An age-old question which many people have pondered at some point in their lives is "why am I the way I am?" We can't say with absolute authority how much of our personality is innate and how much develops early in life, because we don't yet know how to prove these things. But it's logically consistent to consider personality as largely innate—certainly Jung believed that to be the case.
Recent studies done around temperament suggest that we are each born with our temperaments intact, and that our innate personalities begin to develop when we're quite young.
W. Harold Grant, a Jungian pastor, concentrated on Jung's ideas around the development of personality, and concluded that personality type has an ongoing developmental process that can be observed throughout an individual's life. The early phases of our lives help determine the dominance ordering of the four functions (Sensing, Intuition, Thinking and Feeling), and the development of our dominant and auxiliary functions. The later phases help us develop our tertiary and inferior functions.
Let's take a look at Grant's phases of development, using the INFJ personality type as an example.
At this early age, we use all four of the functions in an indiscriminate fashion. We "try on" the different functions for size, determining which ones work best for us. The little INFJ has not yet emerged as any particular personality type, although their parents may notice the dreamy quality associated with the NF temperament.
During this phase, our dominant function begins to develop and assert itself. Our young INFJ becomes more obviously introspective - they begin to prefer to use their iNtuition to take in information, and choose to store data internally and then ponder it in an Introverted fashion. The dominant function of "Introverted iNtuition" begins to show itself as the prevailing aspect of their personality, and we can now determine that the child is an Introverted Intuitive Judger. Intuition is an information gathering function, so by definition its auxilliary will be a decision making function. We don't yet know if the auxilliary is Thinking or Feeling, but we know it must be an extraverted decision making function, since the dominant function is introverted. We determine Judging versus Perceiving by the whether the highest decision making function is extraverted or introverted, and we can now determine that this child is a Judger, and falls into the INJ type.
The auxiliary function asserts itself as a powerful support to the dominant function during this time. Since all recent studies point toward the importance of a well-developed team of dominant AND auxiliary functions, this is an important time of "self-identification." Research suggests that people without a reasonably strong auxiliary function to complement their dominant function may have problems functioning effectively. We need both information gathering and decision making processes at work for us.
In our INFJ example, we see the auxiliary Feeling function come to the front during this phase as a support to the dominant iNtuitive function. As the auxilary Feeling process comes forth, the INFJ begins to develop the ability to make decisions based primarily on the value system of the society in which they live. Our INFJ personality type is now pretty firmly set in place, and we know the dominance ordering of all four functions—1. Introverted Intuition, 2. Extraverted Feeling, 3. Introverted Thinking, and 4. Extraverted Sensing (shadow).
We begin to use our tertiary function more frequently and with better success. Our INFJ begins to use their Introverted Thinking function. They continue to make judgments with Extreverted Feeling, but now also begin to make judgments based on logic and reason, which they will work through in their own mind, rather than discussing it with others.
We pay attention to our fourth, inferior function. We feel a need to develop it and use it more effectively. Our INFJ begins to use their Extraverted Sensing function. They become more aware of their surroundings and begin to take in information from others in a more literal, practical sense. They continues to rely on dominant Introverted iNtuitive to take in information, but they are more able to use Extraverted Sensing than in prior years.
Some researchers have attested that the appearance of our inferior functions at this phase of life may be responsible for what we commonly call the "mid-life crisis."
From this age until our deaths, we have access to all four functions, and we use them in a more disciplined, differentiated manner than when we were young. Our basic personality type continues to assert itself, but we are able to call upon other functions when needed, and the personality type we consciously identify with becomes more fluid. It's common for older extraverts to think they're now introverted, and vice-versa. Their personality type hasn't actually changed, they've just individuated beyond it. Our shadow functions will likely remain a weak spot for most of us.