AmandaLouise, criterion 'a' in the DSM-IV is "The presence of two or more distinct identities or personality states (each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self." It says nothing about a 'host' or 'original' personality at all. It doesn't talk about a host person +1 or more alters, nor an original personality invaded by other personalities, but a person with *more than one* enduring identity or personality state. Why practitioners in New York have chosen to interpret the DSM-IV to mean a 'host' personality with coexisting alters that I could not say. From the information given in the DSM-IV I don't see how they could make that assumption. Do you have any insight?
You are right that the structural theory of dissociation is just that - a theory. You are also probably correct about it being 'bounced around by college majors doing papers'. There are several high level researchers (Ellert Nijinhuis, Kathy Steele and Onno van der Hart to name a few) who devote their research to learning about and understanding dissociative processes, and I think it is a very good thing for any college major to examine the work of these respected researchers in any papers they need to write for their own studies. Increasing knowledge and awareness in the professional community can only be beneficial for all people with dissociative disorders.
There are several other theories about dissociation that are currently being studied and examined too. What is common to all of these emerging theories is that they are contributing to our growing understanding of dissociation and they are reforming previous ideas about it.
The treatment guidelines given on the ISST-D website (for those who don't know this is the International Society for the Study of Trauma and Dissociation, where all the latest research and knowledge from experts all over the world is pooled) state that
"The DID patient is a single person who experiences himself or herself as having separate alternate identities that have relative psychological autonomy from one another. At various times these subjective identities may take executive control of the person's body and behavior and/or influence his or her experience and bevavior from within. Taken together, all of the alternate identities make up the identity of the human being with DID."
Again, they don't suggest there is a 'host' with other personalities living within him or her (although I know only too well that to to the identity states at the front that is what it really feels like) but that all identity states are parts of the same original whole. I personally have never seen any scholarly literature that suggests there IS a 'host', although I know that for many years this was thought to be the case. All the outcomes of current research and study point to the 'presenting identity state' being a part of the whole like any other, and that is why it is presented as such both in the DSM-IV and in international guidelines for treatment of people with DID. It can sound scary to the DID person, but it really isn't - and when considered in terms of overall theory it actually makes a ton of sense.

Why practitioners in New York have chosen to interpret the DSM-IV to mean a 'host' personality with coexisting alters that I could not say. From the information given in the DSM-IV I don't see how they could make that assumption. Do you have any insight?