Have you ever heard someone introduce themselves as an INFJ or ESTP? Those letters come from the Myers-Brigg personality system, which deals with how you perceive and interact with the world around you. Myers and Briss were a mother-daughter team who adapted the philosophies of Carl Jung into a more readable form. Interestingly enough, the daughter, Isabel Briggs Myers, was a mystery writer.
According to the Myers-Brigg website,“Seemingly random variation in the behavior is actually quite orderly and consistent, being due to basic differences in the ways individuals prefer to use their perception and judgment.”
Myers Brigg takes four different traits, and assigns people one of two possible options, as follows:
I vs. E: Introvert v. Extrovert (social interactions)
N vs. S: Introspective vs. Sensing: (information and observation)
F vs. T: Feeling vs. Thinking. (decision-making)
P vs. J: Perceiving vs. Judging: (structure)
Bonus: A vs T: Assertive vs. Turbulent (handling stress)
An introvert who absorbs information as they receive it, who bases their decisions more on feelings than thoughts, and who is open to change, would use the letters INFP.
From what I've seen from informal surveys on social media, around 75% of writers are Introverted, while maybe 90% are Introspective over Sensing.
When I was developing my character Perrin Andres, the main character of The Seventh Clan, I used the Myers Brigg personality system. I knew that he needed to be introverted, because he would need to be comfortable spending long times alone. He'd be Sensing instead of Introspective, because I wanted the story to have a survival-type feel. He had to be Feeling, because he is motivated by his empathy for others. I also wanted him to be open to change, so that led to Perceiving over Judging. That makes him ISFP. I ran that type through the Personality Database, and found that Perrin shares a personality type with Zuko from Avatar: the Last Airbender, Harry Potter, Eragon, and Tyson the cyclops. It looks like, as a young man protagonist of a YA fantasy novel, Perrin is in good company.
"Unlike Myers-Briggs, which is a “neutral” system focused primarily on the differing ways people take in and use information, the Enneagram is often called an “ego-transcendence tool.” Sounds all lofty and new-agey, but it’s really just code for “this-is-gonna-hit-you-where-it-hurts.”
--KM Weiland, 5 Ways to Use the Enneagram to Write Better Characters - Helping Writers Become Authors
“It is unique amongst personality tests in that it doesn't try to pigeonhole you based on who you happen to be at this very second; it accounts for personal evolution, both in the past and in the future, and gives suggestions for how to improve… Your greatest weakness is your greatest strength pushed too far.