squares nicely with her point about represented experience, as the left hemisphere, according to Iain McGilchrist, is well-viewed as the hemisphere of representation and abstraction. McGilchrist suggests that the left hemisphere operates as “an abstract visual-form system, storing information that remains relatively invariant across specific instances, producing abstracted types or classes of things.” The right hemisphere, by contrast, “deals preferentially with actually existing things as they are encountered in the real world.”

This too may seem a bit confusing, as the Myers-Briggs community has historically associated intuition with abstract perception and sensing with concrete perception. While true to a certain extent, the abstract vs. concrete distinction is most useful when comparing INJs, the most abstract of all types, with ESPs, who are the most concrete. The abstract-concrete distinction seems to carry less weight for SJ and NP types. If it’s true that ISJs spend much of their time engrossed in the abstract / represented world of the left hemisphere, then it’s hard to see how they’re significantly more concrete than NP types.

IJ’s penchant for abstract representation helps explain their facility with language, as well as their interest in careers such as law, education, research, etc. IJs love learning and are typically good at retaining information, especially that which pertains to their personal interests. They also appreciate opportunities to share what they’ve learned with others, motivating their work as teachers, advisors, ministers, counselors, and the like.

Consistent with my earlier remarks about IJs’ penchant for conserving physical energy, they also have a reputation for being “home bodies.” Since their preferred mode of perception is introverted, IJs are less driven than other types to venture out in search of new experiences. Add to this the fact that staying home is particularly convenient in our high-tech world, which offers a smorgasbord of new experiences at the touch of a button.

That being said, as IJs mature and take interest in their inferior extraverted perceiving functions (Extraverted Sensing for INJs, Extraverted Intuition for ISJs), they may grow a bit restless and start stretching themselves experientially. I know a number of INJ foodies, for instance, who are constantly scoping out new gastronomic experiences. I’ve also encountered a number of ISJs and INJs who have come to enjoy travelling.

As is true of all types, IJs are curious about, even idealistic toward, their inferior function. Hence, many of their careers or hobbies can be understood as unwitting attempts to marry abstract and concrete forms of perception. ISJs, for example, commonly take up knitting or other forms of handiwork, especially later in life. Similarly, INJs may develop interests in cooking, fashion, painting, sculpting, etc., which furnish the sensory novelty and connectedness (Se) that Ni, on its own, fails to provide.

More on Si, Ni & the Brain

Most typology enthusiasts would likely concur with Thomson’s classification of ISFJs and ISTJs as left-brained types. While I’m also inclined in that direction, if we’re willing to acknowledge Si as a mediator of internal bodily awareness (i.e., the type of awareness cultivated in mind-body practices), then Si would appear to possess a more concrete, right-brained element. However, since NP types are generally more interested in developing bodily awareness than SJs seem to be, one might question the utility of discussing this aspect of Si with respect to SJ types.

Although students of type will probably agree that INFJs and INTJs, like other J types, exhibit certain left hemisphere traits, classifying Ni as left-brained may still seem a bit of a stretch. Insofar as INJs deal in the currency of “represented experience,” I think Thomson’s theory is on the right track. Sandoval’s association of Ni with the establishment and perpetuation of a worldview lends further support.

What seems harder to reconcile is the actual process of Ni, which is suggestive of an integrated, whole-brain phenomenon. While Heiberg was admittedly wrestling with left-brain abstractions prior to his flash of insight, such a revelation surely demanded the creative and synthetic powers of the right hemisphere. We might therefore be wise to delineate between Ni’s role in maintaining / referencing a stable worldview (left hemisphere), which may in some respects resemble how ISJs operate, and the intuitive process itself, which requires greater collaboration between the two hemispheres.

That said, Ni intuitions are typically more convergent than those of Extraverted Intuition (Ne), with Ni proffering what it sees as THE solution rather than multiple potential solutions (Ne). And in this sense, one could argue that even the Ni process is giving the left brain the last word. While Ni is rightly considered a perceiving function, its convergent operations in some regards resemble a judging process (e.g., reaching singular conclusions), which is something we can’t say about Ne.

In the end, even if these sorts of hemispheric classifications are over-simplifying things, they appear to contain at least a kernel of truth that can help us better understand the cognitive propensities of the respective types and their neuropsychological underpinnings.

Learn more about the types and functions in our books:

My True Type: Clarifying Your Personality Type, Preferences & Functions

The 16 Personality Types: Profiles, Theory & Type Development

Related Posts:

The 8 Functions: Roles & Characteristics

Type Profiles: INFJ | INTJ | ISFJ | ISTJ

The 8 Functions