I remember reading somewhere that children's personalities are much more fluid than adults (which would fit with how we know the brain to develop. ie. that it's a fluid learning machine when you're young, but that the older you get the more "static" it becomes and the harder it becomes for us to learn and unlearn things), which is part of why my current opinion is that type develops gradually, but that once it's settled in it's probably settled in for life.
If the research by Katherine Benziger that Peppermint mentioned is right, then it seems likely to me that the primary function is the first to "fix" because it is so massively more efficient for us to use than the others, which fits with at least some of PersonalityPage's theory about type development in children. Also, if Benziger's research is correct then this is why Feeling is always the "inferior" (in the MBTI model) of Thinking, Sensing the "inferior" of Intuition etc., because it's hardest for us to "communicate" with the part of the brain that lies diagonally opposite the area where our primary function resides, so it takes the most concentration and energy.
It would also seem to explain why people can be either INFP or ISFP, ENFJ or ESFJ etc., it's because "communication" with the parts of the brain that are "next door to" our primary require equal levels of effort, at least at first - because I think that what leads to the "fixing" of the secondary function is frequency of use. We know that the more we perform a particular act, the stronger the neural connections in the brain that deal with that act become, making it easier for us to perform it. I think then that the secondary function usually "fixes" during late childhood/early teens because we have simply used it more often than its alternative (eg. I'm an INFP instead of an ISFP because I used Ne more than Se as a child, and so the neural pathways that deal with Ne modes of thought are more developed and therefore easier for me to use than the ones that deal with Se modes of thought. And because my brain is becoming increasingly "static", the effort required to develop my Se pathways more than my Ne pathways is by now probably too much to manage, at least within my lifetime).
Because of all that, I think that in most people once you hit your 20's or so then rather than becoming a different type, it's all about developing the type you already are, by the process that 44sunsets mentioned (ie. people go through phases where they develop their tertiary function during their 20's or 30's, and their "inferior" function during their 30's or 40's).
If the research by Katherine Benziger that Peppermint mentioned is right, then it seems likely to me that the primary function is the first to "fix" because it is so massively more efficient for us to use than the others, which fits with at least some of PersonalityPage's theory about type development in children. Also, if Benziger's research is correct then this is why Feeling is always the "inferior" (in the MBTI model) of Thinking, Sensing the "inferior" of Intuition etc., because it's hardest for us to "communicate" with the part of the brain that lies diagonally opposite the area where our primary function resides, so it takes the most concentration and energy.
It would also seem to explain why people can be either INFP or ISFP, ENFJ or ESFJ etc., it's because "communication" with the parts of the brain that are "next door to" our primary require equal levels of effort, at least at first - because I think that what leads to the "fixing" of the secondary function is frequency of use. We know that the more we perform a particular act, the stronger the neural connections in the brain that deal with that act become, making it easier for us to perform it. I think then that the secondary function usually "fixes" during late childhood/early teens because we have simply used it more often than its alternative (eg. I'm an INFP instead of an ISFP because I used Ne more than Se as a child, and so the neural pathways that deal with Ne modes of thought are more developed and therefore easier for me to use than the ones that deal with Se modes of thought. And because my brain is becoming increasingly "static", the effort required to develop my Se pathways more than my Ne pathways is by now probably too much to manage, at least within my lifetime).
Because of all that, I think that in most people once you hit your 20's or so then rather than becoming a different type, it's all about developing the type you already are, by the process that 44sunsets mentioned (ie. people go through phases where they develop their tertiary function during their 20's or 30's, and their "inferior" function during their 30's or 40's).
