First of all, this still assumes that DID is the underlying cause of your observations. Also, we need not talk about history when we have diagnoses of DID in the present. The Merck Manual says:
Source
I'm not sure i necessarily agree with Merck that the number is that high. My guess would be closer to half of that. I do know several people, however, that show classic characteristics of severe DID; only one of whom has ever seen a therapist. I've also noticed that DID and PTSD diagnoses seem to be more or less concurrent.
The disorder appears to have been first described as a discrete class of phenomena during and after WWI, and was part of the DSM-1, classed as a "hysterical personality disorder". The earliest clear description was in Germany in 1791.
So why don't we see this in Japanese or Australian psychiatric hospitals?
We don't?
A brief Google search turned up:
http://www.sidran.org/refs/ref4.html
http://www.blackwell-synergy.com/doi/abs/10.1046/j.1440-1819.1998.00394.x
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=10084343&dopt=Abstract
http://www.siari.co.uk/Dissociation...ociative-identity-disorder-in-adults-2005.htm
http://www.empty-memories.nl/abstra...www.fortea.us/english/psiquiatria/history.htm
Many historical descriptions of "demon" or "spirit" possession (particularly many of the Greek examples) bear strong resemblance to currently DID diagnoses. And it's highly likely that some "true" mediums, ie. those who were not intentionally deceptive charlatans, may have been DID sufferers as well.
It's generally accepted by DID researchers that many cases have been incorrectly diagnosed as schizophrenia; often "atypical" schizophrenia, in part because of the lack of response to normal treatments. The key difference now know is the lack of "affect" abnormalities characteristic of schizophrenia.
There is no such known physical etiology for schizophrenia. There are no physical tests for schizophrenia. We cannot even confirm that the diagnostic category of "schizophrenic" includes only one condition instead of many.
Sorry, that was poor wording. While the actual mechanism itself is still incompletely understood; the evidence for a physical etiology and genetic predisposition is clear. Particularly since schizophrenic and schizotypal disorders follow a distinct onset and development pattern, and significant changes in cerebral functioning have been measured. Unfortunately, I don't have an online source, and all of my print references are in storage. (I really need to get a bigger apartment so I can have all this crap closer to hand.)
But PET scans appear to show distinct functional abnormalities in the limbic system and frontal lobes. Should the actual mechanism be determined, that would provide a clear way to separate schizoid disorders from dissociative ones.
http://www.studentbmj.com/issues/00/12/news/443a.php