I was browsing books on DID in Amazon. I came upon a bunch of books about how our Intelligence Agency experimented with DID around the time I was a teenager. I don't know how long it went on or whether they are still doing it.
They called it MKUltra or something like that. They had several names for the effort. It was supposedly a big deal. Their goal was to be able to produce DID in some people who would then have an alter (alternate personality) not even known to them. The goal was to have such a person respond to a secret "trigger" not even known consciously to them. Upon receiving that signal, they were supposed to murder some targeted person. Then they would switch back to their previous self and not even know they did anything. They called the person the Manchurian Candidate.
They actually recruited some important psychologists to help with the effort. They expended considerable resources and even tried various combinations of drugs to try to enhance the effort. They actually experimented with real people in these efforts (according to the books I looked at).
In order to produce a Manchurian candidate, they had to use various combinations of torture. Some people believe that DID can only be effectively produced in a small child and after that it becomes quite difficult. That agrees with what I have read. The people I know of who have DID all had it produced in them when they were small children.
It seems that when we are very young and subjected to severe abuse, the personality can split off. I think it's something like when a small child has the ability to make up an imaginary friend. And then it progresses. When the abuse comes, they become the other child to absorb the abuse. When it's over, they revert to the previous personality. They have amnesia toward the abusive event. They don't want to know about it. They don't want to have it in their memory. The more the abuse happens, the stronger this mechanism becomes. If it involves physical torture beyond sexual abuse then the DID structure can become more and more elaborate. It becomes the main coping mechanism of the child and then later the adult. Some people (rarely) can have over 100 alters or alternate personalities. Most of these would be "children" or even fragments. A fragment consists of an alter who didn't get "lived in" very much and so they aren't very well developed as personalities.
The adult person who has such DID sometimes realizes they need therapy to undo the damage done to their personality. They still have that same mechanism they had as a small child. If they were subjected to horrible abuse, the child alter may not even mature. That is what happened to me. I had a 12-year-old alter named Buzzy. He (I) remained 12 for a long, long time. When they are put in a tight spot or under a lot of stress, they tend to actually create another personality or alter to handle the situation. DID becomes their main coping mechanism. To undo this, the T (therapist) has to actually work with them to teach them new coping skills. They have to "meet" the wounded alters. When they begin to realize what's going on then the different alters can become unified and the person moves toward being an integrated personality.
The story is told in the book and movie, Sybil
In the excellent book: A Fractured Mind
, by Robert B. Oxnam, he describes in a most interesting way how he was sitting in his psychiatrist's office and he suddenly became an 11-year-old boy, Bobby. As his therapy progressed, his T found that he had a number of other "alters". It is most interesting. He never gets very much into the abuse that produced this condition in himself - just enough to tell the reader the outline of what happened. So the book isn't about abuse, it's about the therapy and the various alters that were produced in one very intelligent man and how he dealt with life with having that condition.