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ISTJ (introversion, sensing, thinking, judgment) is an abbreviation used in the publications of the Myers-Briggs Type Indicator (MBTI) to refer to one of sixteen personality types. The MBTI assessment was developed from the work of prominent psychiatrist Carl G. Jung in his book Psychological Types. Jung proposed a psychological typology based on the theories of cognitive functions that he developed through his clinical observations.
From Jung's work, others developed psychological typologies. Jungian personality assessments include the MBTI assessment, developed by Isabel Briggs Myers and Katharine Cook Briggs, and the Keirsey Temperament Sorter, developed by David Keirsey. Keirsey referred to ISTJs as Inspectors, one of the four types belonging to the temperament he called the Guardians. ISTJs account for about 10–14% of the population.
By using their preference in each of these areas, people develop what Jung and Myers called psychological type. This underlying personality pattern results from the dynamic interaction of their four preferences, in conjunction with environmental influences and their own individual tendencies. People are likely to develop behaviors, skills, and attitudes based on their particular type. Each personality type has its own potential strengths as well as areas that offer opportunities for growth.
The MBTI tool consists of multiple choice questions that sort respondents on the basis of the four "dichotomies" (pairs of psychological opposites). Sixteen different outcomes are possible, each identified by its own four-letter code, referred to by initial letters. (N is used for iNtuition, since I is used for Introversion). The MBTI is approximately 75% accurate according to its own manual.
ISTJs are faithful, logical, organized, sensible, and earnest traditionalists who enjoy keeping their lives and environments well-regulated. Typically reserved and serious individuals, they earn success through their thoroughness and extraordinary dependability. They are capable of shutting out distractions in order to take a practical, logical approach to their endeavors, and are able to make the tough decisions that other types avoid. Realistic and responsible, ISTJs are often seen as worker bees striving steadily toward their goals. Despite their dependability and good intentions, however, ISTJs can experience difficulty in understanding and responding to the emotional needs of others. [dead link]
Although they often focus on their internal world, ISTJs prefer dealing with the present and the factual. They are detail-oriented and weigh various options when making decisions, although they generally stick to the conventional. ISTJs are well-prepared for eventualities and have a good understanding of most situations. They believe in practical objectives, and they value traditions and loyalty.
For the Keirsey description, see Inspector (role variant).
ISTJs learn best and apply themselves to subjects that they deem practical and useful. They bring painstaking attention to detail in their work and will not rest until a concept is fully learned or a job is well completed.  As learners, ISTJs tend to need materials, directions, and teachers to be precise and accurate if they are to trust the information that is presented. They prefer concrete and useful applications and will tolerate theory only if it leads to these ends.
They like learning activities that allow them time to reflect and think. Material that seems too easy or too enjoyable leads ISTJs to be skeptical of its merit. Because of their practical outlook, ISTJs clearly delineate between work and play. Therefore, their ideal learning environment is task-oriented, has a clear schedule, and has a clear and precise assignment.
ISTJs respect facts. They hold a tremendous store of data within themselves, gathered through their Sensing function. They may have difficulty valuing a theory or idea that differs from their own perspective. However, if they are shown the importance or relevance of the idea by someone whom they respect or care about, the idea becomes a fact that the ISTJ will internalize and vigorously support.
ISTJs often work for long periods, devoting their energy to tasks that they see as important to fulfilling a goal. However, they resist putting energy into things that don't make sense to them, or for which they can't see a practical application. They prefer to work alone but can work well in teams when the situation demands it. They like to be accountable for their actions, and they enjoy positions of responsibility. They have little use for theory or abstract thinking, unless the practical application is clear.
In general, ISTJs are capable, logical, reasonable, and effective individuals with a deeply driven desire to promote security and peaceful living. They can be highly effective at achieving their goals—whatever those may be.
Drawing upon Jungian theory, Isabel Myers proposed that for each personality type, the cognitive functions (sensing, intuition, thinking, and feeling) form a hierarchy. This hierarchy represents the person's default pattern of behavior.
The Dominant function is the personality type's preferred role, the one they feel most comfortable with. The secondary Auxiliary function serves to support and expand on the Dominant function. If the Dominant is an information gathering function (sensing or intuition), the Auxiliary is a decision making function (thinking or feeling), and vice versa. The Tertiary function is less developed than the Dominant and Auxiliary, but it matures over time, rounding out the person's abilities. The Inferior function is the personality type's Achilles's heel. This is the function they are least comfortable with. Like the Tertiary, the Inferior function strengthens with maturity.
Jung and Myers considered the attitude of the Auxiliary, Tertiary, and Inferior functions to be the opposite of the Dominant. In this interpretation, if the Dominant function is extraverted, then the other three are introverted, and vice versa. However, many modern practitioners hold that the attitude of the Tertiary function is the same as the Dominant.
Using the more modern interpretation, the cognitive functions of the ISTJ are as follows:
Si collects data in the present moment and compares it with past experiences, a process that sometimes evokes the feelings associated with memory, as if the subject were reliving it. Seeking to protect what is familiar, Si draws upon history to form goals and expectations about what will happen in the future. Using Si, ISTJs thrive on deep analysis of their surroundings.
Te organizes and schedules ideas and the environment to ensure the efficient, productive pursuit of objectives. Te seeks logical explanations for actions, events, and conclusions, looking for faulty reasoning and lapses in sequence.  ISTJs use this function to actively process and evaluate their perceptions.
Fi filters information based on interpretations of worth, forming judgments according to criteria that are often intangible. Fi constantly balances an internal set of values such as harmony and authenticity. Attuned to subtle distinctions, Fi innately senses what is true and what is false in a situation. Fi allows ISTJs to turn their analysis to themselves and others, to understand their feelings and the causes thereof.
Ne finds and interprets hidden meanings, using “what if” questions to explore alternatives, allowing multiple possibilities to coexist. This imaginative play weaves together insights and experiences from various sources to form a new whole, which can then become a catalyst to action. While ISTJs are capable of rapid and dogged information processing and number crunching, they often have difficulty with, or simply dismiss, abstract concepts without immediate concrete applications.
Later personality researchers (notably Linda V. Berens) added four additional functions to the descending hierarchy, the so-called "shadow" functions to which the individual is not naturally inclined but which can emerge when the person is under stress. The shadow processes "operate more on the boundaries of our awareness…We usually experience these processes in a negative way, yet when we are open to them, they can be quite positive." For the ISTJ these shadow functions are (in order):