This post by Redeemed explains the definitions of DID, MPD and how the mechanism works.

DISCLAIMER: I am not a professional psychologist, psychiatrist, or therapist in any form. The definitions and descriptions that follow represent my understanding of these disorders based on years of reading and being given verbal definitions and descriptions by professionals.

Dissociative Identity Disorder (DID) used to be known as Multiple Personality Disorder (MPD). You may be wondering, why change the name of a disorder after it has become widely recognized by its first name?

I suspect there were more reasons than the one given, but since that is purely speculation, there isn’t much point in expressing those. The reason given is really adequate in itself.

You see, in the professional field of psychiatry, there are classifications of psychiatric disorders and illnesses (illnesses are abnormalities that that can be inherited and have a clear list of symptoms stemming from one source, where as disorders are disabling conditions characterized by a group of several common symptoms that are recognized as being present in a patient but the causes of those symptoms can be unrelated to each other).

One of those classifications is Personality Disorders (including Schizophrenia) and another classification is Dissociative Disorders (including Post Traumatic Stress Disorder or PTSD and Dissociative Identity Disorder or DID).

You see, the process that causes there to be more than one distinct and complete personality in one body is known as dissociation.

Everybody dissociates. It’s a necessary life skill.

You know that everybody has an Intelligence Quota or IQ, and each person’s IQ is determined by a combination of potential present at birth (or before) & opportunities and experiences that either develop or hinder that potential.

In a similar way, everybody is born with an aptitude for dissociation and the level of dissociation a person eventually develops depends on experiences that either enhance or hinder that aptitude. You know that there is a range or scale of intelligence that a person’s IQ can be charted on. In a similar way there is a range of dissociation where different disorders can be charted on. DID falls on the highest extreme of the dissociation scale, with PTSD coming a little lower on the scale.
When you were learning to walk, you fell down often, and that hurt. A caring adult would pick you up, sooth your boo-boo with a kiss or gentle touch, and then distract you by getting you to ignore your pain and focus on something else. That ability to ignore/forget/and in doing so minimize the level of pain you feel is dissociation.

Later, when you were a teen, lets say you had a boyfriend or girlfriend and the two of you enjoyed an intensely passionate relationship for a long time. Then your boyfriend/girlfriend breaks up with you and you are devastated. At home in the privacy of your bedroom the emotional pain you feel is intense. You cry until you can hardly breath and you have no interest in anything else. But at school or at work you function as if nothing was wrong. That’s using dissociation.

If you have a highly developed skill of dissociation, it’s possible no one would ever know by looking at you or your performance that anything was wrong at all. If it’s not so well developed, people who were paying attention would see that something wasn’t quite right and they might ask you about it. As long as you are not focused on it, you don’t feel the pains so intensely; but the moment someone asks you what’s wrong the pain comes back with a vengeance. Get the idea?

When people who are adults experience a sudden trauma like a car accident, they will often have the sensation of floating above their body and looking down on it. While they are floating above their body they don’t feel the pain or limitations that are experienced in the body. Sometimes they can’t remember anything that happened for a period of time just prior to or immediately following the accident. Other times they clearly remember small details about what happened to them while they were unconscious and can even remember conversations that took place between the people who came to help them even though they were completely unconscious at the time. These are all evidences of a high level of dissociation, but it isn’t a disorder.

If a tragedy occurs to an adult that happens over a period of time – like going to war, they can develop episodes when dissociation starts unexpectedly, triggered by any of a number of things, and suddenly their minds think they are back in that traumatic experience again and they experience all the terror and confusion and even body sensations that were present when they were in the midst of their traumatic experience. These are called flashbacks and they are a symptom of dissociation that can be very disabling to the ones who experience them. It’s common for people diagnosed with DID or PTSD to experience flashbacks, but a person doesn’t have to have one of these two disorders to experience them.

If a child who has not yet developed a strong sense of identity is exposed repeatedly to severe traumas, and if this child has a high IQ, a high aptitude for dissociation, and a very creative mind, they may develop different personalities to cope with the traumas they live with.

It is not an intentional act or a strategy that is planned out, any more than a person in an accident can strategize how to float above their body, yet it is a mechanism utilized to survive and cope with a traumatic event. When a child experiences traumas at a young age, it can severely impair their ability to develop a normal healthy psyche.

In order to protect the psyche of the child and preserve his/her health, a personality is developed who absorbs the full impact of the traumas on the child’s mind, emotions, will, and body. That means the new or alter (alternative) personality, has to have a completely separate self. They absorb the full impact of the trauma so that the birth personality can grow and mature unaffected by the traumas.