This is a really interesting question!

I am an INTP. My most recent psychiatrist was kind of on the fence about how to diagnose me--I have traits of both Schizoid and Avoidant PDs, but he decided to go with Avoidant because of my fear of confrontation and sensitivity to criticism. Those traits are, I believe, linked to my PTSD/upbringing, and Schizoid PD just seems to make more sense to me; I relate to it better, especially as I work through my other problems. I also have Asperger's, which I believe relates to both my INTP-ness as well as my Schizoid tendencies...I don't know much about how these things can interact with each other clinically (outside of my own experience), but that is something I'm very interested to find out.

Anyway. I've seen professionals sometimes doubt whether Schizoid personality is a valid disorder, per se. It can seem to be more a personality type (i.e. the traits involved may not necessarily be "disordered" on their own), which may be why we see such a close relation between it and INTP. Some of the smartest people in history (who were also INTPs) were considered "schizoid" because they occupied a remote end of the intelligence bell curve, says similarminds.com. Then again, I suppose the diagnosis is for when these traits become pathological, or interfere in unwanted ways with one's daily functioning. Still, it has never excessively bothered me that I have these traits; I've always seen them as just a part of my personality, much in the same way as I am an INTP. Are all/most INTPs Schizoid, though? I think there are at least some shared traits...maybe Schizoid can sometimes manifest as just an extreme of INTP-ness?