I'm not sure if this has already been said, but an idea crossed my mind today. I've heard the cliche, "if the brain cannot take an occasional holiday, it will take a mental one". Is something similar going on with SPD? Schizoids want to be alone, to be completely seperate from society and not have to interact with other people at all. Though quite often it's just not possible or practical. If one wants to be alone (physically) but can't do so, perhaps the response is for the brain to shut down mentally and emotionally. Or at least it could explain why schizoids feel the need to create these barriers.
Of course, it could be the other way around too - one feels mentally and emotionally disconnected from the outside world, and this causes them to feel uncomfortable around other people, and ultimately to crave solitude. I'm not sure. I see the cause and effect working both ways.
AC