DID or Dissociative Identity Disorder (formally known as MPD or multiple personality disorder, "split personalities", all outdated terms) is a dissociative disorder where a victim's brain forms atleast one other seperate identity/personality state(s) to help cope with severe complex trauma before the ages of 7-9. These personality states are often now referred to as alters or parts.
Often, victims of the disorder would have little to no memory of these traumatic events as it is a way for their brain to help cope with it. You cannot get this disorder when you're older than 7-9 as it often forms in very early childhood.
Victims with the disorder typically go through repeated trauma and/or multiple events that may cause this to form. For instance, children who endured terrifying child abuse at a very young age are more prominent to have it. Of course, it all varies. Everyone's trauma is different.
Despite this, this disorder is rare. There's under 200,000 cases of it yearly, and according to statistics only 2-4% of the world's population may have DID. That's the same amount of people who may have red hair!
Symptoms include: - derealization/depersonalization - dissociation - age regression - depression - dissociative amnesia/large gaps of memory - identity confusion - confusion - impulsiveness - self-harm or suicidal thoughts/actions - intrusive thoughts - hearing voices - out of character behaviour
DID is a very complex and misunderstood disorder, and these people aren't evil.