Why do people experience "MULTIPLE PERSONALITIES"?

The disturbance is not due to the direct psychological effects of a substance or of a general medical condition. Why some people develop DID is not entirely understood, but they frequently report having experienced severe physical and sexual abuse, especially during childhood. Though the accuracy of such reports is disputed, they are often confirmed by objective evidence. Individuals with DID may also have post-traumatic symptoms (nightmares, flashbacks, and startle responses) or Post-traumatic Stress Disorder. Several studies suggest that DID is more common among close biological relatives of persons who also have the disorder than in the general population. As this once rarely reported disorder has grown more common, the diagnosis has become controversial. Some believe that because DID patients are highly suggestible, their symptoms are at least partly iatrogenic, that is, prompted by their therapists' probing. Brain imaging studies, however, have corroborated identity transitions.

you may want to research " overshadowing "

Overshadowing occurs when a person feels that their actions are being affected by an outside influence. It is very common for poets, artists, musicians and others to say that their work had been given to them by other sources.

there are two different types of overshadowing

one is almost just like inspiration

the other is more like possession ( although not in the demonic sense )

I recommend reading this book for answers to many of the questions you have asked

" Archives of the Mind " by professor Archie Roy

http://www.survivalafterdeath.org/reviews/aroy.htm

Trauma causes some people to develop another personality to deal with its effects. In this growing hectic and dangerous world we live in will almost certainly lead to an increase in multiple personality disorders.

I believe a person forms these other personalities under extreme duress to protect themselves. For instance a child being molested may block out what is happening by going inside themselves and making believe it isn't happening. Our psyches are vulnerable to manipulation.

Human nature.

i believe they are brought on by trauma, and that it the minds way to cope. we create an alternate personality to deal with what is going on so we can lie to ourselves and say that it was the other who experienced it. Ever see the movie Sybil(based on true events)?

marriage will cause that