Multiple Personality Disorder (MPD), or as it is now known, Dissociative Identity Disorder (DID), is quite simply, a situation in which more than one person appears to live within a single body. At different times these alternative, or ‘alter’, personalities take control of the body and live their lives.
Within a single body, the different personalities may differ in their age, their gender, and in their attitudes and knowledge of life. For example, in the body of an adult woman one alter may describe themselves to be a nine year old girl, and another might describe themselves to be a grown man.
In some situations one personality may be aware of what happens when another personality takes control of the body. But in other situations they have no idea what has happened.
DID is almost always associated with trauma in childhood. This is often in the form of sexual abuse, but this is not always the case. It may be linked to other things such as physical abuse, extreme neglect, confinement, and the witnessing of someone being killed.