There is no consistent definition for Shadow Personality Type. The word Shadow has been co-opted in multiple different formats. Many people have used the term in contexts that have nothing to do with Myers-Briggs or personality typology.
I’ve heard it used to represent “Shadow Cognitive Functions” which would be the 4 functions that would not be in your stack or Car Model.
Some people refer to the “back seat passengers” as the Shadow Personality Type.
Others believe it is the exact reverse of your personality type (so if you’re an ENTP – your Shadow would be ISFJ).
According to John Beebe (Jungian analyst), the Shadow is the reverse of the 8 cognitive function stack. So, an ENTP in its shorthand function stack would be: Ne – Ti – Fe – Si. The mirror image of this stack would be Se – Fi – Te – Ni, or ESFP. Beebe may not actually refer to this as Shadow Type, but it has been referred to as such by others.
Basically, people like to apply the term “Shadow Personality Type” to the parts we all have that are in the shadows – or the parts of us least represented. Because of all the various contexts used to apply to Shadow Personality Type, we cannot absolutely say what it is or is not.
However, there is some merit to the idea, since we can gain a lot of different insights when we – for example – reverse our dichotomies. (Ideally different terms would be employed to differentiate between concepts.)
One ENTP likened his reverse type (ISFJ) to a meek, little Asian woman hidden within him who was very giving and caring. That was his archetype for his Shadow. What is interesting is he swapped genders, nationality, and language in relating to this part of him.
The shadow part of you is the part that is within you. It is there but you aren’t really checking in with it or
understanding it very well. It influences you and is a part of who you are. It will rear its head in odd moments. So, it is a “Shadow” in that it is still a part of us even when we don’t acknowledge its presence.
Naomi Quenk has a book called “Was That Really Me?” where she discusses the Myers-Briggs dichotomies in reverse. She also has a book called “In the Grip” where she reveals how we get in the grip of the 3 Yr Old process – the part some people refer to as the Shadow part of our type.
Personality Hacker doesn’t use the term “Shadow” very often because of its nebulous definition and application. We talk about how the back seat affects us but we don’t explore the more profound concepts of Shadow. We have our own take and look at typology through the lens of the highest possible leverage for genius style development. We don’t want people to get so bogged down in subtle nuances that they lose sight of the highest leverage points of personality typology. However, for those who are ready for a more abstract approach, an exploration of the above referenced works may help.
— Charis Branson: Inspired by PHQ | QUESTIONS: Shadow Personality Types podcast