I am with Otis, belljr, and bostonfred in the 15% "freak" category. Just stumbled across this concept yesterday and asked a woman who works for me and she said she had the inner monologue. I told her that I don't and her reaction was "that explains alot"
Had a similar convo with my wife yesterday. I brought it up at dinner and she said she had heard about it earlier in the week. Said she has a running monologue and upon hearing some dont immediately thought of me.
This is kinda freaking me out as I have been very consciously inspecting my thoughts and actions and while my head is "quiet" it is definitely filled with thoughts, concepts and ideas. The diff between me and the 85% seems to be the lack of complete sentences. My thoughts are much more conceptual and harder to articulate. As such I seem to have less agency than the 85% describe and yet no one would call me impulsive. Quite the opposite, they'd say I am overly planful and deliberate. I am also well above average in intelligence, yet I do not think I am very good at speaking extemporaneously.