Q:I didn't see anyone ask you this yet, so could you tell me who are some fictional ISTJs please? And how common is this type? I'm one and I don't see too much information about it. Thanks! :)

wondersuntold

ISTJs are very common, one of the most common types. 

Theresa Lisbon, The Mentalist
Dr. Sheldon Cooper, The Big Bang Theory
Dr. Temperance “Bones” Brennan, Bones
John Watson, Sherlock Holmes

- theideathief 

Q:Regarding twin comment: entirely theoretical, but perhaps it has to do with the cell splitting? Since ISTJ and ENFP are at a glance the opposite in all 4 letters. I know that it doesn't necessarily mean the two will be completely opposite in function or whatever, but maybe this is sort of like the left hand/right hand difference that identical twins often have.

Anonymous

Hm. This would best be aimed at a geneticist with some background in psychology. 

Genetics has a surprising amount to do with the outcome of personality- but let’s not discredit environment. 

It could go anywhere from one twin taking on learned behaviors from a young age to compensate for the other (rule-follower v. rebel)- this also appears in sibling behaviors- to pure genetics/ more subtle influences. 

I would argue that it has a lot to do with compensation, and that personality types are far more flexible than we give credit. One might say: your personality type is who you are when you are not influenced by people/ are in a normal, stress free environment. But let’s be honest: we are never free from the influence of other people- in every situation we are called to compensate (or not) for some instance.